Position tolerance calculations for non-fastening parts 1

[r1m]

I am wondering if there are best practices or techniques to calculating position tolerance for non-fastening cylindrical parts. The particular part involves a cylinder that goes into a hole, and is then welded in place. The cylinder is stock part and I have the ability to apply position GD&T tolerance on the hole only. Do I just go with the fixed/floating fastener calculations?

 

[Burunduk]

What is the specific industry or government standard that specifies the tolerances on the geometry of the stock part you are using? Do you have access to it?

 

[r1m]

For most of the parts I have been working with, which is stock tubing, the OD tolerance is the only one that would matter for such an assembly and that is typically say +/- 0.01" on 0.5" nominal - if that helps.

 

[drawoh]

r1m,

If your cylinder is welded in place, it is equivalent to a screw. Calculate your positional tolerances based on screws, and apply them to the hole and the welded cylinder.

--
JHG

 

[Burunduk]

You also need to know the straightness tolerance or other form tolerances, since rule #1 from ASME Y14.5 doesn't apply to stock parts. Therefore, your 0.5"+/-.01" bar may be produced at .501" diameter but also bent, thus acting bigger. So what you need is to determine some acceptable worst case boundary that includes size, form, and position (location+orientation), for the stock cylinder once it's already welded in place and then apply the appropriate tolerance to the hole, accordingly.

From ASME Y14.5, 2009:
"2.7.2 Form Control Does Not Apply (Exceptions to Rule #1)
The control of geometric form prescribed by limits of size does not apply to the following:
(a) stock, such as bars, sheets, tubing, structural shapes, and other items produced to established industry or government standards that prescribe limits for straightness, flatness, and other geometric characteristics. Unless geometric tolerances are specified on the
drawing of a part made from these items, standards for these items govern the surfaces that remain in the as furnished condition on the finished part."

If you use a stock part the way you described, you better know how it's controlled geometrically by the manufacturer.

 

[r1m]

I understand, thank you for both of your comments and the note about the additional geometrical control!

 

[drawoh]

r1m,

I don't specify stock sizes on my drawings unless I am doing a welded frame. I specify dimensions and tolerances, which often are based on known stock sizes and tolerances. Let the fabricator figure out how to do everything.

--
JHG