True Position Calculation

[[Ro...]]

Hello All ,

We are having big discrepancies between measurements from our customers to us on the same parts.

This is our part drawing and our area of concern is the 20 dia true pos and perpendicularity





Please if someone could help us out or walk us through if we are doing it the right way or not, for our base alignment we use

That 3D line was constructed using 4.9 radius circle and 20 dia circle(as per the print)







Our customer is using a 321 Alignment on a mititoyo CMM and we use standard base alignment technique.




This is how we are calculating our true pos on 20 dia


Will the base alignment effect the way we check our true position? I do not know what would be the correct way to check, from what I think our program should give us good readings.

Please correct me if I'm wrong?

[[Ky...]]

You might try reversing you Planar and your Spatial. This might give you more area for you Spatial. I was reading another post and Owen Long said to use the biggest area to set your Spatial.

[[Br...]]

Your Position characteristic has nothing to do with the Base alignment except for the base alignment is what guides the measurements the CMM takes when physically making contact with the part. It also looks like you are not utilizing any of the MMC and MMB modifiers. Everything in your Position characteristic is referencing RFS. This could make a big difference. Make sure you are actualy using Datum feature C and Datum feature B in your Datum reference frame as well. I cant tell what "Cylinder3" and "Cylinder4" are, but If you plug those in, that trihedren representing the coordinate system for the datum reference frame should origin on the opposite "ear" if I assume you measured datum feature A on the top in this view.

[[Br...]]

One other thing, make sure Datum Feature A is measured only in those datum target areas.

[[Ro...]]

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Brett thank you so much for your reply and much needed guidance, Cylinder 4 and Cylinder 5 are datum B and C respectively. I forgot to mention that in my post.

Yes datum A is on the top of this view, but I was taking datum A on the whole area not just those two ears as you mentioned.

[[SH...]]

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Dia 20 true position X value must be 11.55,but your given 11.4 , how 11.4??
I can't cope with your base alignment algorithm, if I was in your case,datum a plane would be spatial rotation, intersection point of datum cylinder b and plane A should be my X,Y origins, and intersection point of datum cylinder c and plane a could be planer rotation for X minus.

As mentioned early you are missing bonus tolerance of MMC.

Dia 20 true position alignment coinside my base alignment, just need to add MMC.

It is good practice take base alignment according to designer intention. I always use primary datum for spatial rotation, secondary datum for location and territory datum for planer rotation. It is the general case, definitely there should be execption...

[[Ro...]]

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Hi Shabu,

Thank you so much for your insight on this part,

This is an old picture I had, where these values were based off of the CAD model provided to us. I am aware we have made changes to X,Y dimensions for true position, I have also added the MMC condition to it.

I understand how you are coming to the base alignment and it makes sense to me but as Brett mentioned my base alignment shouldn't matter as i'll be providing my True position with the correct values and these areas are easier for me to probe as compared to the ones you've mentioned.

Thank You.

[[SH...]]

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I was told that datum target are used for machine tool fixture designing, usually I use polyline scanning for plane like this. If I take three point in specified datum target,it would be accurate as scanning using polyline strategy??
I think he is not using RPS alignment.

[[Br...]]

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The datum targets establish where the datum features exists. they can be single points, lines, or specific areas like we see here. They can be useful on things like castings that have a lot of irregularities. Sometimes the mating part may only contact a portion of a surface that is a larger plane like this, and so the designer wants the drawing to reflect the mating interfaces more precisely through the means of datum targets.

The idea that they are only for a machine tool setup flies in the face of one of the central purposes of the Y14.5 standards, namely that it is to convey "Design Intent", leaving the means of manufacturing completely up to the manufacturer. This makes sense with the rapid change in technology, the last thing you want to do is paint yourself into a corner of how a part is manufactured when there could be a more efficient and cost effective way to make the part.

ASME Y14.5-2009 para. 1.4(e) even states:
"The drawing should define a part without specifying
manufacturing methods. Thus, only the diameter
of a hole is given without indicating whether it is to be
drilled, reamed, punched, or made by any other operation."

Sometimes it may make sense to use datum targets that correlate with a gauge or something, but you would inspect it like the drawings states.

[[Ro...]]

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If I were to use RPS alignment on this part, how do you suggest I take points or use it? I do not have much experience with RPS alignment