NO169666B - METHOD OF CHEMICAL MILLING OF METALS - Google Patents

Description

The invention relates to an improved automated method for chemical milling of metals and metal constructions. Chemical milling is widely used in the aerospace industry to remove excess metal from metal parts when the removed metal is not of significant importance to the strength of the component part. In the chemical milling process, a number of masking and metal removal steps are normally used. The metal is removed in an etching bath which can be alkaline or acidic depending on the metal or alloy being etched. Chemical milling can be used to produce one-piece structures with skin and load-bearing ribs or stiffeners that provide lightweight alternatives to traditional airframe and beam structures.

Within the state of the art, chemical milling has been used to reduce the weight of metal parts intended for use in aviation or space travel for over twenty years. Chemical milling is widely used to increase the strength-to-weight ratio for component parts in aircraft airframes. Chemical milling traditionally involves the steps of masking and chemically milling a metallic workpiece, and this sequence can be repeated several times to further change the shape of the workpiece.

In US patent 4137118 a method for chemical etching of an efficient lightweight structure is described, where excess metal is removed to form the ribs and skin for an aircraft structure. The etching step is repeated to successively undercut and cause the ribs to take on an "I" or "T" shape and to reduce the thickness of the skin.

In US patent 3745079, a method for chemical etching of titanium alloy<g> for use as structural parts in an aircraft is described.

In US patent 2888335, a method for chemically etching a workpiece is described, and a method is described for sequentially etching the workpiece with several cuts in the masking material to form a series of etched elevations in the workpiece. Between each etching bath, part of the mask is removed so that the final product has an etching pattern with varying depth throughout the workpiece.

In US patent 338863, a masking material for use in chemical etching is described, where the masking material consists of a styrene/butadiene block copolymer. This material is widely used in chemical milling processes to mask the part to be protected in the etching bath.

Currently, the known methods include the step of marking the aluminum or titanium starting material with a reference mark or a tool hole for transporting the starting material through the etching solution. The metal part is then covered or coated with the butadiene/styrene copolymer masking material. A template is placed over the masking material, and the masked material is hand-cut along the template line. The masking material is then marked with a marking pen along the cut or score line. These steps are repeated with separate meshes when the aim is to achieve separate etching or milling levels. If it is desired to use several etching baths, the mesh cut line is again covered with a sealing material. After all stencil markings have been cut out and the secondary cuts have been coated, one part of the masking material is removed. The metal plate is then etched and rinsed in rinse water in a counter current. The second mesh area is removed, and the workpiece remains on

newly immersed in the etching bath. The workpiece is rinsed again, and the process is repeated for the desired number of etching steps. At the end of the etching step, the workpiece is "degreased". A typical "degreasing" agent is described in US patent 3988254.

There are two problems with the known method of chemical milling, and these problems are solved by means of the present invention.

The problems are:

a) control of the cut depth through the masking material. Currently, the mask is drawn by hand by experts. If

the cut is too deep, the cut will allow the etchant to etch into the metal and undercut the mask. If the incision is too shallow and the mesh material is not completely split, the mesh material will "blow up", form blisters or tear into pieces when that part of the mesh is removed. This

necessitates a time-consuming repair of the stencil mask.

If, moreover, the<*>"bloat" is not detected, the workpiece will be undercut by the etching material. b) The time taken to place each stencil on the workpiece, to mark each line to be cut,

and to average each line by hand, is essential. A typical workpiece of 0.9 x 1.2 m requires 6-8 hours of work by hand to hand mark and cut each of the areas to be etched. The automated method according to the invention can perform the same marking and sectioning within eleven minutes. In addition, it can perform cuts that cannot be performed by hand.

If the chemical milling is carried out for a three-dimensional workpiece, all of the above-mentioned problems are accentuated. Moreover, it is necessary to preform the metal part around a master mold in a forming or die-punching step to achieve the desired three-dimensional shape.

Each of the stencils must be provided with the appropriate compensation

for three-dimensional positioning. In the former practice, the metal sheets, after being previously formed into an approximately three-dimensional shape, are stapled to a master mold, and the individual stencils were also stapled to provide intimate contact between the stencil and the workpiece. In addition, the three-dimensional shape of the workpiece makes it even more difficult to accurately cut the stencil to the desired depth.

The present invention comprises the new application of two existing devices which have hitherto been used for other purposes.

In the areas of drafting and mapping, large computer-operated drafting machines have been used to mark blueprints, copies and to score plastic stencils intended for use in photo reproduction processes. These machines are quite large and have a drawing area that can be 2.4 m wide and 10.2 m long. A motorized carriage passes across the drawing table in both the x and y dimensions and supports a number of markers. One such device is the "Kongsberg 180OS Series Flatbed Drafting Table". This drawing table can be equipped with a number of drawing tools, including a tangentially controlled drawing tool. This tool uses a single knife or chisel and is normally used to cut and remove material used in the photo reproduction of integrated circuits. The knives are used to scratch coated films.

US patent 3555950 relates to a device for automatically cutting a photomask for use in the manufacture of integrated circuits. In this device, the aluminum foil is cut and the plastic laminate material is retained to define an optically transparent negative for the production of an integrated circuit board.

