MMC on Metric Threads

[MrComment]

I have a drawing witch (seasonally speaking) has a few Metric Threads with true position callouts to them. They also have a Maximum Material Condition modifier on them.

My question is how to you measure an M6X1.0 or M8X1.25 (internal) thread to apply your bonus tolerance to the true position. What is the nominal and the tolerance for these sort of features that should be used when one isnt specified.

With the CMM I can only measure the Minor Diameter, so I found the tap drill size for the M6 and M8 to be 5.0 and 6.8mm accordingly, but Im unsure what the tolerance would be on this sort of hole, and I guess I dont even know if Im on the right train of thought in this case.

Im sure someone out there probably does this sort of thing everyday, so I was hoping I could use a "phone a friend" on this one.

Thanks

[William Johnson]

There is no way that I know of to get bonus from a thread with a cmm. The bonus comes from the pitch dia. You can buy id pitch mic's (expensive) and add what you measure to your caculation.

[#2#]

I alway use the values found in Machinery's Handbook. This will give you all the dims you need. Does your drawing specify MMC at pitch dia, at minor dia, or at major dia?

To tell the truth, I usually just ignore the MMC and report RFS as it is difficult to apply MMC to a pitch dia. (what our drawings usually call out). I always make a note that this has been done. Most of the time the holes are withing tol. at RFS so it really doesn't matter.

[\v/inston]

WGLS

the bonus will be so small (minor dia tol) it probably won't make much of a difference.

[MrComment]

I'll just report it out RFS then, your right the bonus would have to be rather small if any.

After thinking about this part some more. (With food) It likely would have made more sense to give me bonus the more perpendicular the threaded hole was to the surface, rather then how big or small the threaded hole might be.

Thanks again for the advice guys.

[david phillips]

Pitch tolerance

M6 x 1 6H Pitch 5.35 - 5.5
M10 x 1 6H Pitch 9.35 - 9.5

[Ravenmaniac]

Our "little" board group tossed the issue of mmc modifiers on threads around before.....trust me...stick with rfs, it's cleaner and easier to explain. If your tolerance on the thread positions is that tight as to cause you worry then get some threaded jo-plugs or evaluate the part prior to threading with the pilot holes. Trust me, as said by others above, you're not going to get anything extra from the thread pitch.

[tking]

You can measure it with a pin gage to get the size, use the min max from the tables for an internal thread and apply it to determine if an individual hole will pass with bonus. I would not measure for size with a CMM. Go with RFS and double check with a pin gage for holes that need bonus. As you've already surmised, the bonus will seldom make the difference.

TK

[John Kingston]

TP on threads is from the pitch diameter UOS. You can't use minor diameter for the bonus. Most people I know don't bother with bonus on threads. Should be RFS.

[craiger_ny]

WESTRRFS
(what everyone said that recommended RFS)

[cmmguy]

Exactly what John K said. Plus dont expect the CMM to give you an accurate measurement of minor Diameter either - it can't happen.