Customized Datum Refererence Frame 1

[pmarc]

One of the new ideas presented in 2009 standard is "Customized Datum Reference Frame" concept. The standard quite precisely describes what to do on a drawing if one wants to use such concept, however I have difficulties in finding any information about how the concept is realized during inspection. I mean, I could not find any example how datum feature simulators should look like to achieve the effect of overriding constrained degrees of freedom.

1. Taking fig. 4-46 as an example, I can imagine datum feature simulator for square 12x12 hole as a diamond pin which allows rotation of whole part around its center and therefore unconstrains rotational DOF "w", am I right?
2. What about fig. 4-45? How should the datum feature simulator look like for conical datum feature A if one wants to use different part feature for constraining translational DOF "z"?
3. Can this concept be used without hard gaging at all?

Thanks for any hints.

 

[axym]

pmarc,

You said:

"The standard quite precisely describes what to do on a drawing if one wants to use such concept, however I have difficulties in finding any information about how the concept is realized during inspection."

Can I use that? Your statement could be applied to about 100 different concepts in Y14.5. ;^)

Seriously, you make a good point. Your description of the square simulator for Figure 4-46 is pretty much what I had envisioned. One thing that concerns me about 4-46 is that datum feature B is referenced at RMB, which means that its simulator is not a fixed size. It would have to expand or contract to become the "maximum inscribed square". Y14.5-2009 does not really address the simulator behavior for irregular datum features referenced at RMB.

I don't have any answers for your question about the simulator for conical datum feature A in Figure 4-45. We're working on this in the mathematization subcommittee. My take on the concept of customized datum reference frames is that the simulators have to operate as some sort of mechanism. The individual simulators need to be able to move relative to each other. This is in contrast to "natural" DRF's in which the simulators have basic location relative to each other.

Customized DRF's can be created relatively easily if CMM inspection is used. The software generally allows the user to constrain the CMM coordinate system's six degrees of freedom to whatever combination of features they wish. This was always a pro and con of CMM's - they allow you to create coordinate system alignments that are physically difficult or impossible.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

Evan,

Thanks for the input. Nicely noticed that datum feature simulator for square hole from 4-46 example would have to be adjustable in size. I missed that.

If I understood you correct, in both examples (4-45 & 4-46)from 2009 standard it would be much easier to simulate customized DRF with the use of CMM than by hard gaging, right?

 

[fsincox]

That is my impression it was more CMM driven it is where I first noticed the need, working with CMM operators, to be able to say basically: line up to this "stable framework" and then offset "0" to this feature. I do not remember the exact situation it first came to me. Using "tooling" or "construction balls" (spheres)and such either had to be the primary datum or they got overridden by other features for "0" if you did not choose them right. This modification has actually been needed a long time, some here have expressed that they are not quite ready for it yet and are concernd of the effect it will have on confusing the shop, OH WELL, change confuses us all till we learn it.
Frank

 

[fsincox]

Actually tapers is another good example, the apex of a cone can be considered a "0" but not the one people usally want to use, for those "radical souls" that are brave enough to use tapers as datums. I love 2009.
Frank

 

[MechNorth]

Actually, the idea of specifying the DOF constrained by each datum is core to many real-world fixturing situations as well as CMMs. This addition to the standard nicely standardizes the means of communicating that intent. The conical taper is a classic example. Establishing the datum axis from a conical datum feature is fairly easy; you use a taper simulator (there's lots of issues inherent in that, too, but not for this thread). The issue arises when you try to repeatably find the point in space which is the apex of the cone; now, even the smallest errors in the conical simulator and the actual surface will manifest and magnify the error on the location of that point. Since it's not a "real" (i.e. physical) point, people tend to want to use a real surface to constrain the longitudinal dof along the datum axis.

This is also used for tooling centers used as multiple datum features. You want them to establish a datum axis, but not a longitudinal origin. The simulator can be a conical tip in the head stock and tail stock of a lathe, for example, where you move one of them until you come into contact with the tooling center cone.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com