GB2357339A

GB2357339A - Measuring the depth of a workpiece formation comprising accounting for shortening of the forming tool

Info

Publication number: GB2357339A
Application number: GB9929732A
Authority: GB
Inventors: Edward Christopher Williams
Original assignee: Leamount Ltd
Current assignee: Leamount Ltd
Priority date: 1999-12-16
Filing date: 1999-12-16
Publication date: 2001-06-20
Anticipated expiration: 2019-12-16
Also published as: GB2357339B; GB9929732D0

Abstract

An apparatus for forming a bore or other formation in a workpiece 21 comprises a forming tool in the form of an electrode 14. The position at which the electrode 14 contacts the surface of the workpiece 21 is determined prior to forming the workpiece 21. Once the workpiece 21 has been formed to a desired depth, the position at which the electrode 14 contacts a fresh point surface of the workpiece 21 is determined. The difference between the first and second position readings provides an indication of the degree of shortening of the electrode 14 during the formation procedure. A reliable verification of the actual depth of the formation can be determined by subtracting the amount by which the electrode 14 has shortened from the desired formation depth.

Description

2357339 Workpiece Forming This invention relates to forming workpieces

with apertures, bores and surface hollows and more particularly to a method and apparatus for verifying that the same have been formed adequately in the workpiece.

It is well known to form a sequence of apertures, bores or surface hollows etc. in a workpiece using a forming tool by automated processes such as mechanical machining (e.g.

drilling), electrical machining (e.g. electrical discharge machining (EDM)), electro-chemical machining (e.g. capillary drilling), or a combination process.

In such processes it is often essential to verify that the apertures etc. have been correctly formed. It is a well known problem that inadequate apertures are formed if the mechanical drill or EDM electrode has broken or worn during successive operations.

High speed tubular EDM electrodes used in the manufacture of aircraft engine combustion chambers wear substantially in use. obviously, in aircraft engine manufacture it is an essential quality requirement to verify that cooling apertures formed comply to design specifications. Hitherto, such verification requirements have been met by mechanical measurement, manual inspection or by automated visual inspection. It will be appreciated that these techniques add to manufacture time, increase cost and increase the time before a problem is found.

We have now devised a method of verifying the formation of an aperture etc. in a workpiece which alleviates the above mentioned problems.

In accordance with this invention there is provided a method of verifying. a formation formed in a workpiece, the method comprising the steps of positioning a forming tool in contact with the surface of the workpiece to be formed, recording the position at which the tool contacts the surface 2 of the workpiece, forming the workpiece to a desired depth by advancing the tool towards the workpiece or vice-versa, removing the tool from the workpiece, repositioning the tool in contact with a new point on the surface of the workpiece to be formed, recording the position at which the tool now contacts the surface of the workpiece, computing the actual depth of the formation formed by determining the difference between the first and second recorded positions and subtracting the difference from the desired depth of the formation.

The difference between the first and second readings provides an indication of the degree of shortening of the tool during the formation procedure. Thus, the actual depth of the formation can be determined by subtracting the amount by which the tool has shortened from the desired formation depth. The current invention therefore provides a highly reliable verification of the actual depth of the formation that is formed without significant extension of cycle times and at substantially reduced cost compared with alternative methods.

It is known to dress the tip of the tool between subsequent forming operations and it will be appreciated that this can have the effect of shortening the tool. Accordingly, the amount by which the tool is shortened during dressing is preferably determined and deducted from the difference between the first and second recorded positions.

The surface of the workpiece may not be level or flat and thus the tool may be arranged to advance or retract between subsequent forming operations. Accordingly, the amount by which the tool is advanced or retracted between subsequent forming operations is deducted from the difference between the first and second recorded positions.

Preferably the formation is formed by electrode discharge machining (EDM). The point at which contact is made between the tool and the surface of the workpiece is preferably determined by detecting the position at which a spark first occurs as the tool approaches the workpiece.

3 Also in accordance with this invention, there is provided an apparatus for forming a workpiece, the apparatus comprising a forming tool, means for moving the tool relative to the workpiece to be formed or vice-versa and means for recording the position of the tool, wherein the recording means is arranged to record the position of the tool, once the moving means has positioned the tool in contact with the surface of the workpiece, and wherein the recording means is arranged to record the position of the tool again, once the moving means has positioned the tool in contact with a new point on the surface of the workpiece following formation of the workpiece to a desired depth, the apparatus further comprising means for computing the actual depth of the formation formed by determining the difference between the first and second recorded positions and subtracting the difference from the desired depth of the formation.

Preferably, the apparatus is an electrical discharge machining (EDM) apparatus, the tool being an electrode.

