Bracket Drawing Tolerance Stack and Datum Scheme
A small fabrication unit in Guwahati is making 60 mild-steel mounting brackets for a tea-processing conveyor. The bracket has four M10 clearance holes that must line up with tapped holes on a gearbox flange; hole pitch is 120 mm × 80 mm. A newly recruited CAD drafter issued a third-angle orthographic drawing with chain dimensions taken from three different edges, repeated overall dimensions, and no datum feature symbol. The CNC operator programmed from the drawing, and six brackets were scrapped because the hole pattern was offset and the diagonal distance between holes was 3 mm outside the mating flange. The shop has a manual milling machine, a drill jig, and a CMM but no portable co-ordinate measuring arm. The customer wants replacements in four days and has asked for a corrected drawing and a short root-cause note. The drafter argues that ±0.2 mm on every chain dimension is 'good enough' and that the CNC machine repeats accurately.
1. Identify the drawing-practice errors and explain, with a tolerance-stack calculation, why the hole pattern could be out of position even when every chain dimension was within ±0.2 mm. Propose a corrected dimensioning scheme.
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Drawing-practice errors: chain dimensions taken from three different edges; repeated overall and pitch dimensions; no datum feature symbols; no third-angle projection symbol; no position control on the hole pattern; and no single defined programming origin. In a chain, the location error of the far hole is the sum of the intervening tolerances. For three links at ±0.2 mm, worst-case stack is ±0.6 mm in one axis; in two axes the diagonal error can reach about 0.85 mm, before machine, fixture or set-up error. That does not by itself explain 3 mm, so the CNC program origin, scale and edge selection must also be checked. Correct scheme: machine the mounting face as datum A, use two perpendicular machined edges as B and C, dimension all four holes by baseline dimensions of 120 mm and 80 mm from B and C, delete duplicate dimensions, and add one position callout such as Ø0.3 M A B C. Add the third-angle symbol and a first-article CMM requirement.
2. Recommend the datum scheme and a position tolerance for the four M10 clearance holes so the bracket assembles with the tapped gearbox flange. State assumptions and explain the MMC modifier and bonus tolerance.
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Assumptions: M10 bolts, clearance holes 10.5 mm minimum, gearbox tapped pattern has its own position tolerance of Ø0.2 mm at MMC, fixed-fastener condition. The available clearance is H_min - F_max = 10.5 - 10.0 = 0.5 mm diametral. Allocate T_bracket + T_gearbox = 0.5 mm; with gearbox at 0.2 mm, bracket gets Ø0.3 mm at MMC. Datum A is the mounting face, B and C are the two perpendicular edges; this matches the gearbox location and inspection set-up. The MMC modifier allows bonus tolerance: if the hole is produced at 10.8 mm (LMC), the bonus is 0.3 mm, so the permitted position tolerance becomes Ø0.6 mm. Use a functional gauge or CMM to verify the pattern at MMC. Do not simply tighten each chain dimension; that raises cost without controlling the pattern relative to the mating datums.
3. What must happen before the corrected drawing is released and the remaining 54 brackets are cut? Include the CNC program origin and steps to prevent recurrence.
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Containment: stop cutting, quarantine the 54 blanks and the six scrapped parts, and retrieve the CMM records. Verify the CNC program origin, tool offsets and fixture position against datum A, B and C, not against an arbitrary edge. Mark up the drawing, have an independent checker review it, issue revision B under an engineering change note, and update the drill jig and inspection sheet. Run a first article and obtain customer approval before the remaining parts. Prevention: add a drawing-check checklist covering datums, duplicate dimensions, projection symbol and position control; train the drafter on tolerance stack-up; require program prove-out on a witness part. Tell the drafter that CNC repeatability does not correct an ambiguous drawing, and that ±0.2 mm on every link is not the same as a ±0.2 mm position tolerance.