Turning between centers. Need help.
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  1. #1
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    Default Turning between centers. Need help.

    Going to turn a .937 shaft 20" long down to .815, my question is what is the best way to turn this all in one? I have a haas sl20 with a tailstock. My thoughts are put a dead center in chuck and a live center in tailstock, but has anyone tried this without a lathe dog? With the tolerances ive been given, I cannot flip or do a 2nd op turn on it, has to be all in one.

    Thanks

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    My way, center drill both ends. Have 10.2" stick out of chuck. Turn to size with a fillet or radius of at least the tool nose. Flip and do the same.

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    Quote Originally Posted by sknies View Post
    Going to turn a .937 shaft 20" long down to .815, my question is what is the best way to turn this all in one?
    You are not going to turn down a noodle like that unsupported.

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    Quote Originally Posted by sknies View Post
    Going to turn a .937 shaft 20" long down to .815, my question is what is the best way to turn this all in one? I have a haas sl20 with a tailstock. My thoughts are put a dead center in chuck and a live center in tailstock, but has anyone tried this without a lathe dog? With the tolerances ive been given, I cannot flip or do a 2nd op turn on it, has to be all in one.
    Like Phil said, that is just not going to work. You could look into "Box Turning Tools". You could do it in halves like athack said. You could do it by incrementally pulling it further out from the Chuck in sections.

    OR you could "no quote" it, because you don't have a CNC Lathe that isn't equipped for it. You have to support the middle.

    OOC; why no Lathe Dog?

    R

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    Like the others have said, there's no way that's going to work.

    All I have to add is, when you want to turn between centres on a cnc lathe and make any kind of speed at it a driving centre is the way to go, not a dog. Much safer, can go faster, no balance issues etc.

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    I think you will end up roughing it out by pulling out a few inches at a time and then sending it out to a centerless grinding shop.

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    Quote Originally Posted by Dualkit View Post
    I think you will end up roughing it out by pulling out a few inches at a time and then sending it out to a centerless grinding shop.
    Bingo. Exact reason we bought a centerless grinder.

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    I've sent a few items to be centerless ground, but I was under the impression, that centerless grinding a part that had to be concentric with... the center, was a nogo?

    How to do they do that?

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    With the apparent qty of just one, just send the shaft to the C/L grind shop and have them just grind it down.
    No point you screwing around with it.

    Option 1)
    Buy from a Swiss shop - but 1 pc will likely be cheaper from grinder.


    Option 2)
    Use .015 rad inserts - and preferably 35* points if possible, and take small cuts and slow RPM to fight chatter.
    It might be dooable, but not worth it if you can find a [centerless] grind shop.



    --------------------

    Think Snow Eh!
    Ox

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    I think it is doable. As long as your material is balsa.

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    Quote Originally Posted by sknies View Post
    but has anyone tried this without a lathe dog?
    Disregard this post
    Last edited by Mtndew; 09-06-2019 at 07:25 AM.

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    Quote Originally Posted by Mtndew View Post
    Just to add on what others have said about not being supported, how do you intend to drive the part if you're not using a dog?
    The second your tool touches the part it's gonna spin and you're gonna have a bad time.
    See Gregor's post. I've never used one, he swears up and down by them. I have to admit, if they work as well as he purports, they're pretty slick.

    R

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    Fancy new fangled contraptions can be seen here:

    https://www.riten.com/wp-content/upl...DUserGuide.pdf

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    Quote Originally Posted by litlerob1 View Post
    See Gregor's post. I've never used one, he swears up and down by them. I have to admit, if they work as well as he purports, they're pretty slick.

    R
    I do, and they do.

    They've been around a long time, absolutely not a new idea. I first came across them years ago when I was learning the ropes at my dad's shop. He bought an old Rohm one on ebay as an impulse buy for peanuts more out of curiosity than anything else I guess.

    Turned out to be an extremely useful piece of equipment.

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    What material and what tolerance, did I miss that?

    Also, can you tap the ends? You can put a bolt in there and hold onto that as an alternative...

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    I've only done that sort of thing on manual lathes with a follower. It can take a few tries to get it just right.
    There are ways to do it cnc, a 1 off isn't ideal, if its many you can spend some $ and time to rig up a proper follower or at least a steady.
    30+" rifle barrels get turned like that all day every day.

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    Very sharp tool bit so a light finish cut aluminum sharpness might help.
    Closer than .0005 and better have a very high quality live or a dead center...

    Agree I don't know beans about CNC lathes.

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    Use a follow-rest

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    Driving centers work cool, but with very high length to diameter shafts the driver dog spring pressure tends to aggravate chatter or help the part to spring away from the tool near the center of the shaft.

    Follow rests & box tools when set well will work hard all day long.

    Adding 3” to the part length with a collet or soft jaw chuck will work using good tool geometry & reasonable machining parameters (then cut the 3” drive end off).

    Good luck,
    Matt

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    fwiw, the slenderest shaft I've successfully turned with a centre, otherwise unsupported, in a slant bed cnc, was 15mm bar down to 1/2" over 13" long. 316L. I had one end in the chuck with excess material, as suggested by Matt above.

    On the advice of our sandvik rep, I did it in a single pass using a VBMT with a .2mm rad. I was extremely skeptical, but it worked a lot better than I expected, barely noticeable chatter and about .003" big in the middle.

    That was a long time ago on very nice Gildemeister lathe, with an exceptionally smooth, fast, ceramic bearing spindle and fantastic accuracy. I would not like to attempt the same on any of the lathes I have now. I was spoiled by that lathe, the new lathes that we have bought since have been bit of a harsh reality shock in terms of accuracy and capability.

    These days I put such parts directly to the sliding head guys, don't even think about doing it myself.


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