Key takeaways
- Two locating features plus one rotation stop are enough; adding a second round hole over-constrains the part and the panel distorts when the bolts are tightened.
- A 20 mm tab needs a 21 mm slot: with ISO 2768-1 medium tolerances of ±0.2 to ±0.3 mm per part, anything under 0.8 mm of clearance becomes a rework operation.
- An M6 clearance hole to ISO 273 is 6.6 mm, which gives 0.3 mm of radial float; a 46-fastener enclosure takes 5–8 minutes of driving time before any handling is counted.
- Make the wrong assembly physically impossible: offset one hole by 10 mm or widen one tab by 4 mm, because colour marks and labels do not survive pretreatment and powder coating.
An enclosure with 46 fasteners, a locating tab that fits either way round and two M6 screws 12 mm from a bend takes 14 minutes to assemble. One unit in twelve leaves the line with a cross-threaded insert, and the rework bin holds panels whose holes have been elongated by a fitter with a drill.
None of that is a labour problem. It is a drawing problem. The part was designed to be manufactured, not to be put together, and the assembly time is the cost of that omission.
Design for assembly on sheet metal comes down to three decisions: what locates the part before anyone picks up a tool, how many fasteners the joint actually needs, and what stops the part going together the wrong way. Each has a number attached.
What does design for assembly change on a sheet metal part?
It changes the first ten seconds of the build. A part that lands on two locating features and closes with six fasteners assembles in a repeatable time. A part that needs a second pair of hands, a mallet and 46 screws does not.
The reference part is a 1.5 mm CRCA enclosure, 600 × 400 × 200 mm, 46 M6 fasteners, a locating tab at each end and a removable access panel. It is the shape of a data centre rack panel, a genset canopy door or an elevator controller box. The numbers below apply to that class of part and to the sheet-metal assemblies that bolt onto it.
Locate before fasten
A part should find its own position before the first screw is fitted. That means two locating features and one rotation stop, not four.
The 3-2-1 principle still applies: three points define a plane, two define a line in that plane, one stops rotation. On sheet metal the plane is the mating face, the line is a tab in a slot plus a datum hole, and the rotation stop is the second tab or a second hole that is slotted.
Two round holes on two round pins is the classic error. The first hole fixes position, the second fixes rotation, and then the distance between them has to match the distance between the pins exactly. It never does. The panel is forced over the pins, the mating face bows, and the fasteners that follow pull the distortion into the assembly.
Use one round hole and one slot. The round hole locates; the slot takes up the variation in centre distance and still stops rotation. The same logic applies to a hinge: one round hole and one slotted hole per leaf, or the pin binds.
The tab and slot numbers
A tab needs clearance on three sides, and the clearance has to cover the tolerance stack of both parts plus the coating.
ISO 2768-1 medium, the general tolerance most drawings fall back on, gives ±0.2 mm for a length between 6 and 30 mm and ±0.3 mm for 30–120 mm. Two parts made to that tolerance can be 0.6 mm apart at a locating feature before anything else is added. A 20 mm tab in a 20.2 mm slot has 0.1 mm per side, which is less than the variation, so the tab either will not enter or enters with a hammer.
The working rule is 0.8–1.0 mm total clearance: a 20 mm tab in a 21 mm slot. That feels loose on a drawing and is right on the line. If the tab also carries the joint load, add a weld or a fastener; clearance is not a structural connection.
Count the fasteners
Every fastener carries three costs: the part, the hole and the time. The time is the one that hides.
An M6 screw with a washer takes 6–10 seconds with a nutrunner and 20–30 seconds by hand, plus the torque check and the mark. Forty-six fasteners is 5–8 minutes of driving per unit before any handling, and a 500-unit batch is 40–65 hours at the assembly station.
The alternatives are cheaper when the joint is permanent:
- a blind rivet sets in 2–3 seconds and needs access from one side;
- a tab and two spot welds replace two screws and remove the holes;
- a clinch stud replaces a loose bolt and washer and cannot be lost;
- a quarter-turn latch replaces four screws on a panel that opens monthly.
Keep screws where the joint is adjusted, serviced or preloaded. Use rivets and welds where it is not. A service panel with 12 screws is a design choice; a fixed panel with 12 screws is 12 holes, 12 inserts and 12 opportunities for a cross-thread.
Fastener clearance and float
ISO 273 defines clearance holes by series. The medium series is the default for general fabrication: 6.6 mm for M6, 9.0 mm for M8, 11.0 mm for M10.
