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GD&T: Hole Patterns and 100% Fit 1

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amanz

amanz

Mechanical
Mar 23, 2021
6
Hello,

I currently work for a small company, and it has no acknowledgement of GD&T. I recently took an online course for GD&T, and I think it can help a lot in production because supervisors constantly get inspected parts that don't fit even though drawings meet specifications. I believe its the issue with plus and minus tolerancing without any bonus tolerances. So I want to solve the problem by including some GD&T for the company. Keep in mind I am still learning--in no way have I perfected GD&T.

All images are mockups of parts--not exactly the parts due to proprietary reasons (obviously).

I have a rear cover/plate that is used to mount various subassemblies. The subassemblies are mounted by four holes in a rectangular pattern. I want to control the hole locations relative to each other, but I don't care too much about the overall placement of the hole pattern, so what I used was a composite position tolerance. Here is an image of the drawing.

housing_part_parn1b.png


Does this appear to be correct on what I am trying to do? Is there any issue with Datum Feature C? Would it be difficult to determine the center of the part due to the curve sides? I also read somewhere that if feature control frames have the same datum features, then the tolerancing is required simultaneously. Does that work in the drawing for both patterns? I don't want to have both patterns toleranced simultaneously; I want the two hole patterns to be independent from one another.

Since there is a mating issue. I also redid the mating assembly part. I just made sure the virtual condition equaled the hole pattern's virtual condition. Essentially, its the same tolerance as the cover.

mating_part_vocosa.png


This should ensure 100% fitment, right?

Thank you for anyone helping me out. I really do appreciate it!

EDIT 1: Fixed datum feature in image two. (2020-03-23 @ 7:23pm)
EDIT 2: Fixed feature control frame in image one. (2020-03-23 @ 7:59pm)
EDIT 3: Did not change original post to save change history. Edits to drawings have been made below that were suggested by Greenimi. (2020-03-24 @ 10:05am)
 
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Just few things here:
- missing Ø symbol in the feature control frames (composite)
- datum feature C symbol is incorrectly placed in the second/lower figure. That is datum feature C? The pattern of the 4 holes?
- what is the size of datum feature C in the upper figure ? 10.00 "largest width" is not completely defined in my opinion.
- GD&T purist will claim that datum feature B and C are missing perpendicularity controls to the higher order precedence A and A|B| respectivelly. So, here is that.

amanz said:
I don't want to have both patterns toleranced simultaneously; I want the two hole patterns to be independent from one another.
Click to expand...
Add SEP REQT near the upper segment of the composite (separate requirements is already default for the lower segment anyway)
 
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Thanks greenimi for your input.

- I can't believe I missed the Ø symbol; thanks for catching that.

- Ah, yes! I realize that the axes of holes are not considered a feature of size. I was tying to establish a center plane datum feature relative to the size of the part. I shall move datum feature C to the dimension 4.00.

- I can see how it's not fully defined? I think what you're trying to get at is that the largest width can be a diagonal across the cover while satisfying datum features A & B. What I intended is the largest width dimension parallel to datum feature B. Is there a way to say that in GD&T? Would it be just a parallel callout underneath the 10.00 dimension with "largest width"? I think "largest width" is needed because I don't want to confuse the width dimension between largest width and largest width colinear to radius centers (if that makes sense).

- Actually, good point. Completely forgot about form control.

- Okay, so there is note that says that. I'll add it.

I uploaded revised images regarding what you said (above). I really do appreciate you helping me. I learned like 5 new things! If you can, then let me know if there are still issues.

new1_eyjxu8.png


new2_qhmdfk.png
 
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There is a little detail forgotten on the way to 100% fit.

Exactly what fastener are you trying to "fit" thru the holes.

Use this little formula and the Force will be with you:

Capture_rlunpc.png


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
 
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amanz,

You have placed [±][ ]tolerances on your outline radii. This is practically uninspectable. I strongly recommend profile tolerances for the outlines, especially if they are anything other than straight, orthogonal geometry.

I try very hard to avoid using features of size (FOS) as datums, unless they are accurate. Your outline datum[ ]C is not. If this were my part, I would use one of the hole patterns as the secondary datum, and I would apply a profile tolerance all around the outline.

--
JHG
 
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