Stamped enclosures and chassis parts: design rules

A stamped enclosure is usually lost at the assembly bench, not in the press. The panel that bows 0.8 mm across its diagonal, the earth stud that reads 12 Ω through a cured powder film, the flange that will not sit flat on a gasket: each one is a forming or edge decision that never reached the drawing.

Stamped enclosures and chassis parts carry three decisions that a folded sheet metal box does not. Whether the shell can be formed in one stroke, how the earth path survives the coating, and what happens to the cut edge after it is trimmed. Get those three right and the tool pays for itself. Get one wrong and the part is reworked at final inspection, or worse, installed and returned.

Can the shell be formed in one press stroke?

Yes, when the drawn depth stays under about 0.6 times the shorter side of the box and the wall radius is at least 1.5 times the sheet thickness. Past that ratio the metal cannot feed from the flange fast enough, the wall thins at the punch nose, and the part needs a redraw station.

Test the ratio before anything else. A 400 × 300 mm footprint with a 120 mm wall gives 120 ÷ 300 = 0.4, which is a comfortable single draw. The same footprint at 200 mm deep gives 0.67 and needs two stations: a shallow first draw to about 110 mm and a second draw to depth. That second station is the difference between a single-stage tool and a transfer or progressive tool, and it is most of the tooling quote.

Corner radii follow the same logic. An inside corner at 1.5t on 1.6 mm sheet, so 2.4 mm, draws without tearing in CRCA D grade. A 90° corner with a 0.5 mm radius concentrates strain at the corner and splits on the third or fourth thousand parts, after the tool has been signed off.

What blanking tonnage does the shell pull?

Blanking force is blank perimeter times thickness times shear strength. For CRCA at 400 MPa tensile, shear sits near 300 MPa. A 400 × 300 × 120 mm enclosure in 1.6 mm sheet develops to a 640 × 540 mm blank, a perimeter of 2,360 mm. That gives 2,360 × 1.6 × 300 = 1,133 kN, which is 115 tonne-force of pure cutting.

Add 25% for stripping, friction and press losses and the job asks for 1,416 kN, or 144 tonne-force. A 100 T press is not enough at that number and a 160 T press is. The blank also weighs 4.34 kg, which decides how the part is handled between stations as much as it decides the press.

Piercing changes the answer more than drawings usually admit. Twenty-four 6 mm holes have a cut length of 24 × π × 6 = 452 mm, and at 1.6 mm and 300 MPa that is 217 kN added to the same stroke. The blank and the holes together are 1,350 kN before the stripping allowance, which pushes the same part from a 160 T to a 250 T press. The press tonnage method runs the same arithmetic with your own perimeter and thickness.

What limits apply to stamped enclosures and chassis parts?

Sheet thickness governs draw, blank and coating in that order, so it belongs at the top of the table. The values below are shop practice for CRCA and mild steel; stainless and aluminium move the shear strength and the springback rather than the geometry rules.

The column that gets argued about is flatness. A drawn shell releases residual stress when it is trimmed, and a 400 mm panel that was flat in the tool can bow 0.5 mm after the trim station. Either the drawing tolerates that, or the tool carries a restrike and the price carries the restrike. State which one before the quote is compared.

Why does an earth stud fail continuity after powder coating?

Because cured powder is a dielectric and 60–80 µm of it sits between the stud and the panel. A stud welded before coating is coated with everything else, and a continuity test at 10 A reads open, not 0.1 Ω. The part is dimensionally perfect and electrically useless.

There are four ways to fix it, and only one of them is free:

• Weld the stud before coating and mask a pad around it, at least the diameter of the washer plus 5 mm. This is the standard answer for a chassis that carries a protective earth.
• Fit the stud after coating as a clinch or rivet nut, with the coating removed locally. It moves work to the assembly line and adds a part number.
• Use an internal-tooth washer that cuts through the film. It works on 40 µm and becomes unreliable at 80 µm, so it is a fallback rather than a design.
• Press a dedicated earth boss with a through-hole and a bare annulus, formed in the same tool as the shell.

Whichever route is chosen, the drawing needs the resistance limit and the test current, because both sides of 0.1 Ω are defensible without them. Material pairing matters as well: a zinc-plated steel stud in an aluminium panel is a galvanic couple that corrodes in a damp plant room, and a stainless stud with a plated washer removes the argument.

What edge condition does a cut enclosure need?

A trimmed edge needs a limit, not a habit. Sheared edges on an enclosure typically carry a burr of 0.05–0.15 mm and a rollover of about 0.3 mm on the cut face, and both are invisible under a coat until the part is installed. Specify a maximum burr height of 0.10 mm, state the burr side, and add a 0.2–0.3 mm radius where the part is handled, gasketted or slid into a rail.

The cut face has no coating of its own. Powder wraps a rounded edge and bridges a square one, so a sharp corner is where a blister starts in a wet plant room. On an enclosure that sees condensation, either deburr and radius the edge or move the panel to a galvanised or pre-coated blank, and in both cases name the salt spray test to ISO 9227 and its duration. Without a number, two suppliers can both claim salt spray testing and mean 96 hours and 480 hours.

Hole position relative to the edge is the other edge rule. A hole closer to a trimmed edge than about 1.5 times the thickness distorts the edge on piercing and leaves a thin sliver that the tool cannot cut cleanly. The hole-to-bend and edge distance rules explain the geometry behind both limits.

How does a chassis part differ from an enclosure shell?

