another one, holes on angled surfaces 1

[bxbzq]

In this sketch, datum feature A and B are equal important. Each of them sits on a surface at same time, then bolted down through the two clearance holes. My question is:
1. Is the datum setup OK?
2. Given the datum hole pattern C and D, how to define their interrelationship?
3. Is the position callouts for the two dia 28mm holes and 30mm thick bar correct?

Thanks.

 

[Belanger]

OK, I'll buy that. (I think the gravity bit was the cause of my resistance to the given situation.)

So let me push this just one bit more: Is it really wise to have so much hanging on the fact that the print shows a certain orientation, which makes such a huge difference? If we rotate his view 40º then suddenly the meaning changes! Though I see what you're saying, it just seems like a shaky foundation to hang our hats on.

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

http://www.gdtseminars.com

 

[pmarc]

But I think we are not hanging on the orientation of the part on the print in OP's case. The part could be shown at any angle and nothing would change functional requirements and the procedure of datum feature reference frame establishment if the datum features stayed the same.

 

[Belanger]

pmarc, in your prior post you said that gravity makes the difference. So how is the angle unimportant?

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

http://www.gdtseminars.com

 

[MechNorth]

It goes back to design intent, documented on the print, rather than drawing layout ... definitely. Our perceptions/biases of orientation based on drawing layout may lead us astray.

Jim Sykes, P.Eng, GDTP-S
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[bxbzq]

I have another example, imagine an exhaust pipe on a car. At each end of the pipe there is a flange and 4 bolt holes perpendicular to the flange. A catalyst is in the middle of pipe. The pipe is typically skewed/curved. Let's say the upstream flange surface is datum feature A, and downstream flange surface is datum B. What would be the datum scheme, A-B or A|B?

 

[MechNorth]

It's only supported at A and B? No straps/hangers? Where is the flexibility in the system? In reality, this is not a simple scenario because of the interfaces. Perhaps start a new thread to deal with it?

Jim Sykes, P.Eng, GDTP-S
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[bxbzq]

Jim,
I don't have more details. It just occurred to me that the exhaust pipe may be in a similar situation.

I've seen "surrogate" couple of times and I searched "surrogate" in this forum and found out most of the posts were from you. Can you introduce this concept a little bit?

 

[Belanger]

There is an SAE paper # J2370 that endorses exactly what you propose (2 ends or flanges of a curved hose/pipe which become a hyphenated datum), but I think the key thing is that the example I'm mentioning has a restraint note. I suppose the restraint concept resolves the earlier concern I had.

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

http://www.gdtseminars.com

 

[MechNorth]

I think you're right about the constraint note, J-P.

bxbzq (btw, what's it stand for?),
I (usually) use the term "surrogate" to indicate a feature on a fixture which is not actually a datum simulator (i.e. contacting the datum feature), but which is accessible and convenient for measurement origins. A surrogate datum (feature simulator) is at a known, constant, repeatable distance and orientation wrt the datum feature simulators.
Such simulators are commonly used in metrology and in-process gauging where datum fixturing makes it difficult or impossible to return to the datum feature simulators. Apologies if I interchange surrogate datum and surrogate datum feature; surrogate datum feature would be meant.

Jim Sykes, P.Eng, GDTP-S
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[bxbzq]

It's not English. Some emotional words when I lost money from stock market. Nothing special.

If possible, would you show me a picture or something explaining the surrogate datum feature idea? Take Fig. 4-25 in '09 std as example, I still have problem to picture the inspection process where datum feature A and B controlled relative to datum A-B.

 

[MechNorth]

Can't post any graphics right now; I can never successfully post to this forum directly, and my site is down for a bit while I get problems resolved on the new server.

For Fig 4-25, I would use tooling centers as surrogate datum features. They are common conical tapers machined into both ends before any of the other features are turned from stock; they are how the machines hold the workpiece. For inspection, you first engage collets or chucks on the A and B features simultaneously, constraining the part in all but the axial direction. You then engage conical tapers in the tooling centers and subsequently release the collets/chucks on the A and B datum features. The conical tapers (tooling centers) are not your actual datum features, and thus the datum established them is not your true datum; it is, however, adequately precise wrt the actual datum to act as a surrogate.
An example that I concepted for a training client was for a hull-shaped fabrication with about a half-dozen datum targets on the undersides of the hull. They were having problems with repeatability among other issues, using a mobile-arm CMM; there was not enough reach on the arm to hit more than 1/4 of the hull at any one time, so there was no way to get to all the datum targets to establish the datums. I concepted a large fixture that would hold the entire workpiece within a cradle. By establishing a series of "pads" on the fixture that were at known locations/orientations wrt the original DRF, we established surrogate datum features that could be reached at any time using the arm. So, the measurements were not from the actual datums, but from the surrogate datums. The difference between the two is a "basic" distance.

Jim Sykes, P.Eng, GDTP-S
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TecEase, Inc.

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