Ordinate 0,0 NOT at a datum?
Ordinate 0,0 NOT at a datum?
(OP)
This is not how I would prefer to detail a part, but there is a specific motivation I won't go into.
What I have is a large part with many features. I have 3 datums selected (a plane, and two bores). The part is round and neither of the bore datums has axis at the center. Now, for a few reasons, people would like to see ordinate dimensions with 0,0 at the center of the part even though the positional tolerances reference A, B, and C which are unrelated (except by basic dims) to the center. Is this OK? I'd rather put 0,0 at my Datum B, but that causes other logistical problems. Any help is appreciated
What I have is a large part with many features. I have 3 datums selected (a plane, and two bores). The part is round and neither of the bore datums has axis at the center. Now, for a few reasons, people would like to see ordinate dimensions with 0,0 at the center of the part even though the positional tolerances reference A, B, and C which are unrelated (except by basic dims) to the center. Is this OK? I'd rather put 0,0 at my Datum B, but that causes other logistical problems. Any help is appreciated





RE: Ordinate 0,0 NOT at a datum?
RE: Ordinate 0,0 NOT at a datum?
RE: Ordinate 0,0 NOT at a datum?
Chris
SolidWorks/PDMWorks 08 2.0
AutoCAD 06
ctopher's home (updated 10-07-07)
ctopher's blog
RE: Ordinate 0,0 NOT at a datum?
RE: Ordinate 0,0 NOT at a datum?
RE: Ordinate 0,0 NOT at a datum?
It does seem a bit senseless, however, to not have 0,0 correspond w/ primary datums.
RE: Ordinate 0,0 NOT at a datum?
I was curious what your inspection and machine shops would say.
Chris
SolidWorks/PDMWorks 08 2.0
AutoCAD 06
ctopher's home (updated 10-07-07)
ctopher's blog
RE: Ordinate 0,0 NOT at a datum?
It seems to me that the wrong functional primary DRF is being specified here since there is such a strong desire to have an origin separate from the current specified DRF. Additionally, maintaining an assembly clearance implies to me a functional requirement.
So I ask a question and suggest a possible solution. The question is whether you have considered adding an axial datum to the main bore feature defining the center of the part and utilize multiple DRF's or not? If not please see, Para 4.5.11 and 4.5.11.1 in the standard.
Last but not least, combining methods will surely lead to problems. I suggest pick one or the other, never both. My preference is GD&T.
RE: Ordinate 0,0 NOT at a datum?
The reasons for requesting the XYZ origin at a certain place often have nothing to do with part function - they are usually something to do with convenience for NC programming, fixture reference, or something like that. But that's not a problem - put the XYZ origin wherever they want it. That's the beauty of GD&T and basic dimensions - you can lay them out in whatever coordinate system you want, and it's all equivalent.
Having said that, all of this only works if the distinction between datum features and features at the (0,0,0) origin is understood properly by all concerned. If the inspection report shows a hole that is nominally (0,0) with actuals of(0.003, -0.005), this can be counter-intuitive if you're expecting datum features to always be at the origin (as some machine shops might).
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Ordinate 0,0 NOT at a datum?
RE: Ordinate 0,0 NOT at a datum?
One of the functions of your datum is to provide fixturing for manufacturing and inspection. If you use a sloppy feature as a datum, you make fixturing difficult.
Definitely, you should use your bores as datums.
JHG
RE: Ordinate 0,0 NOT at a datum?
Also if the center of the dowel hole pattern then was symetrical with the other coaxial diameters (although it does not need to be) then the origin 0,0 produced from the dowels (midpoint between the spread) would not need to be translated and may more functionally reflect the assembly location and orientation between the structures.
This is how many powertrain component dowel interfaces are specified.
Paul
RE: Ordinate 0,0 NOT at a datum?
pinsNpads I really don't believe your alternatives are worse, I know if I had more information that this could be easily defined unambiguously using GD&T. I am wondering if this DRF that you speak of (a plane and 2 bores) observe the mutual perpendicular plane requirement. If it does, possibly use of PaulJackson speculation of your situation having a pattern, to use as a datum feature could be utilized by you.
drawoh, be aware the variable size used to simulate the datum on jigs/fixtures/gages comes from the actual FOS relating to the datum feature, not the size of the mating component. Unless MMC/LMC is used in which case a fixed datum feature simulator is used; if it fits it's functional, end of story.
RE: Ordinate 0,0 NOT at a datum?
There seems to be a LOT of confusion propagated here. I'm NOT avoiding using GD&T. I am, in fact, using it. If I had my choice, I'd have 0,0 at Datum B and move on with life. Several people where I work don't want this as it make checking drawings for basic fit and "checklist" reviews more difficult. The unfortunate result of this thinking is drawings with datums chosen really really poorly. Datums which have NO functional importance whatsoever.
RE: Ordinate 0,0 NOT at a datum?
RE: Ordinate 0,0 NOT at a datum?
I've been having similar headaches and at the moment I've pretty much given up trying to make people happy and am instead leaning toward standards compliance, technical veracity etc.
May not make you popular though.
If you do this it's even more important that you're technically correct, so don't make a mistake like I did on the part in my thread the other day thread1103-207146: Question on Perpendicularity
KENAT, probably the least qualified checker you'll ever meet...
RE: Ordinate 0,0 NOT at a datum?
The following is then my take on a potential solution. The way I see it, if the two (Dowels) are mating to locating holes and share equal function. I am not necessarily sure that you still can't use the pattern datum here; you would just need to be a bit creative with the GD&T language. To me the standard is very gray in this particular area. It does not say that the pattern of features must be the same size, therefore I would propose that:
• you leave the hole sizes and tolerances as they are
• that you create a position FCF, toleranced w.r.t. Primary "A"
• use two leaders from the FCF pointing one to each hole
• attach the “B” datum symbol to the FCF
• Make the "bolt circle" Dia a basic dim
• rotate the part so that the two holes are at 0 & 180 degrees respectfully as viewed
By doing this I can not find where the standard would be violated and you now have locked the component in six degrees of freedom with a DRF at the center. Additionally, use of the projected tolerance zone modifier is recommended here.
RE: Ordinate 0,0 NOT at a datum?
The one exception I have with your points is the comment on rotation of the view so that "the two holes are at 0 & 180 degrees respectfully as viewed." pinsNpads explained earlier that "Several people where I work don't want this as it make checking drawings for basic fit and "checklist" reviews more difficult" and Evan commented that the way the views are laid out and coordinates are detailed..."they are usually something to do with convenience for NC programming, fixture reference, or something like that", so I would not advise rotating the view if it is oriented to other details for ease of checking, manufacture, CMM programming, inspection, etc.
Paul