RPS Alignment

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Racinslack20
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RPS Alignment

Post by Racinslack20 » Fri Jul 26, 2019 1:09 pm

Hello,
I have some questions on aligning some parts and wanted to see what other peoples opinions were. I work in an Automotive R&D facility and deal a lot with in car coordinates. I was told that my best option for most of my parts is to perform a manual (CAD) alignment and then follow up with an RPS alignment. The part I am working on in particular is an automotive from sub frame. It has four datum targets to establish Z datum (offset between the four), and a circle and slot for establishing X,Y datum. My question is for picking up the datum targets would I measure those as points (say 5 for example) on each datum target? Then once I pick up all of those surfaces (20 points for example) I need to create a best fit plane between the four, but I cannot use a plane in RPS alignment for locking Z. I guess my question is what do i need to create/construct to use in my RPS alignment? I cant use all 20 points I don't believe. Maybe someone with some experience with this can help. I have attached an example drawing to try to help visually of what i am dealing with. Any help would be greatly appreciated.
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chuck swanson
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Re: RPS Alignment

Post by chuck swanson » Fri Jul 26, 2019 3:17 pm

Maybe I'm over simplifying this, or not understanding what the problem is. I would simply call up the cad model and perform my alignment with a four point plane where the datum targets are shown. Then align the part off the hole and slot, set my datum on the "XY" hole.

cmmpro1
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Re: RPS Alignment

Post by cmmpro1 » Fri Jul 26, 2019 3:31 pm

As the guy mentioned above. You can take those 4 points to establish the z.

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CrashN8
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Re: RPS Alignment

Post by CrashN8 » Mon Jul 29, 2019 9:53 am

Even considering the 4 points constraining Z, RPS Alignment is certainly the way to go for this example, see details here - viewtopic.php?t=173

Keep in mind, CMM-Manager allows for overconstraint on Primary Datum for RPS Alignment - i.e. 3-4 points or even more is acceptable.

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Re: RPS Alignment

Post by Racinslack20 » Mon Jul 29, 2019 11:59 am

Thanks for the responses guys. I guess the main question is that i have a datum target area of 35 mm, so only pick up one point in that datum target area? I understanding using just 4 points to create my "Z" datum but i assumed you would want to pick up more points in each datum target.

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medupriest
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Re: RPS Alignment

Post by medupriest » Tue Jul 30, 2019 10:03 am

Typically, it is just one point per target area.

Racinslack20
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Re: RPS Alignment

Post by Racinslack20 » Tue Jul 30, 2019 12:46 pm

OK i was just over thinking this. Most of the datum targets have separate call outs ex. profile or flatness i would just have to measure separate from datum structure. On another note, ( I know this has been brought up before) but my part sits on 4 stands clamped to my CMM fixture plate. Is it a bad idea to probe the fixture for those datum targets?

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medupriest
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Re: RPS Alignment

Post by medupriest » Wed Jul 31, 2019 10:10 am

I don't usually do this, especially if there are inexperienced operators that will be running the part. If material gets between the holding fixture and the part, that will of course change the measurement. If you have a motorized probe head, I suggest articulating the head to A90 and physically touching those target points.

On some parts it makes sense to measure the surface the part is resting on to replicate a mating surface, but it comes with that risk of not being flat on the fixturing.

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Re: RPS Alignment

Post by Racinslack20 » Wed Jul 31, 2019 12:40 pm

medupriest,
Thanks for the help. I have typically probed the part itself in the past mainly because of the reasons you have stated. thanks for helping out.

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CrashN8
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Re: RPS Alignment

Post by CrashN8 » Thu Aug 01, 2019 9:15 am

ASME Y14.5 does make some distinction between Datum Target Point and Datum Target Area.

See Paragraph 3.3.3.3

Datum Target Areas. Where it is determined
that an area or areas of contact is necessary to assure
establishment of the datum (that is, where spherical
or pointed pins would be inadequate), a target area of
the desired shape is specified. The datum target area is
indicated by section lines inside a phantom outline of
the desired shape, with controlling dimensions added.
The diameter of circular areas is given in the upper half
of the datum target symbol. See Fig. 3-9, illustration (a).
Where it becomes impractical to delineate a circular
target area, the method of indication shown in Fig. 3-9,
illustration (b) may be used.


ASME Y14.5 also talks quite a bit about physical constraint of part that cannot be replicated easily with a CMM. If this is truly a concern for you I would suggest building CMM holding fixtures that replicate the Datum Ref Frame perfectly. Really it's not a problem as long as your Datum features do not exhibit a lot of Form deviation.

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