confused by lathe behavior

Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:


Hmm forgot about this. I've been fiddling with toolpost height, setting the cutter so that a facing pass doesn't leave a nub at the end of something. Too low definetely makes the parting blade very unhappy as the work starts to climb and chatter like crazy.
I'm able to compensate for this behavior, it just seems opposite of what I'd expect.
My first guess is that a deeper cut would tend to be less than you dialed in as the machine everthing is flexing apart. I'd then assume a wimpy fine cut would be about what the handwheel would indicate there's less stress on the machine, work and everything in between. Going over what I dialed in would be possible due to backlash if the cutter is being pulled INTO the part.
Is that about right?
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There's an awful lot that can go wrong on a light "hobby" lathe. Banquer said it right, about having to "baby" it to prevent flex and spring. The other responses about spring in the system were also spot-on.
Also, if you're using a lantern toolpost, it's hard to control the rake without re-grinding the cutter. Raising and lowering the tool only serve to alter it's "cut per increment".
It's not "kosher" on aluminum to use a zero back-rake, but it will tell a tale if you try it. The finish won't be all that great unless you use high speed and have a VERY sharp cutter, but if the problem goes away, it indicates that hogging (due to spring in the system) is the problem.
A zero back-rake springs away from the work -- if it's precisely on the centerline.
You might also check your mounts. If the whole bed can spring and twist, it will give similar disgusting results.
Lloyd
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Oh... and also snugging up the gibs on your cross-slide as much as you can stand. Lloyd
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On Thu, 4 Apr 2013 19:06:19 +0000 (UTC), Cydrome Leader

Perhaps your cutting tool has too much "draw" and is pulling into the work. Try locking the gib screws on the cross-slide before making the cut.

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Reminds me of turning a 1" rod of 6060 in the lathe - dry as a bone - and as it heated up Al expands and you cut more. Then you come off it and it soaks into the chuck and air - shrinks back to where it was and A slot I cut not a smoothing cut!
Al needs coolant and plenty of it to stay cool so it won't expand.
Martin
On 4/4/2013 2:06 PM, Cydrome Leader wrote:

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