Datum Target quick question

[brandnew1]

Sorry for the simplicity of this question, but i have a few parts that need to be measured before we machine them.
i understand that datum targets are, "used to establish the datum points, axes and planes". So just for clarity, when i inspect a part (forging) can i just utilize the areas noted as Datum's or do i need to base off the datum target points used to establish the datum’s?
i figure the datum’s are fine since the datum targets are used by the manufacturer to establish these datum’s...but then i wonder is this correct if i want to verify the forgings are good?

There’s an attached drawing (not a complete drawing) to help clarify my question (its cylindrical in shape)

thank you as always

 

[Belanger]

Hi -- you really do need to set up on the exact targets, rather than just the full surface. A traditional datum (if you only used the type circled in red) would feel the highest points. But the datum target idea is to feel only the specified points, and this can result in a different zero plane, and result in different measurements.

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

 

[ptruitt]

Since it seems that you do not have an inspection procedure to follow, you are in a tough spot. Since there are targets, you may eventually be directed to use them. Perhaps you should do what seems best and provide data with a note that explains how you measured the part and explain that that you do not have a procedure to follow.

 

[drawoh]

brandnew1,

Your drawing's datum symbols conform to ASME Y14.5M-1982. The datum targets conform to no standard that I am aware of. Datum targets have no meaning unless you call up and follow a standard.

As per ASME Y14.5, the datum targets are your fixturing points. If the drafter had meant for you to use the diameter as a datum, they would not have used the datum targets.

If we continue to assume that the standard is ASME Y14.5, then either you show the datum targets, or you show the datums. The fact that your datum targets are scattered around a diameter is not relevant. The targets themselves are your datum.

I generally use datum targets for parts that do not have accurate, rigid base features to fixture off of. The fact that four datumn targets are called up rather than three tells me that your part is not rigid, and that it will be bent or twisted so that it engages target A4.

I am puzzled by your datum target B5. When you use datum targets for datum_B, you start numbering from one again, at least if you are using some form of ASME Y14.5 that I am aware of. I have access to 1982, 1994 and 2009 here.

Probably the best explanation for all this is that your drafter does not understand whatever standard they are using. Having a chat with them is a far better idea that trying for a correct intepretation as per the standards.

JHG

 

[brandnew1]

Thanks everybody,

Just to clarify, we only deal with the machining of the part not the forging (which the datum targets are located on) and datum A does not get touched from us. So in other words, Datum A noted on the drawing changes to another datum label on the machined drawing but still forging.

Datum target A1 and A2 are located on a smaller diameter (1.610) compared to Datum Target A3 and A4 (1.690). So from what i can tell if i want to verify that these forgings are good i will need to align off the Datum targets...So here is the question:

Datum targets have basic dimensions off the theoretical center of the those diameters (1.610 & 1.690). If i want to hit those datum target points, will it be alright to measure out the diameters to center the origin so that i can hit the datum target points and thus create an alignment?

According to the drawing it says the following:
-Interpret dimensional tolerances per USASI Y14.5-66 (not sure if this is the same as ANSI Y14.5)
-A1 thru A4 are datum targets establishing Datum A. C6 and Axis of Datum A establish datum planes A1 thru A3; Circle B and B5 establishes datum plane B.

 

[ptruitt]

Ask for USASI Y14.5-66, but be careful when you take it out of it's urn and try to unroll it. You can't inspect it without the controlling document.

 

[SteveMartin]

I pretty much agree with drawoh, except for the statement "either show the datum targets, or you show the datums."

I'm only looking at the 1994 version of the standard at the moment, but most of their figures regarding datum targets show the targets and the datum plane defined by said targets.

Other than that there aren't enough points (would require six - in two sets of three equally spaced points) it looks like an attempt to define datum axis A via targets.

I concur with the suggestion to discuss the intent with the drafter or other responsible party, if at all possible. Especially with only a partial drawing, the best we can do from here is educated guesses.

 

[Belanger]

The 1966 standard! (The subsequent ones were 1973, 1982, 1994, and now 2009.) Suffice it to say that I wasn't around in '66, but I suppose some of those drawings still linger....

Not 100% what you're asking in the main question, but datum targets are meant to represent perfect locations (basic dims) for where the holding device shall be. If the actual diameters are different, then the part may sit differently on the fixture; that's OK.

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

 

[SteveMartin]

Replies came in while I was formulating mine, so some updates...

I'm not familiar with USASI, but from a quick search it appears to be the predecessor name of ANSI (name change in 1969). 66 is likely the year of release of that version of the standard. Good luck getting your hands on a copy of it for interpretation of the drawings you have, but unfortunately that's what you ultimately need to interpret against.

To your newer question: I'm not sure how to define datum A (axis? plane?) from four points, especially in accordance with the referenced standard. Per Y14.5-1994 it would NOT be ok to measure out the diameters, you would use two sets of three (not two) "...equally spaced contacting features capable of moving radially and an equal rate from a common axis." Of course, this means exactly nothing for your application, since it's referencing a different standard.

 

[drawoh]

brandnew1,

I need to correct one of my earlier remarks.

Your two-step diameter is not located by three points. It requires four datum targets, as shown on your drawing.

SteveMartin,

The four points now make sense to me. Everything is located from the four datum points. We all understand that there is a diameter, but it is not a very round or accurate diameter. If you fixture to the diameter and I fixture to the diameter, the part will wind up in two different positions.

My first experience with datum targets was when I prepared drawings of a casting that required subsequent machining. I needed everyone to fixture to the same points.

JHG

 

[brandnew1]

Thanks again for all the input; i must apologize on my part as i further looked into datum targets.

Let me explain the situation i'm in...i need to create a CMM program to verify that the forgings we have in house from the forging manufacturer actually do come within spec.
The drawing i provided was not the complete drawing, there are dimensional callouts and the like not shown. All that was shown was the datum targets and associated dimensions with them.

My question was whether the alignments for the CMM measurement required me to use the datum targets. So for datum A there was Datum Target A1 thru A4, would this need to be included? From what i gather, Datum targets are used to establish datum’s which are utilized by datum target simulators (i'm not quite sure if this is the case all the time)

From what i gather, i can go ahead and plan the alignments based on the drawing's Datum A and B as normally done, since i can actually align to features such as datum A. This was my original intent...i apologize for not communicating this more accurately. i don't deal with datum targets on the CMM for the most part, but i just wanted to make sure the results attained would be accurate with alignments used.