Computer-controlled cutting devices have been widely used in the clothing industry to cut or cut one or more fabric sheets to a desired pattern size. These devices then also have drive motors to move a cutting tool in the x and y directions. Examples of computer-controlled cutting devices are described in US patents 3803960, 3805650, 3895358, 3991636 and 4171657.

Although the above-mentioned devices have been used for computer-controlled cutting of cloth and photomasks within the relevant technique, they are . not been used or used for the chemical milling process. The chemical milling process has remained essentially unchanged for over twenty years. The computer-controlled drawing tables with a flat surface and cutting tables have been around for over ten years. As far as the patent applicants are aware, these devices have not been used in the chemical milling process, and their use in this area offers significant advantages both in terms of speed and accuracy.

The above devices, although they are suitable for use in connection with chemical milling of flat starting materials, are not suitable for use in chemical milling processes for three-dimensional workpieces. For three-dimensional milling, the tangentially controlled scoring tool is mounted on a robotic device that can be computer controlled via the x, y, z dimensions to provide an accurate scoring depth as the robotic device passes over the three-dimensional contoured surface. A robotic device that can be modified for use in the chemical milling process is manufactured by ASEA, Inc. and is described in ASEA brochure YB 11-101 E.

Both the ASEA robotic device and the drawing board from Kongsberg are able to pass over an existing template to derive a series of point-by-point measurements along the perimeter defined by the template. These point-by-point measurements can be recorded and stored on magnetic tape. These point-by-point measurements can then be used for

to scribe the mesh-covered workpiece with the tangentially controlled scribing tool which is arranged on the Kongsberg plotting machine or the ASEA robot.

Another means of providing the instructions to control the movements of the robotic device or flatbed drafting device is to provide a new template geometry on a CRT via an existing computer program currently sold under the trademark "CADAM". This program will define the x and y coordinate values for the newly created mask before these are digitized and stored on magnetic tape.

The present invention relates to an automated chemical milling process for metals, and the method is characterized by the features indicated in the characterizing part of claim 1.

The present method includes digital automation and the use of two separate and existing devices for an application area within which they have not previously been used. The present invention includes the steps that the metal to be milled or etched is coated with an etchant-resistant mask, such as a mask of a styrene copolymer. The perimeter of the area to be etched is then digitized to define a series of point definitions along the x, y axis for the flat base metal or via a set of x, y, z axes for three-dimensional workpieces. If more than one area is to be etched by a step-by-step etching process, the values for all coordinates are defined and stored on a magnetic storage medium, such as a magnetic tape, a disk drive device or a bubble memory device.

Method includes the step of automatically scoring the metal and the coating with a scoring tool, the scoring tool scoring through the resist coating (coating agent coating) along the perimeter defined by the x, y coordinate values or by the x, y, z coordinate values for three-dimensional workpieces. After each of the mask areas has been scratched, the scratch lines are marked so that they become visible and make it easier for the operator to remove areas of the resist coating before the etching step. If the workpiece is to be subjected to step-by-step etching,

the step-by-step incised lines are coated again with a temporary sealant to prevent the etching bath from penetrating the crack line.

After the resist coating has been removed from the area or areas to be etched, the workpiece is immersed in an etching solution for a predetermined time to remove a predetermined amount of metal from the uncoated area or areas. After the etching step has been completed, the etching bath is neutralized with a countercurrent water bath. If sequential etching steps are used, the second resist coating covering the second area to be etched is removed, and the partially coated metal is then re-immersed in the etching solution for a predetermined time. The following sequence of steps is then carried out until the workpiece has been etched so that it has obtained its final shape. After the last etching bath, the workpiece is "de-soiled" to remove the layer of metal residue that has remained after the etching bath.

The present invention includes digitizing existing meshes to provide x,y coordinate values for each of the perimeter lines defined by an existing mesh in terms of two- or three-dimensional workpieces. A new template geometry (template geometry) can alternatively be created on a CRT (display unit) which is connected to a CPU (central unit) with the appropriate software. One software that is particularly suitable for creating a new template geometry is the CADAM software which is a commercially available licensed software package.

In the present invention, an electronically controlled scoring tool is used which exerts a predetermined pressure against a cutting knife which engages with the resist coating and the base metal to be etched. The cutting pressure can be regulated very precisely so that an even and accurate cutting depth is obtained through the resist coating and 0.0254 mm into the workpiece.

The present invention can also be used in conjunction with "nesting" software to etch a number of metal parts from a single large piece of metal starting material, where each of the parts is etched simultaneously by the etch bath. An example of a type of software capable of "nesting" the parts on a sheet metal is the CAMSCO software which is a commercially available licensed software package.

The present invention therefore provides an automated chemical milling process for chemical chamfering of metal or metal parts.

