Inspection with Pattern of Holes 1

[dthom0425]

Hi all,

I have a simple plate with four holes (rectangular hole pattern). Functionally, the part is located on these four holes...so I made the pattern of holes a datum feature. The primary datum is the larger flat face of the plate.

From a hard gauging perspective, I understand that if I were to make a functional gauge, I would make a plate with 4 pins which are sized at the virtual condition of the holes from the 4 hole pattern.

The flat face locks 3 DOF, the 4 pins take care of the rest.

Let's say we did this without a hard gauge though. How can (or how does) a CMM create the two theoretical planes created from the pattern of the holes (datum reference frame is essentially in the center of the part)? Are the planes established in the software prior to actually probing the part? Do you have to probe the part to some degree to establish the center of it?

Sorry - not an inspection expert.

Thanks for the help

 

[Belanger]

First, since you mentioned the possibility of a hard gage, then I assume the datum created from the 4 holes will be referenced downstream with the MMB modifier. That is a key thing which will drive the discussion.
But if that's true, and you want to do it the CMM way instead of a hard gage, then I believe you'd have to do some sort of best-fit algorithm with the CMM.
Here are some previous threads that touch on the subject. They might not be exactly your situation (I think one of them deals with position the holes back to A, B, C), but if you really dive into these discussions it will give you some insight. Especially helpful will be some comments by PaulJackson and Axym (Evan J.).
[URL unfurl="true"]https://www.eng-tips.com/viewthread.cfm?qid=350412[/url]
[URL unfurl="true"]https://www.eng-tips.com/viewthread.cfm?qid=283384[/url]

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems