One Million Datum Targets?
One Million Datum Targets?
(OP)
I have a plastic part that attaches to another part by using several hoop snaps. The coworker I am working with is insisting on using a datum target at each hoop snap location to establish datum A since the snaps are functional features. The resulting drawing ends up having 12 datum A targets. Has anyone heard of doing it that way instead of using three target and profile of a surface control frames on each hoop snap?





RE: One Million Datum Targets?
Can the snaps not be tied back to the main datums?
KENAT, probably the least qualified checker you'll ever meet...
RE: One Million Datum Targets?
Having 12 datum A's will cause issues with the mfr/shop and inspection. It is not standard practice.
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion
RE: One Million Datum Targets?
RE: One Million Datum Targets?
RE: One Million Datum Targets?
If your primary datum is a plane, it is defined by three datum targets. I do not think a snap is very good for this. A snap would define a centre would it not? Twelve datum targets have no geometric meaning to me.
You should use some other feature as your primary datum. Probably, you can use snaps as your secondary and tertiary datums. Locate your snaps with positional tolerances.
JHG
RE: One Million Datum Targets?
http://
http://gd
http://www.tec-ease.com/tips/june-07.htm
http://www.tec-ease.com/tips/april-98.htm
http://www.draftingzone.com/shoppingzone/dmpgd.pdf
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion
RE: One Million Datum Targets?
Take a look at Ctophers links.
I tend to agree with drawoh look at using another feature if appropriate, or just pick 3 of the snaps if you must use them.
KENAT, probably the least qualified checker you'll ever meet...
RE: One Million Datum Targets?
When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - Thomas Jefferson
RE: One Million Datum Targets?
RE: One Million Datum Targets?
RE: One Million Datum Targets?
KENAT, probably the least qualified checker you'll ever meet...
RE: One Million Datum Targets?
RE: One Million Datum Targets?
Wouldn't a note on affected dimensions about the restraining be more appropriate, see section 6.8 of ASME Y14.5M-1994 and figure 6-54?
KENAT, probably the least qualified checker you'll ever meet...
RE: One Million Datum Targets?
It seems although that designing the target areas to reflect the mating part's attachment features and engaging the hoop snaps for restraint might better reflect the functionally "installed" condition of the part for inspection scrutiny.
RE: One Million Datum Targets?
It seems a common misconception that you need to have exactly 3 points of contact to establish a primary datum plane... it is a MINIMUM of 3 points of contact to establish the plane, not a MAXIMUM! If it were a maximum, you could not legitimately use an entire planar surface to generate a planar datum, you'd be obliged to use datum target points.
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: One Million Datum Targets?
Anyway I'm confused so I guess I should shut up, sit back & learn.
KENAT, probably the least qualified checker you'll ever meet...
RE: One Million Datum Targets?
RE: One Million Datum Targets?
I'm glad you raised the question that moves this to the next consideration ... what is your origin of measurement? If all 12 target simulators (let's say pads for convenience sake, though also applicable for line or point contact) are created at the same level (i.e. are coplanar within gagemaker tolerances), then you could measure from any of the pads. If the pads are at different levels (i.e. not establishing a single plane, but rather a multiplicity of planes) then you need to indicate your origin of measurement on the drawing. Again for convenience, you would typically indicate the surface of one of the pads; you get to choose which one. On the drawing, you would put a datum callout on an extension line from that feature in the profile view. So, on your drawing you would then have your 12 datum target callouts (A1 thru A12), and a traditional datum callout symbol (A). The datum plane is thus identified as originating at that single datum target. Less conveniently, you can establish your origin of measurement at a basic offset distance from any of your datum targets, and label it such as "ORIGIN OF MEASUREMENT FOR DATUM A"
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: One Million Datum Targets?
This has really been helpful. Thanks for all the responses.
RE: One Million Datum Targets?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: One Million Datum Targets?
On a cast, planar surface with flatness that I do not trust, I would define three datum targets. This means that the fabricator and inspector for the casting, and the fabricator and inspector for the subsequent machining, all are working from the same nominal plane.
Twelve datum targets are a solution for a non-rigid structure in which the datum shape ought to be defined by the snaps.
JHG
RE: One Million Datum Targets?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: One Million Datum Targets?
RE: One Million Datum Targets?
RE: One Million Datum Targets?
Next, the surface profile only applies to the top surface in each of those cutouts...is that the intent, or is it to be the entire profile of the cutout?
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: One Million Datum Targets?
RE: One Million Datum Targets?
That's a good point, the entire top surfaces of the two tabs at targets A1 and A10 are in contact with the other part and should be target areas. The rest of the points contact small ribs so maybe axis targets should be used.
Regarding the origin of measurement, Datum A is the entire back edge of the part. In further discussions with the engineer for that project I found out there are more contact ribs that touch the edge of the wall where Datum A is where additional targets should be added.
The surface profile is intended only for the top surface. The rest of the square hole has sufficient clearance to the receiver ramp feature on the mating part.
Thanks for all your help everybody!