Aligning tailstock and headstock BORES

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jskeen

jskeen

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I've suspected that there is something gone afoul with my lathe for a while now, but never really spent too much time thinking about it till tonight, when in the midst of replying to another thread about Out of Round blanks, I had to stop and spend a solid hour lying in a dark, silent room trying to get my 3 year old daughter back to sleep. So here's my Idea, please help me disprove, improve, or remove it as you are able.

As we all know, if the tips of the dead and live centers on your lathe don't touch when you move the tailstock all the way up, it can make your pens end up out of round. But, if they do happen to touch perfectly tip to tip, that doesn't guarantee that you won't still have problems for two reasons. The first is that just because the tips touch with the tailstock all the way up is no guarantee that they still would with it down in it's usual spot for turning. The second is that even if the tips do touch perfectly, there is no way to tell or adjust how the actual centerline of the bores align all the way down the axis of rotation.

I found a couple of places that sell these little widgits:

http://www.packardwoodworks.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=lathes-acc-mrstool

and

http://sierramold.com/WMorse_Taper.htm

for $18 And $24 respectively, but i'm not really sure even that will really fix the problem totally. I guess they would help you accurately shim your headstock so that the bores are concentric at one point on the ways, and that may be enough to eliminate most of the OOR issues when TBC, but you still have to question if the tailstock is moving that bore out of alignment somewhere down the ways.

SO..... Here's my thought. I know this is not going to be accurate down to the last whillionth of a skillimeter, but it may give some indication as to what's going on with your lathe without having to lay out a big chunk of cash. If you were to take a piece of 3/8 or about drill stock, or even just cold rolled from the borg about 18 to 24 inches long, and check it to see if it's bent by rolling it on your tablesaw or jointer bed, and chuck it in your mt jacobs chuck in the headstock, so that it extends through the bore of the tailstock chuck it seems that it would give you a fairly accurate gauge as to how centered the axis of rotation in the headstock is to the centerbore of the tailstock AS YOU MOVE IT down the ways. I know, I know, intrinsic runout in jacobs chucks and linear deviation in a steel rod that length, ect. ect. ect. but would it be accurate enough to let you know if something is out of whack enough to be causing a 2 inch or less pen blank to turn out of round for some reason?

What say the cognoscenti among us?
 
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I guess you and I are on the same wave length. I posted that link in the other thread as well. I aslo mentioned the rod trick. But like I said in the other thread there are other factors that come into play. The male and female ends of a taper must be clean and no scratches. When locking the tailstock you will always get some play and the farther away from the headstock the more it is amplified. One other tidbit I added in the other thread if using that double morse taper do not run the lathe with it connected on both ends. :eek:
 
Sorry to say that alignment of the bores won't give an out of round situation , but it will give a tapered cut . On a metal lathe the tailstock is designed to be moved from side to side to allow for making tapered cuts .
If you are having an out of round condition your problem lies elsewhere . Something is causing the stock to wobble along the axis , like a bad bushing or runout in your spindle or live center or something in the MT bore causing the dead center to wobble around the axis .
 
Butch; I understand what your saying about tapering, that's standard machining practice. If you take a metal lathe, where the bores are assumed to be concentric and aligned the full length of the bed, and offset the tailstock, you will produce a tapered turning. BUT, the assumption is that the bore of the tailstock remains PARALLEL with the bore of the headstock, even though it is no longer concentric. Also, in machining metal, the live center is generally used to contact the workpiece with it's point, in at most a small dimple, not usually in a hollow tube, at some point a half inch or so up the angle of the center. (I know, machinists do taper tubes as well as bars, but not as often, and from my limited understanding, if they need to do so with extreme precision, they will use an endcap in the tube, rather than letting it ride on the center on it's ID) I don't guarantee that part however, it's just second hand info.

Geometry says that if a plane (like the correctly squared end of a blank) intersects a cone (like a live center) at a 90 degree angle to the centerline of the cone, the resulting coincidence is a circle. But if that plane intersects that cone at other than a 90 degree angle, the coincidence is an oval.

Now what I don't have any FACTS on, is just how bad that misalignment would have to be to produce a noticeably OOR pen. I would suspect that it is a lot easier to incorrectly square the end of the blank without noticing it than to have the bores that far out of alignment, but again, that's supposition on my part, not direct measurement.

Other issues that might need to be eliminated before alignment could be proved the culprit would be burrs in the end of the tube, different hardness, density, and thermal expansion of different parts of the blank.

Lots of fun things to talk about here :-)

Thanks for the input.
 
I think that ldb2000 is right: any misalignment in the tailstock will ONLY result in a taper, not a OOR. if you see that the tail stock does not wobble, if your live center's point is always in the same spot when turned (and that's very easy to check) than the problem will be always with the lathe's head.

I've used a dial caliper on a magnetic base to find out the runout.
The first test you can do is by resting the probe on the morse taper, inside the lathe.
this can be done in both axis.

you can then pretty much pinpoint the source of your trouble, by removing the quill (what's it called?) and measuring any runout on the inside of both bearings etc.

I've realized my rockler's excalibur chinese lathe had something wrong with the quill itself (what's it called?) and I was lucky to have a friend with a machine shop re-boring the taper.

but I suspect that my outside thread is still whacked.
caliper.JPG

https://dl.dropbox.com/u/17249321/caliper.JPG
 
Another thing to check is OOR of your center's cone. I go by feel - at the point where the tube contacts the center. I clean CA off my center with lathe tools, and have had occasion that although I still had a point that aligned with the tail center, I had inadvertently introduced OOR further up the center, and had to re-surface the center more carefully.

I also mark the center make sure I insert it with the same orientation every time. If you make your own drive center, you can compensate for run-out this way.

Another thing I notice as my inexpensive live center ages, that if I give the tool a little too much pressure, it can deflect the live center slightly. If this happens I reverse the blank so final sizing always happens on the drive center side.

Lastly, I often have problems with wood being OOR, I think due to density differences. But, if I build up CA and then take a few thou off with a carbide scraper, then I can final size with good accuracy.
 
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