Bearing-housing flange drawing with conflicting fit note and hole pattern
Al-Rawda Metal Works has received a customer drawing for 25 bearing-housing flanges to be machined from castings that are already on site. The drawing specifies a 40 mm H7 bore with a general note reading: clearance fit 0.020 to 0.060 mm with mating shaft g6. The flange face carries six 9 mm through holes equally spaced on a 120 mm pitch circle. The mating plate drawing, issued in the same package, shows six M8 tapped holes on the same 120 mm pitch circle, but the pattern is clocked 30 degrees from the flange drawing centrelines. No projection symbol appears in either title block, and the front and section views are arranged so that the section sits to the right of the front view. The estimator has already priced the job at 4.5 hours per flange, and the customer design engineer is travelling for two days. The workshop foreman wants to start rough boring immediately to protect the six-day delivery date.
1. Separate the three drawing issues into genuine design conflicts and documentation gaps, and state what the workshop should and should not do before metal is cut.
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Issue 1 - the fit note versus the tolerance letters: a genuine conflict. The letters 40 H7/g6 are a complete specification on their own; the note adds a second, different specification. Both cannot be satisfied, so the machinist cannot choose. Do not bore.
Issue 2 - the clocked hole pattern: a genuine conflict with a hard consequence. Both plates have six holes at 60 degree spacing on the same 120 mm pitch circle. If the flange pattern is rotated 30 degrees relative to the mating plate, each flange hole sits midway between two tapped holes - the parts physically cannot bolt together. This is not a tolerance stack issue; it is a 30 degree indexing error on one of the two drawings. Do not drill the pattern.
Issue 3 - missing projection symbol: a documentation gap, not a design conflict. Without a symbol, a section placed to the right of the front view is the right-side view in third angle and the left-side view in first angle. This changes the direction of the counterbore or chamfer, not the hole positions. It still cannot be guessed, because a reversed counterbore is a scrapped casting, but it can be closed out by the drawing office against the CAD model much faster than the hole pattern.
Recommended action: raise one consolidated drawing query (RFI) listing all three items with marked-up extracts, and ask for a written reply within 24 hours. Meanwhile, cut the castings to length and prepare tooling only. Compare the cost of two days of idle machine time against 25 scrapped castings plus the delivery penalty - the delay is the cheaper option every time.
2. Calculate the clearance range produced by the 40 H7/g6 callout and decide which of the two specifications in the drawing package is achievable with standard tooling.
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Use the ISO limits for the 30-50 mm size band: IT7 = 25 micrometres, IT6 = 16 micrometres.
Hole 40 H7: EI = 0, ES = +25 micrometres (+0.025 mm). Shaft 40 g6: es = -9 micrometres, ei = -25 micrometres (-0.009 / -0.025 mm).
Clearance = hole size minus shaft size:
- Maximum clearance = ES - ei = 25 - (-25) = 50 micrometres = 0.050 mm
- Minimum clearance = EI - es = 0 - (-9) = 9 micrometres = 0.009 mm
So the letters give a clearance band of 0.009 to 0.050 mm.
Compare with the note, which demands 0.020 to 0.060 mm:
- The minimum is wrong: 0.009 mm is below the 0.020 mm the designer asked for. A fit this tight on a 40 mm bore risks galling and makes assembly a press job.
- The maximum is inside the note but does not reach it.
Conclusion: the callout is tighter than the note at the low end, so the two specifications are not interchangeable. Note also that H7/f6 would give 0.025 to 0.066 mm - it satisfies the lower bound but overshoots the upper bound, so no common ISO fit reproduces the note exactly. The choice between revising the note and changing the shaft tolerance is an engineering decision with a cost consequence, not a shop-floor decision. The drawing office must issue a revision.
3. Draft the content of the drawing query and explain why a verbal agreement with the customer inspector is not acceptable evidence for this job.
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A usable query contains, for each item: drawing number, revision letter, view or zone, the exact text or dimension in dispute, the two possible interpretations, the consequence if the wrong one is chosen, and the date by which an answer is needed to hold the delivery date.
Suggested entries:
- Fit note. Drawing 4471-C rev B, note 3 vs bore callout 40 H7. H7/g6 gives 0.009-0.050 mm; the note requires 0.020-0.060 mm. Confirm which governs, or issue revised shaft tolerance. Consequence: bore cannot be sized until resolved.
- Hole pattern. Flange view A-A and mating plate 4472-A view B: both six holes on 120 PCD but clocked 30 degrees apart. Confirm which part is correct. Consequence: 25 flanges scrap if drilled as drawn.
- Projection. No projection symbol in either title block. Confirm first or third angle. Consequence: counterbore direction reversed.
Why verbal is not enough: the fit, the pattern and the projected view all affect a characteristic the customer will measure on receipt. If an inspector says over the phone that third angle was intended, and the parts are later rejected, the workshop has no evidence and carries the loss for 25 castings. A written, revision-controlled reply (a revised drawing or a signed concession) transfers the risk back and tells the drawing office to update the master. Log the query number on the route card and keep the reply with the inspection record.