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Help with Simple GDT Question

Help with Simple GDT Question

Help with Simple GDT Question

I'll make this brief.

Both Cross-holes to be centered to each other
Axis of the Cross-holes to be centered, controlling both X and Y component in relation to Datum E. The Y component is more critical to control than the X component.

What I have so far:
Datums A & B True Position to each other, within .001 diametrically. (Easy Enough, first goal is accomplished I think.)
Axis A-B (Which is made from Datums A & B) to be within .004in to Datum E in the Y direction, and .015in from Datum D in the X Direction. I can't think of a clean way to do this. I really want a linear .015" tolerance from the axis to Datum D Surface, along with controlling Datum E in the Y direction.

To Control the Axis Position:
X Component- The tolerance of the basic dimension 1.63in to be plus minus .015in
Y Component- The tolerance of the basic dimension to be 1.378in plus minus .004

I think I almost need a symmetry call-out, along with a parallelism? I hear symmetry is a big no-no due to the complexity of measuring.

Attached a print for clarity (I hope)

RE: Help with Simple GDT Question


I'm going to assume ASME Y14.5-2009 is the relevant drafting standard, but you should indeed specify.

You can't apply position tolerances to "AXIS A - B" as you have shown in your drawing. The closest option would be to call the two diameter 1.06 holes a single continuous feature (see para. 2.7.5) and apply position tolerances to that. For the position tolerances themselves, it sounds like bidirectional positional tolerancing (see para. 7.4.4) is what you need. It would probably make sense to use the same datum reference frame for both tolerances. You'll have to decide based on functionality whether D should be primary and E secondary, or vice versa.

Calling the two holes a single continuous feature might also address your first goal, though in a rather different way. Can't say much more without knowing the actual diameter tolerances involved.

Why have you added the 1.378 dimension? If you want to control the relationship between feature axes, it should not be there.


RE: Help with Simple GDT Question

You guys are awesome. Right now we're technically using the 1994 standard, however, everything is pretty open over here. I read both sections that both of you referred me too, and it looks like for this situation, the solutions are very similar to each other. I'm going to attempt to stick with 2009, but if there is an easier 1994 solution I'm not opposed to that. Also, cross-holes are really held to tenths, and diameters to a few thousandths.

Attached an updated spec that I have so far. What exactly is holding the 2 cross-holes to each other though? I don't want one cross-hole not true to the other cross-hole.

I'm going to quote you pylfrm:
"It would probably make sense to use the same datum reference frame for both tolerances. You'll have to decide based on functionality whether D should be primary and E secondary, or vice versa."

What I really want is the Cross-holes in the Y direction held to Datum E. When I positionally tolerance that cross-hole in the X direction, I can't picture holding that to Datum E, being an axis? If anything, E would be primary.

I also made a small rectangular theoretical tolerance zone I was shooting for Section F-F, not to scale.

RE: Help with Simple GDT Question

To the OP:

I have attached my recommendations for your latest revision based upon your choice to use bi-directional position. I know you said datum E was your primary but I used D on my drawing. You can easily switch the order of the datums in the feature control frames to suit your needs.

1) I removed the dual dimensioning. There is a fundamental rule in 2009 (1.4d) that states dimensions shall not have more than one interpretation. Some people (myself included) interpret this to mean that dual dimensions are bad and should not be used. Totally up to you to use dual dimensions or not.

2) I put a flatness callout on datum D (my primary). The primary datum may need a form refinement if rule #1 allows for too much slop in the form.

3) I put a perp. callout on datum E (my secondary). The secondary datum is usually related back to the primary datum.

4) I put extension lines between the continuous feature ∅1.06 holes for clarity. This is not required but helps, imo.

5a) The ∅1.06 dimension does not need to be repeated for the bi-directional position feature control frames. If this dimension is repeated it should be as a reference. Reference dims are put in parentheses. The FCFs on my drawing can be read to say that the axis of the continuous feature hole must be within a rectangular tolerance zone that is located at true position. True position is described by the basic dimension of 1.6300 from datum D and an implied basic 0.000 from datum E. This rectangular tolerance zone is .015 tall and .004 wide, and centered at true position.

5b) Please keep in mind that this tolerance zone controls location and orientation of the axis of the hole feature. The holes are allowed to be less than perfectly parallel to datum D as long as the axis is inside the rectangular tolerance zone. Also, this tolerance zone is as wide as the part. If the allowed tip of the axis is too much then you should lessen the .015 value.

6) I added two notes which are mandatory per ASME Y14.5-2009, if you are going to invoke this standard.

RE: Help with Simple GDT Question


The composite callout on the right side of your image is not correct.
FRTZF (lower segment) should repeat the datum features in the PLTZF (upper segment), therefore datum feature C in the lower segment is wrong.

RE: Help with Simple GDT Question

I think what Vishal2015 posted isn't wrong or anything. I just think the bi positional method, in this case is easier to visualize (For the machinist, CMM operator, everyone involved). I thought about going that route, but with the different primary datums (Although the way you have the datum setups might be better.) I like the larger flat surface. I may end up stealing that one from you.

I loooove the feedback everybody. I'm going to run by a couple ideas by the manufacture, then the CMM operator (Who is very picky about what the machine can do) And I promise I'll post back what we went with.

RE: Help with Simple GDT Question

The composite position tolerance proposed by Vishal is, in fact, incorrect per the format allowed by ASME Y14.5 (1994 and 2009). The lower segment is inane due to the datum referenced.

RE: Help with Simple GDT Question

Please provide your views.

RE: Help with Simple GDT Question

Vishal, attached is a multiple single segment FCF that I believe will accomplish what the OP wants. Your composite version will allow the smaller tolerance zone to float anywhere inside the larger t.z. The multiple single segment version retains the implied basic zero dimension to datum B.

Also, after re-reading the OPs first post he is asking for +/- .015 (∅.030 tolerance zone) and +/- .004 (∅.008 tolerance zone).

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