Measurements on Datum Reference Frame 2

[ntuedutw]

Hello,

If you have a part with only one datum reference frame(DRF){A,B,C}. A is a planar surface which has a form control, B is a external shaft which has a position geometric control reference to Datum feature A under RFS condition and C is also a external shaft which has a position control reference to datum features A & B under RFS condition. Then, we put the part on the specified fixture and start building the DRF{A,B,C} first on CMM and go for the measurements.

How could we measure the position controls of datum feature B & C?
(1) The position control of shaft B is to refer to Datum Feature A. It's just only one Datume feature( Plane A) in its own feature control frame, not the whole DRF{A,B,C}
(2) The position control of shaft C is to refer to Datum Feature A & B only in its own feature control frame, not the whole DRF{A,B,C}.

Does any one konw this concept? It doesn't make sense for me to imagine that we could setup the DRF{A,B,C} first and go for measuring the position controls of shaft B & C under this DRF in CMM metrology.

Many thanks,
Ntuedutw

 

[aardvarkdw]

Can you post a picture? I'm not sure that I understand the exact situation, but right off I don't see how you can control the position of cylindrical feature B exclusivly by planar feature A. By controlling only to A you leave 3 degrees of freedom open, you control "Z"linear movement, "X" rotation and "Y" rotation. This leaves "X" linear, "Y" linear, and "Z" rotation. cylinder C can be controlled by A and B assuming that you don't care about the orientation of the cylinder in "Z". If you only care about controlling cylinder B to plane A, you would be better off using perpendicular.

 

[caseynick]

I would say that calling out a positional tolerance on the datum B shaft to only the datum A plane is a drawing mistake. It should have a perpendicularity control. A positional tolerance would control the perpendicularity, but there is no locational relationship being established to any other feature at this point so position is a misnomer. A positional tolerance can be used to locate the datum C feature with only datums A,B in the feature control frame. The basic dimension establishes the location from B. It's just that at this point there is no tertiary, clocking datum so you are checking only the distance from B to C.

 

[ntuedutw]

Hi Caseynick, Aardvarkdw,

Both you are right, it's my mistake. I think I should take a example shown on Y 14.5 to describe my problem. Please have your time to check out the Fig 4-15(Page 64) that describes the establishment of datum feature RFS.

In this picture, we could setup Datum Reference Frame from these Datum features - A, B and C. Then go for measuring the four holes - Dia 7.7 to 7.8. But if we want to verify the Datum Feature B and C that are the Functional Critical Dimensions, how could we make it?

Q:
Datum Feature B is only related to Datum Feaure A for pendicularity control. Could feature B be measured under the datum reference frame establishing from Datum feature A, B and C?

As same question with Datum Feature B, how could we measure the datum feature C?

I think it's so strage to me. We couldn't use datum feature A, B and C to setup the Datum Reference Frame(DRF) of the part, then use the DRF to go back measure the Datum Feature B and C.



Many thanks,
Ntuedutw

 

[aardvarkdw]

If you make feature "B" into Datum B, you cannot use datum B to control feature "B". Feature "C" can be controlled by datums A and B but not C if feature "C" is to be come Datum C.

If "B" is controlled only by perpendicularity to A. Then the location of "B" is controlled by the other dimensions and tolerances listed on the drawing and in the tolerance block. "B" can then become datum B and control "C" relative to B and A. This would control perpendicularity to A and location relative to B but not location relative to the edges of your part for "C". So "B" and "C" float perpendicular to A and relative to each other and "B" is controlled by the other dimensions on the drawing.

Again a picture would let me explain better, I think.

David

 

[ntuedutw]

Hi David,

Thanks for your quick reply. I really want to attach a picture for your infotmation but I can't make it. I don't have any web server to post the picture.
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From TGML:
Use the tag to show images from any web server. Be sure to use the entire url to the image you want to use including the "[URL unfurl="true"]http://"[/URL]
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Maybe, I could send the picture into your email box. My email:ntuedutw@hotmail.com

Thanks. :)

 

[aardvarkdw]

http://www.tinypic.com/

Upload your picture here and then post the resulting link.

