CMM plane construction method types and accuracy differences for measuring Pos Tol

V

vinmech32

I have a sheet metal bracket in which i am measuring a true position tolerance wrt to the plane. The inherent variation in the plane flatness is 0.5 (-0.25 to + 0.25mm). I am taking 4 points (+0.2,-0.25,+0.22,_0.21). If we ask the CMM to construct a plane with these 4 points. What would be the best method to have a plane which would be close to 0. I have a true position of a hole which needs to be measured from this plane. I fear if there is a variation of 0.1-0.2 in plane construction then the true position values would be deviated from the spec even the hole is machined at the right position.

I would like to have an understanding of the different methods that are employed in CMM measurements for plane construction and how do the accuracies vary between them. Which one is the preferred one.

I tried to look for similar posts which discuss about plane construction methods (least squares, average, best fit etc) but i was not able to spot any. If there is any previous discussion which talks abt these areas, do post a link and i will refer it.

Expecting your technical expertise for my understanding

Stijloor

Staff member
Super Moderator
Re: CMM plane construction method types and accuracy differences for measuring posi t

A Quick Bump!

Can someone help?

Thank you very much!!

Michael_M

Trusted Information Resource
Re: CMM plane construction method types and accuracy differences for measuring posi t

You may have better results asking the question 'here'.

If I understand your question, I will attempt to answer it, I am not an expert, I just program our CMM when it needs it.

Try taking more than 4 points and take the points at close to the hole and on the edge of the sheet metal. If I am checking for flatness, 16 points is the minimum suggested (by the software I use) to get an accurate reading. Before the CMM I am currently using, I always thought 8 points was a good number so take your pick.

RSEGRIGGY

Involved In Discussions
Re: CMM plane construction method types and accuracy differences for measuring posi t

What software are you using?

I'd say regardless of what you are using, 4 hits is probably not enough to establish a good plane, especially one with that much deviation.

What tolerance are you trying to hold?

V

vinmech32

Re: CMM plane construction method types and accuracy differences for measuring posi t

Thanks to all for their responses. Will try CMMGuys forum too.

I think its PCDMIS, i am not sure. The part is 300mm height 200mm length 70mm width. Its a bracket a kind of C shape. The plane which i am talking about is on the 70mm side and the points are taken 50mm apart. I have two datum target areas both separated by a distance of 190mm along the height. So its 2 points on one target area and another 2 points exactly below in the another target area which is 190mm apart. I have a position tolerance requirement of 0.4 with this plane as primary one.

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