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Geometric Tolerance - a second opinion

Question asked by Kevin Andrews on Oct 12, 2017
Latest reply on Oct 13, 2017 by S. Casale

I haven't really had to use geometric tolerancing for a few years and I am a little bit out of touch.

 

We have a part that is constantly coming in from our supplier with the mount holes misaligned (slots in the mount tabs). The slot locations are relative to the location of the main connection point - which is the 4" NPT shown on the drawing below. This part mounts to the legs of a C12 channel (legs facing the part) - there is a larger hole for the 4" NPT to fit through on the inside face of the channel.

Note that I have applied a 0.03 (1/32") parallel geometric tolerance, to both legs of the mount bracket, in relation to the center line of the 4" NPT connection. If memory serves me correctly, this could lead to a max of 1/16" tolerance on the slot alignment - please correct me if I am wrong with that.

If that is the case, I am OK with that because the ones we have been receiving have been a full hole diameter(+) off - as in, you could get bolt #1 in, but couldn't see the hole at all on the other side for bolt #2.

 

This unit is supplied to us from a 2nd party manufacturer. This is their own design - meaning we did not do any of the manufacturing drawings for this unit or any of it's pieces. I created this drawing just so that we could make sure that we stay on the same page with the manufacturer and they know what we are expecting to receive from them. With that information, I did do a REFERENCE design for the mounting band (the part with the slots) that is included with this drawing on the second page.

 

Our speculation is that, whenever the mfr. rolls the center part of this bracket, they, somehow or another, twist the piece - so that the slots no longer line up. I created the reference drawing just to drive home the point that we want these legs parallel - or at least, we want the holes to line up as close as possible. Here is the detail on the second page:

My questions are:

1) Is the geometric tolerancing on the second page superfluous?

2) Will my .03 variance on each page combine to allow a greater/looser tolerance than what I am after in the assembly? My gut tells me NO, because, regardless of the second GDT, the assembly tolerance holds priority.

 

Thoughts??

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