I'm relatively new at my position so please forgive me if this is a newb uestion...
I have a callout for profile of a surface reference to the datum scheme with MMC bonus. How does maximum matarial condition apply to a datum surfce?

I'm relatively new at my position so please forgive me if this is a newb uestion...
I have a callout for profile of a surface reference to the datum scheme with MMC bonus. How does maximum matarial condition apply to a datum surfce?I'm going to give you some of the best advice you'll ever receive:
When a customer or employer or anyone else gives you a document or instruction or anything else you don't understand, the best and most accurate answer you will ever get is by asking that person or entity exactly what it means. Do not let pride or embarrassment come between you and the correct answer. If he doesn't know, then you can continue on the "search for the truth" together and result in building a better personal bond between you.

If I or anyone else gives you an answer to your question, it may be technically correct, but at variance with the meaning or interpretation of the person who first presented the problem to you. You could then be setting yourself up as seeming to call the problem presenter "confused" or "inaccurate" - not generally conducive to appeasing customers or bosses.

I agree with Wes wholeheartedly. It is good advice, and too many of us are afraid to ask our customers. But, I have found that when you follow it, your customer usually respects you for it and life is easier.

Having said that, I'll give the technical answer. MMC, when applied to a datum is typically applied to a feature such as a hole that may be used as the origin or for alignment. Your gage would typically be made using a pin that is made to the smallest datum hole diameter minus any allowances that might have been applied to the feature.

Your question was not clear on what the MMC modifier was applied. The more specific your question, the better the answer.

That is definitly sound dvice and it is appreciated however the problem that i run into when i try to do exactly that is that the people i am working with give me an I don't know answer. I am not in a customer contact position and when the engineer in charge of a job does not have a clear explanation for what i am looking for I am still responsable for completing the task.
Is there perhaps any GD&T resources that you could direct me to so I can research the issue?

I agree with Wes wholeheartedly. It is good advice, and too many of us are afraid to ask our customers. But, I have found that when you follow it, your customer usually respects you for it and life is easier.

Having said that, I'll give the technical answer. MMC, when applied to a datum is typically applied to a feature such as a hole that may be used as the origin or for alignment. Your gage would typically be made using a pin that is made to the smallest datum hole diameter minus any allowances that might have been applied to the feature.

Your question was not clear on what the MMC modifier was applied. The more specific your question, the better the answer.

Print spells out the feature as follows (Profile of Surface 1.5(mmc)|A|B|C|)

I think that I would buy the standard (ASME Y14.5M-1994) and/or visit tec-ease.com

The datum must be a hole or pin to have MMC apply. MMC of a hole is the smallest allowable size.

Now we do have a problem though. Profile of a surface is an actual measurement and is the range of a bi-lateral tolerance in most cases. If you have 0.5 mm profile of a surface, it means that from the theoretical surface (called true profile), we have a + & - 0.25 mm.

Having the datum at MMC blows the mind especially if one is on a CMM (co-ordinate measuring machine). You really can't do that so the datum hole will be picked up at RFS (regardless of feature size). Just obtain the centre of the hole.

MMC on datum holes is most applicable in positional tolerances and NOT profiles.

Hope this helps.

Shy poster:

Go to my web site for a better explanation.

Are there any folks out there using GD & T symbols in Process Control Plan files and if so, whose software is being used. We currently use Dyadem and have ran into some issues getting the symbols in the work sheet cell

I would never place GD&T symbols in a Control Plan but their respective meaning.

The Control Plan is really inspection instructions in a manner of speaking and placing positional tolerances to datums A B MMC and C MMC using the feature control frame will blow everyone's mind on the shop floor. How about stating "positional" in a Control Plan and then reference in some manner how the checking fixture functions and how to use it. I think this is much better.

The datum must be a hole or pin to have MMC apply. MMC of a hole is the smallest allowable size.

Now we do have a problem though. Profile of a surface is an actual measurement and is the range of a bi-lateral tolerance in most cases. If you have 0.5 mm profile of a surface, it means that from the theoretical surface (called true profile), we have a + & - 0.25 mm.

Having the datum at MMC blows the mind especially if one is on a CMM (co-ordinate measuring machine). You really can't do that so the datum hole will be picked up at RFS (regardless of feature size). Just obtain the centre of the hole.

