English thread position at LMC

[aniiben]

An internal thread 5/32 position is modified LMC and has MAJOR note beneath. How to read and understand this geometrical call out?

I've seen a straight thread modified at MMC for gaging purposes, but never one modified at LMC.

Is it correct?

 

[3DDave]

Sure. And it makes sense. It establishes the maximum amount the thread can drift from perfect location in contrast with MMC which tends to prevent a feature from encroaching on the middle of the feature.

 

[pylfrm]

aniiben,

To interpret this, you need to know the maximum allowed major diameter of the thread. Does the drawing provide that? If not, does the thread standard?


pylfrm

 

[aniiben]

Pylfrm,
Yes the thread is standard and its applicable information ( min max values) found on the thread standard (B3.1 or equivalent) or the machinery's handbook.

How to measure or qualify this position on LMC for an internal thread? Any acceptable solutions?

 

[drawoh]

aniiben,

The fact that somebody is calling out 5-32 is a sign of trouble. There is 5-40UNC and 5-44UNF. A quick search of McMaster Carr's website does not reveal 5-32[ ]taps.

I would say that 5-32UN with the major diameter at LMC is completely interpretable, and damn near uninspectable. In addition to fabricating a custom tap, you need a custom helical probe that simulates the LMC condition of a 5-32UN hole. Fun? Wow!

The nominal major diameter of a #5 thread is .125".

--
JHG

 

[pylfrm]

aniiben,

If the tolerance is large enough, it might be possible to prove conformance based on measurement of some other feature of the thread.

What is the exact specification on the drawing? What you've posted so far ("English thread", "5/32", and "B3.1 or equivalent") doesn't mean much to me in this context.


pylfrm

 

[aniiben]

Sorry for the delay. I was out of the office for a couple of days.

Here is the spec I am questioning

5/8 -32 UN
Position within diameter .005 inch at LMC to A primary and B secondary.
With MAJOR note beneath
MAJOR diameter specification: .6250-.6275.

(Spec per ANSI/ASME B1.1-1989 (R2001)

Could you, please, help me to understand how to qualify this specification? Not sure how to measure a spec for LMC on an internal thread as above with MAJOR diameter shown.

How to get there inside the thread?

 

[pylfrm]

5/8 -32 UN
Position within diameter .005 inch at LMC to A primary and B secondary.
With MAJOR note beneath
MAJOR diameter specification: .6250-.6275.

(Spec per ANSI/ASME B1.1-1989 (R2001)

This doesn't make much sense. Can you post an image showing the relevant portion(s) of the drawing?

It's difficult to make useful suggestions based on the information you've provided. A detailed explanation of your situation and goal would be helpful.


Nothing you've said so far would seem to rule it out, so how about X-ray computed tomography?


pylfrm

 

[drawoh]

aniiben,

When you say "qualify", do you mean "inspect"?

--
JHG

 

[SeasonLee]

aniiben

Position tolerance applies to pitch diameter by default, if the minor diameter or major diameter is required to apply, as I know the minor diameter is specified on the internal thread while the major diameter is specified on the external thread, so are you sure it's the major diameter specified on an internal thread hole?

Season

 

[drawoh]

SeasonLee,

He has stated that it is an internal hole. If the LMC applied at the minor diameter of the thread, there would not be a problem.



--
JHG

 

[aniiben]

Drawoh,
Yes, I meant inspect. Qualify=inspect and confirm that the part/thread is per specification.

SeasonLee,
As drawoh said: internal thread with MAJOR note and position modified at LMC.

Pylfrm,
Not sure what X-rayed computed is and how can be used on stainless steel parts, but for sure our inspector does not have this technology available.

A port with the above specification must be inspected for position to make sure the hole is not too close to the edge of the valve body to prevent any burst pressure to be detrimental to the valve or the wall thickness to be under the minimum calculated value per the maximum working pressure multiplied by the safety factor chosen per the design.

 

[pylfrm]

aniiben,

Is there really no tolerance class specified for the thread?

Is "MAJOR diameter specification: .6250-.6275" actually on the drawing? That's much tighter than the standard tolerance of 0.625000-0.634108 for 5/8-32 UN-2B or 0.625000-0.633008 for 5/8-32 UN-3B.

Does the inspector have any control over (or even knowledge of) the manufacturing process and tools used to create the thread?


pylfrm

 

[drawoh]

aniiben,

You should be able to make a tool that will engage your thread and indicate its major diameter. Something like below...

--
JHG

 

[aniiben]

Pylfrm,
Yes there is no class tolerance specified for the thread and that is intentionally made like that because it is a modified MAJOR diameter of the thread therefore standard allowances are no longer applicable.
Not sure if our inspector care too much of their manufacturing processes even he should. His question has been: how to inspect the positional callout?

Thank you drawoh for the tip in making a stand-alone tool/gage specialized to inspect this requirement.

 

[drawoh]

aniiben,

It is a weird thing to do on the drawing, especially if you are only concerned about leaving "meat" between the thread and edge of your part. This is the usual reason for specifying LMC/LMB. A larger clearance specified from the thread's minor diameter would be way easier and cheaper to inspect.

--
JHG