A chassis part is usually a channel rather than a box, and the tonnage moves from the blank to the sequence. Take a 900 × 150 × 80 mm U-channel in 3 mm CRCA. The developed blank is 900 × 310 mm, so 279,000 mm² and 6.57 kg at 3 mm. The blank perimeter of 2,420 mm gives 2,420 × 3 × 300 = 2,178 kN, or 222 tonne-force, and 2,722 kN with the stripping allowance. That is a 300 T press, at the top of the range rather than in the middle of it.

Add sixteen 10 mm holes and the same stroke cannot carry them. The hole cut length is 16 × π × 10 = 503 mm, which at 3 mm and 300 MPa is 453 kN, taking the combined load to 2,630 kN, or 268 tonne-force before allowance. With 25% added the job asks for 3,287 kN and no press in the shop can do it in one stroke. The route then splits: pierce on the 250 T punching press, form on the 300 T, and the quotation says two operations rather than one.

Chassis parts also carry a camber limit that enclosures do not. A 900 mm channel formed from coil has a natural bow from residual stress, and 1 mm over the length is a normal commercial limit while 0.5 mm needs a straightening pass or a restrike. That number belongs on the drawing next to the hole pattern, because a rail that will not sit flat on a frame is a weld fit-up problem before it is a dimensional one.

What does assembly add after the press?

Fasteners and joints are decided at the drawing stage, because every one of them changes the tool or the coating sequence. A clinch nut in 1.5 mm sheet needs a hole and a press stroke at the right setting; a weld nut needs a projection weld schedule and survives the oven; a tapped hole in 1.5 mm sheet holds about three threads and strips at the torque a maintenance fitter applies.

• Clinch fasteners suit 1.0–2.5 mm sheet and are placed before coating. Specify the fastener standard and the sheet thickness on the drawing, not just the thread size, because the same M6 nut has different body lengths for 1.0 mm and 2.0 mm sheet.
• Spot welds on a coated enclosure are made in the white and touched in afterwards. Spacing below 25 mm on 1.2 mm sheet shunts current between electrodes and produces a cold weld that passes a visual check and fails a peel test.
• Weld distortion pulls a flat panel into a dish. Sequence welds from the centre outward and hold the panel in a fixture; on a 400 mm cover, two parallel seams at the ends will bow the middle by 0.4–0.8 mm without one.
• Masking is a process step with a cost. Every masked pad, stud and thread adds handling at the paint shop and a re-mask on touch-up, so consolidate earth points to one pad per enclosure where the electrical design allows it.

A stamped part that arrives at assembly with the right fasteners already in place removes a workstation from the line. That is the argument for buying the enclosure as a welded and coated assembly rather than as a painted shell plus a bag of hardware.

How is this run at Ganesh Multitech?

Enclosures and chassis parts are pressed on mechanical presses from 20 T to 300 T with 250 T punching, in single-stage and progressive tools designed and built in the in-house tool room, with checking fixtures made alongside the tool. Formed tolerance is ±0.5 mm typical, tighter on critical features by agreement, and the custom-part range runs 0.8–40 mm. First article inspection and PPAP documentation are available when the programme needs them.

Coating follows the same route as every other pressed assembly: seven-tank pretreatment, a 16 ft × 16 ft shot blast chamber, an 18 ft × 14 ft electrostatic powder booth and a wet paint booth with a curing oven. Dry film thickness and cross-hatch adhesion are measured and recorded, which is what turns the earth-path note on the drawing into evidence on the inspection sheet.

The company has manufactured since 1992 and now runs five units across Mumbai (Vasai) and Nagpur under ISO 9001:2015 and ISO 14001:2015. Customers include Mahindra, Mahindra Powerol, Godrej & Boyce, TKE, Eros Elevators, Minda, Gunnebo and Indian Railways, and the sheet metal supply to Mahindra's Farm Division carries an Annual Commodity Award.

The material side of the same part is covered in the CRCA stamping guide for OEM brackets and panels, and the fold-only alternative in the box bending guide for electrical enclosures. Both use the same flat pattern logic as a stamped shell.

What belongs on the enclosure drawing?

• Material and grade, for example IS 513 D at 1.6 mm, not the word CRCA alone.
• Thickness with its tolerance class, because a 0.10 mm step moves blank force by 6%.
• Blank outline, wall radii and depth, with the depth ratio stated if it is near 0.6.
• The critical assembly features: mounting hole pattern, stud positions, door hinge line, gasket land width.
• Earth path: pad diameter, mask requirement, stud material, resistance limit and test current.
• Edge condition: maximum burr, burr side, radius size, and whether deburring is included.
• Coating system: pretreatment, powder or paint, colour, dry film thickness range, adhesion test.
• Salt spray method and hours if the enclosure is exposed.
• General tolerance class from ISO 2768-1 at the title block, plus any tighter geometric callout.
• Annual volume and release pattern, because the tool decision depends on both.

When stamping is the wrong route for an enclosure

Stamping is wrong below roughly 300–500 assemblies a year, where a press tool cannot repay its cost and the same drawing is better served by laser cutting plus bending on the 200 T, 4,450 mm brake. It is also wrong for a shell deeper than about 0.8 times the shorter side, for a blank whose perimeter and hole pattern together exceed 2,600 kN, and for a part whose flatness requirement is tighter than 0.2 mm per 100 mm without a restrike.

We do not galvanise in-house, we do not run presses above 300 T, and we do not heat treat after forming. Where the drawing needs any of the three, the route changes and the quotation should say so rather than quietly absorb it. Send the current revision and the volume through the RFQ form and the stamping and press tools hub covers the tool and tonnage rules that sit behind the quote.