MMC on datum holes is most applicable in positional tolerances and NOT profiles.

Hope this helps.

So essentialy you can not have MMC of a surface profile an there is an error in the part drawing?

So essentialy you can not have MMC of a surface profile an there is an error in the part drawing?:topic:See what I meant about "variation" in answers? You really do need to work your way through the food chain to the guy with the definitive answer for which your product must meet requirements. If not, you may just waste time with what we used to call a "circle jerk" forty years ago.

Referencing a profile of a surface back to a hole at MMC is legal per ASME Y14.5M-94 but cannot be measured accurately.

The only possibility of use would be in the making of checking fixture for positional and also contour. The checking fixture would have inner and outer boundaries and one would just look to see if the surface falls between them. This is only practical if we have maybe a profile tolerance of 3 mm or .120 thou. It is NOT practical for 0.25 mm or .010 thou.

Imagine trying to measure a height of a surface from the centre of a hole. Yes, one can do that. Now imagine measuring from the exact location of the surface from a pin of the smallest allowable hole size that goes into the datum hole. One has movement and getting an accurate measurement is tough if not impossible.

Some people think that they add the difference between the smallest hole size and the actual size of the datum hole to the tolerance and sometimes it does work out. Sometimes it doesn't work out.

Profile of a line or of a surface should reference the datum hole in RFS. We can measure it that way.

Design personnel should be discouraged to applying MMC on datum holes when using profiles.

Does this help clear it up. I hope so.

I would never place GD&T symbols in a Control Plan but their respective meaning.

The Control Plan is really inspection instructions in a manner of speaking and placing positional tolerances to datums A B MMC and C MMC using the feature control frame will blow everyone's mind on the shop floor.

Using GD&T symbology in control plans is common practice; the best way to avoid confusion is to transfer drawing requirements verbatim whenever possible. Why would anyone's mind be blown by a feature control frame on a control plan, but not on a drawing? As to aa8vs's question, you can use a GD&T font, one of which is available here (http://elsmar.com/Forums/showthread.php?p=78703#poststop).

Print spells out the feature as follows (Profile of Surface 1.5(mmc)|A|B|C|)
This appears to be an error in the callout. I do not have a copy of the ASME Y14.5M-1994 standard available, but am referring to a Ford pocket guide. It states that the MMC modifier may be applied to the datum of reference only, not to the Profile tolerance itself.

An example of a legitimate callout: (Profile of Surface 1.5|A|B|C(mmc)|)

Referencing a profile of a surface back to a hole at MMC is legal per ASME Y14.5M-94 but cannot be measured accurately.

The only possibility of use would be in the making of checking fixture for positional and also contour. The checking fixture would have inner and outer boundaries and one would just look to see if the surface falls between them. This is only practical if we have maybe a profile tolerance of 3 mm or .120 thou. It is NOT practical for 0.25 mm or .010 thou.

Imagine trying to measure a height of a surface from the centre of a hole. Yes, one can do that. Now imagine measuring from the exact location of the surface from a pin of the smallest allowable hole size that goes into the datum hole. One has movement and getting an accurate measurement is tough if not impossible.

Some people think that they add the difference between the smallest hole size and the actual size of the datum hole to the tolerance and sometimes it does work out. Sometimes it doesn't work out.

Profile of a line or of a surface should reference the datum hole in RFS. We can measure it that way.

Design personnel should be discouraged to applying MMC on datum holes when using profiles.

Does this help clear it up. I hope so.

The ASME Y14.5M-1994 standard was created with part functionality in mind, not variable measurement and process control. Trying to measure some callouts on a CMM is a quick road to frustration.

A Profile callout using MMC on a datum must be checked using a functional gage.

From Miner

"The ASME Y14.5M-1994 standard was created with part functionality in mind, not variable measurement and process control. Trying to measure some callouts on a CMM is a quick road to frustration.

A Profile callout using MMC on a datum must be checked using a functional gage."


I would love to see someone check a profile of a surface of 0.1 mm with a checking fixture. Yes it can be done if the tolerance is large but not if the tolerance is small.

I think that this is one area where ASME is not quite correct. Would we ever be able to check positional tolerances in RFS when the secondary and tertiary datums are holes at MMC. It may be legal but not practical.