 

[ntuedutw]

Hi David,

Thank you very much. I am done as follow.

http://www.dropshots.com/day.php?userid=177546&cdate=20060912

http://www.dropshots.com/day.php?userid=177546&cdate=20060913

The first link is about the Fig 4-15 shown on Y14.5.
The Second link is about my original question.

From 2nd link, you could check out the DRF(XYZ labels) built from Datum feature A, B and C on CMM.

Then we could measure Dia 20 bole under this DRF due to there are 3 datum reference letters in its feature control frame. And my question is that why we could use this DRF(coordinate) to measure Datum Feature B and C.

I hope my pictures posting on web could work.
Many many thanks.

 

[aardvarkdw]

It doesn't matter so much that there are 3 datums as that the degrees of freedom are controlled.

The problems that I see are:

1. (and this is a big one) Datum B is not located. You don't have any dimensions locating the position of B. In order for you to locate C or the hole you must first locate B.

2. Your origin axis (DRF?) is located at datum C. This is fine if you want that to be the controlling feature. In this case I would swap the GDT from datum B and datum C.

There are others but they mostly are contingent upon these two.

In the current configuration, you are telling me to locate Feature "C" and the hole relative to B. That is okay except that they are fixed relative to B and is free to rotate 360deg and be at any location on the flat feature. Also your axis, if I am understanding you correctly, is where you intend to define your datums from. You can't have it centered on datum C if datum C is dependent on datum B.

I drew up 2 ways of accomplishing this. I may be missing dims and I left off the tolerances so you will need to double check me.

http://i5.tinypic.com/4bz4fi9.jpg

David

 

[ntuedutw]

Hi David,

Thanks for your reply. But I still have some questions in my mind. Could you have your time to take a look at the first link I cut from Y14.5 and focus on Datum feature C?

How could we evaluate datum feature C which has the position control with 0.2 parallel planes apart tolerance zone at MMC while referencing A & B?

I am thinking could we use DRF which is a 3 mutually perpendicular planes built from Datum A, B and C to evaluate datum feature C "directly"? I guess we can't in my mind. Because...The datum reference letters located in datum feature C's feature control frame are only A & B, not A, B, & C(itself).

It's so hard for me to solve this question. Perhaps, I should transfer my focus on datum feature "simulator" of datum feature C.

Many thanks. :-3)
Ntuedutw

 

[dingy2]

On the drawing that was scanned from the Y15.4M -94, reflects both datum B and datum C at MMC. Whenever MMC is applied, a checking fixture is most conducive to measuring and qualifying each datum.

On datum B, the checking fixture would have a flat plane (for datum A) and a a pilot hole in it for the checking pin. The part drops on the plate and over the hole. A checking pin of virtual condition size with a pilot pin is placed through the hole on the part locating on the pilot hole. If the pin goes all the way down (bottoms)one now has datum B.


Datum C is pretty easy too. A checking fixture with plate fro datum A and a perpendicular pin of datum B's virtual condition size. There will also be a retangular pin that will travel parallel to datum A and center on datum B. The width of this slider pin will be the virtual condition size of datum C slot.

The part is located on datums A and B. Rotate the part until the slot lines up with the retangular slider for checking the position of C. If one can push the slider all the way into slot datum C, then C will qualify as the datum. One would gain a little extra tolerance from datum B.

Any time MMC is applied, an attribute checking fixture is always best but only if there is high volume production.

I could do this on CMM but there is a lot of playing around - kind of tough.

Hope this helps.

 

[aardvarkdw]

I am probably not getting you the specific answer you want as to how to make what you have work, and I'm sorry if that is the case. I wish I could help more as far as the actual CMM part of this goes, but what I am saying is that how you have things set up right now from what I can tell from the drawing, from a drafting point of veiw, arn't going to cut it. Here is a drawing to demonstrate what I mean. I have applied the same GDT you have on your drawing to this one with the exception of adding the dimensions for locating datum B.

http://i2.tinypic.com/2uj5sap.jpg

The other two features a free to rotate around B and still meet all the criteria for the drawing. B needs to be controlled in it's Z rotation in order for C and the hole to be located from it.

