INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

datum translation questions

datum translation questions

(OP)
The "Means this" portion of figure 4-32 (b) states that the datum translation modifier for datum B means "adjustable location within location tolerance".
1) Is this saying that the datum feature simulator can translate 0.1 in either direction?
2) If a gage was made for this part how much travel is allowed for the translating datum B simulator?

My understanding of datum translation is that it is used to make sure clocking datum simulators only stop rotation of the part and do not do the job of locating the part.
3) If, in figure 4-19, datum C had been modified at MMB in the hole position FCFs (∅9.2 & 4X ∅5.1) would the translation modifier not be needed anymore?
4) Same question for figure 4-32(b). If datum B had been modified at MMB for the slot position FCF would the translation modifier not be needed anymore?
5) So really, I'm asking if datum translation only gets used (or mainly gets used) when the clocking datum feature is specified at RMB?

Thank you.


RE: datum translation questions

AndrewTT:

Quote (AndrewTT)

3) If, in figure 4-19, datum C had been modified at MMB in the hole position FCFs (∅9.2 & 4X ∅5.1) would the translation modifier not be needed anymore?

Please refer to figure 4-49. In this case the left sphere does not control clocking but controls position via coaxiality. So translation datums are not limited to clocking.

Certified Sr. GD&T Professional

RE: datum translation questions

I know, it is very late in a stressful week (and I am glad the week is over), but what the translation modifier (triangle symbol) has to do with the movable datum target spheres specified in fig. 4.49. (bird beak symbol)
Not sure I understand.
If I am missing something I will stand corrected.
Thank you for your help in understanding.


RE: datum translation questions

Here is what James Meadows had to say: "This is the result of a long drawn out fight between individuals and committees. It's known as the "tertiary datum problem". Some of us said (as it currently says in the Y14.43 standard) that since the tertiary datum feature of size is only to give angular orientation to measurements taken from the secondary location datum axis (of a hole or shaft) that the datum feature simulator (real gage or fixture element) and the TGC (imaginary datum feature simulator) should translate (move) toward or away from the secondary simulator. Others said that both simulators should be represented as stationary, separated by their basic dimension, as is currently done for pattern datums. The stationary folks won the vote.

Then some of us pointed out that the result of this vote is that we aren't really measuring distance from the secondary hole or shaft, but are really measuring distance from both the secondary and tertiary datum features of size. What are we supposed to do when we want to actually have the secondary locate and the tertiary give us an angle of measurement? They came up with the translation modifier. It says that the tertiary (or sometimes secondary) datum feature it is attached to can be represented by a simulator that moves toward or away from the secondary (or sometimes primary) datum feature that precedes it in the feature control frame. That way the tertiary datum feature of size only gives orientation to the measurements.

It's a mess created to solve a problem that rarely existed. But the faction got what it wanted and that was a default rule that all datum feature simulators (real) or true geometric counterparts (imaginary) are stationary and separated by their basic dimensions (unless the translation modifier is used).

Since you didn't mention the planar datum features shown with a profile tolerance that can be referenced at MMC and represented at their virtual condition distances/sizes or the RFS ones that can grow from their basic dimensions until they contact the part, I won't either.

Things have gotten out of control, but there is no stopping this runaway train."

Thanks Jim

RE: datum translation questions

(OP)
Thanks for that info. Very helpful.

My thought was that if the third datum is specified at MMB then you almost always would have some slop between the datum feature simulator and the datum feature so the translation modifier might not be necessary. I don't know how often this situation arises (use of the translation modifier) so maybe not worth too much thought/conversation. It seems rare based upon what James writes.

Anyone have an answer for the first question from my original post?

RE: datum translation questions

geenimi,

The problem is that a datum scheme should represent what the mating part will be like. It doesn't have to, but there is no reason it should not. The examples of 'translation' in 4-32 aren't realizable mechanisms in any meaningful way and unless those pressing for the translation modifier were able to show drawings of production items where they had supplemented the D&T symbols with notes describing the desired behavior, they should have been shot down.

The problem with standardizing ad-hos solutions is they are often not well thought out, as the example diagram exposes.

RE: datum translation questions

I agree wholeheartedly with 3DDave. The Y14.5 folks pride themselves on having rules that are function-driven (whereas ISO is said to be more math or theory based). Yet the translation modifier doesn't seem to relate to a functional datum scheme. It's fine to have the modifier -- it makes for interesting academic discussion -- but it seems to be a solution looking for a problem.

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

RE: datum translation questions

greemimi:

I am curious: How did you contact Jim Meadows to get input on this topic? The details he describes are priceless.

Certified Sr. GD&T Professional

RE: datum translation questions

3DDave:

Quote (3Dave)

The problem is that a datum scheme should represent what the mating part will be like. It doesn't have to, but there is no reason it should not. The examples of 'translation' in 4-32 aren't realizable mechanisms in any meaningful way and unless those pressing for the translation modifier were able to show drawings of production items where they had supplemented the D&T symbols with notes describing the desired behavior, they should have been shot down.

Just an observation: A similar disconnect between functionality and manufacturing can exist with datum targets.

Certified Sr. GD&T Professional

RE: datum translation questions

Quote (mkcski)

I am curious: How did you contact Jim Meadows to get input on this topic? The details he describes are priceless.

I did not. Copy-paste from different thread where this issue has been previously discussed.

http://www.eng-tips.com/viewthread.cfm?qid=363878

RE: datum translation questions

OK Thanks. I have attempted to contact Committee members I have met at meetings via e-mail with little success.

Certified Sr. GD&T Professional

RE: datum translation questions

Quote (3DDave)

The problem is that a datum scheme should represent what the mating part will be like. It doesn't have to, but there is no reason it should not. The examples of 'translation' in 4-32 aren't realizable mechanisms in any meaningful way and unless those pressing for the translation modifier were able to show drawings of production items where they had supplemented the D&T symbols with notes describing the desired behavior, they should have been shot down.

The problem with standardizing ad-hos solutions is they are often not well thought out, as the example diagram exposes.

Quote (J-P)

I agree wholeheartedly with 3DDave. The Y14.5 folks pride themselves on having rules that are function-driven (whereas ISO is said to be more math or theory based). Yet the translation modifier doesn't seem to relate to a functional datum scheme. It's fine to have the modifier -- it makes for interesting academic discussion -- but it seems to be a solution looking for a problem.

Just a loose thougth... As explained by Jim Meadows, the translation modifier was added to the '09 standard, because majority of the committee members voted (rightly or not) for changing the default requirement for relationship between tertiary and secondary datum feature of size simulators from fixed orientation only (as defined in the '94 standard, figs. 4-8 and 4-9) to fixed orientation and translation, yet the minority wanted to have a tool working similarly to the previous default requirement.

So to me, guys, it looks like your concerns are rather about functional validity of the default requirement in the '94 standard than about the translation modifier itself.

RE: datum translation questions

I think the point is - it's an ad hoc solution and it doesn't work right. Voelker's work on this did not suggest anything about 'location tolerance.'

I would never use the translate modifier - if there is ever a need I would create a note that explains how to set up the part on a CMM. Since no one has ever supplied an example of a need, it's a note I expect will never be needed.

There was no sense to the 1994 diagram. Either the two holes are fixed in location relative to each other or they are not, but the diagram indicates they are both. It adds an unnecessary glitch.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close