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[Alexandre Dubois]

hi, I have an aluminum housing for a small gearbox where the bearing seats are, of course, critical. I need to hold tight tolerances on the ID and OD, but only over a short engagement length of about 15 mm at the front end of the bore, so the bearings can be a light press fit. The rest of the part doesn’t need such high precision. What’s the best way to notate this on the drawing so it’s clear to manufacturing and inspection that the tolerance only applies to that localized section?

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[Sophie Noël]

You need to use localized GD&T. For the bore, you can dimension the full ID/OD nominal size and then apply a feature control frame indicating the tighter tolerance specifically over the bearing fit length. You can do this with a “zone” or “length” modifier, often noted as “L = 15 mm” next to the tolerance frame.

The rest of the bore can have a looser general tolerance, and you can either leave it at standard machining tolerance or specify it separately. Just make sure the tight tolerance is clearly flagged with a note or GD&T callout, so manufacturing and inspection know exactly where it applies.

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