Question about LMC

Guys, having a brainfart here...

Have a part, which is a hex nut and I have to put 6 wireholes in it.
The wirehole dia is .05, with the UOS being +/-.01.
The true position callout is .010-LMC to the minor dia and face of the nut.

Does this make sense???

You haven't told us all about a complete tolerance callout however, ignoring the missing info, what you have told us make reasonable sense to me.

Basically, if a hole dia. is .06 it must be located within .010 of the nominal true position.
If a hole dia. is .04 it may be located within .030 of nominal true position.

That explanation does not take into account any effect of the actual size of the minor diameter.

I'm guessing that the holes should be clocked relative to the faces of the hex. Maybe that's your question? Without seeing the drawing I can't help. . . too many options.

Please define "UOS" as I've not heard that acronym before. I took it to mean the title block tolerance for 2 place dimensions.

extropic said:

Please define "UOS" as I've not heard that acronym before. I took it to mean the title block tolerance for 2 place dimensions.

Click to expand...

Yes, UOS = Unless Otherwise Specified

extropic said:

That explanation does not take into account any effect of the actual size of the minor diameter.

Click to expand...

I do not believe the size of the minor dia has anything to do with true position, as the datum is the C/L of the minor.

extropic said:

Basically, if a hole dia. is .06 it must be located within .010 of the nominal true position.
If a hole dia. is .04 it may be located within .030 of nominal true position.

Click to expand...

Exactly the part that does not make sense.
If the hole is "small", it can be almost anywhere, but if it's "big" it must be w/in .01?

That flies completely against what I'm guessing about mating parts.
IOW:
If the hole is "small", it should be no more than .01 out of TP to mate with an other hole that is "small", or a shaft that is "big".
At the same time, if it is "big", then it's center can be out by as much as .04 and would still mate with the same part mentioned above.

SeymourDumore said:

... the datum is the C/L of the minor.
If the hole is "small", it can be almost anywhere, but if it's "big" it must be w/in .01?

Click to expand...

Could you please explain the second sentence in the context of the first?

If the datum is the C/L, how does the hole diameter affect position at all??

- Leigh

Call whoever drew it. Maybe they meant to say the hole has to be where it is in 2 directions but the face has stock? Hard to see without a picture.

The real Leigh said:

Could you please explain the second sentence in the context of the first?

If the datum is the C/L, how does the hole diameter affect position at all??

- Leigh

Click to expand...

Leigh, those are two separate statements referring to two different features.


For the first statement, the datum is the C/L of the minor diameter, which by definition is RFS. IOW no matter what the ACTUAL minor diameter is, it's center line is the datum.

The second statement refers to the wirehole, which can be .05 +/-.01, where "small" = .04 dia, "big" = .06 dia.

dstyr

Can't call the "picture maker" as this part is a hand-me-down from a regular customer who broke 7 ( count: SEVEN ) drills into 12 parts attempting to do it with HSS drills and no fixture.

The main question is actually kind of rhetorical as I was told to just get them holes drilled and the drill-remnants removed as they need to ship in the AM.
Location doesn't matter much, ignore print, ignore look, ignore everything, just git'r done. That, I already did do.

I'm only asking because I cannot think of an explainable reason to define wirehole locations this way.

As for a real blueprint, if anyone ever seen the drawing of a hex nut with wireholes, then you've seen them all. Plain vanilla.

Hex size, thread size, length and location of 6 x wireholes with TP callout in respect to thread minor [A] and one face . That is it.

Similar to this:
Redirect Notice

Only to add a TP callout to dia L.

I believe they are merely trying to control the wire holes distance from centerline (dimension J in drawing you linked) and distance from the face of the part. I am no expert but it seems they should also have something controlling where the wire holes are rotationally with the flats.

They may be using LMC to keep a minimum thickness between the wire hole and the outside of the part. Could be a strength issue.

-Kurt

SeymourDumore said:

I do not believe the size of the minor dia has anything to do with true position, as the datum is the C/L of the minor.

Click to expand...

As I stated, you haven't provided a complete callout. The size of the minor diameter would affect the total tolerance IF the datum reference were modified by either MMC of LMC.

SeymourDumore said:

Exactly the part that does not make sense.
If the hole is "small", it can be almost anywhere, but if it's "big" it must be w/in .01?

That flies completely against what I'm guessing about mating parts.
IOW:
If the hole is "small", it should be no more than .01 out of TP to mate with an other hole that is "small", or a shaft that is "big".
At the same time, if it is "big", then it's center can be out by as much as .04 and would still mate with the same part mentioned above.

Click to expand...

You have written that the subject hole are "wireholes". Wireholes aren't "mating" with anything except a length of safety wire. Right?

The use of LMC in this case makes perfect sense to me and I would interpret it as the designer allowing the loosest reasonable tolerance while maintaining a minimum edge distance from the hole(s).

If it makes you more comfortable to have less tolerance = brainfart?

I agree with Knaz85 and extropic. The LMC spec is ensuring that the holes don't extend "beyond" a certain area. If they are smaller, you have more room to play with. If they are larger, less margin for error.