I do agree the positional in MMC relating to datum holes in MMC is certainly a checking fixture or functional gauge but not when the requirement is in RFS or actual measurement. Of course, that is my opinion.

When there are datum holes at MMC, CMM Operators set them up in RFS.

I would love to see someone check a profile of a surface of 0.1 mm with a checking fixture. Yes it can be done if the tolerance is large but not if the tolerance is small.

Agreed, but arguably putting an MMC on a datum when the tolerance is .1 mm may be questionable engineering judgement.

I think that this is one area where ASME is not quite correct. Would we ever be able to check positional tolerances in RFS when the secondary and tertiary datums are holes at MMC. It may be legal but not practical.

I do agree the positional in MMC relating to datum holes in MMC is certainly a checking fixture or functional gauge but not when the requirement is in RFS or actual measurement. Of course, that is my opinion. Agreed, but the question was about MMC, not RFS.

When there are datum holes at MMC, CMM Operators set them up in RFS. This is common practice, but you are giving up valuable tolerance. If your process is capable without it, great. If not, get a functional gage.

Wes Great Advice!

Print spells out the feature as follows (Profile of Surface 1.5(mmc)|A|B|C|)

Coleman,

The specification is wrong because according to your quote the (mmc) modifier is applied to the surface tolerance... not the datum features.

Datum shift [applying (mmc) to datum features of size] is permitted by the standard but applying it to the tolerance is not!!!

Dave,

I respectfully disagree with you again and again. Datum shift is permitted with profile tolerance specifications by the standard because it can functionally be permitted occasionally. Furthermore it can be inspected in a number of different ways from the most rudimentry (paper graphical layouts) to more sophisticated methods (point cloud iterations on solid models). It can even be done with hard gage registry of the datum features and variables data collection of the profiled surfaces.

Coleman,

If your description of the callout is accurate it is wrong. Good luck on resolving it!

Paul

"Pushing up on the rope" is never easy

From Paul Jackson
"
Dave,

I respectfully disagree with you again and again. Datum shift is permitted with profile tolerance specifications by the standard because it can functionally be permitted occasionally. Furthermore it can be inspected in a number of different ways from the most rudimentry (paper graphical layouts) to more sophisticated methods (point cloud iterations on solid models). It can even be done with hard gage registry of the datum features and variables data collection of the profiled surfaces."

Paul

Yes we seem to disagree in a couple of areas for sure and I guess that I am addressing this thread from my experience in Quality and primarily as an old CMM Operator (we were called Precision Inspectors at that time at GM).

When one measures a dimension and reports an outcome, we are not biased whether the part is in spec or not. We report the actual and if it is nonconforming, someone will make a disposition.

Having a profile of a surface relative to a secondary and tertiary datums that are holes at MMC is a problem although it is legal using ASME Y14.5M-94.

Do we want the feature to be conforming? Maybe we are having a bad day and want to feature to be nonconforming? If we have datum pins at MMC in the datum holes and we try to indicate the feature's face if it is a plane, should we push the part so that the feature is higher or maybe let it drop down on the pins so the feature is lower.

We know that the tertiary datum is for anti-rotation. Maybe we could push up on the part on the pins on datum B and down on datum C rotating the feature. What should we do here?

Since parallelism or angularity is a component of profile of a surface if the feature is a plane, should we have angularity (perpendicular, paralelism or angularity) also referencing datum holes at MMC?

Now comes a favourite subject of yours. Let's do a capability study on the surface. Wouldn't we have fun with this? We could move the part north on one part since we get a better reading and then south on another one. We cold also try to tilt the part by pushing down on datum B and up on datum C pins. Does this make sense?

All of the feedback has been very helpful but has brought up a couple points of confusion for me. Perhaps it is unique to the software that i am using but when i run a part on the cmm alignment is established for each part taken from the datum scheme. If i were to move the part north or south as described above the alignment would be re-established and the readings SHOULD not be manipulated any differently. The points of measuremeant that i am taking are established by zones on the print so it is outide of my scope to move the measuremeant zone.
Since capability was also brought up... I am finding confusion here as well. Customer is asking me to show capability of the true position of a hole but how can you do a 30 pc. study when the tollerance is dictated by the actul feature and is different on every part? Is it fair to add maximum bonus to all samples or are we expected to incorporate no bonus to the study which could make capability unachievable by our process?