 

[ntuedutw]

Hi Dingy,

Thanks for your help. It does help me out. The description you explained seems like the functional gage using. It's great help for me to catch. Per my knowledge, functional gage could be used for volumetric production but it just could judge the parts pass or unaceptable in my ideas . Need to bother you one more, have any ways to evaluate the "exact" numetric value on datum feature B and C if we use CMM's mathmetical way?


Thank you very much,
Ntuedutw

 

[dingy2]

Using a CMM to check perpenciularity of the hole will depend upon the CMM software. Assuming you may primative software one would have to calculate the tolerance since the feature is at MMC. More up to date software would calculate this automatically.

Calculate the virtual condition size of the hole from the drawing and then measure the actual hole size. Subtract the difference and one has the tolerance in a RFS mode. The fact that it is diametrical means any direction or quadrant.

Using your CMM, touch 3 points on datum surface A to create a plane making sure that the points are towards the edge of the part and about 120 degrees apart. Now touch a number of points around the hole at the top to create a C/L. The larger the number, the more actuate one is. Now that you have the center of the hole, dial in 90 degrees on the software and take the same number of points at the other end of the hole. The software may ask for a tolerance which you have calculated. The CMM equipment will reflect exactly how far off true 90 degrees the hole is at the far end. One can then tell if the hole is within specifications or not.

If this is the info you want, I can reflect how one would check the tertiary slot to this datum if requested.

 

[thundair]

This is a pet peeve I have with quality, as they ignore Primary, Secondary and Tertiary datums.

We have agreed to allow them to set and check from an axis but to best fit to the datums after inspection to allow for any size limits of Secondary and Tertiary datums, IE... RFS, MMC, LMC.

Your part will need more definition as others have mentioned. You will also need to realize your secondary datum will use two points to identify it and the way it is set up you will rotate around the secondary to one point on the tertiary.

Cheers

I don't know anything but the people that do.

 

[ntuedutw]

Hi Dingy,

Your explanation is so clear and that is what we need. In the past, we just measured the hole only one time then hand out the reports. No detail data on top and bottom surface end of hole. Could you go for how to verify the Datum Feature C?

Thank you very much.

 

[dingy2]

Datum C is much more difficult to measure since it does reference B at MMC.

Calculate the tolerance by measuring the actual size of datum B hole and subtract that from the virtual condition size. Next , measure that width of the slot that will eventually be used for orienting the part. Subtract the actual size from the virtual condition size of the slot. Now add both tolerances together and we get the total calculated tolerance converting the situation to a RRF mode.

Set the part up on an index head (chuck) using the hole datum B. Make sure that datum A is perpendicular to datum B by using an indicator on the face of datum A and rotate the part. This would be similar to checking cirular runout on the face (perpendicularity). If the face has a FIM or TIR of say .010, tap the face until it is running relatively true (or less than .002.

The part is now set up on datums A and B.

I will assume that you have a height gauge and know the height of the center of the index head (chuck). Subtract half the actual width of the slot from the chuck center height. Rotate the part in the chuck until you have the slot bottom and the height gauge just touching. You now have the slot ready to qualify as datum C.

Measure the distand from the top of the slot that you have aligned and the bottom. Does it exceed half the calculated tolerance? If not, it meets the requirement. If the slot is on such and angle and exceeds half the calculated tolerance, it does not conform.

This is so much easier done on a checking fixture.

If you have a CMM, simulate what I did using tool room equipment.

Hope it helps

 

[ringman]

Ntuedutw,

Just curious, but Is this a molded or machined part?

Aardvarkdw,

looks like you did a good job of correcting The orig drawing.

Ringman

 

[ntuedutw]

Dingy,

Thank you very much to help me out. I will follow your steps on CMM.

RingMan,

It's a plastic part.