Question about Projected Tolerance
Question about Projected Tolerance
(OP)
On positional tolerancing of a threaded hole we are taught that the projected tolerance zone should project the distance of the maximum thickness of a mating piece. I've also been told, "Don't worry about usint a projected tolerance zone if the mating part is sheet metal," implying that a thin piece doesn't need that.
My question: At what thickness of the mating part should we begin using a projected tolerance zone? Is there a formula or a table that shows this? Or even a rule of thumb that can be applied?
Thanks
My question: At what thickness of the mating part should we begin using a projected tolerance zone? Is there a formula or a table that shows this? Or even a rule of thumb that can be applied?
Thanks





RE: Question about Projected Tolerance
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Question about Projected Tolerance
RE: Question about Projected Tolerance
RE: Question about Projected Tolerance
Obviously, not using projected tols, even for fairly thin mating parts, does risk interference at worst case. I suppose by not putting it we're taking advantage of statistics, it's unlikely that all the parts will be at worst case in every relevant dimension at the same time.
KENAT,
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RE: Question about Projected Tolerance
A locational tolerance can be a mixture of translational and angular misalignment. Without a projected tolerance zone, there's no restriction about the mix. Projected tolerance zones, however, restrict the magnitude of the angular component.
For example. Let's say you have a .750 inch thick flange having threaded thru-holes holding interference fit studs that project 3 inches above the flange surface. If you fail to use a projected tolerance, any angular misalignment in the threaded hole will be multiplied 4 times (3.0/.750). Even a sheet metal mating flange, otherwise properly toleranced for location, wouldn't fit. In this case, the project tolerance zone should be three inches.
2nd example. Same flange as above. Mating flange has clearance thru holes. If the mating flange is .750 thick or less, don't project the tolerance zone, you don't need to worry about angular misalignment being multiplied. If the mating flange is thicker, project the tolerance zone for the length of the thicker part.