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Thread: turning problem

  1. #21
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    Jun 2009
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    san jose, ca. usa
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    Quote Originally Posted by old mart View Post
    I bought a test bar on ebay, made in India which has a MT2 at one end, centres both ends and is 11" long overall. It runs better than 0.0001" tir and has been very useful in getting the alignment and height of the tailstock corrected. Don't make the same mistake as me in expecting live centres to be spot on, but use dead centres for testing. I put a piece of 3/8" diameter bar in a collet and turned a 60 degree tip (using a chuck would be ok too), which must be true to the axis of the spindle as long as it is undisturbed. Try the alignment test with the tailstock quill retracted and also with it fully extended to see if there is a difference.
    I didn't think to extend the tailstock ram while adjusting, I'll recheck with it all the way out too.

  2. #22
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    Quote Originally Posted by BCRider View Post
    I'm surprised no one has asked the main question yet. Has the lathe bed ever been tested to ensure it is straight and parallel? A key part of any bench lathe setup is that it be trued up (often called "leveled") before you can trust it to do long parallel cuts.

    Now some Logan lathes have a motor and primary drive cabinet. But that does not mean you can just plunk it onto a floor and KNOW that the bed is in flat from end to end. It still needs to be checked.

    A good and surprisingly easy way to do this is chuck up a long and stout bar in the lathe that sticks out about 4 to 5 inches. Relieve the middle of the exposed bar by about .04" leaving two full diameter "collars" at the end and in close to the chuck jaws. Now skim cut the two collars and mic them. Using shims under the bed mounting bolts or the adjustable feet on the cabinet twist the bed until you measure the same size on both collars. Note that this does NOT involve using the tail stock. You don't want the TS in the equation at this point.

    Once you remove any twist in the bed you can now use the identical size collars to check for any bowing by running a DTI over the tops of the collars. Any difference there is an indication of the bed sagging down or bowing upwards and that too can and should be corrected. Confirm you don't affect the zeroing of the twist by taking another skim cut and adjust as needed until you get same size test collars and a DTI reading the top of the collars shows no difference between them.

    NOW bring the TS up to the end collar. If you chose your bar stock well it is roughly the same size as the TS ram. So with the mic'ed size of the collar and the TS ram you can use a dial gauge between them to set the ram to center side to side using just a little math so you know the step it should be. And while you are at it run the dial gauge over the tops and see if your TS ram is also centered for height. If needed put shims between the base and upper piece. And again while you are there extend the TS ram a few inches and check that it's both at the correct height and not angled up or down.

    After all that the bed should be as true as it can be. If it still cuts a taper or a barrel or wasp waist long shaft then it truly is bed wear.

    But unless you can see the wear quite easily I'm guessing it's just the bed has a bit of twist to it and you need to run this "collar skimming" test and true up the bed.
    I recently moved my shop so the lathe has been leveled and rechecked several times over a two month period. I can borrow a level and recheck. the collar thing will probably be next.

  3. #23
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    Aug 2009
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    Quote Originally Posted by gambler View Post
    with two new dead centers. as close to dead on as I can tell.
    I found the most accurate way was to mount a dial on the chuck. You can pick your mounting method, magnetic base etc. I made a fixture that I mount in the chuck so there is no dial rod sag when the dial is rotated under the TS quill. Only draw back is you have to chase the dial around the quill.
    You can sweep the OD of the quill or the inside taper, compare them. You can mount a good dead center and sweep the taper of the center.
    The TS can be OK front to back but can also be sitting high or low. This dial test will tell you where it is.

    Others have mentioned putting a center in the spindle and bringing it up to the TS center point to point, some do the cigarette paper trick or even use a thin piece of flat shim stock.
    But I like the dial method better.

    JL...............

  4. #24
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    May 2008
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    Quadra Island, BC, Canada
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    A Co-Ax indicator would be a convenient way to check the tail-stock quill, trying to follow a dial test indicator all the way round isn't the easiest thing. When checking the tail-stock it's probably worth testing with the quill retracted and extended, to see if the tail-stock has any sag.

  5. #25
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    Sep 2009
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    Sacramento, CA
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    Quote Originally Posted by MattiJ View Post
    Easy: re-adjust tailstock so that both ends come out same diameter. (You still might get slight barrel/ huor-glass shape but it will be a lot smaller error)
    That's what I do. Just shift the tailstock .005" away from the tool and measure by watching an indicator on the tailstock end of the work.

  6. #26
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    Quote Originally Posted by JoeLee View Post
    I found the most accurate way was to mount a dial on the chuck. You can pick your mounting method, magnetic base etc. I made a fixture that I mount in the chuck so there is no dial rod sag when the dial is rotated under the TS quill. Only draw back is you have to chase the dial around the quill.
    You can sweep the OD of the quill or the inside taper, compare them. You can mount a good dead center and sweep the taper of the center.
    The TS can be OK front to back but can also be sitting high or low. This dial test will tell you where it is.

    Others have mentioned putting a center in the spindle and bringing it up to the TS center point to point, some do the cigarette paper trick or even use a thin piece of flat shim stock.
    But I like the dial method better.

    JL...............
    I intend to do this soon.

  7. #27
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    Jun 2009
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    Quote Originally Posted by MattiJ View Post
    Easy: re-adjust tailstock so that both ends come out same diameter. (You still might get slight barrel/ huor-glass shape but it will be a lot smaller error)
    ok so I did this, got the shaft to within .001, good enough for now. thanks.

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