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Single-Segment tolerance for single hole

Single-Segment tolerance for single hole

This is a simple question that I'm embarrassed I can't find an answer for. Given a simple hole in a part where I'm interested in more tightly controlling deviation in the 'x' direction than the 'y' direction can I simply do a single segment position tolerance where the first segment one establishes overall deviation in the x-y plane and then a tighter tolerance while omitting the datum corresponding to y?

e.g. ( Datum corresponds to a normal plane of the same direction)
| Pos | Dia .010 | X | Y | Z |
| Pos | .004 | X | Z |

Alternatively, is it permissible to establish the X and Y seperately (effectively creating a linear X/Y tolerance) e.g.
| Pos | .010 | Y | Z |
| Pos | .004 | X | Z |

I can't seem to find references to single segment or composite tolerancing when it relates to a single feature. Linear tolerancing isn't desired as the location is on two centerlines.


RE: Single-Segment tolerance for single hole

You could look at bidirectional positional tolerancing of features (section 5.9 of ASME Y14.5M-1994 7 fig 5-41 if that's the standard you work to).

This will give you a rectangular zone when arguably function by justify an obround zone but I'll leave that up to you to consider.

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RE: Single-Segment tolerance for single hole

Yeah... bidirectional is what you want (I can provide a picture if you don't have a GD&T reference book handy).  The two-single-segment examples you give aren't the best because the datums are being jumbled around. Think of a physical gaging scenario: the inspector would then have to jiggle the part against different edges for each positional check.  That's not what you want; you want him to set the part in a fixture only once, but then measure it in two directions to two tolerances.

IOW, the datum references given in the positional tolerance don't have to correspond on a one-to-one basis with the tolerance you are after.  They merely tell everyone how to hold the part.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Single-Segment tolerance for single hole

Thanks John, I completely let that portion of the meaning slip my mind.  

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