The float is half the difference between hole and bolt: 0.3 mm radial for M6, 0.5 mm for M8, 0.5 mm for M10. That float is what absorbs the positional tolerance of the holes in both parts. If each part is drilled to ±0.2 mm, the combined error is 0.4 mm, which fits inside the 0.6 mm diametral float of an M6 medium hole. If the drawing calls ±0.1 mm on both parts, the hole can be smaller and the joint tighter, but the cost of holding it usually exceeds the value of the clearance.
Tool access is part of the locating scheme
A fastener the tool cannot reach becomes a hand-tool operation. A nutrunner socket needs about 25 mm of radial clearance around the bolt head and 60 mm of axial clearance for the tool body; a 90° angle head needs about 40 mm. An M6 screw 12 mm from a return flange cannot be driven with a socket, so the fitter uses an open-end wrench, adds 15–20 seconds, and the torque is whatever the wrist delivers.
Put the tool envelope on the drawing next to the fastener, the same way weld access is put next to a weld symbol. A row of 46 fasteners 20 mm from a flange is not a tolerance problem; it is 10–15 minutes per unit that nobody quoted.
Make the wrong build impossible
A symmetric part will be fitted backwards. The only question is when.
Mistake-proofing is geometry, not instruction. Offset one hole in the pattern by 10 mm, widen one tab by 4 mm, put the locating slot on one end only, or make the left and right panels differ by one hole position. The error becomes impossible rather than discouraged.
Colour marks, labels and paint dots do not survive the process. Pretreatment removes them, powder covers them, and the person assembling the part never sees the drawing. Laser marking after coating survives, but it is an extra operation and it can be masked off by mistake. A 4 mm difference in tab width costs nothing and cannot be removed by a tank.
The stack-up that decides the slot
Take the enclosure's access panel. The tab is positioned at 100 mm from datum A with a tolerance of ±0.3 mm. The slot in the body is at 100 mm from the same datum, also ±0.3 mm. Powder coating adds 80 µm per face, or 0.16 mm across the joint if both faces are coated and neither is masked.
Worst case, the mismatch is 0.3 + 0.3 + 0.16 = 0.76 mm. A 1.0 mm slot leaves 0.24 mm of margin, which is thin but workable. Add weld distortion of ±1.0 mm from the body fabrication and the total becomes 1.76 mm, so the slot needs to be 2.0 mm wide or the tab needs to move to a feature that is machined after welding.
That arithmetic is the whole argument for locating on a datum rather than on a folded edge. A tab positioned from a bend inherits the bend tolerance, the bend deduction and the springback; a tab positioned from a datum hole inherits only its own hole tolerance.
| Feature | Design value | What it absorbs | Ganesh limit or note |
|---|---|---|---|
| Tab to slot clearance | 0.8–1.0 mm total | ±0.2 to ±0.3 mm per part plus coating | 20 mm tab in a 21 mm slot |
| First locator | Round hole, H12 | Position only | One per assembly, never two |
| Second locator | Slot or diamond pin | Centre distance and rotation | Slot length ≥ 2× the stack-up |
| M6 clearance hole | 6.6 mm | 0.3 mm radial float | ISO 273 medium series |
| M8 clearance hole | 9.0 mm | 0.5 mm radial float | ISO 273 medium series |
| M10 clearance hole | 11.0 mm | 0.5 mm radial float | ISO 273 medium series |
| Fastener pitch | 150–200 mm | Panel flatness and gasket load | Closer pitch if the panel is under 1.5 mm |
| Rivet setting time | 2–3 s per rivet | Permanent joint, one-side access | 30+ manual stations and spot welders |
| Fastener driving time | 6–10 s per screw | Labour and torque check | 46 fasteners is 5–8 min per unit |
| Coating on a mating face | 0 mm, masked | 0.16 mm across two 80 µm films | Mask datums and machined faces |
| Mistake-proof offset | 10 mm hole shift or 4 mm tab width | Wrong orientation | Geometry survives every tank |
| Assembly flatness | 1 mm per 500 mm | Weld distortion and clamping | Checked in the fixture before dispatch |
The Ganesh column is where the process sets the limit. We build welding and checking fixtures in-house, so the locating scheme on the drawing becomes the locating scheme in the fixture. If the drawing asks for two round holes on two pins, the fixture will hold the part in the distorted shape and the checking fixture will pass it. That is a consistent way to make a part that does not fit.