Coleman:

When I refer to moving the part on the datum holes at MMC, I mean that there is a checking fixture that have pins of MMC size when the datum holes locate in. The part "floats" around the datum pins.

Capability studies on positional is a separate thread and has been extensively approached in the past.

Well, Coleman, I'm an experienced old hand at interpreting GD&T, and was pretty sure I knew the "proper" solution to your problem. Now, I'm a little confused and unsure of the certainty of my original thinking.

If I'm confused, I sympathize with what you must be thinking about now.

My original answer still holds true - you have to get to the ONLY guy whose opinion matters and get the answer straight from him. If you have to work through channels, that's what you have to do. There is no reason to go forward until you have a definitive answer.

All of the feedback has been very helpful but has brought up a couple points of confusion for me. Perhaps it is unique to the software that i am using but when i run a part on the cmm alignment is established for each part taken from the datum scheme. If i were to move the part north or south as described above the alignment would be re-established and the readings SHOULD not be manipulated any differently. The points of measuremeant that i am taking are established by zones on the print so it is outide of my scope to move the measuremeant zone.

It is good to always keep in mind the fact that the whole concept of MMC and LMC was developed for part functionality and must be checked using a functional gage.

CMMs always measure RFS. Some can handle MMC when applied to a positional tolerance of a hole, but none that I am aware of can handle MMC applied to a datum feature. You can stil measure the feature RFS, but cannot take advantage of the bonus tolerance.

Coleman:

Capability studies on positional is a separate thread and has been extensively approached in the past.
Do you have a link?

Yes I do have a link and I know that Paul Jackson, who disagrees with me on this one, also has a link.

Wes - Absolutely - attribute gauges on Positional at MMC referencing datum holes at MMC.


http://www.qmsi.ca/capabilityindex.html

Coleman,

The advice given by Wes is very wise. No matter what you get from this discussion the dialog will continue with the customer untill one side relents or both agree.

It appears that I am out-numbered in my assertion... that variable tolerance material condition datum feature modifiers may occasionally be functionally appropriate with profile tolerances...and that one does not need to have attribute gages to inspect features with variable tolerance limits or variaable datum shift allowances.

I do not have a link but there are a few threads here in the cove that you'll find rigorus debate on these issues.
http://elsmar.com/Forums/showthread.php?t=18195
http://elsmar.com/Forums/showthread.php?t=16607
There are other forums as well CMMTalk.com, eng-tips.com

Virtually all CMM software can analyze a variable tolerance on a given feature from its estimated size but very few CMM software packages can properly apply datum shift allowances to all "identically referenced" features in compliance with the constraint for "simultaneous requirements". None-the-less it "can be" and "is" done daily. I know that Valisys and Checkmate among a few others can do the analysis correctly.

It may be costly or impractical to include "datum shift" in a continuous data inspection of the parts but including a feature's "variable bonus" from ordinary CMMs and variables gaging can be quite advantageous in reducing sample frequencies, providing defect prevention feedback to the process, and enabling one to see process optimization opportunities in the parameters of a variable tolerance...as opposed to discreet "attribute" gaging.

See those other threads for a lively debate on these issues.

I just want to reiterate that the spec "as you described it" (Profile of Surface 1.5(mmc)|A|B|C|) is wrong...not because of anything having to do with variable datum shift since there are no (mmc) modifiers attached to the datum features ...it is wrong because the profile tolerance itself cannot have a (mmc) modifier according to the rules in ASMEY14.5-1994.

Paul

I just want to reiterate that the spec "as you described it" (Profile of Surface 1.5(mmc)|A|B|C|) is wrong...not because of anything having to do with variable datum shift since there are no (mmc) modifiers attached to the datum features ...it is wrong because the profile tolerance itself cannot have a (mmc) modifier according to the rules in ASMEY14.5-1994.


That's it in a nutshell. The discussion here has been interesting, but if it proves anything, it's that most designers either have only a vague familiarity with GD&T or apply it strictly as theoreticians. They've never stood at a surface plate with part and drawing in hand and had to try and figure out what was intended. They also tend towards being offended when a lowly inspector questions their work. In my own experience, in the majority of cases where elaborate GD&T schemes are used, good ol' x-y coordinate dimensioning could have been used without losing anything except all of the confusion. As Miner pointed out further up the thread, the whole idea behind GD&T was to facilitate assembly, and frequently we see parts where GD&T has been applied, but there is no analogous dimensioning for mating parts. This is a clear indication of egregious ignorance on the part of designers.