The advantages of the present invention will be more easily understood by the person skilled in the art from the following detailed description of the many preferred embodiments of the invention, considered in connection with the accompanying drawings on which like elements are given the same reference number, and of which

Fig. 1 shows a schematic flow chart for the method according to the invention, Fig. 2 schematically shows the tangentially guided scoring tool, the resist coating and a metal workpiece to be etched, Fig. 3 shows a cross-section through the metal workpiece shown in Fig. 2, after the etching step,

Fig. 4 shows a cross-section and a schematic elevation

which shows the step-by-step etching process,

Fig. 5 shows a curve for the depth of the cut made with the tangentially guided scoring tool, in relation to different "power settings", Fig. 6 shows a perspective view of the carriage assembly which carries the tangentially guided scoring device, the markers and the recoating device to seal previously made scratches, Fig. 7 shows a perspective sketch of the drawing board with a flat surface supplied by Kongsberg Våpenfabrikk A/S, Fig. 8 schematically shows a drawing of the robotic device from ASEA and a three-dimensional workpiece, Fig. 9 schematically shows x, y, z and coordinate axes,

Fig. 10 is a partial cross-section and perspective view of

a part of a three-dimensional workpiece formed according to the present invention, and

Fig. 11 shows a perspective view showing the registration or tool holes which can be formed by means of the present method.

In Fig. 1, the automated chemical milling process according to the invention is shown in the form of a flow chart. The aluminum alloy or titanium alloy starting material 11 is first cut to the correct dimensions for the part to be produced or for a series of parts when a series of parts is to be produced from a single piece of starting material. If the starting material is a flat starting material, it is then transferred to a float coating process 12 where it is float coated with the styrene/butadiene copolymer resist mask sold under the trademark "Turco Mask 522". If the starting material is intended for use as a three-dimensional workpiece and is not provided in the form of a three-dimensional workpiece, the starting material is transferred to a contouring or punching step 13 in which the flat starting material is given a contour that corresponds to the desired three-dimensional shape.

After the starting material has been coated with the styrene/butadiene copolymer, it is transferred to a mesh cutting step 14. As indicated above in connection with the state of the art, the mesh cutting was performed by placing a template over the starting material and by marking the circumference of the template on the starting material. The marked perimeter is then cut by hand so that the cut goes through the resist coating and faintly marks the surface of the metal piece. The desired score depth into the metal workpiece is 0.0254 mm. Currently used military specifications for use in aircraft intended for procurement by the United States Government prescribe a narrow kerf into the workpiece of 0.1016 mm. If the cut is too deep, the subsequent etching step will undercut the resist coating, and if the cut is not deep enough, it will cause a "blowout" when the center of the template is removed for etching. The "blowing out" must then be retouched by hand, and this is a time-consuming and labor-intensive operation.

In the present invention, the chemical milling process is automated in one of three separate initial steps. As indicated at 15, an existing template can be placed

on top of the workpiece, and the template can be scanned optically. All of the original output markings, the workpiece size, the template count, and other desired information are fed into a memory along with the optical scan to provide the data necessary to align the cutting device at the appropriate point on the workpiece when the mesh cutting operation 14 begins. Instead of using an optical scanning device, a pen can be used to draw up the circumference of the template, while each of the reference points along the template is fed into the memory via the CRT used. For a simple, flat, two-dimensional workpiece, each of the straight lines can be registered by placing the stencil at the corner and locking the position data and section orientation. The stencil will then be moved to the end of the particular straight line, and the second lock point set of data will be entered. These steps will be repeated around the overall perimeter of the template until the desired area to be etched has been completely defined by the reference point data.

If the workpiece is to be subjected to an etching process carried out in sequence or in several stages, the template defining the second area to be etched will then be superimposed on the original reference marks, and the template will be scanned optically or drawn up with a pen to provide the keypoint data for the present template.

Simultaneously with an optical scan and the original key data steps, the applied CPU 16 will digitize the x, y coordinate values for each of the key points entered at 15. The digitization step is indicated as a separate step 17

in Fig. 1 as a number of methods for digitizing an existing template are available. Two methods for digitizing an existing template are described above, but a person skilled in this technical area will understand that a number of methods can be used to define the x, y coordinate values and to convert these into numerical form for use in a normal CPU 16 .

After the template has been converted into numerical expression, an impression is taken as indicated at 18. This impression can be made with a drawing table with a flat surface supplied by Kongsberg Våpenfabrikk A/S similar to the drawing table that will be used in connection with the mesh cutting operation 14 in a later stages of the process. Any impression device will be able to be used, provided that it is capable of producing a full size template that can be physically checked against the template that was entered via the optical scan or key data entry indicated at 15.

The input of the x, y, z coordinate values for the robot device is similar to that used to number existing templates for flat output materials.

As indicated above, it is common to begin with

to pre-form a flat stock to its desired three-dimensional shape, as indicated at 13. The pre-formed flat stock is then placed on a master mold which has a very precise contour to the desired final shape. A three-dimensional template is then placed over the starting material, and the resist coating is then cut.

As indicated above, it is even more difficult and time-consuming to cut the three-dimensional workpiece by hand than to cut a two-dimensional workpiece. A constant pressure must be applied to accurately draw up the contours of the workpiece to completely cut through the mesh and lightly score the metal surface without causing undercuts or blowouts.

