another one, holes on angled surfaces
another one, holes on angled surfaces
(OP)
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.
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.





RE: another one, holes on angled surfaces
Similarly, D can't have a true position wrt B, but it could have half of a true position wrt A.
The thickness dimension of the diagonal bar cannot have a true position wrt to anything.
Mike Halloran
Pembroke Pines, FL, USA
RE: another one, holes on angled surfaces
I wouldn't hyphenate datums A and B (co-datums, or whatever you want to call it) because they don't act in the same direction of space.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: another one, holes on angled surfaces
By applying perpendicularity callout to datum feature B wrt A, you somehow made A of higher importance than B. I would recommend profile of surface callout without any datum reference applied to both surfaces simultaneously. Mutual relationship between profile tolerance zones would be defined through basic 100 degrees dimension.
Why not to assign all 4 holes as datum feature C through position callout? This would solve the problem of interrelationship between 2 pairs of holes.
For position of 2 dia. 28 holes basic dimension is (are) missing. Either one between the axes or two identical from center plane of the part to each axis.
For position of 30 mm thick bar another basic dimension is missing from the center line passing all 4 holes to the center of the bar.
RE: another one, holes on angled surfaces
There are more questions:
1. When no datum referenced in profile control, it is form control. Specifically, a flatness control. So how are the datum feature A and B related to each other by form control?
2. How does the datum reference frame look like?
3. How to locate the 2 ø28mm hole and the bar vertically in the bottom view?
RE: another one, holes on angled surfaces
1. No, if you do something like on the attached picture. I mean form separately as well as mutual orientation of both surfaces will be controlled.
2. DRF is a 3 mutually perpendicular planes (see the picture):
- #1 - plane passing through the center of V;
- #2 - plane passing through the vertex of V, parallel to the bar, perpendicular to #1;
- #3 - plane passing through the center of 4 holes, perpendicular to #1 & #2.
3. Locate them from vertex of V.
I am just wondering if my initial idea of positioning all 4 holes together was functionally correct. It rather seems that each pair of holes should be controlled separately wrt to proper planar primary datum.
http://files.engineering.com/getfile.aspx?folder=9...
RE: another one, holes on angled surfaces
About the 2 pairs of holes, I think would I go as my first sketch shows, add interrelationship, which is not easy.
Something more.
Functionally I think using A-B as one datum makes excellent sense, but in the std I don't find something supports the usage of two angles surfaces as ONE datum. The '09 std only shows coplanarity application. I think this is also Belanger's doubt. Similarly, use the 2 non-parallel pairs of holes as ONE datum.
RE: another one, holes on angled surfaces
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: another one, holes on angled surfaces
It is not only that "2X" does the trick. It is also the Profile of surface callout that makes those two surfaces tied together. That is why in standard coplanarity example (fig. 4-23) Profile of surface is used and not Flatness (which is reserved for single unrelated features). I agree that my idea is not standard-like, but I do not think there is anything in Y14.5 that prohibits it. It is just an extention of principles. Maybe this will sound too absolute, but in my opinion there is no other way to define a relationship between those inclined surfaces without making one of them of higher importance.
As for interrelationship between 2 pairs of holes, you can always apply additional positional callout to all 4 holes without any datum reference. This should do the thing.
J-P,
I am having troubles in picturing of what you said. Why are you saying that the surfaces are at right angle? Aren't they at basic 100 deg or am I missing something? I also do not really get why those features cannot contact datum feature simulators at the same time, regardless of actual value of the angle between them - especially that they are primary datum features. And I would rather compare it to the situation with datum targets B1 and B2 in fig. 4-47 of Y14.5-2009, with the difference that the targets are planes and not lines.
RE: another one, holes on angled surfaces
Attached sketch shows the difference between A-B and A|B to my understanding. In my case, datum feature A and B are of equal importance and make contact with mating surfaces simultaneously.
RE: another one, holes on angled surfaces
So we have to ask: what is the true, theoretical datum that you are after? A single vertical plane?
Also, (to bxbzq): I think you're relying on gravity to be what equalizes those two hyphenated features. But suppose we rotate the picture 40º in either direction. Should we still assume that the part will equally hit both surfaces? That was the issue I was getting at (despite my goof about the 90º).
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: another one, holes on angled surfaces
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: another one, holes on angled surfaces
I agree with the stuff about never having 100% contact with the simulators, of course, but I'm trying to consider what the true theoretical datum is once this is done, and whether that's really what we want to dimension everything from.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: another one, holes on angled surfaces
Also essentially the same as using two coaxial datum features to establish a single datum; which one has precedence? Are you guaranteed stability of setup?
I believe we've also debated the idea of a piping system with end datum features aligned at some basic compound angle wrt each other; same issue, and same validity.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: another one, holes on angled surfaces
Jim, do you mean this thread?
http://www.eng-tips.com/viewthread.cfm?qid=237732
RE: another one, holes on angled surfaces
I agree with the A-B, but the C-D isn't necessary. The only dof not constrained by A-B is translation into the depth of the sheet; only one of the two holes is necessary to constrain that dof, so adding another feature would add complication without value (that I can see). For datum feature C, use option #2. For all other holes, use A-B/C or A-B/C(M). For profile controls, A-B/C (no (M) on the C because it's not used in a profile simulation anyway).
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: another one, holes on angled surfaces
You say that in Fig. 4-2 the datums are already mutually perpendicular. Of course, but I'm posing that the datum features might not be mutually perpendicular. That's where we need to know the precedence. Thus, if we hyphenate E-F, they become multiple datum features -- why is that suddenly not a good way to go?
Same with the coaxial examples. Sure, two coaxial diameters can form multiple datum features. Now suppose those diameters are designed to be at a 20º angle -- still OK to make them multiple datum features? Then what about two diameters at 90º?
I didn't mean to divert the OP's stuff. The other suggestions are just fine and I think he's on the way to a solid GD&T scheme.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: another one, holes on angled surfaces
No problem. I'm here to learn more than just solving specific problems.
By reading your posts, I'm actually a bit lost as to whether to go A-B or A|B.
My idea was even the two surface were at right angle, the primary datum should still be A-B. But now I would think it over.
RE: another one, holes on angled surfaces
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: another one, holes on angled surfaces
Gravity, or actually assembly configuration, makes the difference.
RE: another one, holes on angled surfaces
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
RE: another one, holes on angled surfaces
RE: another one, holes on angled surfaces
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: another one, holes on angled surfaces
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: another one, holes on angled surfaces
RE: another one, holes on angled surfaces
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: another one, holes on angled surfaces
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?
RE: another one, holes on angled surfaces
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: another one, holes on angled surfaces
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
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: another one, holes on angled surfaces
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.
RE: another one, holes on angled surfaces
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
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com