Preferably, the apparatus further comprises means for sensing the position of the tool.

An embodiment of the present invention will now be described by way of an example only and with reference to the accompanying drawings, in which:

Figure 1 is a sectional view through an electro discharge machining apparatus in accordance with this invention; and Figures 2a-2g are schematic diagrams to illustrate the method by which the apparatus of Figure 1 verifies a formation machined in a workpiece.

Referring to Figure 1 of the drawings, there is shown an electro-discharge machining (EDM) apparatus for forming a single point aperture in a metal workpiece.

The apparatus comprises a L-shaped body portion 10 to which an EDM electrode tool assembly 11 is mounted for vertical movement. The tool assembly 11 is comprises a support 12 4 slidably mounted in a rail 13 attached to the side of the L shaped body portion 10 of the apparatus. An internally threaded aperture is formed in the support 12, through which a vertical ly- extending externally threaded shaft 14 extends. The upper end of the shaft 14 is connected to a motor 15 which drives the shaft 14 about its vertical axis and thereby raises or lowers the tool assembly 11 according to the direction of rotation. The motor rotation is measured by rotary encoder 16, from which the linear motion of the tool assembly 11 can be determined by knowledge of the thread pitch of the shaft 14.

The tool assembly 11 comprises an elongate tubular electrode 17 which extends vertically upwards through an electrode pressure seal 18 and into an electrode storage housing 19. The electrode storage housing 19 typically provides a pressurised water feed to the tubular electrode as part of the well known EDM technology.

The lower end of the electrode 17 extends through a contact clamp 25 which provides electrical power to the electrode 17 and clamps the electrode against vertical movement relative to the assembly 11. The electrode 17 extends through a second clamp 20 fixed to the L-shaped body portion 10 of the apparatus. The clamp 20 can be actuated to grip the electrode 17 whilst the contact clamp 19 is released and the assembly 11 moved upwards, so that further amounts of the electrode 17 can be extended from the electrode storage housing 19, in order to compensate for wear.

The lower end of the electrode 17 is aligned above a workpiece 21 on a fixture 22, which is typically mounted to a machine base 23 by a horizontal slide (not shown) so that the workpiece 21 can be moved horizontally under the electrode 17.

The electrode 17 is supported and guided by a nose guide 24.

Referring to Figures 2a-g of the drawings, in use the workpiece 21 is moved so it is at drill position 1 and presented at the correct entry angle and stand-off from the workpiece. The EDM dress cycle is then performed to remove any taper from the tip of the electrode 17 of to form the tip to a desired profile: this is achieved by energising the electrode and advancing it towards a template (not shown) or the workpiece 21. The length of electrode removed is determined from rotational data obtained from the rotary encoder 16 and stored in a memory.

The electrode 17 is advanced until it contacts the surface of the work piece 21. The slide position is recorded as EDM start position 1. At this position the drill depth position is set to zero. Next, the EDM cycle is performed to drill an aperture of correct form to the desired depth. During EDM drilling there is a certain amount of wear of the electrode 17 which reduces its length and also forms a taper at its tip.

Therefore, normally the final depth is set a distance beyond the desired depth to compensate for the normal wear and taper allowance expected. The settings are usually determined by prior experimentation to produce a data sheet for the operation designating all the EDM process parameters necessary to specify a repeatable process. The final depth is called the set depth.

Following drilling, the EDM electrode is retracted and the workpiece moved to drill position 2. The EDM dress cycle is then performed again to shape the tip of the electrode 17 to the desired profile. The length of electrode removed is recorded as Dress Amount 2.

The electrode 17 is advanced until it contacts the surface of the work piece 21. The slide position is recorded as EDM Start Position 2. At this position the drill depth position is again set to zero.

The actual electrode wear can now be calculated as:

(EDM Start position 2 - EDM Start Position 1 - Dress Amount 2) The depth drilled can now be calculated as:

(Set Depth - Actual Electrode Wear at Position 1) 6 If desired, appropriate action can be taken at this point if the wear is above a designated limit. For example, the drill may be moved back to position 1 and the process repeated, an alarm may be given or the error recorded prior to the next aperture being drilled. If the wear is in limits the above steps are repeated for each subsequent drill position.

The actual electrode wear at drill position j (where j = position 1, 2,..., (n-1)) is thus calculated as:

EDM Start Position (j+l) - EDM Start position j - Dress Amount (j+l) The actual depth drilled at drill position j (where j = position 1, 2,..., (n-1)) is thus calculated as:

(Set Depth - Actual Electrode Wear at Position J) The last position n can be checked by indexing to an final position and the calculations. Once the results and positions detailed above have obtained they may optionally be saved in the system memory, printed, transferred to an external system by electronic or other means for use in statistical analysis systems SPC Quality reporting etc..

