GD&T practices

is there anyway you can scan in an image?

NTS

This is what needs the GD&T. The holes need to be aligned with the groves/slots, but the slots can float somewhat within the width of the part.

Thanks

Ron

you say the slots can float but the holes have to move with the slots?

If I under stand this, I'd make the surface where the holes are drilled at my primary datum and the center of one slot as my secondary and the center of the other slot also as a secondary datum. center of the part itself as the Tertiary datum.

then it's a simple true position call out |0.0xx | A | B-C | D |

that means they have to establish a datum through b-c to get the part centerline.

Interesting, why use the drilled surface as datum A?

Thanks,

Ron

The drilled surfaces need to be called out as the primary datum to keep the holes perpendicular to that surface Ref ANSI 14.7-1994

You want a true position call-out. Holes are the controlled featured restrained by slot position. Slot to be dimensioned to datum of your liking, center or edge of part?

You have ref dims to keep from double dimensioning. Often times a simple part likes this can be adequately controlled by using non-standard tolerances. That is to say incrementally decreasing +/- allowances without feature control frames. This part would be very easy to over-dimension. Of course if you need those restraints (especially the turning/rotating of parts) then by all means, restrain away.

But for the matter at hand let's say slot position is controlled from edge of part
+.002/ -0.000. Slot width is +/- .001 and hole is true position .002 MMC to datum A (slot width).

Crap this is hard to explain!

Hope it's a little help. My numbers are arbitrary and in no way assume proper form, fit or function of your component.

cr3

Thanks,

I see where your going, I can think I can take it from here.

Ron

"We have a yoke attached to a piston and the yoke rides on guides."

Either the features used for attachment to the piston OR the features that ride on the guides should become the basis for the datum reference frame. Which is assembled first, yoke into guides or piston to yoke?

You also say "This is the part that needs the GD&T." Do you mean to leave the other mating parts and the resulting assembly without GD&T?

It is often a failure to appreciate the functioning of parts / assemblies that results in faulty GD&T.

Disagree Guest.

GD&T is used for each part so it can stand alone. The tolerance that is applied should come from the assembly in some way, but in no way should the tolerance of the assembly belong on a piece part. If you handed me a print for a single part, I should be able to make that part to the print without further information. And then hand you that part and if you dimensioned the drawing correctly, the assembly should all fit and function correctly.

My understanding is that GD&T is used for assemblies. The manufactured part does nothing by itself, it's the assembly that matters. Both have valid points. In this case the assembly IS the reason for the tol. Thanks for your help.

Ron

In general, assembly variation is the result of part variation (with some process variation thrown in as well).

Equally important to tolerancing is datum structure, which is almost always best when representative of how the part fits into its assembly.

How do you expect someone to dimension and tolerance a part without knowing where it fits and how it functions?