When numbering an existing template for the robot device, a similar process is followed. The ASEA robot is provided with its own computer control system and is normally programmed using the "learning method" for which a robot tracking method is used to mark the stencil with a pen to "teach" the robot the desired contour to follow. The stencil attached to the robot is placed by hand

in the original reference starting position, and this key point data is input. This operation "teaches" the robot that this position should be taken with respect to the programmed operation for all future starting reference points. The numerical positioning values for all three axes and the necessary traversing speeds are then stored in the computer memory. The robot is then moved by hand along the template to the next position, and the second set of x, y, z coordinate values are entered. The second set is then stored in memory, and these steps are carried out until the perimeter of the template has been completely traversed by the stencil. For long, straight sections along a two-dimensional plane, only two entry points need to be entered. Along a complex curve or a three-dimensional curve that traverses all three x, y, z axes, it is necessary to enter each coordinate value that requires a shift in the robot's rotation axes. As soon as the entire perimeter of the three-dimensional template has been fed into the robot's memory, this can be transferred to a magnetic storage medium, such as a magnetic tape or a magnetic disk.

The reference point data is used in a simple way to achieve coordinated transformation in straight line position for the robot. The reference point entered using the key point data at 19 does not cause movement of the robot.

This simply defines the first, the second, that

third up to n points of the movement pattern to be traversed. The distance and direction to the next reference point is calculated between the reference point and is carried out in relation to the point in the room at which the robot is located before the next reference point is entered.

The ASEA robot will work on Sn in two ways. If point-to-point control is selected, each of the many axes that must be moved to reach a new point begins immediately, and the track path is not controlled. All axes will start in the same position, and each motor will stop when its driven part has reached its new programmed position.

The robot device is also provided with a separate instruction function, where the motor speed for each of the axes is selected so that all axes reach the new position at the same time. This is particularly useful when it comes to following contours formed by a series of closely related keyI data points along a complex x, y, z coordinate curve.

As indicated by dashed line 24, the keypoint data from the robot tracking of the existing template can be read out onto magnetic tape, as indicated at 23, or onto any other form of permanent magnetic storage media. When it is desired to initiate a production run, the magnetic tape created for this part to be produced is read back into the robot's memory for execution of the three-dimensional template.

A third method of forming meshes for the automated chemical milling process of the invention is shown at 20 and 21, where a new mesh geometry is formed on a CRT using a program entitled "CADAM". The CADAM program is commercially available, licensed software.

When the CADAM program is used, the part to be formed is displayed on the CRT used, and a number of key points are entered along the part to define the geometry of the new mesh. The key points are then entered by the operator into the CRT used to completely enclose the perimeter of the new template's geometry. If it is desired to carry out etching in sequence in an etching bath, each of the templates is developed by the operator at the CRT used. by entering the desired key points to define the x, y values for each of the points along the perimeter. The new template geometry is numbered as it is developed by the applied CPU 16, and if desired a printout can be made, as indicated at 18, to check the geometry of the mask against the starting material example or against an original true copy model of the part that may have been made in the model workshop.

If the method involves forming a number of parts from a single piece of starting material, a separate braiding subroutine 22 is used to stack the various template perimeters in the most efficient manner for the particular size and shape of the starting material. This

The "f lettings" subroutine indicated at 22 is a software program designated "CAMSCO".

The numbering of the present template, as indicated at 17, or the formation of the new template geometry, as indicated at 21, is stored in magnetic form in a temporary or permanent storage medium as it is formed. If it has been stored in a temporary memory, it is then transferred to a magnetic tape or disk, as indicated at 23, for future use in the manufacturing process. Since large aircraft manufacturers may have thousands of templates used for chemical etching of aircraft parts, it is desirable to have a permanent magnet plate, as indicated at 23. This plate can be used any time it is desired to begin a production run for a particular part or a series of parts.

As shown in Fig. 1, a separate CPU 25 has been shown for use in production line environments. For the apparatus according to the invention, CPU 16 is a computer that is normally used in drawing and construction departments, while CPU 25 is a separate computer that is used to operate the drawing table with a flat surface from Kongsberg Våpenfabrikk A/S. However, it will be possible to use the same computer for both functions if this is desired. The ASEA robot has an integrated CPU which can be used in line with the CPU 25 or as a replacement for this, if the template geometry has been developed via the "learning" function.

Instructions from the CPU 25 to the score control unit 26 use only the x and y axes for flat output materials. For the mentioned drawing table with a flat surface, a motor is used for the x-axis and a motor for the y-axis. These are the only two values that are numbered and used to control the positioning of the scoring tool. The scoring tool is controlled by an analogue signal developed by the scoring control unit 26, which will be described in more detail below.

When the method is used in connection with three-dimensional workpieces, the development of the command signals for the scratch control unit is significantly more complex. As indicated by the dashed line 2 4a, the magnetic tape: 23 formed by the robot device during the "learn" function can be reintroduced into the CPU controlling the robot device, and each of the rotational motions will be developed in the same sequence and order as they were " learned" by the device under key point tda take step 19.

The robotic device contains from 3 to 6 motors for traversing the x, y, z axes, while the flatbed plotter has two motors for traversing the x, y axes. New three-dimensional templates can be developed via a "CATIA" program, as indicated at 21, to obtain a new template geometry. "CATIA" is commercially available. A separate set of rotation command signals must also be developed, as indicated at 26, for each of the motors in the robotic device. This step controls the direction and speed of each of the motors necessary to traverse the x,y,z profile defined by the numbered x,y,z values at 21. This can be performed as a separate routine between the applied CPU and the robot controller or it may be developed at the time the new template geometry is developed by the applied CPU 16.