A specific requirement of this method is to detect the first electrode contact with the work piece. This may use proprietary techniques such as first spark detection, or other techniques such as capacitance, inductive pickup, increase in flow of electrical current (e.g.. during capillary drilling) force or torque (e.g.. mechanical drilling) The technique is particularly simple in the case of single point drilling, but is also applicable to multi point EDM tools by detecting the onset of first spark for each electrode in turn. This can be done irrespective of if a reverse dress cycle is needed or if straight into a drill. If a dress cycle is performed the individual amounts removed can be determined by detecting the positions of first spark for 7 each electrpde being dressed in turn.

The method and apparatus in accordance with this invention enable an extremely reliable test to be performed that all apertures have been formed to the desired depth with no extension of process cycle times. The present invention is far more reliable than vision inspection techniques which have severe problems achieving accurate viewing down tapered holes etc.. It cannot on its own guarantee that taper errors are eliminated, but combined with other known techniques such as detecting fast electrode advancement during breakthrough in piercing operations.it can be reliably verified that taper is satisfactory.

The method can be applied to mechanical drills to detect a broken or damage drill. Variants allow a check that is a capillary drill comb is not damage d by detecting the positions at which current begins to flow through an electrode.

The method can be used to generate data to allow the Drill Depth Positions at which EDM Process Settings are changed to be adaptively modified. Usually during an EDM drill cycle the EDM Process Settings are adjusted with Drill Depth Position in order to optimise process and cycle times. In particular, the settings during breakthrough are usually very important.

Adjustments can also be made to optimise electrode run through and prevent back wall contact when drilling into cavities or other obstructions. The method does not require apertures to be uniformly arranged or the required depth or material thickness to be identical at each drill position, although these would need to be accounted for in the calculation procedures.

The method can be generalised to account for several electrode dressing operations at intermediate stages in a drill cycle.

A wide variety of methods can be used for providing the EDM slide positions including using the rotary encoder 16 or using a separate position sensor.

8 In a typical embodiment of an EDM machine where the CNC functions are provided by a Delta Tau Controller fitted to an PC the software could be: a Delta Tau CNC program (s); a PC resident program (s); or any combination of these. The software can be integrated with the main EDM control software or be stand alone - possibly performing a post process/down load operation to standard part programs.

It is not necessary for the measurements and calculations to be performed in software. A suitable electronic circuit of discrete devices and/or integrated circuits such as:

up/down counters; registers; latches; etc. can readily be devised to perform the some or most of the came functions, albeit with less flexibility.

9

Claims

1) A method of verifying a formation formed in a workpiece, the method comprising the steps of positioning a forming tool in contact with the surface of the workpiece to be formed, recording the position at which the tool contacts the surface of the workpiece, forming the workpiece to a desired depth by advancing the tool towards the workpiece or viceversa, removing the tool from the workpiece, repositioning the tool in contact with a new point on the surface of the workpiece to be formed, recording the position at which the tool now contacts the surface of the workpiece, computing the actual depth of the formation formed by determining the difference between the first and second recorded positions and subtracting the difference from the desired depth of the formation.

2) A method according to claim 1, wherein the formation is formed by electrode discharge machining.

3) A method according to claim 2, wherein the point at which contact is made between the tool and the surface of the workpiece is determined by detecting the position at which a spark first occurs as the tool approaches the workpiece.

4) Apparatus for forming a workpiece, the apparatus comprising a forming tool, means for moving the tool relative to the workpiece to be formed or viceversa and means for recording the position of the tool, wherein the recording means is arranged to record the position of the tool, once the moving means has positioned the tool in contact with the surface of the workpiece, and wherein the recording means is arranged to record the position of the tool again, once the moving means has positioned the tool in contact with a new point on the surface of the workpiece following formation of the workpiece to a desired depth, the apparatus further comprising means for computing the actual depth of the formation formed by determining the difference between the first and second recorded position and subtracting the difference from the desired depth of the formation.

5) Apparatus according to claim 4, wherein the apparatus is an electrical discharge machining (EDM) apparatus, and wherein the tool is an electrode.

6) Apparatus according to claim 4 or claim 5, wherein the apparatus further comprises means for sensing the position of the tool.

7) A method of verifying a formation formed in a workpiece substantially as herein described with reference to the accompanying drawings.

8) Apparatus for forming a workpiece substantially as herein described with reference to the accompanying drawings.