Spare parts and service: fitting the installed base

A replacement canopy door that will not close is rarely a manufacturing error. It is a specification error: the drawing dimensions the part, and the machine it bolts to was toleranced somewhere else, years earlier, by a different supplier. On a twelve-year-old genset enclosure the frame has been repaired, the hinge pillars have been re-drilled once, and the new panel is expected to land in a hole that no CAD model has ever described.

Spare parts and service work is decided by three numbers: the clearance in the fastener holes, the coating thickness on any bore the fastener passes through, and the angular tolerance on the flanges that set the gap. Everything else on the panel can be free. Once those three are controlled, a part that was never designed for service can still fit an installed base that has moved.

Why does a like-for-like spare part still fail to fit?

Because the installed base has moved while the drawing stood still. A part made to revision C in 2014 came off tooling that has since been re-ground, from coil at the top of the thickness band, coated to a film that has been repainted twice, and bolted to a frame that a fitter straightened in the field.

Each of those changes is small and each one is additive. Cold-reduced sheet at 1.6 mm carries a thickness band of roughly ±0.10–0.15 mm. Bend allowance scales with thickness, so with a 1.6 mm inside radius and a K-factor of 0.44, the allowance for one 90° bend is 4.87 mm at nominal and 4.98 mm at the top of the band. That is 0.11 mm per bend, and 0.44 mm across a four-bend panel before anything else is counted.

Then the coating adds its own layer to every mating surface, the weld pulls the frame, and the hole that was cut round in 2014 has been reamed oval by a bolt that was never quite tight. A service part that is geometrically correct and interface-blind will fight all of it.

Separate the interface from the free profile

A service part drawing should mark six to ten features as interface and leave the rest alone. The interface is what has to land on the machine: the hole pattern, the mating face, the hinge centreline, the latch strike position, any bore that carries a shaft, and the earth stud. The free profile is the skin, the returns, the stiffening beads and the cosmetic edge.

This split changes what the supplier has to control. A fully toleranced panel asks the shop to hold every dimension at once, which means a fixture, a checked first-off and a slow inspection on a single unit. An interface-controlled panel asks the shop to hold the pattern and the mating face, cut the rest to general tolerance, and inspect the features that matter. The cutting cost is identical; the fit rate is not.

The general tolerance on the free profile should be stated rather than assumed. ISO 2768-1 class m covers linear dimensions without an individual tolerance indication at ±0.3 mm from 30 to 120 mm, ±0.5 mm from 120 to 400 mm and ±0.8 mm from 400 to 1,000 mm, with angular tolerances of ±0°30′ on the shorter side of a feature between 10 and 50 mm. Those values are looser than the ±0.5 mm a laser-cut and press-braked part normally holds, which is the point: the general tolerance is a floor, not a target.

The tolerance budget on a service panel

The table below is the budget for a coated steel service panel that mounts on four fasteners and carries a hinged door. Read it as a list of what consumes the clearance, not as a set of shop limits; the Ganesh column states what we control and where a tighter callout needs to be agreed before quotation.

The pattern is the one row that should never be opened up to buy tolerance. If the pattern is right and the skin is loose, the panel fits. If the pattern is loose and the skin is right, the panel rocks, the door binds at one corner and the gasket fails at the other.

Five failure modes and the physical cause

Revision drift between the model and the machine

The most common cause is a service part specified from the current CAD model of a product that has been through two revisions since the installed unit was built. A harness clip moves 8 mm, the drawing follows it, and the replacement panel now has a hole where the old loom used to run.

The fix costs one page. Add a supersession table to the drawing pack: model, year of build, revision, and the interface dimensions that changed. Where the base is mixed, specify the worst case and widen the adjustment rather than splitting the part into variants nobody will stock correctly.

Tool wear on the original part

A progressive die wears. Cutting clearances open, punch edges round, and hole positions drift outward by 0.02–0.05 mm per 100,000 strokes on a heavily run tool. A service part cut on a laser to nominal is therefore not identical to the parts the die is still making, even when both are "to drawing".

The size of the drift is usually inside the positional tolerance the drawing already allows, which is exactly why it goes unnoticed until a mating part with a tight pattern arrives. The answer is not to chase the die; it is to make the mating part's adjustment carry the drift, and to state the drift allowance in the interface note.

