Releasing a Bracket Drawing with a Tolerance Conflict
A cadet is checking a fabricator's drawing for a steel mounting bracket before it is released. The drawing is in third-angle projection, plotted at a scale of 1:2.5, and shows a base plate with two fixing holes and an upstand. The dimensioning is mixed: hole positions are given as 85 plus or minus 0.3 and 115 plus or minus 0.3, both chained from the same left-hand edge, and a separate overall dimension of 200 plus or minus 0.5 spans the same features. The holes are called up as '2 x dia 10 THRU'; the mating pins are drawn on a separate part drawing as dia 9.96 plus or minus 0.02, and the bracket is to be painted after machining. The dimension for the upstand height is missing and the drawing office is closed. A colleague measures the upstand on the paper with a rule, gets 34 mm, multiplies by 2.5 and writes 85 mm on the shop copy. The cadet must decide whether the drawing can be released.
1. Analyse the dimensioning of the hole positions against the overall dimension. Is the drawing over-constrained, and what is the worst-case result?
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Worst-case stack of the chain:
- Upper limit: 85.3 + 115.3 = 200.6 mm
- Lower limit: 84.7 + 114.7 = 199.4 mm
- Result: 200 plus or minus 0.6
Overall stated: 200 plus or minus 0.5 (199.5 to 200.5).
Conflict: if both chained dimensions land at their upper limits, every dimension is individually within tolerance but the overall is 200.6 mm, which is 0.1 mm outside its own tolerance. The dimensions are also redundant - two chained dimensions already fix the span, so the third is over-constraining the part and creates an inspection argument: an inspector can reject a part that meets the chain, or accept one that fails the overall.
Recommendation: do not release as drawn. Dimension both holes from one datum (the left-hand edge) and either delete the overall dimension or mark it as reference, untoleranced. Baseline dimensioning from a single datum also stops tolerance accumulation between the two holes, which matters if the pins are a fixed pair.
2. Evaluate the hole and pin fit as specified, using the callouts '2 x dia 10 THRU' and the mating pin size of dia 9.96 plus or minus 0.02. What will assembly be like, and what must be added to the drawing for the painted part?
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Hole range: assume the holes are dimensioned dia 10 with a plus/minus tolerance, or take the usual shop assumption of a nominal 10 mm drilled hole; state the assumption explicitly, because the drawing does not. Taking holes at dia 10.00 to 10.15 (a drilled H-class range):
Pin range: 9.96 plus or minus 0.02 gives 9.94 to 9.98 mm.
Clearances:
- Minimum: 10.00 - 9.98 = 0.02 mm (just clearance - no interference)
- Maximum: 10.15 - 9.94 = 0.21 mm
So the fit is always clearance, but the minimum is only 0.02 mm. That is effectively a location fit: it will assemble, but the pins will fight if there is any burr, swarf or distortion, and the two-hole pattern tolerance matters because the pins are a fixed pair.
Painting: a paint film of 0.06-0.10 mm per surface will close a 0.02 mm clearance completely. The drawing must state 'mask holes before painting' or specify reaming/Drilling after paint - otherwise the bracket will not go on the pins on site. Also add the hole tolerance itself; a nominal 'dia 10 THRU' with no tolerance is not a releasable specification.
3. The upstand height is missing. The colleague's method gives 85 mm. Should the shop copy be used to cut the part? Explain the rule and the correct action.
Read the worked answer
No - do not cut to a scaled dimension.
Why the arithmetic is not the problem: at 1:2.5, actual = 34 x 2.5 = 85 mm, so the sum is correct. The problem is the input.
Why scaling off a drawing is not permitted:
- The stated scale is nominal. Prints shrink and stretch with paper humidity, and plotters are not exact.
- Line width and the position of the feature's edge can shift a measured value by a fraction of a millimetre; at 1:2.5 that fraction is multiplied by 2.5.
- The view may be foreshortened, or the feature may be drawn in a view where its true length does not appear.
- Above all, a measured dimension carries no authority and no tolerance - nobody has signed for 85 mm, and if it is wrong the fabricator owns the scrap.
Correct action: hold the drawing and the job; raise a drawing query or a technical deviation to the drawing office with the dimension requested, and release only when the missing dimension is added with a tolerance. If a schedule forces progress, get the figure authorised in writing by the responsible engineer and record it on a marked-up copy - but a measurement does not become a dimension because it is written on the shop copy.