surface grinding on three points
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    Default surface grinding on three points

    this has been mentioned several times. how is it done? how high the shims? what surface area? plain steel or magnetic?

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    These guys use it all the time. They use machinist jacks with a pointed top, so they can get it indicated in straight and then they block the part in around it. They are excellent rebuilders / builders and are taking over for me in teaching classes in mainland Europe.

    Maschratur - Schaben & Maschinenservice

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    so there is no magnetic force pulling part down at all?

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    "Three points" takes the wiggel-waggel-tilt out of the part to be ground or measures so you have a straight line going point to point, one that is not obscured by any high places in between those points.

    For exanple ..002 high place might tilt a part .020 high on one side if allowed to rest/set on that high place..and so not a very good idea of the true shape or error so not how so set or measure a part.

    Setting on shims a block-in block is/may be place at the go direction.

    likely Richard uses 3 points for other things in machine rebuilding but that is how it relates to my grinding.

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    Quote Originally Posted by dian View Post
    so there is no magnetic force pulling part down at all?
    It would bbe similar to grinding aluminum. You block it in all around with steel blocks so it can't move, and the wheel pressure pushes it down...

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    doesnt matter if clamp is magnetic or applies mechanical clamping force. you usually want to check for "soft foot" you usually put indicator on part, apply clamping force, see if part is distorting from clamp force when applied using a indicator. many magnetic chucks have magnetic force control to be able to turn it down, to not distort parts so easily
    .
    many a part is adjusted .0001" or more by varying clamping force

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    On bigger parts on the single point side you can put stabilizer jacks with finger tightness on the outside edges. Or say .0001" up pressure. It take a little practice, but the more blocks around the part the better. Better safe then sorry. I bet Cash does this a lot too. On most parts I set the 3 points at 30% from the ends. If they are rectangle I find the balance point and split that in 1/2 on both ends to locate the 3 point locations.

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    many a part each time material removed when rechucked part goes bong or distorts when free of clamp force. thus why rechucking at lighter pressure and removing less material often done many times. also temperature unevenness can easily cause part distortion. big part if coolant only used on one end that end of part might change height .0003" after 1 hour.

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    sometimes due to temp change when part unclamped you hear a thump..... part when free it can expand or contract. when clamped if section gets longer between supports it ends to curl as excess length being constrained
    .
    hard to describe but often why part on surface plate measures different than when in the machine. of course also gravity sag or where supports are and orientation effect things too

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    Quote Originally Posted by DMF_TomB View Post
    doesnt matter if clamp is magnetic or applies mechanical clamping force. you usually want to check for "soft foot" you usually put indicator on part, apply clamping force, see if part is distorting from clamp force when applied using a indicator. many magnetic chucks have magnetic force control to be able to turn it down, to not distort parts so easily
    .
    many a part is adjusted .0001" or more by varying clamping force
    Did you miss a decimal Tom?!? I know you *normally* work to .00001" tolerances with your giant castings weighing 10 tons or more...

    (almost forgot, long endmills, runout, hard spots, etc etc...)


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