The robot control device, indicated at 27, performs the process of controlling the relative axes of rotation for each of the motors in the robot device.

It is clear from Fig. 1 that the scratch controls, indicated at 26, can be adapted to the three-dimensional mode of operation from the robot control 27 or to the two-dimensional mode from the Kongsberg table 28.

The slitting control 26 is capable of performing three separate operations for each of the pieces of the starting material 11 which are flow coated as indicated at 12. These three steps are the mesh cutting indicated at 14, the marking step 29 and the resealing step indicated at 30.

If a simple one-step etching process is used, the applied CPU 25 will drive the x, y motors of the Kongsberg table, as indicated at 28, while the score controller 26 regulates the pressure of the score knife and the orientation of the score blade during the mask score operation 14. Since the mask is somewhat springy, it is difficult to see the tear lines in the mask. After the mask has been cut, it is therefore common practice to mark the score line, as indicated at 29, with a marking device to make it easier for the workers removing parts of the mask to find the area to be removed. Since several thousand types of workpieces can pass through the production line for the etching bath 31 and each workpiece has its own special shape and each has its own number of marked templates, it is highly desirable to use the marking step, although this is not absolutely necessary, in order to achieve a flawless etching or chemical milling.

If several etching baths are desired for the work piece 11, the scratch guide 26 will then recoat all but one of the template lines that have been scratched during the mask scratch operation 14. Normally, the innermost template is then removed, as indicated at 32, and the work piece is then immersed in the etching bath 31. If several masks and several etching steps are used, the workpiece is recycled, as indicated at 32a, to remove the second mask and to be returned to the etching bath along 31a. After each of the areas has been etched on the work piece, it is then cleaned in a cleaning bath 2 3 and sent to a milling machine, as indicated at 34. The part separation step 34 can be used when a number of parts are formed on a single piece of starting material or when a part of the starting material is used for positioning the reference or line orientation holes that guide the workpiece through the various production steps to which it will be subjected.

As shown schematically in Fig. 2, a single knife 35 is used for the tangentially controlled scoring tool, which is held firmly in a central cylinder 36 by means of a set screw 37. The cylinder 36 can both rotate and move back and forth by means of an air bearing 37 which completely surrounds a rotary piston 38. The piston 38 reacts to two orthogonal stator windings 39 and 40 which in turn are connected to analog sine and cosine voltages respectively. 41 and 42 received from the scoring control 2 6 shown in Fig. 1. These voltages turn the rotation receiving piston 38 and thus the knife edge to ensure that the knife 35 will always have its normal cutting edge facing the material in step with changes in the tool's direction of movement. A third winding 43 is regulated by the operator via a potentiometer or another signal device 44 to control the downward pressure exerted by the piston 38 against the knife 35. The effect of this downward pressure will be described in more detail below with reference to Fig. 5 .

As shown in Fig. 2, the thickness of the aluminum or titanium workpiece 11 has been greatly exaggerated in relation to the size of the tangential scoring tool in order to show the relationship between the thickness of the starting material and the thickness of the flow-applied coating mask 12a. In practice, the aluminum or titanium workpiece stock has a thickness varying as shown by arrows "A" from 0.31 cm to 1.27 cm. The thickness 12a of the mask resist coating, which is indicated by the arrows "B" in Fig. 2, is approx. 0.2 5 4 µm. The reciprocating distance of the knife 35, indicated by arrows "C" in Fig. 2, is approx. 0.31 cm. It thus appears that even with a two-dimensional, flat starting material, the tangentially controlled scoring tool 14a is able to compensate for any waves or variations in the surface thickness of the starting material as the scoring tool scratches the surface.

The starting material 1a can be any metal suitable for etching or other chemical milling. According to the preferred embodiment, this is made of aluminum or of an aluminum alloy or of titanium or of a titanium alloy that is commonly used in the aircraft and aerospace industry. Such an aluminum alloy is generally referred to in the trade as 20-20-4 and satisfies a federal US specification entitled Q-A-250/4 or Q-A-250/5. One of the titanium alloys used according to the invention is referred to in the trade as a 6-6-2 titanium alloy and satisfies a rail specification with the title T-9046.

In addition to titanium, it contains 6 parts aluminium, 6 parts vanadium and 2 parts tin.

These metals are etched in separate etching baths though

it appears from the state of the art that a number of different etching solutions can be used. According to the preferred practice in carrying out the present invention, the aluminum alloys are etched in an alkaline sodium hydroxide bath, while the titanium alloys are etched in a hydrofluoric acid bath. Other desirable baths for aluminum may include caustic soda or potassium hydroxide, and another chemical bath that may be very beneficial for titanium is nitric/fluoric acid.

Although aluminum and its alloys and titanium and its alloys are the main metals used in accordance with the invention, it will be clear that any metal or material that can be etched with an oxidizing acid or with a strong alkaline solution can be used in accordance with the invention.