What Ganesh runs
Sheet metal assemblies are cut on the 12 kW fibre laser or the 250 T press, formed on the 200 T, 4,450 mm brake, and welded in three CLOOS robotic cells or at 30+ manual MIG, spot and SPM stations. Fixtures and checking fixtures are designed and built in-house, which is what makes a locating scheme real rather than nominal.
The welded assemblies page covers the cell mix and the fixture route. Assembly-ready fabricated metal parts covers the hand-off from fabrication to the line, and how fixtures improve repeatability explains why a fixture built to the drawing can still produce a part that will not assemble.
Enclosures and frames for Godrej & Boyce, Minda and Eros Elevators go through this route, with PPAP and first article inspection where the assembly is safety-related. The custom parts page shows the range of assemblies that leave here ready to bolt on.
What to put on the drawing
- Datum scheme A, B, C, with the datum features marked on the view they control.
- Locating features: which hole is round, which is slotted, and the slot direction and length.
- Tab and slot dimensions with the clearance stated, not left to the fabricator.
- Fastener callout: size, property class, torque and thread class after coating.
- Assembly sequence: locate, clamp, weld or fasten, check. If the sequence matters, say so.
- The mistake-proof feature, dimensioned and identified as orientation-critical.
- Masking on mating faces and datum pads, with the film limit at the joint.
- Flatness and door-gap limits for the finished assembly, measured in the checking fixture.
The design and CAD hub indexes the rest of the drawing-note guides. For the welded side of the same assembly, welded sheet metal enclosures covers distortion and sequencing, and the welded assembly tolerances article gives the numbers that this locating scheme has to live inside.
When this route is wrong
Tabs and slots are not a structural joint. On a part that sees vibration, a tab locates and a weld or a rivet carries the load; a tab alone will fret, elongate and come loose. If the joint is safety-related, the drawing needs a positive fastener, not a fit.
Rivets are wrong where the joint is serviced. A pump access panel, a filter cover or a controller door needs a screw, a latch or a hinge, even though the rivet is faster to assemble.
Below 1 mm, tabs shear and slots tear. The locating scheme needs a thicker gauge, a folded tab or a different joint. And if the assembly tolerance is tighter than ±0.3 mm, the answer is not a tighter tab; it is to locate from a machined feature or to assemble and then machine.
Send the assembly, the fastener schedule and the mating parts to request a quote. The locating and fastener review is part of the manufacturability check, and it is cheaper than a rework station at the end of the line.
Frequently asked questions
How much clearance does a tab and slot need?
Plan for 0.8–1.0 mm total clearance on a 20 mm tab, so a 21 mm slot. Each part carries ±0.2 to ±0.3 mm under ISO 2768-1 medium, and two parts stack to 0.4–0.6 mm before coating. A 0.2 mm clearance looks neat on the drawing and becomes a file-and-hammer job on the line.
Why does my enclosure only fit with the bolts loose?
The parts are over-constrained. Two round holes on two pins fix position and rotation twice, so the panel has to distort to close the gap. Use one round hole and one slot, or a round hole and a diamond pin, and let the second feature control rotation without fighting the first.
How many fasteners should a sheet metal enclosure have?
As few as the load and service access allow. Each M6 screw costs 6–10 seconds with a nutrunner and 20–30 seconds by hand, so 46 screws is 5–8 minutes of driving per unit. A tab and two spot welds can replace two screws on a permanent joint; a quarter-turn latch can replace four on an access panel.
What is the best way to mistake-proof a left and right panel?
Use geometry, not labels. Offset one hole by 10 mm, make one tab 4 mm wider, or put the locating slot on one end only. Colour marks and paper labels disappear in pretreatment and powder coating, and a symmetric outline with a symmetric hole pattern will be fitted backwards eventually.
Should mating faces be masked before powder coating?
Mask them if the joint is a datum or a close fit. An 80 µm film on each face adds 0.16 mm across the joint and changes the preload on a bolted connection. On a locating tab, coating is usually acceptable if the slot clearance already allows for it; on a machined mating face it is not.
Sources and further reading
- ISO 2768-1 — General tolerances: linear and angular dimensions · reviewed 3 Nov 2026
- ISO 273:1979 — Fasteners: clearance holes for bolts and screws · reviewed 3 Nov 2026
- ISO 286-2 — ISO system of limits and fits: tables of tolerance classes · reviewed 3 Nov 2026
Want the assembly checked before the drawing is released?
Send the assembly, the fastener schedule and the mating parts. We review location, clearance and fastener count alongside the manufacturing route.
Request a quote