IES78832B2 - Manufacture of metal casings - Google Patents

Abstract

Casings are produced by interaction (10-12) of a CAM controller (20) with a CNC machine 21, deburring and linishing (13), casing assembly (14), surface treatment (15), and coating (16). Bends for a casing part (40) which are closer together than 6 mm are formed simultaneously by a tool (35). Surface treatment involves alkaline degreasing, rinse, iron phosphate degreasing, rinsing, warm water rinsing and overhead drying. This allows particularly effective powder coating. There is mechanical removal of powder between adjoined casing part side edges. This avoids the "wrap around" effect.

Description

The invention relates to high-precision manufacture of metal casings such as containers for switching equipment or panels for computer equipment.

One object of the invention is to provide for manufacture of casings in which there is a very comprehensive, uniform and durable paint coating. A further object is that the coating should be of a quality whereby the integrity is maintained during the life of the casing both on flat surfaces and at joints such as where side edges of a cover and a container meet. Another object of the invention is to provide this level of quality in an efficient manner.

According to the invention, there is provided a method of manufacturing metal casings comprising the steps of:recording design drawing data and converting said data to punch press machine commands and transmitting the commands to a punch press machine; the punch press machine punching a workpiece to produce a set of flat casing parts; performing bending and assembly operations on the parts to produce un-coated casings, in which bends in casing parts which are closer together than 6mm are formed in a single press operation using a die and press tool having corresponding tapered shoulders to allow simultaneous double bend formation^ surface treating the casings by degreasing in an alkaline solution at a temperature of 60 to 70°C and pH of 11.3 to 11.8, rinsing in a water bath, and subsequently drying the casings; - 2 S 78 8 3 2 coating the casings by:applying a charge to each casing; spraying an epoxy polyester powder which is oppositely charged onto the casing; removing particles between adjoined casing part side edges by mechanical separation; and curing the casings at a temperature in the range of 185° to 190°C for a time period of 8 to 10 minutes; and inspecting the coated casings.

In one embodiment, the method comprises the further step of mounting a mask behind apertures of the casing, the mask being separated by a distance of 0.5 to 2.0mm behind the apertures.

In one embodiment, the mask separation is approximately 0.5mm.

Preferably, the surface treatment step comprises the additional sub-step of dipping in an iron phosphate degreasing bath and subsequently rinsing before drying.

In another embodiment, the iron phosphate degreasing bath temperature is 60° to 70°C.

In a further embodiment, the time period for alkaline cleansing is 4.5 to 5.5 mins and for iron phosphate degreasing is 3.5 to 4.5 mins.

Ideally, the drying is performed by dipping in a warm water bath and subsequently dipping.

Preferably, the water temperature is in the range of 45°C to 55°C.

In one embodiment, the method comprises the further step of deburring and linishing the casing parts after operation of the press punch machine.

In another embodiment, the method comprises the further step of pin insertion or welding during casing assembly.

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:Fig. 1 is a flow diagram illustrating a manufacturing process of the invention; Fig. 2 shows production of a flat casing part; Figs. 3(a) and 3(b) show bending of casing parts for assembly of a casing; Fig. 4 shows surface treatment of casings; and Fig. 5 illustrates powder coating and curing of a casing.

Referring to the drawings there is shown a manufacturing process 1 for production of metal casings. The process 1 has an initial step 10 which is recordal of design drawings by a computer automated manufacturing (CAM) controller 20, which controls a CNC punch press 21. This step involves inputting drawings using a graphical interface and automatic conversion of the drawing commands to CNC machine commands. The step of converting the drawing commands to CNC commands is indicated by the step 11 of Fig. 1 and in step 12 the punch press 21 operates to produce casing parts 27. The punch press 21 comprises a clamp 22 which clamps a flat sheet metal workpiece 23 which overlies tables 24 which provide the Y direction of motion, the clamp 22 providing the X direction. Punch tools 25 are mounted on a rotating head 26 which is automatically controlled according to the received CNC commands .

The punch press 21 punches out the casing parts according to the relevant design using a minimum margin of 5mm in the workpiece 23 for optimum material usage.