While I often offer the same advice as Wes has in this thread--talk to the customer--in point of fact that's often a futile strategy because of a number of factors. Sometimes there's just no time; sometimes getting in contact with the responsible party is nearly impossible, and sometimes you're left to deal with an ignorant but well-meaning purchasing person who can't answer the question. This means that guessing is the last resort, and more often than not an experienced inspector will guess correctly, but not always. Contract review, contract review, contract review! Let the people who actually have to use the drawings see them early enough in the process to be able to understand what the customer actually wants, and listen to them when they say the drawing makes no sense.

Hi everyone,

Could you please help me again.
I have a hole called out in the drawing (datum). The size of the hole is 5.6 +/-0.2. The positional tolerance is 0.8 (+/-0.4).
I need to know what should be the size of the PIN in the check fixture.
In fact, for RFS, the customer was saying that the PIN size should be 5mm BUT the fixture designer says it should be 4.6mm
So, iam between the two and don't know what to do as I am not experienced in fixtures.

COULD anyone please explain what should be the Pin size?

Thank you very much.

Caro.

Hi everyone,

Could you please help me again.
I have a hole called out in the drawing (datum). The size of the hole is 5.6 +/-0.2. The positional tolerance is 0.8 (+/-0.4).
I need to know what should be the size of the PIN in the check fixture.
In fact, for RFS, the customer was saying that the PIN size should be 5mm BUT the fixture designer says it should be 4.6mm
So, iam between the two and don't know what to do as I am not experienced in fixtures.

COULD anyone please explain what should be the Pin size?

Thank you very much.

Caro.

Caro,

Could you please provide us with the information in the feature control frame?
(The box that contains the geometric characteristic, the tolerance information + possible modifiers, and the datum references)

That postitional tolerance callout does not seem right to me.....

Even better; can you scan a portion of your drawing and attach it to your post?

Stijloor.

Hi everyone,

Could you please help me again.
I have a hole called out in the drawing (datum). The size of the hole is 5.6 +/-0.2. The positional tolerance is 0.8 (+/-0.4).
I need to know what should be the size of the PIN in the check fixture.
In fact, for RFS, the customer was saying that the PIN size should be 5mm BUT the fixture designer says it should be 4.6mm
So, iam between the two and don't know what to do as I am not experienced in fixtures.

COULD anyone please explain what should be the Pin size?

Thank you very much.

Caro.

If the positional tolerance is shown at MMC (M with a circle around it), the gauge hole size is 5.4 (MMC) - 0.8 which is 4.6 mm diameter.

If the positional tolerance is shown at RFS, then it is not conducive to a checking fixture. It means that one has a 0.4 mm radial tolerance of the centre of the hole from its theoretical centre. No checking fixture here.

I need the size of the pin to locate the part in one direction. It is exactly a square slot. Attached is a picture of that slot.

Thanks.

Caro.

I need the size of the pin to locate the part in one direction. It is exactly a square slot. Attached is a picture of that slot.

Thanks.

Caro.

The slot is rectangular rather than square (5.3 X 5.2) and the drawing datums are not correct either. The way they reflected the positional tolerances appear to be in one direction (again not shown correctly there either) so the pin size will be 5.3 - 0.2 - 0.8 = 4.3.

Primarly datum A should not have been the other axis for the slot but the mounting surface of the part but that is not your problem. The positional tolerances reflecting one way should have been at the end of the arrow heads in that direction rather than on the side of the slot.

The slot is rectangular rather than square (5.3 X 5.2) and the drawing datums are not correct either. The way they reflected the positional tolerances appear to be in one direction (again not shown correctly there either) so the pin size will be 5.3 - 0.2 - 0.8 = 4.3.

Primarly datum A should not have been the other axis for the slot but the mounting surface of the part but that is not your problem. The positional tolerances reflecting one way should have been at the end of the arrow heads in that direction rather than on the side of the slot.

Sorry but I see it as a rectangle 5.3 +/- .02 x 5.6 +/- .02.

Yes you are correct. I stand corrected on the dimensions of this rectangular slot.