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Using focus position as a Z location

afotion
Hey all,

I have a simple one for you which may have some complex answers. As a rule of thumb I have always avoided using any kind of focus points/positions to do any kind of Z location/height measurements.

Do any of you use focus points with high confidence? I have had decent experiences and sometimes I have had complete failures with it. Is it safe to assume it comes down to material and lighting and the application? I found that when trying to perform this on a very centralized locations within your FOV it then becomes very difficult to make repeatable.

I would love to hear your thoughts.

Thanks!
  • There is one family of parts we used Ring Light to focus in on a surface that is 0.1-0.3mm wide. But to make sure that the point is focused on the center of that surface, it requires variables based on other features to make sure that the vision probe goes into the right spot to pick up the depth in Z. It can be done. To increase your confidence in doing Z depth with vision, run a Gage R&R.
  • in reply to bfire85
    Thanks B,

    We are actually doing a Gage R&R with a supplier and seeing if we can correlate for that exact reason. I was very interested in the experience you guys have with this. Although I have done it myself as well we have always tried to just revert back to our WLS or TTP. I have also thought about making the machine take multiple points in the same exact location and report all of the points over and over again to check repeatability. I also wanted to compare the measurements between all 3 of my probes.
  • I made some tests, comparing touch probe and CMMV. The focus can be repeatable less than 0.01 mm with a fixed light... If you change the light, the focus position can change more than some 0.01... and be repeatable at this position !!!!!
  • Hey Jef,

    Can you elaborate just a little bit? Are you saying your range of focus is 10µm? or are you saying or focus position range is 10µm?

    Also, are you saying that if the lighting settings change then the position itself with change but then if you use those settings it will be repeatable within µm of that position?

    I am asking because we would want to use TTP to set a reference plane and then use Vision to find a groove that comes to a perfect valley. It is actual a cross weave type pattern like you would see for a grip on a tool, we want the depth of it. This would normally be done using our Mitutoyo Profilometer but a supplier is insisting on this method since it saves them time and they are convinced it is repeatable and gives accurate readings.
  • The focus postion depends on the size of the target.
    I think it calculates the position on an average of net pixels in the target area. If the defect is narrow, or with a V form, I think you can't measure accurately the depth.
    You can try to do it by comparison with the profilometer, and find the right parameters, but if your supplier change the color of the material, or the surface treatment, the result won't be valid !!!!!
    What i said about 0.01 mm was the repeatabilty range of focus on 10 measurements, compared to a contact measured plane.
  • in reply to JEFMAN
    Very useful Jef thank you very much.
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