Coating build-up at the joint

Coating is a dimension. At 60–80 µm per face, a two-face joint closes by 0.12–0.16 mm, a bolted lap loses clamp load by the same amount, and a bore shrinks as the callout above describes. On a service part that is coated and then fitted to a coated frame, the film is on both sides of the joint, so the effect doubles.

Masking is the control, and it has to be on the drawing as a masked-feature note with the faces called out. A general note that says "mask where required" will be interpreted differently by every shop that quotes it. The coating masking article sets out which features are worth masking and which are cheaper to ream after coating.

Springback on a part straightened in the field

A bracket that has been bent back into shape in a vice has been cold worked at the bend and lost its original angle. When the replacement is formed on a brake to 90°, the release takes 1–2° out of it, and at the end of a 250 mm flange that is 4.4–8.7 mm of movement. The fitter sees a strut that will not reach and blames the part.

Two controls fix it. Form critical angles with compensation built into the program rather than by hitting the same stroke as the last job, and dimension the service part by the position of the flange end relative to the interface, not by the angle alone. The bend allowance calculation covers how the flat pattern has to change when the angle target moves.

Fasteners, slots and the direction of adjustment

A service part needs exactly one datum hole and one slot. Put the round hole at the corner nearest the features that set the door gap; put the slot on the diagonal, aligned with the direction in which the frame is most likely to have moved. Slot length should cover the summed positional tolerance of both parts plus thermal movement plus a handling allowance.

For a 2,000 mm steel frame, 20 °C of temperature difference moves 0.48 mm. Add ±0.5 mm on the panel pattern, ±0.8 mm on the frame, and a 0.5 mm handling allowance, and the slot needs about 2.6 mm of travel. A 12 × 20 mm slot in a 10.5 mm hole position gives that with margin and still leaves a washer bearing face.

What Ganesh runs for service and aftermarket parts

A service part does not need a tool, which is the reason it can be made economically at a quantity of one. Ganesh cuts service parts on the 12 kW fibre laser with a 40 ft × 16 ft bed, forms them on the 200 T five-axis brake with a 4,450 mm bed that handles material to 10 mm, and welds them on three CLOOS robotic cells or at any of the 30-plus manual MIG, spot and SPM stations. The same route produces the production panels for TKE, Eros Elevators and Mahindra Powerol, so a replacement and its production equivalent come off the same machines and the same inspection.

Where the volume justifies it, the same part moves to the 20–300 T press line with tooling built in the in-house tool room, which is how a service part that becomes a recurring item stops being a laser job. Material thickness for custom parts runs from 0.8 mm to 40 mm, and the finishes run through the seven-tank pretreatment line, powder booth and oven, with dry film thickness checked on a gauge and adhesion by cross-hatch. First article inspection and PPAP documentation are available where a service programme needs the same evidence as production, and the whole route sits under ISO 9001:2015.

The applications and cases hub collects the field problems that drive this kind of specification, and the custom parts capability covers what a one-off replacement can be quoted against.

What to put on a service-part drawing

1. The installed machine: model, year of build, and the drawing revision the machine was made to.
2. Six to ten interface features with individual tolerances, marked as interface; everything else to ISO 2768-1 class m or a stated general note.
3. The coated condition of every mating feature, with masked faces called out by name.
4. The fastener schedule: size, clearance hole, and whether the bore is coated or masked.
5. One datum hole, one slot, and the direction of adjustment.
6. The supply condition: coated or bare, hardware fitted or loose, and whether the part is marked with its revision.
7. Any interface dimension that changed since the installed unit was built, stated as a supersession note rather than left to the shop to discover.

A drawing pack built this way can be quoted without a site visit and manufactured without a trial fit. That is the whole return on the exercise: the part arrives and the fitter bolts it on.

When a like-for-like replacement is the wrong answer

Sometimes the frame has moved far enough that no panel will close the gap, and the honest answer is to redesign the interface rather than reproduce the original. We do not survey installed machines on site, we do not certify structural weld repairs to a frame we did not build, and we do not hold finished spare stock against a call-off we have not agreed. Where the measured frame is out by more than the adjustment can absorb, we will say so at quotation and quote a slotted interface instead of a copy.

Send the sample photographs, the machine model and whatever interface dimensions can be measured, and the engineering team will return a drawing for approval before anything is cut. The quote request takes a PDF, DXF, DWG or STEP file if the original drawing still exists.