Question about Projected Tolerance

[randy64]

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

 

[axym]

One rule of thumb that I've heard of is to use a projected tolerance zone when the mating part thickness is equal to or larger than the depth of the hole.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[WHITMIREGT]

The reason we need the height stated on the drawing for thin mating parts is to tell inspection how high to measure above the thread. If they measure too high, they could be rejecting good parts.

 

[randy64]

Thanks axym, that makes sense to me.

 

[KENAT]

I will admit I don't use projected tol zones a whole lot unless the fit is really critical with fairly tight tolerances (such as alignment pins or similar) or if the mating part is fairly thick. While I hadn't thought of it that way axyms rule of thumb is probably close to what I use.

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,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[MartinSr00]

A projected tolerance when the tolerance zone needs to be extended to a length longer than the indicated feature.

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.