Bracket Drawing: Tolerance Stack-Up and Fit
A steel mounting bracket is bolted to a frame through two 11 mm clearance holes, using M10 bolts. The mating frame has two tapped holes whose centrelines are dimensioned at 60 ± 0.2 mm. The bracket drawing dimensions the same hole spacing as 60 ± 0.1 mm, and the drafter has applied a general tolerance note of ± 0.3 mm to all untoleranced dimensions, including the hole positions. The assembly is assembled dry on the line, and fitters report that some brackets will not fit over the bolts without reaming the holes. The drafter argues the drawing is correct because each dimension is within its own tolerance.
1. Explain, using the concept of tolerance stack-up, why components that individually conform can still fail to assemble.
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Each dimension carries its own tolerance band, and when two parts are mated, the relative position of the features is the sum of the two independent errors. The worst-case separation between the bracket holes and the frame holes is the sum of half the bracket spacing tolerance and half the frame spacing tolerance: 0.1 + 0.2 = 0.3 mm per side, so the two hole patterns can be offset by up to 0.6 mm relative to each other in the worst case.
The clearance available is the difference between the hole diameter and the bolt diameter: 11 − 10 = 1.0 mm, giving 0.5 mm of radial float per bolt. That float must absorb the pattern offset, the bolt straightness, the hole position error within the bracket, and any angular misalignment from the bracket not sitting flat.
The key point is that conformance to individual tolerances does not guarantee assembly - interference between toleranced features is governed by the combined stack, not by any single dimension. The drafter's argument is therefore wrong in principle: a drawing is only correct if the worst-case stack leaves the required clearance, or if a positional tolerance with the correct datum scheme and a maximum-material modifier is used so the clearance is properly accounted for.
2. Recommend specific drawing and manufacturing changes, and state how you would verify the fix without a full production run.
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Drawing changes.
- Replace the ± tolerances on hole positions with a true-position tolerance (e.g. Ø0.4 mm at maximum material condition) referenced to clearly defined datums A, B and C, so the tolerance zone is circular and the bonus tolerance from the clearance hole is available.
- Make the bracket hole spacing dimensionally identical in nominal value and tolerance basis to the frame, or deliberately offset the nominals so the stack is centred rather than one-sided.
- Increase the clearance hole to 12 mm if the joint is not a locating joint, which raises the radial float to 1.0 mm and absorbs the stack comfortably.
Manufacturing changes. Control the hole pattern with a drill jig or a single set-up on a CNC machine rather than marking out and drilling each part, which removes the largest source of position error.
Verification. Build a small pilot batch (say 10 brackets and 10 frames), measure the actual hole positions with a calliper or CMM, compute the real stack, and assemble them dry. Compare the measured distribution with the predicted worst case. If the measured worst case still leaves positive clearance, the fix is validated; if not, the tolerance or hole size must be revised again. This is cheaper and more convincing than arguing from the drawing alone.