Part fixture: Vacuum on a diameter
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    Default Part fixture: Vacuum on a diameter

    Hello!

    We do lots of round parts of different shapes and sizes, but they all tend to have a similar .500 +/-.001 OD in one way or another. We've got a few variations on fixtures, but there's always a trade off.

    Is there a method of madness to hold a small OD using a vacuum?

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    Quote Originally Posted by thunderskunk View Post
    Is there a method of madness to hold a small OD using a vacuum?
    Vacuum provides a pressure of 14.7 pounds per square inch or about 1kg per square centimeter. Your part has such a small area that the "clamping forces" from a vacuum would be about 3 pounds = 1.5kg. That's not nearly enough for any standard machining operations, not even grinding.

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    Quote Originally Posted by ballen View Post
    Vacuum provides a pressure of 14.7 pounds per square inch or about 1kg per square centimeter. Your part has such a small area that the "clamping forces" from a vacuum would be about 3 pounds = 1.5kg. That's not nearly enough for any standard machining operations, not even grinding.

    Unless he applies vacuum on 180į of the the side Then it depend on how long the 0.500" feature is
    But I agree A collet would perform better You could go fancy with a air or hydraulic operated colletcloser

    Peter

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    Default Part fixture: Vacuum on a diameter

    Quote Originally Posted by ballen View Post
    Vacuum provides a pressure of 14.7 pounds per square inch or about 1kg per square centimeter. Your part has such a small area that the "clamping forces" from a vacuum would be about 3 pounds = 1.5kg. That's not nearly enough for any standard machining operations, not even grinding.
    To your point, 3# isnít enough, but keep in mind itís just going in a CMM after machining is 100% complete.


    Now that Iím rereading my original post, Iím realizing I didnít actually say this was for a CMM, just posted in metrology, so thatís on me.

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    I'm not really sure how a vacuum would work holding an od. It can neither expand the part nor shrink the fixture, so what would it really achieve? The vacuum itself is not what holds the part on a vacuum plate...

    I assume you're struggling with physical workholding methods distorting the part?

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    Not quite. Weíve got a few ways we currently hold the parts; if you want to measure the front, you use a datum simulator on the back. If you want to measure the back, you use a v-block on the front. If you want to measure both, weíve got a thin clamp that sort of works, but recent data made me thing itís causing issues. On top of that, it limits measurement of certain features.

    My thought is just to use a vacuum to pull the part into a low-profile locating fixture. More access, and to your point less influence.

    The other advantage is around timing. We have front and back features that arenít necessarily timed to each other, so thereís thoughts around a mechanical clocking mechanism to orient each feature. Thatís a tall order. Iím well aware itís a silly idea, but Iím curious whether anyoneís done it.

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    You could certainly make a vacuum fixture that would hold on an O.D. from the side, but locating might be interesting.

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    A hand sketch showing the part and your proposed fixture would help. Or a photo of your part.

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    Quote Originally Posted by ballen View Post
    A hand sketch showing the part and your proposed fixture would help. Or a photo of your part.
    cmm-widget-snip.jpg

    The two flats on either end represent unclocked features, though I clocked them here because it was easier to sketch quickly.

    cmm-widget-fixture-snip.jpg

    This is my "I've got 60 seconds" thoughts about what the fixture would look like. The posts would just be old short CMM probes. The rearmost post would be a locator along the axis while the other 4 are essentially V-blocks.

    I found this thing on McMaster Carr: McMaster-Carr

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    Are your parts non-magnetic?

    A small four-bearing inspection vee block with a magnet in the centre might achieve the same result?

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    Quote Originally Posted by gregormarwick View Post
    Are your parts non-magnetic?

    A small four-bearing inspection vee block with a magnet in the centre might achieve the same result?
    I wish. Copper.

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    It would be held around the full diameter, but a place I used to work had a test fixture that you dropped a spool into, was clamped, tested, then lifted out. The fixture was set up like a hydraulic tool holder, so the walls just swelled a bit. I don't think that would be able to cover a full 0.002" range, but perhaps it spawns a few new ideas.

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    Well, the little rubber vacuum cup works. Iím not sure how reliable it would be on a fixture, but itís nice to know it functions.


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    Quote Originally Posted by thunderskunk View Post
    Well, the little rubber vacuum cup works. I’m not sure how reliable it would be on a fixture, but it’s nice to know it functions.

    This led me to search for UK or European sources for these, and I found this place, they have a lot of variety of these things, the bellow type ones look like the solution to your problem.

    Vacuum Suction Cups | Schmalz

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    Quote Originally Posted by gregormarwick View Post
    This led me to search for UK or European sources for these, and I found this place, they have a lot of variety of these things, the bellow type ones look like the solution to your problem.

    Vacuum Suction Cups | Schmalz
    I did end up getting a few with the bellows so itíll pull it into whatever itís locating to. This one is fairly restrictive. Might work well on the end of a robot or something though.

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    Could you mill a trough 1/4 or 1/2 diameter into a block fitting the side of your part diameter and drill several vacuum ports going to a central port.
    Maybe line the trough with a split piece of tubing [ nylon, Teflon, rubber ] Something with a durometer that gives the surface a texture to help seal against the part

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    I think vac will work for your application as it's only CMM pressures.
    But possibly....V block as you say and slide a top "sprung finger" over the top instead of vac?
    Simple fixture with a V and a piece of 1.5mm thick stainless captivated in a slot with a top to stop lifting would do.

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    Quote Originally Posted by barbter View Post
    I think vac will work for your application as it's only CMM pressures.
    But possibly....V block as you say and slide a top "sprung finger" over the top instead of vac?
    Simple fixture with a V and a piece of 1.5mm thick stainless captivated in a slot with a top to stop lifting would do.
    Thatís pretty much the fixture they came up with, but Iím really doubting our GRR results lately. Itís a fairly loose fixture, which just comes down to their design, but even perfectly rigid, the groove isnít a very tight tolerance. Hard to make a jaw that keeps it snug.

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    Quote Originally Posted by thunderskunk View Post
    Well, the little rubber vacuum cup works. I’m not sure how reliable it would be on a fixture, but it’s nice to know it functions.



    They use suction cups in the packaging and printing industry, that could be a source for different cup styles.

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    I have a bunch of this cmm fixturing if you are interested in some.
    Don


    Sent from my iPhone using Tapatalk Pro


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