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Author: Subject: Complex tolerance stack advice
bi22le

posted on 13/2/18 at 11:16 AM Reply With Quote
Complex tolerance stack advice

Hi all,

I am working on something at work that my normal adhoc simple GD+T method can not solve.

Essentially I have 3 sheets of metal, each with 2 holes. Considering only 1 dimension the holes have a position tolerance from the edge of the sheet and pitch between the holes, the other tolerance is regarding clearance.

so, if you could imagine a bolt going through these 3 sheets of metal, that have holes that could move in position, pitch and yet have a clearance from the fit to 'soak' up any miss-alignment. Now consider a 2nd dimension and how a hole reduces 'soak' as it moves in a perpendicular axis.

Now you see the complexity!!

Any techniques or tools in completing this task that I may not already be aware of? I am currently use a max \ min matrix but this is leaving gaps in the analysis that need to be considered.





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dinosaurjuice

posted on 13/2/18 at 11:45 AM Reply With Quote
Basic (box) dimensions with a positional GT on the hole position. Calculate the minimum clearance between shaft and hole (based on there tolerances), and use this for the positional target zone.
If you want to get fancy, assign datums to two orthogonal edges and the face of the plate (A,B,C), then reference A&B on the positional, and C on a perpendicular (only worth doing if thickness is > diameter normally). Add an MMC to help the supplier out.

Googling 'GD&T hole in plate' explains this better with pictures than i ever can with words!

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nick205

posted on 13/2/18 at 01:00 PM Reply With Quote
Can the 3 sheets of metal be stacked one upon the other and have the holes made in them together?

If done this way alignment of the holes should be much much better.

During assembly can you not butt the 3 sheets against a straight edge when the bolts are inserted?

If done this way the edges of the 3 sheets of metal should be aligned whilst the bolts are inserted.


Are the 2 holes to be drilled, punched, laser/water cut?

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bi22le

posted on 13/2/18 at 03:51 PM Reply With Quote
quote:
Originally posted by nick205
Can the 3 sheets of metal be stacked one upon the other and have the holes made in them together?

If done this way alignment of the holes should be much much better.

During assembly can you not butt the 3 sheets against a straight edge when the bolts are inserted?

If done this way the edges of the 3 sheets of metal should be aligned whilst the bolts are inserted.


Are the 2 holes to be drilled, punched, laser/water cut?


LOL, These are not actually 3 pieces of sheet metal. I just used this as an example.

In fact it is a metal extrusion with inserts, a gas filled structural plastic injection moulded part and punched stainless steel plate!!!





Track days ARE the best thing since sliced bread, until I get a supercharger that is!

Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1

Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I

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nick205

posted on 14/2/18 at 09:49 AM Reply With Quote
Understood.

We have sheet metal parts made for us by a supplier at work. They used to be punched and folded, now they're laser cut and folded. When faced with the issue the way I interpreted yours the supplier ended up stacking the sheets to ensure they all turned out the same.

Given what you're actually working with I'm not sure how I'd approach the issue. From experience I do know it's often best to talk with the people who'd be doing the work. They often have the best understanding of how their equipment works and how to get the desired results.

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