Use of M modifiers on threded faetures

[Madhu454]

Hi All,

I am beginer to GD&T subject, I need your help in understanding the use of GD&T to threded fatures.

Queation is
1) Can we use M or L modifiers on threaded fatures?
2) What is really the LMC and MMC of a thred fature? Example If I say M10 X 1.25 what is the MMC or LMC?
3) How to check the LMC or MMC of the threded feature?
4) Does really play exists between the threded fatures when both produced at LMC.
5) When a positional control is used to locate the threded feature, does the axis of pitch dia should be with in the specified tolerance zone?

There are lots of such stupid questions in my mind for which i dont know clear answer.


Madhusudhan Veerappa
Mechanical Engineer

 

[ptruitt]

Have you purchased and read the Standard to which you are trying to comply? I am not familiar with the ISO Standard, but the ASME Y14.5-1994 or -2009 Standard explains the answer to this question better than most people who might respond here. This should be your first step.

Peter Truitt
Minnesota

 

[KENAT]

1. Yes. However, there are arguments about what it gains you. One theory relates to allowing use of inspection fixtures.

http://www.tec-ease.com/gdt-tips-view.php?q=188

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[fcsuper]

Question 2 (and maybe a bit of 3):
L increases locational variance as feature moves away from LMC. This is generally a bad choice when defining any type of internal feature, such as holes. This is because (in very simple terms) the feature is allowed to jump around an increasing tolerance zone as it gets smaller, drastically affecting its actual interface location.

M increases locational variance as the feature moves away from MMC. This is general preferred for most holes, but isn't of much value to a threaded hole. M would apply to the pitch of the thread (unless otherwise specified), which is not directly measureable.

Might I recommend simply using the default RFS method for your threads?

Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
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[KENAT]

L is generally bad for mating holes (so hence threads) but for many other applications entirely appropriate.

I use it sometimes on vent holes lightening holes etc. where about my only concern is that the holes don't get so close to each other that the webs in between fail.

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[drawoh]

Madhu454,

1) This is done by people, but I do not see the point. I regard threaded fasteners as self centreing. The whole point of MMC and LMC is that you have a bonus tolerance, which in my opinion, is not available in tapped holes.

2) Read the standards. Do you have a Machinery's Handbook?

3) It depends on which feature of the thread you inspect. You can easily inspect the minor diameter, but do you really care about the minor diameter? The pitch diameter and the major diameter are more challenging.

4) Of course, until you tighten them. In the Unified National system, two untightened class_3 threaded parts will have zero nominal clearance at MMC. I did not bother looking up ISO threads.

5) Yes.

JHG

 

[SeasonLee]

You may get an answer from the search from the forum, here is the latest thread regarding the modifier on threaded feature about half year ago.

http://www.eng-tips.com/viewthread.cfm?qid=285951

Thanks

SeasonLee

 

[fcsuper]

KENAT, Could get you to write an brief article (on SolidWorks Legion) about L mod use on vent holes? (I can make any images needed to illustrate the article.) This is a very good and informative topic to cover, and a use of L that some experts may not appreciate (I've talk to a couple of GDTPs who pretty much dimiss L for internal holes).

Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
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[drawoh]

fcsuper,

I specified LMC on a hole in a casting once. My intent was to machine it out in a subsequent process. My objective was to ensure that there was material to remove to create the final, accurate hole.

LMC is not a useful concept for mating features. That is not the only reason we specify tolerances.

Go ahead and add this to your article.

JHG

 

[SDETERS]

drawoh
We use this same LMC for the same reason on our castings. Doing this Allows the die caster more tolerance on cores that end up being machined.

 

[KENAT]

Matt, if I thought I could articulate the issue well enough and was confident enough in my grasp of some of the intricacies I might but I'm not sure I'm quite qualified.

Also, I'm definitely weak when it comes down to the implication on inspection so someone with that experience might be able to give useful input.

I suspect it will be a situation where many argue the little advantage you get from increased tolerance is outweighed by the extra inspection effort. My argument that you can always inspect RFS and if if passes you're good no more inspection, if it fails you have the option to allow for the LMC, seldom convinces folks so it may be that I'm missing something.

That said, I've used it several times on sheet metal and cast components with apparent success.

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[Belanger]

It's simple: use the LMC modifier when you're most concerned about the position of the biggest hole (such as drainage holes or maybe a vent hole) and use the MMC modifier when you're most concerned about the location of the smallest hole (such as a clearance hole for a bolt).

Don't use either modifier (meaning RFS) if you're equally worried about a hole at ALL possible sizes (such as a press-fit dowel hole).

But it's true that there are other considerations such as the desirability of using a fixed (hard) attribute gage.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems