Hemispherical Probe Use?

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  • Hemispherical Probe Use?

    Couldn't find much discussion on Hemispherical Probe usage, so I've got to ask. We've got a new part that "on paper" would be ideally suited to use a Hemispherical Probe. It would be a Renishaw A-5000-3614, Ø18mm.

    Problem is I've got to have a 120mm worth of extension. (Renishaw said carbon fiber was the best for these long applications). I've got that feeling that I'm going to lose accuracy or even have not work at all due to length / weight / gravity causing triggers before touches (I'll be using Extended Force module). And I've either got to use A45 thru A105 (the dreaded 45° cosine error), with the majority of the hits being at A90, or have a second setup program using A0 thru A30.

    Feature is a tapering torus that cuts heavily under other features, hence why I'm looking into the hemisphere to actually get good hits. And it's a pretty tight tolerance for what it is, almost 9" diameter that I've got to inspect 300° around, with a profile requirement of .030" to itself on a casting, so I can't afford to have inaccuracy in my probe system.

    Any insight from someone who's used them before we spend $500 on something that might only work in theory? Thanks.

  • #2
    What probing system are you using? The length you can go will depend on that answer.
    Remembering my beautiful wife Taz who's life was lost on 6-13-2020. I love you and I miss you.

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    • #3
      Originally posted by A-machine-insp View Post
      What probing system are you using? The length you can go will depend on that answer.
      My thoughts exactly. Depends on your probe system.

      I have worked with that exact hemispherical probe before with a TP20 probe system. It is great because the probe diameter is bigger than the diameter of the probe body. Using extensions between the probe head and the probe module, you can measure some pretty deep features. I was mostly using it at A90 angles and saw negligible probing error compared to 3mm probes, so I can tell you that it is possible. But, it depends on your probe system.

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      • Cris_C
        Cris_C commented
        Editing a comment
        Also beware, that hemispherical probe is very delicate. It huts to spend around $300 any time someone drops it while cleaning it. And, I'm not talking dropping it on the floor, I mean a 1 foot fall will do it.

    • #4
      Are you talking about 120mm worth of extension between wrist and module? it should be no problem and .030 is not really that tight of a profile tolerance for that. I have TKA140mm that I use with standard force modules and have no issues with false triggers(with a star probe on it). I use that m140mm extension to check .005" concentricity specs on parts... datum checked in A90 wrist position and measured feature in A-90 position and consistently get values around .001" so your accuracy would not be an issue at all for .030 profile IMO

      If you mean 120mm between the module and tip that is a different story and you should rethink that and try put the extensions before the module... if your probing system allows that, as mentioned a lot depends on what you are running

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      • #5
        Thanks for all the feedback, sorry for the late reply, I work a strange schedule.

        PH10MQ with TP20

        We do have the pre-module extenders, never used them before, was considering it, but I am definitely going to use them now. I had been talking between module and tip and that's why I posted, didn't seem like a solid strategy, so was throwing it out for those with hands on experience.

        And I never considered that the sphere is larger diameter than the module, that's awesome info Chris_C, makes for an efficient probing system with that module having a safety envelope.

        I'm sold, now time to sell my boss, hahaha. Thanks so much gents! Ya'll keep rocking this forum!

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        • #6
          They cost $$ but check out the EM1 and EM2 extensions as well (these are in effect extended SF modules).

          The PEL extensions (which go between the probe and the TP20) are okay but eat up machine volume - also you obviously need to re-calibrate everything if you add/remove them from your system.

          The EM extensions allow you to have just one longer probe whilst leaving everything else as is.

          Automettech - Automated Metrology Technology

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          • #7
            Originally posted by NinjaBadger View Post
            They cost $$ but check out the EM1 and EM2 extensions as well (these are in effect extended SF modules).

            The PEL extensions (which go between the probe and the TP20) are okay but eat up machine volume - also you obviously need to re-calibrate everything if you add/remove them from your system.

            The EM extensions allow you to have just one longer probe whilst leaving everything else as is.
            I have used the Renishaw A-5000-3614 Ø18mm Hemispherical probes with an EM2 extensions and I can tell you that the setup works, but can it be a little fickle. At the A90 angles I had to really cut back on the acceleration to avoid false triggers with vertical movement. Also, there were some problems of having the EM2 module unseat from the TP20 before there was enough force on the tip to trigger a hit. Having the tip so far out from the magnetic connection put a lot of leverage on it. I got around that by adding a 20mm extension on the stylus and slowed down the touch speed. Tests with ring gages at various orientations verified that the measurements were as good as it gets with a standard force module even though my setup did not fall within Renishaw's mass specs. I asked a fella at Renishaw about that and he basically said, "if it works, it works". That setup worked very reliably around the clock on two Discovery IIIs for years.

            If you don't need that much length, an EM1 would probably have less trouble to setup - less leverage on the magnetic connection. Or, if you have enough machine volume, PEL extensions would work even better.

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            • #8
              In a previous job (hydraulic equipment) we had a ton of seal grooves to measure - I had a 25mm disc on an EM2 - total life saver that probe!
              Automettech - Automated Metrology Technology

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              • #9
                Originally posted by NinjaBadger View Post
                In a previous job (hydraulic equipment) we had a ton of seal grooves to measure - I had a 25mm disc on an EM2 - total life saver that probe!
                I use a 12MM disk very frequently (for the same thing, seal grooves) and I also have a 30MM ceramic disk that I have to take out on occasions. The 30MM disk is also like ~$300 so I only use it when I HAVE to. Definitely not a daily driver.

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              • #10
                Good stuff here for future generations. Thanks you all. Pretty sure we've got the envelope for the PEL, we've got a cheaper PC-DMIS seat, so I can't simulate to make sure, but I'll make it work. Isn't 90% of programming making stuff work anyways? Ha ha ha ha. Demon tamer deluxe!

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