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Another method is to use functional tolerances, which are a set of tolerance pairs for a shaft and a hole, and define various fits, namely clearance, transitional and interference fits, and is governed by ISO 286. These standards define fits with an upper and lower deviation and can be easily applied to multiple parts to ensure they will all fit together. For example, for a clearance fit you might choose an H7/g6 tolerance which means the hole tolerance will be H7(+15/-0 μm) and the shaft tolerance will be g6 (-5/-14 μm). As you can see the shaft will always fit into the hole with some room for movement.

[Matilda Jones]

Hi everyone! For an assembly with tight concentricity requirements, I’ve run into issues caused by stacked tolerances during assembly. Does anyone have tips for specifying concentricity in such cases? Is it better to define an overall concentricity tolerance for the assembly, or stick with individual part tolerances?

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[lars heringer]

GD&T is a good way to ensure multiple components fit together. The concentricity tolerance has been removed from ASMEY14.5-2018 and has instead been replaced with positional tolerances; this can be used to correctly specify the concentric requirement. With the positional tolerance, you have to select a datum feature, i.e. the main shaft of your assembly, and then specify a positional tolerance on its axis. Then all the other cylindrical features are toleranced to that datum. I.e. all the cylindrical axes need to be aligned to the datum axis within a specified tolerance. You can also use a runout tolerance in tandem with a positional tolerance.

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[Mattias]

Another method is to use functional tolerances, which are a set of tolerance pairs for a shaft and a hole, and define various fits, namely clearance, transitional and interference fits, and is governed by ISO 286. These standards define fits with an upper and lower deviation and can be easily applied to multiple parts to ensure they will all fit together. For example, for a clearance fit you might choose an H7/g6 tolerance which means the hole tolerance will be H7(+15/-0 μm) and the shaft tolerance will be g6 (-5/-14 μm). As you can see the shaft will always fit into the hole with some room for movement.

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