As shown in Fig. 3, the workpiece lia is shown on

Fig. 2 has been immersed in an etching bath to remove part of the metal by chemical milling. The distance "D", which represents the depth of the etchant cut, varies greatly depending on the material, the etching bath, the temperature and the time during which the material is immersed in the etchant. The parameters included in the etching steps, such as the concentration of active agent, the temperature or the etching rate etc, will vary from application to application.

According to the preferred embodiment, when aluminum and its alloys are etched in sodium hydroxide, 0.0203-0.0559 mm of material will be removed per minute immersion in the etching bath.

When titanium and its alloys are immersed in hydrofluoric acid, 0.0152-0.0305 mm will be removed per minute in the flux etching bath.

It should be noted that the entire workpiece is protected

of the masking material applied in step 12, except for the part to be chemically milled. The lower mesh 12c protects the underside or, in the case of an aircraft skin section, the outer side of the feed material 11a.

As shown in Fig. 4, a workpiece 11b has been exposed to a multi-etch bath. Several cuts were made at 14b, 14c and 14d through the protective mesh to delineate separate meshes 12a, 12b and 12c. After the slits 14a-14d were made, the slits 14c and 14d were again sealed with "TURCO" or the styrene/butadiene copolymer masking material as indicated at 30a and 30b. The original part 12a was removed by hand, as shown in Fig. 4, and the first metal layer was etched away, as indicated by the dashed line surrounding 31a. After this initial etching, the sealing material 38a" was removed, the cut mesh material indicated at 12d- was removed, and the workpiece was re-entered into the etching bath. During the second immersion, the metal indicated at 31b was removed as a result of the action of the etching bath.

After the desired amount of metal had been removed, the third section of cut mesh material 12e was removed, and the portions of metal indicated at 31c were removed during the third immersion in the etching bath. It thus appears that a number of possible different surface shapes can be milled using the multi-etch bath process shown in Fig. 4.

The curve shown in Fig. 5 shows the depth of the cut made with the knife blade 35 through the mask 12a and into the metal substrate 1a. The horizontal axis marked "force setting" indicates the signals emitted by the device which are written from the score control 26 shown in Fig. 1. Although this is normally set in advance by the operator at the time the incisions are made in the mask, the signal will be able to be supplied as part of the data processing signal which is stored on magnetic tape 23. As indicated by the slope of the curve 50 and 51, the pressure developed by the torsion piston 38 and the winding 43 is sufficient to easily cut through the mask, while it will only slightly scratch the feed material. As shown in Fig. 5, the maximum depth of cut through the aluminum feed material was 0.051 mm even at the highest "power setting" for the tangentially guided scoring tool. This is well within the 0.102 mm limit prescribed in currently applicable military specifications. As the titanium feed material is significantly harder than the aluminum feed material, the depth of cut into the titanium is even smaller than the depth of cut into the aluminium.

The crack control stage shown at 26 in Fig. 1 is more fully shown in Fig. 6 in the form of a crack control head 26a. The scoring guide head 26a is mounted on a flat layer drawing table.. from Kongsberg Våpenfabrikk A/S, as shown in Fig. 7. This special drawing table can be extended with sections of 1.5 m up to a maximum length of 10.5. The score guide 26a is supported by a gantry-like stand 50 for movement along the x-axis and by carriage devices 51 and 52 which move back and forth along guide rails 53 and 54 (not shown) which are mounted on either side of the flat-layer drawing table. A pair of high capacity DC servo motors are used for accurate x,y position positioning of the scribe guide 26a using rack and pinion drive mechanisms. The pinions for the carriage 51 and 52 are connected to each other via a single shaft and engage with racks mounted in the guide rails 5 3 and 54 for movement back and forth along the table's y-axis. A single pinion in the score guide 26a guides the score guide back and forth along a rack mounted in the portal stand 50. The surface flatness of the layer is - 0.75 mm

or well within 1/8 of the vertical reciprocating dimension c indicated in Fig. 2. The table's ripping speed can be as high as 42 m/min.

The tear guide 26a shown in Fig. 6 has the tangentially guided tear tool 14a and a pair of marking pens 55 and 56, one of which is uncovered for the sake of the design shown in Fig. 6. The tear guide 26a also contains a renewed sealing device 30a which is in conjunction with a supply of masking material, such as "TURCO", or any sealant compatible with the masking material and capable of withstanding the acidic or alkaline bath to which the masking material will be exposed during the multiple setting steps. The step 30 of resealing is carried out, by means of. 30a by means of a rolling ball 58/ a brush or another fluid dispensing device as required depending on the flowability properties of the material to be dispensed. For some applications according to the invention, it may be desirable to combine the sealing agent with an ink or another contrast pigment to achieve a single step both for marking and resealing the cut tear line. A series of compressed air devices 59, 60 and 61 pressurize the containers for sealant 57 and ink containers (not shown). The inks are then delivered to marking pens 55 and 56 by means of tubes 62 and 63 to mark the scored lines after the tangentially controlled scoring tool 14a has scored through the mask.

Electronic control of the scoring tool is achieved for the scoring control 26a via a hanging cable 64 which is shown in Fig. 7. Electronic control of each of the scoring, marking and resealing stages is achieved by means of a coupling piece 65 and a control line 66. The air supply for the tangential controlled scoring tool's air bearing is obtained via a line 67, while a line 68 provides a similar supply for the ball applicator device 30a which is used for renewed sealing of the cut score lines.

