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Straightening bent SB lathe spindle

Posted: Thu Feb 20, 2003 10:35 am
by Orrin
I have a 9" SB lathe spindle that has a 0.0015" bow in it. In a previous lifetime (before I bought it) the compound had a serious tangle with a chuck. Besides the compound being broken in half, the bend must have occured at the same time.

I'm contemplating using spot heating, not force, to pull it back into alignment. The theory behind this is that some elastic deformation takes place in the hot metal, and then it shrinks when it cools off.

I'll start out with a very small, insignificant amount of heating, let the temperatures equalize, map out the bend, and keep increasing the amound of heating, if needed.

Does anyone have any pointers or suggestions?

I feel that this method will give me more control than a hydraulic press. Besides, I'd have to dial it back in on the lathe after each trial bend. Yesterday, it took me the better part of an hour to dial it in and map out the bend. I can do the heating without removing it from the lathe.

I'd appreciate any suggestions or comments. [img]/ubb/images/graemlins/smile.gif"%20alt="[/img]

BTW, the spindle seems to be made of mild steel. I don't think there'll be any problems with hardening. The bend seems to be in the area of the step pulley. There are no bearing surfaces, there.

TIA [img]/ubb/images/graemlins/laugh.gif"%20alt="[/img]


Re: Straightening bent SB lathe spindle

Posted: Thu Feb 20, 2003 11:37 am
by jpfalt
Couple of things:

The process youre talking about is called heat tensioning in the sawmill industry. You want ot keep the heated spot very small at any one time. heating large areas won't work. Usually you will take an acetylene torch with a welding tip and heat one spot without moving the torch until the spot just turns red. What happens is the material around the spot restrains the hot spot and the material yields in compression. When it cools it shrinks and goes into tension. If not enough, then you let the part cool and do another spot near the first one, but not in the sme place.

There are possible problems. The amount of deflection is hard to control and you will probably spend a lot of time with too much or too little. The second is that the tension is very localised and at very high stress. It will relieve itself over time and the part will move over time as a result..

Second item is that you will need to be careful to thoroughly map out the spindle before you start tweaking it. If the bend is at the base of the nose thread, then that is where you have to change the tension. If it's under or behind the front bearing, then you tension at a different location. If the bend is distributed ove rthe length, then the tensioning will also have to be distributed.

For the amount of runout, I question whether you should try to heat tension, but rather make it as best you can on a hydraulic press and then either remachine the chuck backplate to compensate for the wow and regrind the #3 MT in the spindle with an internal grinder to restore the accuracy.

I used to own a SB 9" and between chuck scrollplate wear, chuck jaw wear and so on, .0015 may not be that big a deal.

Now if it's really .015, then that needs some attention. Then I'd press it to as close as possible and then regrind the thread and the taper to get back to tolerance.

Re: Oops. Make that "plastic" deformation

Posted: Thu Feb 20, 2003 12:14 pm
by Orrin
I'm not awake, yet. I should have said "plastic deformation."

Indeed, the bend is 1-1/2 thousandths, not fifteen thousandths.

I've tried straightening in a press, before, and even with the use of a dial indicator to monitor things, it was very difficult to avoid overshooting and then it was necessary to reverse things and bend it back.

I guess we'll see how the heating bit goes. It's now cooling down after the first trial. I'll keep you posted.

Re: Straightening bent SB lathe spindle

Posted: Thu Feb 20, 2003 12:50 pm
by Anonymous
Check out ebay. 9" SB parts are always listed.

Re: Straightening bent SB lathe spindle

Posted: Thu Feb 20, 2003 1:44 pm
by Orrin
[img]/ubb/images/graemlins/frown.gif"%20alt="[/img]I wish that would work. Unfortunately, this machine was built in June of 1930. Somewhere along the way, SB changed their design. Most 9" SB parts out there will not fit my lathe.

I learned this the hard way when I went to install an MLA cross slide. Not only did I have to modify the cross slide, I then had to get a "modern" 9" compound for it.

I've now had a chance to check the spindle. I don't think I can get it any straighter than it is. The max runout is a half-thou.

I'll true it up the rest of the way by grinding.