Fit formulas at LMC

[Tunalover]

I use the floating- and fixed-fastener formulas for designing hole patterns. These formulas apply with the holes and fasteners at MMC. Is there an easy way to get versions of these formulas that apply at LMC? Does that even make sense? I recently had a problem with a hole pattern having holes close to edges. I applied the tolerances at LMC to prevent breakthrough but scratched my head when figuring the in-pattern positional tolerance. TIA for your valuable inputs!

Tunalover

 

[ringman]

In the days of Y14.5-1988, you could have used composite tolerancing for the location of your hole patterns. The pattern locating tolerance zone would effectively establish your edge distance.

Depending on what version you have on your drawing, you may want to consider using that scheme.

 

[drawoh]

tunalover,

Use two holes as secondary and tertiary datums. Specify the hole tolerances to work at MMC. Put a profile tolerance on your outline. You can specify your profile tolerance to slop outside the nominal outline. It is explained in the manual.

Now your ourline is controlled by the hole pattern, which is really what you want, I think.

Applying a positional tolerance at LMC makes no sense to me in your application. I have actually done this, on a casting with two big holes. The LMC condition left me a minimum amount of material in place for a machine shop to cut accurate inside diameters to conventional MMC tolerances.

JHG

 

[Tunalover]

ringman-
I don't think composite tolerancing will do anything remarkable to prevent breakthrough under the 1994 standard or the 1982 standard (not 1988). Thanks for the thought though!


Tunalover

 

[Tunalover]

drawoh-
Thanks for the input. While I'm sure your method will work, it is complicated and not something I'd want to make as a standard practice. I'm sure that there is a simple modification of the fit formulas to make them apply at LMC. I just don't know what it is!


Tunalover

 

[type26owner]

I guess the simple answer is the LMC virtual condtion boundary isn't for fits. If I remember correctly it's just to ensure the wall thickness values don't exceed a worst case minimum. You should use only it for that purpose.

 

[drawoh]

tunalover,

You can apply a positional tolerance of zero, with no material conditions specified, RFS being the default.

For each of your holes, you get a tolerance zone of MMC to LMC. This controls the outline of your holes at MMC and at LMC, which is what you need. The holes can be off a bit if they are between MMC and LMC. You may have to open up your diameter tolerances a bit to keep things fabricateable.

Why was my first suggestion complicated? You need three datums. Your hole positions and your outside outline dimensions are basic. If your outline is complicated, this dimension strategy gets very simple.

Another strategy is to set the positional tolerance to zero at LMC. If your fastener is a bolt and nut, your MMC diameter is...

MMC = (LMC + BoltDia)/2

For a screw with a positional tolerance of SPT...

MMC = (LMC + ScrewDia + SPT)/2

This is more obvious if you draw it out. I think it is rather complicated. I think my first two ideas are simpler and more logical, and my original idea is better design practice. A large allowable variation in hole diameters means a lot of clearance around the bolt, and not much material under the head.

JHG