The casing parts 27 are then de-burred and linished using a grinding wheel and a linishing machine, respectively. It has been found that by linishing after deburring a very high quality finish is achieved both to give high edge accuracy and also material handling safety. The linishing machine has an abrasive belt operating at a feed rate of 3 to 5 m/min, and 5m/min has been found to be particularly effective.

As shown in Figs. 3(a) and 3(b), for assembly of a casing the casing parts 27 are fabricated to the required shape using a press 30. The press 30 has a lower die 31 above which there is a press tool 32 which provides a rightangle bend in conventional manner according to the V configuration of the tool and die. However, a problem arises when using such an arrangement where bends must be close together. As shown in the diagram there must be a minimum gap between right-angled bends of half of the ν'block dimension (V/2) plus a minimum margin (M) of 2mm, plus the thickness (t) of the sheet at each side. For example, if the V-block dimension is 6mm and the material thickness is 1mm, then the minimum separation between bends must be 7mm. It is not possible to reduce this minimum gap by reducing the size of the V-block because this must have a dimension of at least six times the sheet thickness, namely 6mm. Accordingly, major problems arise where the design requires gaps which are less than 7mm apart such as for provision of a shallow groove in a casing part 40 shown in Fig. 3(b). This problem is solved in the invention by use of a press tool 35 and a die 36. In the die 36, there is a square” shoulder 37 in conventional manner. However, the other side of the V” has a right-angled bend at a side edge 38 from which extends a tapered shoulder 39 which is parallel to the corresponding side of the V of the press tool 35. The press tool 35 is shaped to correspond to the die 36 and has a tapered shoulder 39. It has been found that this tool and die arrangement is particularly suitable for forming a double bend with small separations, such as 5mm. It also considerably improves efficiency as set-up time and quality control time are considerably reduced.

Bending of the sheet metal is indicated by the step 14(a) and assembly of the casing may be completed by various sub-steps such as pin insertion 14(b), drilling/tapping 14(c), and welding 14(d).

An important aspect of the process is the step 15 which is treating of the surfaces of the casings to ensure that they may be coated with a durable coating which has both colour and thickness uniformity throughout. The surface treatment plant is indicated by the numeral 50 in Fig. 4 and it comprises the following stations:alkaline cleansing 51, cool overflow rinse 52, iron phosphate degreasing 53 cool overflow rinse 54, and warm rinse 55.

Casings 57 are transported by a conveyor system 59 between the various stations to treat the surfaces. In more detail, the alkaline cleansing station 51 contains an alkaline solution maintained at a temperature in the range of 60° to 70°C. The solution is a powder formulation of alkaline salts, phosphate and surfactants dissolved at 200gms/litre. It has been found that a pH of 11.3 to 11.8 and preferably 11.5 is particularly effective. The cold overflow rinse station 52 contains water at room temperature. The iron phosphate degreasing station 53 contains a water based formulation of phosphates, surface active agents, and activators at 60 to 70°C. It has a density of 1140 to 1150gms/litre. The station 54 contains rinsing water at room temperature. The station 55 comprises a warm water bath at 45 to 55°C, and preferably 50°C.

For surface treatment of the casings 57 they are dipped in the alkaline cleansing bath 51 for a time period of 4.5 to 5.5 mins. The rinsing time for the station 52 is 30 seconds. The dip time for the iron phosphate degreasing bath 53 is 3.5 to 4.5 mins and the rinse time in the bath 54 is 45 seconds.

The casings are dipped for 45 to 90 seconds and suspended overhead for 2 to 3 minutes to drip dry.

It has been found that this sequence of surface treatment steps is particularly effective.

Following surface treatment, the casings are coated using a powder coating system 70 shown in Fig. 5. The system 70 comprises a rail 71 from which the casings are suspended. Powder is applied using a spray gun 72 which has an air supply line, a powder supply line, and a low voltage cable. The powder is an epoxy polyester powder which is charged positively, while the casing is charged negatively. In order to achieve the optimum charging of the powder, there is manual adjustment of automatic feedback current to adjust electrostatic charge. This is important for penetrating Faraday cage areas and for uniform powder deposition across the casing. Excess gun current results in lower transfer efficiency, reduced finish quality and back-ionisation. The latter limits attainable film thickness, inhibits recoating, and has a detrimental effect on the powder's ability to penetrate Faraday cage areas .

