Alignment

[Bernie Weishapl]

I noticed today the my tailstock with a 60 deg livecenter in it is about 22 thousandths higher than the point in the headstock. Is this going to be a problem? Hasn't seemed to bother but was wondering if you wanted to turn something fairly accurate if it would cause problems?

[Bruce Shiverdecker]

Well bernie, when the centers are off, you change the position of the axis. That becomes a problem when doing legs. The axis is not perpendicular to the ends of the turning. This can get REALLY interesting when the points are off by about 1/4". That cost a customer of mine an extra $50.00 on a project.

He had turned two legs on his lathe. Besides the misalignment he had several catches that scared the ... out of him. When I placed those two on my "Perfectly" aligned lathe, they tried to break it. I had to use his lathe to currect those two, then made the other two on my lathe.

All that said, you coud adjust your head stock by loosening the mounting screws and shimming it with a piece or two of paper.

Bruce

[George Tokarev]

Not really a problem turning between centers. Might scrub a bit with a cup center or give a bit of drag, but it's certainly close enough to be called a point with the 60 degree. Hate them anyway. Cup center is much more secure against side loading.

Now if you're using them to wedge into a prebored hole and expecting things to center, a greater difference might give you problems, but wood is pretty elastic when you come down to it. Probably more give than .022 over a half a foot in the wood alone.

[Bill Boehme]

I noticed today the my tailstock with a 60 deg livecenter in it is about 22 thousandths higher than the point in the headstock. Is this going to be a problem? Hasn't seemed to bother but was wondering if you wanted to turn something fairly accurate if it would cause problems?

Bernie, you must be using a micrometer to check alignment. The answer to your question is that it will have no effect whatsoever if you are turning between centers because the axis of rotation is the straight line between the point on the drive center and the point on the live center (NOT the spindle axis). The drive end operates like a universal joint to account for the very slight angular misalignment just as the universal joint on your car works.

Consider this example: You are turning a table leg which is 29 inches long and there is 0.022 inches difference in height between the drive and live center points. This amounts to an angular alignment error of only 0.01745 degrees (or one arc minute). It would be difficult to accurately measure an angle within one degree with woodworking shop tools, much less measuring within 1/60 of a degree.

If you were doing faceplate turning, the point on the tailstock would inscribe a circle with a diameter of 0.44" or slightly more than 1/32 inch. Again, this is not anything to worry about because the tailstock is there in most cases just to keep a bit of pressure against the headstock.

The only area that could present a problem is when using a rigid mandrel in the headstock for things such as pens. However, one thing that gets overlooked in the process of emphasizing the importance of aligning the points on the headstock and tailstock is angular misalignment between the spindle and tailstock axes. The points could align perfectly while at ther same time the spindle rotation axis is skewed away from the tailstock centerline. This could account for one of the problems that some penturners have encountered on mini lathes where the pen mandrel does not line up with the live center point and yet there is no coning motion at the tail end of the mandrel and the points were matched perfectly before installing the mandrel.

Bill

[John Hart]

I did alittle CAD drawing just now and put a 9X6 bowl into a jaw chuck, then placed a 22 degree cone shaped live center .022 high into the bowl blank and simulated the resultant tilt. This situation would cause the tenon of the bowl to pull away from the jaws at the bottom by 10 thousandths and push the jaws into the base of the bowl 10 thousandths at the top....so while spinning, the piece would be rocking. So my question is....In this scenario, would the spinning and resultant tilt, cause a "machining" of the tenon over the duration of the project, because of this 10 thousandths disparity...and when you pull the live center away from the blank to finish turning the bowl....would the grip on the tenon be so compromised that it will pop out more easily?

[George Tokarev]

I did alittle CAD drawing just now and put a 9X6 bowl into a jaw chuck, then placed a 22 degree cone shaped live center .022 high into the bowl blank and simulated the resultant tilt.

In this scenario, would the spinning and resultant tilt, cause a "machining" of the tenon over the duration of the project, because of this 10 thousandths disparity...and when you pull the live center away from the blank to finish turning the bowl....would the grip on the tenon be so compromised that it will pop out more easily?

No tilt. Pushing a point into wood is not even going to budge the tenon, merely enlarge the seating of the point. Then there's the elasticity of the wood to consider. Better to worry about not seating the shoulder on a tenon properly and the assymetry that can work it lose while hollowing.

[John Hart]

...Better to worry about not seating the shoulder on a tenon properly and the assymetry that can work it lose while hollowing.

Good point...and well taken. But I'm curious about the constant upward pressure exerted. If you have elasticity in the wood, then the shoulder will give in to the jaw face toward the top and allow the shoulder to pull away from the jaw face at the bottom. I know we're only talking about 20 thousandths here and it's probably negligible....but if that were exaggerated, wouldn't it then be troublesome? And, by troublesome, I mean a small loosening of the tenon will make smaller amounts of assymetry in the wood, more noticeable and more problematic.

[Paul Engle]

Yep, the constant wabble could make you nuts , best always rule of thumb "shim to make em match" misalignment can cause strain on other parts of the system you may not see until it breaks. In machinery alignment .022 " is not acceptable at all , prime mover to driven part needs to be as close as design calls for . same goes for wood lathe . Granted ,wood will give,but the live cnter will take the beating only so long before you are tearing it apart and replacing the bearing.

[George Tokarev]

If you exceed the elastic limit of the wood, the point will create a blunter than average image of itself - larger hole. Doubting you can exert enough pressure on a small area without exceeding its elastic limit to overcome the broadly distributed load on the shoulder of a tenon or bottom of a mortise. I can't, anyway. The smaller point just rides to one side of the larger hole. Assuming, of course, that it's a "dead" center. A live one would rotate around best axis, a little shy one side, a little fat the other.

[Bernie Weishapl]

Thanks guys. I really appreciate the answers. A machinist friend of mine came over and he measured it with his calibrated micrometer. He said it was actually .0175 inches off. Mine is a cheap Enco micrometer. He told me if I am not doing long spindles don't worry about it. He said like Bill B. said he really doesn't see where this is really a concern with woodturning lathe. I think I will maybe try to shime it as I have a lot of brass shim stock.