Although this flat layer drawing board from Kongsberg shown in Fig. 7 is able to move relatively quickly along the x and y axes, it will normally only do this when cutting straight. The software for the Kongsberg table at 28 of Fig. 1 provides the CPU 25 with a "look ahead" feature which enables the applied CPU to look ahead to the next block of instructions and to slow down the speed of the scoring tool 26a when a change of direction is implied. If e.g. the device is in the mesh cutting stage where the tangentially guided scoring tool cuts through the mesh cutting material 12a, any angle change for the knife blade 35 of more than 7° will lead to a vertical reciprocal movement of the piston 38 shown in Fig. 2. The movement of the scoring tool 26a becomes instantaneous punched, and the blade 35 is raised and repositioned in consideration of its new. angular orientation by means of signal transmitters 41 and 42 which are provided by the scoring control 26. It should be noted that the instructions from the CPU 25 to the Kongsberg table 2 8 to the scoring control 26 are in analogous form as regards the tangentially controlled scoring tool.

The reciprocating cutting feature of this device enables the operator of the device to introduce a very narrow knife point for the blade 35 and to cut very small diameter holes in the mesh material.

A round circular hole having a diameter as small as 6.35 mm can be easily cut in the mesh material using the present invention. Cuts with this radius size have so far been impossible to achieve by cutting by hand. Besides the fact that with the help of the present invention it is possible to make cuts or scratches that were previously impossible to make, with the present invention it is also possible to achieve scoring at high speed and with high precision and reproducibility. As indicated above, using the present invention, it took only 11 minutes to scribe and mark a 0.9 x 1.2 m workpiece with several parts, while for the same workpiece<e> 6-8 hours were required for to draw this by hand and mark this by hand.

The present invention also makes it possible to use the scoring guide 26a in connection with three-dimensional workpieces, as shown in Fig. 8-10. The slit guide 26a is mounted in a robotic device 60 for cutting, marking and masking the interior of a workpiece, as shown in Fig. 8.

On the model mold 70 is a workpiece 11f which has been shaped in advance so that it has been given a three-dimensional contour.

The workpiece 11f has also been coated with a coating agent 12a and is ready for scoring, marking and renewed sealing, as previously described with reference to

Fig. 2-4.

The robot device 60 is provided with a number of rotation axes, each of which facilitates the device in transporting the scribe guide 26 from a point along the x, y, z axes to another point along the x, y, z axes. The pedestal 71 rotates around a pedestal torque 72 which is described as 0. The second robot motion is the input and output torque of the lower arm 72 and is described by the angle The third axis of rotation prescribes the upward and downward movement of the arm 73 around the angle Y • The robot can also be provided with three separate motions for the wrist, although only two are shown in Fig. 8. The first wrist moment is a which is designated as a "wrist-joint flexion", and a second axis of rotation & which is designated as a wrist swing. These five axes of rotation enable the robot to walk from any x, y, z coordinate point to the next x, y, z coordinate point. As previously indicated with reference to Fig. 1, each of the servomotors responsible for the movement of the robot around each of the five axes shown in Fig. 8 can be programmed to run simultaneously so that all axes start at the same time and run with the same engine speed. Each motor will then stop when the part it is driving has reached its new programmed position. Alternatively, it should be mentioned that the robotic device manufactured by ASEA. has a function in the instruction that will give a motor speed for each axis to be selected so that all axes reach the new programmed position at the same time. This ensures a smooth contouring of three-dimensional work surfaces when the robot traverses a curve through three-dimensional space. Furthermore, as indicated above, the tangentially controlled scoring tool exerts a constant downward pressure against the scoring knife over a reciprocal area of 0.32 cm. This reciprocal movement of the knife ensures that the knife is kept in constant force engagement with the aluminum or titanium workpiece at all times during the various movements of the robot's arms. The robot has a position tolerance for all five axes of - 0.004 mm which is compensated by the reciprocal tolerance for the air bearing of 0.32 cm as indicated by "C" in Fig. 2.

Fig. 9 shows the traditional x, y, z axes which are normally used to span a three-dimensional space. A fourth axis is shown to indicate that any set of x, y, z axes may be used provided no two axes are parallel to each other. If a particular set of x, y, z axes is more efficient in terms of calculating the movement through the three-dimensional space taking into account a given surface part design, the x, y, z axes for this particular part can be changed so that a more efficient calculation of the movements from one x, y, z point coordinate value to another. Fig. 10 shows a part of an aircraft which has been chemically milled after the mask had been cut by a robotic device as shown in Fig. 8. As indicated, the aircraft skin 81 remains coated with the resist material on its lower surface during the entire milling operation. The ribs 82, 83 and 84 and the long girders 85, 86, 87 and 88 form one piece with the skin 81 so that a unitary construction is obtained which is light in weight and strong and free of any mechanical joint, rivets, screws or other fastening devices. The natural undercutting effect of the chemical milling process will give a natural T- or I-beam shape so that the metal skin 81 is made strong. Fig. 11 shows a further advantage of the present invention. A template outline has been applied to the metal part 11g which has been cut out and marked along 14a, 29a as shown in Fig. 1. A pair of tool holes 90 and 91 have been defined within the chemically milled areas to grip the part as it passes through the various stages of manufacture to which it will be subjected. The part llg is covered with a mesh material 12a outside the circumferential line defined by 14a, 29a. The circumferential line 14a, 29a indicated on part 11g is typical of a part design that can be shaped. An optional method of providing tool holes is shown by the dashed lines for part llh and tool holes 92 and 93. This type of registration or tool hole design can be used if a part design without holes that had been chemically milled to reduced thickness had been desired. . After completion of the chemical milling and removal of dirt, the flap part llh will be cut along the outer circumferential line 94 with a milling cutter or metal saw at part-