Assembly of the casings involved mounting of a mask plate 60 directly behind any panel 61 of the casing which has apertures 62, as shown in Fig. 5. In this drawing, powder is indicated by the arrows 63. An important aspect of this step is the fact that the mask plate is mounted 0.5mm to 2.0 mm behind the panel. The preferred separation is 0.5 mm. This ensures that during the subsequent coating steps the coating does not bridge the panel and the mask plate, thus causing feathered edges to arise after removal of the mask plate. It also helps to ensure that there is negligible ingress of powder into the casing. The mask plate is removed after coating at a final assembly operation, immediately before inspection.

As also shown in Fig. 5, immediately after application of the powder, joints such as the joint 81 in the casing 80 are prepared by longitudinal movement of a tool 82 along the joint to mechanically remove powder between the side edges at the joint. While this feature is very simple to implement, it has been found that it is particularly important at ensuring that the wrap-around effect does not occur whereby the coating bridges the two surfaces and thereby leaves a feathered edge and possibly particle deposition when the casings parts are separated, such as opening a panel which is attached to a main casing body. This is a very important feature at achieving the necessary high quality finished end product because such edges are unsightly and can also result in deposition of powder which can affect electrical circuit performance in the field. Because this problem is avoided in such a simple manner, efficiency of the process is maintained.

Finally, the casings are cured in an oven 90 on racks 91 at a temperature in the range 185° to 190°C for a period of approximately ten minutes. It has been found that this is a particularly suitable setting to achieve a tough and durable coating.

It will be appreciated that the invention provides for production of casings in a very simple manner in which quality of the end product both in visual and functional terms is of a very high standard. This is very important commercially. The interaction of the CAM controller and the CNC machine provides for very efficient part production. The particular degreasing steps have been found to be very effective for the powder coating steps and an excellent quality finish is consistently achieved.

The invention is not limited to the embodiments hereinbefore described, but may be varied in construction and detail within the scope of the claims.

Claims (5)

1. A method of manufacturing metal casings comprising the steps of:recording design drawing data and converting said data to punch press machine commands and transmitting the commands to a punch press machine; the punch press machine punching a workpiece to produce a set of flat casing parts; performing bending and assembly operations on the parts to produce un-coated casings in which bends in casing parts which are closer together than 6mm are formed in a single press operation using a die and press tool having corresponding tapered shoulders to allow simultaneous double bend formation; surface treating the casings by degreasing in an alkaline solution at a temperature of 60 to 70°C dnd pH of 11.3 to 11.8, rinsing in a water bath, and subsequently drying the casings; coating the casings by:applying a charge to each casing; spraying an epoxy polyester powder which is oppositely charged onto the casing; removing particles between adjoined casing part side edges by mechanical separation; and curing the casings at a temperature in the range of 185° to 190°C for a time period of 8 to 10 minutes; and inspecting the coated casings.

2. A method as claimed in claim 1 comprising the further step of mounting a mask behind apertures of the casing, the mask being separated by a distance of 0.5 to 2.0mm behind the apertures, and wherein the mask separation is approximately 0.5mm, and wherein the surface treatment step comprises the additional substep of dipping in an iron phosphate degreasing bath and subsequently rinsing before drying, and wherein the iron phosphate degreasing bath temperature is 60° to 70°C, and wherein the time period for alkaline cleansing is 4.5 to 5.5 mins and for iron phosphate degreasing is 3.5 to 4.5 mins, and wherein the drying is performed by dipping in a warm water bath and subsequently dipping, and wherein the water temperature is in the range of 45°C to 55°C.

3. A method as claimed in any preceding claim comprising the further step of pin insertion or welding during casing assembly.

4. A method substantially as hereinbefore described, with referenced to and as illustrated in accompanying drawings .

5. Metal casings whenever produced by a method as claimed in any preceding claim. CRUICKSHANK& CO.,