Motor Bracket Weldment: Tolerance Stack and Drawing Interpretation
A pump motor bracket is fabricated from a 12 mm thick mild-steel plate welded to a hub. The drawing gives the hub bore as 40.00/40.05 mm and the motor shaft as 39.94/39.98 mm. The drawing also specifies a positional tolerance of 0.20 mm on the bore axis relative to datum A (base face) and datum B (two bolt holes), and a perpendicularity of 0.10 mm on the base face to datum A. The fabricator reports that after welding, the base face bows by 0.15 mm and the bore axis shifts 0.12 mm from nominal. Assembly is intermittent: some units bind on the shaft, others fit freely. The inspector has measured only the bore and shaft diameters and finds them within drawing limits.
1. Determine the diametral clearance range and explain whether the measured diameters alone can guarantee assembly.
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Clearance extremes
- Minimum clearance = minimum bore − maximum shaft = 40.00 − 39.98 = 0.02 mm.
- Maximum clearance = maximum bore − minimum shaft = 40.05 − 39.94 = 0.11 mm.
So the diametral clearance band is 0.02–0.11 mm, i.e. 0.01–0.055 mm radial.
Can diameters alone guarantee assembly? No. The bore and shaft being within their size limits only establishes the size fit. Assembly also depends on the location and orientation of the bore axis relative to the bolt-hole pattern and base face, which is controlled by the 0.20 mm positional tolerance and the 0.10 mm perpendicularity.
Worst-case stack at the interface:
- Bore axis shift: 0.12 mm
- Base-face bow: 0.15 mm (this consumes the 0.10 mm perpendicularity allowance and leaves 0.05 mm of error beyond the stated tolerance)
- Combined positional error can therefore exceed the 0.20 mm zone when the bowed face tilts the hub.
Because the minimum diametral clearance is only 0.02 mm (0.01 mm radial), any axis misalignment larger than 0.01 mm radial will cause binding. The measured diameters are within limits, but the geometric errors already consume more than the available clearance. The intermittent binding is therefore a geometric-tolerance (orientation and location) failure, not a size failure. The inspector must verify position and perpendicularity with a CMM or functional gauge, and the fabricator must control weld distortion (fixturing, balanced welding sequence, stress relief) to hold the 0.10 mm perpendicularity.
2. Recommend how to allocate the tolerance budget and what inspection method to adopt.
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Budget allocation. The 0.02 mm minimum clearance is the hard constraint. Allocate radial positional error so that the worst-case misalignment stays below 0.01 mm radial - which is impractical for a welded bracket. A better approach is to increase the minimum clearance by opening the bore to 40.10/40.15 mm (giving 0.12 mm minimum diametral clearance) while keeping the shaft unchanged, or to introduce a floating/self-aligning coupling. Then the positional budget can be relaxed to a realistic value.
Practical allocation (with enlarged bore):
- Perpendicularity of base face: 0.10 mm (unchanged, but must actually be held)
- Position of bore to A|B: 0.20 mm
- Weld distortion allowance: 0.05 mm residual, achieved by stress relief after welding
- Remaining clearance margin: ≥ 0.06 mm radial
Inspection method. Diameters alone are insufficient. Use a functional gauge (a simulated mating part with the shaft at MMC and pins at the bolt-hole MMC) to check the combined size-and-position requirement, backed by CMM verification of the datum reference frame (base face as primary datum, two holes as secondary datum). Record base-face flatness before and after welding to isolate distortion. Statistically, plot the bore-axis shift and face bow on an X-bar/R chart; if the process capability index for position falls below 1.33, the welding fixture needs review. Finally, update the drawing to state the maximum material condition modifier where appropriate, so that bonus tolerance from the bore size is legitimately available.