Conflicting Views and an Untoleranced Bore on a Bearing Cover Drawing
A local pump repair workshop receives a drawing for a bearing cover plate. The drawing carries a first-angle projection symbol. The front view shows a circular plate of Ø120 mm with a Ø60 mm central bore, a 100 mm bolt circle and four Ø10 mm holes. The side view shows a 12 mm plate thickness. A note says 'Bore 60 H7, shaft 60 h6'. The section view shows a counterbore 66 mm diameter × 6 mm deep around the bore, but the side view shows the four bolt holes as plain through holes and does not show the counterbore. The dimension 120 has no tolerance, and the bore has no surface-finish symbol. The production fitter has already cut one plate to Ø120 and is about to bore the centre hole when he asks you whether to use the counterbore depth from the section or 'just go through'. The customer wants 25 plates by Friday.
1. Determine the type of fit and the clearance range for the 60 H7/h6 joint. Show the limit calculations.
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For Ø60 H7/h6:
- H7 hole: lower deviation 0, upper +0.030 mm, so 60.000 to 60.030 mm.
- h6 shaft: upper deviation 0, lower −0.019 mm, so 59.981 to 60.000 mm.
- Minimum clearance = 60.000 − 60.000 = 0.000 mm.
- Maximum clearance = 60.030 − 59.981 = 0.049 mm.
So it is a clearance fit, close running or sliding, not a press fit. At the minimum end there is line-to-line contact, so assembly should use light oil and not force. The workshop must not assume that 'H7/h6 means hammer fit'; the limits show zero minimum clearance.
2. The counterbore appears in section but not in the end view. What should the fitter do, and why is 'use the section because it is more detailed' not a safe rule?
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Stop the fitter. A drawing conflict is not resolved by choosing the view that looks more detailed. The section may show the true internal geometry, but the end-view omission could mean the counterbore is on the opposite face, applies only to one hole, or was forgotten by the drafter. A section is a cutting-plane view and may not control the pattern of all four holes. Raise a drawing query or change note, mark the drawing HOLD, and ask for: counterbore diameter 66 and depth 6 confirmation, which face, whether all four bolt holes are counterbored, and the missing surface finish. Cutting metal to a guess creates scrap and a possible wrong bearing seat. The fitter may cut the OD and drill pilot holes only after the query is logged.
3. State the checks you would make on the drawing before releasing the remaining 24 plates, including tolerance and surface texture.
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Before releasing 24 plates, check revision status and the projection symbol; the general tolerance note; the untoleranced 120 dimension; the 60 H7 bore limits; surface texture on the bore, because 'H7' alone does not specify Ra; coaxiality or perpendicularity of the bore to the face; the counterbore callout; material and heat treatment; deburr and marking requirements. Confirm the first plate is measured with a calibrated micrometer or bore gauge and that surface finish is verified against the drawing. If the customer cannot supply the missing data, obtain a written deviation approved before production. Record the query and answer in the inspection record.