Compound datums

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Tabor
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Compound datums

Post by Tabor » Wed Sep 10, 2014 1:43 pm

I have a part that has two parallel but not coplanar surfaces that are being used together as the primary datum for several profiles on the part. When I'm performing my alignment and reporting my profiles,I would like to be able to reference both datums but I'm not sure how to go about it. I tried creating a plane between the two planes and leveling to that but I'm not confident that this is the right way to go about it.
Tabor

JohnK
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Re: Compound datums

Post by JohnK » Wed Sep 10, 2014 2:48 pm

I don't think that drawing is using the intended way to use compound datums, but it sounds like the functional use would be treating the midplane between the two as the datum, as you initially tried. I'm thinking it should basically be treated as the distance between say Datum A and Datum B as a datum feature of size -X- and that defines your orientation for the profile control.

truline1
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Re: Compound datums

Post by truline1 » Wed Sep 10, 2014 3:09 pm

I'm still relatively new at all this, but...

If you had both your measured planes, and constructed points at the center of each, then constructed a line between both points, you would be able to construct a plane either at the Mid point of 2 planes, or at the Mid point of 2 points. You could then construct a line between the 2 points. Create a point at the intersection of a plane and a line. Set your origin to that point, save your alignment. You can do this for each point and just recall your alignment and report your profiles accordingly.

Does this make any sense?
Brad Deliere
Truline Industries
www.trulineind.com

Tabor
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Re: Compound datums

Post by Tabor » Wed Sep 10, 2014 4:14 pm

I can't show the print so I whipped out my mad MS Paint skills for the attached. The datums are the mating surfaces for the part hence the call out.
You do not have the required permissions to view the files attached to this post.
Tabor

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Ryan Christopher
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Re: Compound datums

Post by Ryan Christopher » Wed Sep 10, 2014 7:18 pm

Y14.5 says that in this situation both datums are equally important so if the surfaces are of different size and you simply created a mid-plane, I suspect you would essentially be adding artificial weight (importance) to the smaller surface.

Datum feature simulators are the way to go here but...

If you can't use datum feature simulators like 3-2-1 blocks or the like, I might create a best-fit offset plane or best-fit offset line. I think this is what truline1 suggested.

You could also hunt around for high points using the [construction > point > high/low point] function and build a single representative datum feature from them.
Ryan Tackes
Applications Manager, VMS Inc.
http://www.vantagemeasure.com/

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US_Helpdesk
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Re: Compound datums

Post by US_Helpdesk » Thu Sep 11, 2014 11:24 am

Use Constructed Best Fit Offset Plane or use RPS Alignment.
I've migrated to a new user account, see my other posts here

Tabor
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Re: Compound datums

Post by Tabor » Thu Sep 11, 2014 12:57 pm

Thanks everyone for the input. Bestfit Offset plane works beautifully.
Tabor

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