Guage design to varify parts with L modifiers 1

[Madhu454]

Hi All,

I have a question on designing a cartoon gauge for the drawing shown in the attachment. I got this drawing from one of the book.

I am very beginner in GD&T, I have studied in books that how to design a guage when there is a use of M modifiers on the parts , but there are very less examples on the use of L modifiers in the books.

I have read, a fixed guage can be used to verify the parts which uses L modifiers.

Please go through the attachment,
can a fixed guage can be designed to the part shown?
because the LMVC is dia 69.8, where as the size of the part is dia 70 - 69.9 . The part will not fit in to the gauge if we use dia 69.8 for guage design (LMVC size).

Please help me to understand the concept correctly.

Thank you.





Madhusudhan Veerappa
Mechanical Engineer

 

[Madhu454]

Soory I forgot to mention the datum A, datum A is for the axis of the hole 35 +0.1/-0.

Madhusudhan Veerappa
Mechanical Engineer

 

[Belanger]

Hi -- you can't design nor make a fixed gage when LMC is involved. Your last sentence explains why:

"The part will not fit in to the gauge if we use dia 69.8 for guage design (LMVC size)."

Since fixed gages check the "worst case," a gage designed for the LMC's worst boundary would not allow anything different than that boundary to even fit!

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

 

[KENAT]

You cold make some kind of overlay or something though correct? Indicating the furthest potential extent of the hole edges?

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[axym]

Madhusudhan,

You can design a cartoon gage for tolerances in which LMC is involved. The difficulty is that a physical version (hard gage) won't work. This is because the virtual condition boundary is inside the material, and as John-Paul says the part won't fit into the gage.

Despite this, the concept can still be used. If points on the part surface are scanned using a CMM, the set of points can be compared to the virtual condition boundary in the CMM software or in a CAD system. This is sometimes called "soft gaging".

The cartoon gage in your sketch is correct. But with LMC the concept is that the gage must fit inside the part, without breaking through the surface.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Good point, Evan and Kenat. I was thinking purely of a physical gage, but of course a sketch or cartoon gage is valid.

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

 

[SeasonLee]

Are there any ease ways to detect a bad part on mass production if hard gage cannot be used on L modifier?

SeasonLee

 

[Madhu454]

Thanks Evan,
This is the first time I came to know something called "Soft guaging'. I think I have lot you learn from experts like John-Paul and all members of the forum.

Madhusudhan Veerappa
Mechanical Engineer

 

[Madhu454]

Hi John-Paul, when there is no physical guage can be designed for the part when there is L modifier is mentioned. I am curious to know about the datum shift concept when L modifier is used.

Question
1) Does datum shift is allowed if L modifier is specifed?
(Because when there is no fixed guge, then how the shift can exists?)

2) Does Fixed guage and Datum shift concepts are inter related in any way? ( Because when there is no fixed guge, then how the shift can exists? I have read in book, the shift is allowed when M or L modifiers are used with datums)

Please clarify my doubts.

Thank you.

Madhusudhan Veerappa
Mechanical Engineer

 

[Belanger]

1) Yes, it is legal to use the L modifier on a datum. And mathematically, there can be a resulting datum shift. But from a practical viewpoint it is rarely used for the same reasons given above: if we're building physical parts, and a datum is referenced with "L" (called "LMB" or least material boundary"), then how can any part with more material on that datum feature even fit?

But I keep speaking about parts where this is an actual fit or assembly. There may be other reasons that drive the use of the L modifier (such as wall thickness concerns for LMC).

2) Yes, there is a direct relationship between fixed gaging and datum shift. If we have a fixed gage (meaning a physical gage or fixture that is built to one size), then of course every part that is manufactured with not fit on that gage in the same way. Think of the M modifier on a datum:

A part with the datum feature manufactured at its worst-case extreme limit will just barely fit onto the gage (perhaps even a line-to-line fit, but that's a different discussion about how to apply tolerances to gages).

An acceptable part whose datum feature is not built at the worst case (say that the datum hole is a toward the larger allowable diameter) will certainly fit on the gage, but it won't be a tight fight; it may jiggle around a little. And that's OK: it still assembles properly. This jiggling or looseness is what we call datum shift. It can be calculated from the tolerances on the drawing, and should be accounted for when doing tolerance stacks.

Since the L modifier on a datum reverses the effect, the previous paragraph would be revised to consider a datum hole that is made toward the smaller diameter (away from the LMB). Well of course a smaller hole won't fit over a fixed gage that was designed with a pin meant for the bigger (least material) hole. This is why we have the answer I give in your question #1.

I recommend you consider a basic GD&T class of some kind

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

 

[Madhu454]

Hi John-Paul,
Thank you for taking time to reply to my question.
I had understood the concept of datum shift with the M modifiers. with L modifiers it is was confusiong, and many books have not covered this subject much. Now I am clear with the concept, to summerise your points

For L modifiers :
1) No fixed guage can be produced to inspect the parts.
2) Only a theoretical/mathematical datum shift exists.

I consider your advise of joining GD&T course , but unfortunately here in my place, they dont have training programmes for individuals, they only have corporate training programmes for a batch of 25 or so.

I have purchased few books by Alex Krulikowski, i felt it is a good book which explians the concepts clearly (but less information on application part).

Is there any good books available which tell about the application of GD&T, Tolerance allocation, etc., (to learn how to put these controls into the parts? or it has to come by experience?). Please advise me.

Thank you,






Madhusudhan Veerappa
Mechanical Engineer

 

[KENAT]

Madhu, the first document I'd want is a copy of the relevant standard. Are you working to ASME or ISO?

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[axym]

Madhusudhan,

As John-Paul touched on, the functional reasons for specifying the (L) modifier usually relate to ensuring that there is enough material on the part surfaces. This is often for the purpose of maintaining a minimum wall thickness or edge distance, or for ensuring material removal on workpieces that will be machined. In your example, one possible reason for specifying the Position tolerance of 0.1 at LMC could be that the part is going to be machined with an ID of 35.1 and a coaxial OD of 69.8. The Position tolerance ensures that the part is a suitable workpiece.

This type of functional consideration is very different from clearance for assembly, to which the (M) modifier applies. I would consider the application of the (L) modifier, especially on datum features, to be an advanced GD&T concept. It is obscure and difficult enough that few GD&T books give it much coverage. Most focus on the much more common and physically gage-able MMC applications.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[DeanD3W]

Madhusudhan,
I think you're getting good information from other posts, and there's one additional thing I'd like to mention...

While their part drawings may not always have L modifiers on datum features, the casting and forging industries essentially use them every day as they "balance" a casting before machining. They need to shift the part relative to the coordinate system used for machining operations to ensure that all machined features will have sufficient material present to clean up nicely when they are machined. This is just like shifting the part relative to a datum reference frame, to satisfy tolerances on features that are to be machined, while keeping the "Least Material Boundary" inside the material of the datum feature(s). The point is, as Evan said, to ensure that there is enough material present on part surfaces. Applying an L modifier to the datum features is a way of encoding this "balancing" of a casting or forging (or any part in a "to-be-machined" state) using GD&T.

Dean

http://www.d3w-engineering.com