True Position? 1

[metaldork]

So I'm drafting up some process prints for a run of new tombstones we have in our shop. Aside from the surfaces being machined we are putting a dowel hole pattern on the faces. We are looking to control the location of these holes to the bottom and to the center of the tombstone, which I believe will be our primary and secondary datums. My question is what would be ideal for our third datum?

 

[pmarc]

Okay, maybe instead of "wrong" I should have said: "From theoretical point of view your inspection is not 100% correct by requiring tertiary datum, because theoretically there is a way of living without it and having the tombstone's geometry fully defined and inspectable". Would it be better then?

 

[CheckerHater]

The short answer is "yes".

 

[fsincox]

CH,
I am not really sure what you want to use +/- dims on but I would say it is not well defined once the whole FOS redefinition occured (1994), I am not saying it was perfect before, but, I honestly am not sure what we have now, at least with ISO I know what it is, opposed point measurement.
Frank

 

[CheckerHater]

Frank,
I don't want to use +/- per se. I was using it as example against "it can be done" argument.
Let say I prepared the drawing that can be interpreted 10 different ways. But all 10 will result in good part. Should I consider my drawing "good" because it "can be done", or should I still try to make it more standard-compliant and unambiguous?
After all tomorrow someone may invent the 11-th way.

 

[fsincox]

Are you aware of anyplace in the current ASME standard that gives a definition to a non-directly opposed toleranced dimension? Something everyone thinks they know until pressed. I must have missed it.
Frank

 

[CheckerHater]

Directly toleranced or Basic included?

 

[fsincox]

No basic, No directly opposed (FOS), just the kind of dimensions on the majority of prints I see everyday.
Frank

 

[Belanger]

Frank, the closest example in 2009 would be Fig. 2-5?

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

http://www.gdtseminars.com

 

[pmarc]

The link is showing Tec-Ease's opinion about applicability of directly toleranced dimensions.

http://tec-ease.com/gdt-tips-view.php?q=165

Though no references to the standard given, I like this one.

I also find interesting that chamfers have not been listed there, unlike in Alex Krulikowski's Fundamentals of GD&T. I am not saying they should be, just mentioning the difference.

 

[fsincox]

J-P,
Thank you, that example involves the concept of implied datum reference measurement, so to speak, but definitely, thank you for helping me look.
Frank

 

[CheckerHater]

Frank,
If you insist on dimensioning non-opposing points, the only thing that comes to mind are dimensions with origin symbol, and, by extension, dimensions without dimension lines.

 

[MechNorth]

So, the counterbore hole at the bottom of the tombstone establishes the datum axis? Not great because it's so shallow compared with the height of the tombstone, but so be it. Datum shift also mentioned, but can be eliminated by putting a conical taper instead of cylindrical mating condition. Looking at the layout of holes and faces on this fixture, there is an evident need for a rotational control. Otherwise, the holes on the base can be rotated about the datum axis, causing the tombstone faces to be off-square wrt the tool path on the machining center. Note though, that this doesn't center the dowel holes on the face of the tombstone, but rather wrt the base of the tombstone.


Personally, favor CH's datum layout for centering on the tombstone mating/cutting face. The cutter is adjusted to that position anyway (center of the width of the vertical face) as its center, and that's what where the programs start from (from my background) rather than the center of the tombstone base interface to the machine bed. This minimizes the impact of the dowel pattern being shifted to the side.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

Simply, as I see it now there are FOS dimensions and feature locating dimensions, so to speak, the one that are not really considered a FOS. Under the old versions of the standard I always looked at all dimensions as FOS dimensions, I suspect others did, too. That seems to have been one of the things that definitely was intended to be changed with 1994 with the change in the definition of FOS.
Frank

 

[axym]

CH,

I support pmarc's opinion that it is not necessary for the datum features to constrain all 6 degrees of freedom. There are many examples in Y14.5 in which less than 6 degrees of freedom are constrained (or even none at all).

Degree of freedom (DOF) constraint is a central concept in datum reference frame (DRF) definition. Another central concept in Y14.5 is that the DRF is defined in the datum feature simulators. In other words, the coordinate system is defined in the gage elements. This concept is easy to work with in a hard gaging context, which the Y14.5 standard and most GD&T books tend to focus on. If certain degrees of freedom are unconstrained (or partly constrained), the part can be shifted around (translated and/or rotated) on the gage to get everything to conform. Dealing with the datum shift is hands-on and understandable. It isn't necessary to understand DOF constraint to use a hard gage.

If a hard gage is not available, and CMM or open setup methods are necessary, dealing with the datum shift is very complex. It's very mathematical, and requires full understanding of the gory details of DOF constraint to do properly. Instead of moving the part around relative to the coordinate system (which is in the gage), we have to shift the coordinate system around relative the part. This really clashes with one of the fundamental paradigms of CMM's, in which a fully constrained coordinate system is needed. The coordinate system must be optimized either by trial and error or by special "soft gaging" algorithms (which most CMM softwares still do not have).

So FCF's with unconstrained DOF's creates challenges for inspection. This is not to say that it can't be done, however. I would still resist the temptation to "dumb down" the drawing by specifying extra datum features that add unnessary constraints, just for the sake of making things simpler for inspection. Your mileage may vary.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[fsincox]

Evan,
I does mean more coming from someone who actually has had to do it, or knows it can be done, in my opinion.
Frank