GD&T for a groove machined in a flat surface

[rdpostak]

Which GD&T would be best for control. Profile of surface or True Position with Boundary?


Richard D. Postak
Aerospace Mechanical Designer
Schafer Corporation

 

[drawoh]

rdpostak,

Profile of surface locates your groove from the datums. True position allows you to locate a feature of size. Probably, the true position is very much less critical to you than the size of the feature. I cannot see why I would accurately locate a sloppy feature of size, but maybe you have a good reason.

What are you trying to accomplish?

--
JHG

 

[drawoh]

... I cannot see why I would accurately locate a sloppy feature of size, but maybe you have a good reason.

Oops! That is not true. I love to locate wildly inaccurate holes to zero position at MMC/B.

--
JHG

 

[JNieman]

Vague question.

Equally vague answer: Whichever one best suits the designed necessity of the feature.



Less-sarcastic-answer: Grooves can vary a lot. Are we talking about a groove for an o-ring? Are we talking about a groove to let gravity drain fluid out of a stationary object? Or is it a groove that will allow material from an adjacent member to be staked/deformed into it to resist fastener walking?

One application may require tight profile control in both location and form. Another might not care about size, but the position matters. Another might not so much care about location, but size is more important. Sorry for the non-answer.

 

[rdpostak]

JNieman,

No worries and all good questions, however I am not the designer just the checker and still sort of new at it. I will ask the designer on the intent of the groove and go from there.

Thanks!

 

[Bflo]

I attached a drawing of my attempt to apply GD&T to a profile callout on a curved surface seeing as this is a related subject to this thread. It's the .020 radius in B4 & B5. Any suggestions?

 

[drawoh]

Bflo,

Your datums look weird to me. I am glad I don't have to inspect this. Since I don't know how you will use the part, I am saying weird, not wrong. Try to work out your fixturing, given that your primary and secondary datum feature are FOS holes, and the bottom face is your tertiary datum feature.

I take it your slot wraps around the end? As per ASME Y14.5, your specification is meaningful. I cannot see what you are trying to do.

--
JHG

 

[Belanger]

To the OP: position controls only location. Profile controls form, and often location, size, and orientation. So the key to your question is whether you wish for the GD&T to control form or not.

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

 

[Bflo]

I could be totally wrong in how I'm doing it since I'm a total novice to GD&T. I'm trying to control the form of the profile of the R.020 according to GD&T rules, and wondering if I'm doing it correctly, or even if the Datums are done correctly. I was actually going to post a new thread question about if the Datums were even done correctly or not, but I can just ask the question here...? My boss and I were thinking agreed that it's more important that the inspection fixture would need the pins to be lined up to the holes before worrying about the bottom face, which is why the feature control frames are A (hole),C (hole), and then B(bottom face). As far as how the part is used, it is in an assembly that articulates and so the A datum is most important requiring the tightest tolerances, and C will need to be lined up to another part but will have a little room to float. The B datum will have to not be sticking out too much since it will be sliding near another face but isn't as important as A & C holes. The slot does wrap around the end of the part and a rough guide for a wire to floss along it. Hopefully that makes sense.

 

[drawoh]

Bflo,

I am still trying to understand your fixture. You have an accurate rod (Ø.0604/.0602?) to pick up datum face[ ]A. This controls[ ]X, Y, and two axes of rotation. You have a diamond pin that picks up two sides of datum face[ ]C, but clears the bottom of the hole. This controls the other axis of rotation. You have a flat face that picks up datum face[ ]B, and controls[ ]Z. Note how the flat faces contact at one point, only. There is no reason to assume they are parallel.

If this is a good, functional description of your part, then this is a good datum scheme. You might want to tighten up the diameter tolerance on your primary datum. Either that, or make sure you understand how your part works at anything other than MMB (MMC?).

--
JHG