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indigo.frog
13th Feb 2011, 11:12 PM
I have a Carba-tec lathe, bought October last year, which has been great up until today. I have some dunnage, cut into 30 cm lengths, I put a piece into the lathe and roughed it at a reasonable slow speed, and ended up with the tailstock end wobbly, and a spiral here and there. Tried slowing it right down, but still getting a wobble at the tailstock end, and a bigger wobble in the middle of the piece. This hasn't happened before with this lathe, normally I'll get a perfect piece at the end of roughing.

Everything is tight, headstock and tailstock secure. Do I need to bolt it the lathe to the floor? Any suggestions appreciated :C

Linda

oldiephred
14th Feb 2011, 06:42 AM
Some prefer to secure the lathe but IMO one needs to be careful not to get a false sense of security by doing so. When a heavily unbalanced piece is loaded and the M/C is well secured, it is difficult to "see" the vibration so it is assumed all is well until something more expensive breaks. That vibration forces have to be distributed somewhere, either in the stand or the machine .
In any event, it is difficult to see how the problem you describe can be due to the lathe stand not being secured.
Just some opinions.

indigo.frog
14th Feb 2011, 11:56 AM
Thanks, that at least eliminates one possibility :)

Manuka Jock
14th Feb 2011, 08:12 PM
Linda ,
it could just be that piece of wood , the tension in it , sap content or whatever.
Turn a couple more lengths , cut from different bits of dunnage , using the same process .
See what happens

indigo.frog
14th Feb 2011, 09:47 PM
ok, I'll try that. I did try another piece from the same length but try from something else. Thanks Jock.

munruben
15th Feb 2011, 07:50 AM
Is the tailstock and the headstock in perfect alignment?
You can check this by bringing them together and see if the dead centre and live centre are in alignment, if they are, the points should touch perfectly and not be off centre in any direction. I may be barking up the wrong tree but that's my thoughts on it.

Paul39
15th Feb 2011, 11:25 AM
Linda,

Here is a list of what comes to mind, in addition to the above.

Unbalance at beginning:

Timber not centered, draw a line corner to corner on the ends and put drive center and tail center on the crosses.

One side of timber heavier than the other, use different blank or carefully proceed.

Unbalance part way through:

Drive center or tail center moved sideways because of soft timber or tailstock loosened. While turning, snug up tailstock and be sure to lock.

Thin spindle bows and whips around because spindle is thin and / or tailstock pressure is too much. Ease off tailstock pressure and / or use center rest.

A combination of just a bit of several of the above will cause more vibration than just one larger misalignment, and be harder to find.

If one develops good habits, possibly using a check list as aircraft pilots use before every take off, variables are reduced and the work becomes more efficient and fun.


MAKE A SPINDLE


Set Lathe to lowest appropriate speed.

Mark ends of timber corner to corner, if hard, cut slots on headstock end and drill pilot hole on tail end where lines cross.

Put timber between centers and pull tailstock up to tail end of timber and lock tailstock to bed. Seat drive spur and tail center in place and tighten tailstock hand wheel while turning timber by hand, lock tailstock.

Hold drive end still and attempt to twist timber back and forth, if it wiggles, apply more pressure with tailstock.

Put tool rest in place, spin timber by hand, if nothing bumps stand to side and turn on lathe for 10 - 15 seconds. Turn off and check that timber is firmly held.
If tight proceed roughing.

Periodically shut off lathe and check that timber is firmly held, tighten if needed. I find that a piece will accept snugging up 3 - 4 times before being finished.

Once the above is done a number of times, it becomes automatic and greatly reduces spindles getting loose between centers, vibrating, or flying out of the lathe.

I hope this helps, and maybe more will comment on their favorite system.

Especially , spindle turner extraordinaire.

indigo.frog
17th Feb 2011, 10:02 AM
Thanks for your suggestions. I've been woodturning for a long time, so I understand the principles of finding the centres and spindle turning, although it's not a bad suggestion, as I offer turn pieces that are unbalanced (cut by chainsaw, it's hard to get a big bowl chunk square). Eventually they become round and everything is fine. Just not in this case. I was inclined to try a fresh, unrelated piece of wood (and completely dry, as I'm not sure about the dryness of the dunnage). I just went out and checked and my drive and live centres and they are off meeting by a fraction - approximately 0.5 mm. Is this enough of a misalignment to cause an unbalanced lathe? Considering that I've turned plenty of wood since October, when I got it, and haven't had a problem so far (including turning rather odd shapes which aren't even remotely square).

I'll have a go with the dry wood tomorrow and see if there's a difference.

Linda

RETIRED
17th Feb 2011, 12:19 PM
You summed it up perfectly Paul.

Indigo: Misalignment is nothing to worry about unless you are using mandrels (say for pens) or wanting to hold a bowl blank between centres and even then not a major concern at the amount you have. Bowls I mean, not pens.

Have you checked the spindle lock in the tail stock? Some people wind them out too far and the quill can turn in the housing and never locks in properly.

Another thing to check is that the drive pulleys are tight on the shaft and that the belt is ok.

You didn't say what size (diam) the stuff is you are turning.

Thin stuff can set up its own harmonics, particularly pine.

indigo.frog
17th Feb 2011, 10:32 PM
Thanks , I'm relieved that it's not a major concern. The original wood was 100x100 mm. Not sure what it is now, but it's not too thin yet (it is pine). I will check all the things you suggested, and make sure I follow the process that Paul has mentioned, and try a different piece of wood, and see which one helps :)
Hopefully good news tomorrow when I try it all out.