the separation process.

As described above, the present invention avoids laborious manual drawing of the mask on flat and three-dimensional parts intended for chemical milling. The present invention can eliminate the creation of new templates for new parts to be manufactured and will eliminate the use of existing templates for parts that have already been constructed. The present invention eliminates hand marking of multi-scored areas and chips and eliminates hand application of "TURCO" sealant to the chips to be used in step etching of the part.

The present invention provides a regulated depth of cut through the mask and into the metal part which is very accurate. The cutting can be carried out at high speeds and with high precision and high reproducibility. The tangentially controlled scribing tool makes it possible to cut curves on both flat and three-dimensional surfaces and to define holes and curves that have not been possible until now with hand scribing processes.

Claims (14)

1. Automated, chemical milling process for metals, characterized in that it includes the steps that (a) the metal to be milled is chemically coated (12) with a cover agent coating, (b) the area or areas to be chemically milled are digitized (17) in order to determine at least x- and y-coordinate values for the perimeter of the area or areas to be chemically milled, (c) the metal and coating are scored (25, 26) automatically with a scoring tool (14) that cuts through the coating along the perimeter determined by the x- and the y-coordinate values, (d) the masking agent coating is removed (32) from the area or areas to be chemically milled, and (e) the partially coated metal is immersed in an etching solution (31) for a predetermined time to remove a predetermined quantity of metal from the uncoated area or areas. 2. Milling process according to claim 1, characterized in that step (b) is carried out by determining a three-dimensional space with three separate x-, y- and z-axes, that the area or areas to be chemically milled are digitized (17) with x- , y and z point coordinate values, and that step (c) is performed by scoring (26, 27) the coating and the metal along one or more three-dimensional perimeters determined by said x, y and z point coordinate values. 3. Milling process according to claim 2 for three-dimensional metal parts, characterized in that it also includes the step of determining three or more rotation axes (26a) for the scoring tool, the scoring tool selecting one or more of the aforementioned rotation axes when it passes through the three-dimensional space determined by the aforementioned x-, y-, z -coordinate values. 4. Milling process according to claims 1-3, characterized in that it also includes the steps that (a) more than one area to be milled is chemically scored (14b, 14c, 14d) separately for each metal part, (b) the incised scoring lines (30a) , 30b) for all but one of the areas to be chemically milled, recoat, and (c) the masking agent coating is removed (12a) in sequence from each of the areas to be chemically milled, between separate further immersions in said etching solution, whereby each of the specific areas to be chemically milled is immersed in the solution for different cumulative periods of time. 5. Milling process according to claims 1-3, characterized in that it also includes the step that the circumference or circumferences of the area or areas that have been cut using the scoring tool is marked with a visible marking material that marks the aforementioned coating along the perimeter line or - the lines determined by the x, y coordinate values, as the marking takes place . before the masking agent coating is removed. 6. Milling process according to claim 4, characterized in that in the step for renewed coating a visible sealing agent is used to simultaneously re-seal and mark the incised crack lines. 7. Milling process according to claims 1-5, characterized in that it also includes the step that the perimeter lines for each of the areas to be chemically milled are "nested" (12a, 12d, 12, 12b) when more than one part is to be chemically milled from a single metal plate. 8. Milling process according to claims 1-6, characterized in that it also includes the step that a predetermined constant force is exerted against the scoring tool (35) in order to force the scoring tool into engagement with the aforementioned metal part (lia), the force being exerted perpendicular to a plane determined by at least two point values for each point passed by the risse tool. 9. Milling process according to claims 1-8, characterized in that it also includes scoring and chemical milling of registration marks (90, 91) in a metal plate. 10. Milling process according to claim 9, characterized in that tool set-up holes (90, 91) are then formed from the aforementioned registration marks. 11. Milling process according to claims 1-6, characterized in that it also includes the step of raising the scoring tool as soon as a line or curve described by a future set of point coordinate values differs from a line or curve drawn from a previous set of point coordinate values with more than 7°. 12. Milling process according to claims 1-11 for chemical milling of aluminum or its alloys, characterized in that an alkali metal hydroxide is used as the etching bath (31). 13. Milling process according to claims 1-11 for chemical milling of titanium or its alloys, characterized in that a hydrohalic acid is used as the etching bath (31). 14. Milling process according to claims 1-13, characterized in that a butadiene/styrene copolymer is used as a covering agent coating.