Tail/Headstock misalignment (or something)...

If you have the lathe bolted down, check to see if everthing is level and you have not twisted the bed. This can be fixed by shimming the feet until the tailstock matches with the headstock. Just a thought

Bits which guide by the edge rather than a point run a lot more true. Natural enough for a bit to follow the line of least resistance along earlywood between annual rings. I've had good luck with brad points, but the wood can also twist and flex some, so a steady is a good idea.

Does the point meet the one you made at the tailstock when you chuck up? If not, you've got misalignment. Could be chuck, could be what it's mounted to. A point in the headstock should meet a point in the tailstock.

That is what happens. Actually, never mind the drill thing; it distracts from the problem. If I do this:

in (4) the tailstock live center will not line up with the old point. Now the tail stock has some play (it is not an exact fit between the ways), but even the play is not enough to make line up. In my mind, this would indicate the head is not parallel to the ways; how would one go about fixing this? Of course the chuck could be bad too, but it is much much better built then the lathe.

PK

Not bolted down, but interesting thought. It has cast iron ways; can they actually be twisted? Mind you, my lathe has a cheap stamped steel stand, so I expect the ways tend to force the stand into some "mode", not the other way around, but still I wonder...

Thanks for the suggestion. I might put a couple of winding sticks on there just for fun...

PK

You can discount the information based on prejudice if you want, but what you have here is one of the reasons why I use dovetails, not compression holds. If you squeeze a piece of wood, it'll seek accommodation by compressing or working against the incompressible steel. Can be so gross as to be visible, or so subtle as to pass without notice. Try bringing up the tailstock with the piece only supported in the chuck, then tightening the chuck. May not be absolute, but it will restore the status quo ante.

Your problem might be exacerbated by serrations on the inside of the chuck jaws. They're generally symmetrical, and so as likely to push away as draw toward the established shoulder when you tighten. They also break fibers, making a precise remount less likely. The dovetail wedges itself against the bottom, doesn't chew fibers, and if you use it properly - snug, not cranked to the point of depressing the wood - automatically recenters if you dismount.

Did you put your spur center point matched up against your tailstock point?

Should have read it before sending. It is I who am prejudiced toward dovetails. This type of circumstance is one of the reasons for it. Inside or outside, they grip on a broad, nondestructive face. Lots of folks believe otherwise.

Hey, I can't re-grip the same piece of wood and have it stay true. Perhaps it will in metal working though, but not wood. Even after you remove some it will change shape a bit from further drying out or relieved stress's.

Making pens I have even tried remounting the blanks on the mandrel (tight enough fit) and there was a significant off centre effect.

John

Paul Kierstead wrote:

Hello Paul,

You've received a number of good suggestions, all of which could be the problem. I suspect however from what you say that the headstock is not locked down in perfect alignment with the tailstock. I've used rotating head lathes for several years and finally settled for a double ended Morse Taper to ensure the centering of the lathe headstock and tailstock. When you turn between centers, it does not matter if there is some misalignment between the tailstock and the headstock. Remounting the turned wood into a chuck seldom mounts perfectly as several have stated because the wood fibers compress differently at different locations on the piece of wood. The result can throw you out of alignment. Your problem is not really a newby problem. It is just that experienced turners expect to have the problem and have learned ways to compensate.

If the misalignment is not too great, you can turn on the lathe before bringing the drill up to the wood. Allow the wood to self center the drill and then lock down the tailstock. If the offset is too large for this compensation, take a skew chisel laying on its side and make a "V" indention in the wood large enough to accept the end of the drill. This "V" indention will now be exactly on the center of rotation. Now move the tailstock with the drill mounted in until the drill aligns in this new "V" and then lock it down. Drill your hole and you should be on center. All of this assumes, however, that you have made sure your headstock and tailstock are in alignment before you mount the wood in the chuck.

Good luck, but don't feel this is a newby problem. Only the most expensive lathes have perfectly aligned headstock/tailstock. With more experience you learn how to overcome the problem with lesser quality equipment.

Fred Holder

SNIP ........

=============== Paul, Don't discount the chuck too readily. The jaws are supposed to go on in a certain sequence. If it's new, there's a possibility that the thing wasn't properly set up at the factory. Check on that before going any farther.

Ken Moon Webberville, TX

And some agree with you.

Actually George, having rough turned about 40 bowls so far (I never had a lathe in my life until August of 2004), I very much favor dovetails. It is easy to turn the recess and, as nearly as I can tell, they center a WHOLE lot better than a tenon does. Before I remount it to turn the face, I calculate the maximum depth I want to turn to so I avoid going too deep over the dovetail. Then I'll leave the edge intact so I have a reference point to measure from until I am done machining the interior. From this rim I use a straight edge and a ruler to make a simple depth gauge.

It's simple, works really well and the only expense is a minor amount of thought.

To mount the bowl on the chuck, I place the bowl face down on the lathe bed ways, drop the el-cheapo utility chuck into the recess and expand them by hand until snug. Then I give the chuck a quick twist with the tommy bars and mount the whole arrangement on the headstock spindle.

To make life just a little easier on myself, I ground the edges I need into a piece of HSS bar stock I found at work (From a broken cutter blade. So far as work is concerned, it is junk metal. For my purposes, though, it is 'found gold'.)

Bill C. on the western edge of Detroit, MI

Cheesehead friend and I have been discussing "feet" versus reveal. The interior dovetail makes it easier to have a mild recurve (~1/4) to make the piece look as if it spread from nothing visible. Too many with "feet" look like they were set on top of something. I leave more weight in the bottom when I sneak under 1:4 ratio, though. Can get tippy.

Tip for you if you're turning bark up warp and go. Make a real rim around the recess, stand it an eighth or so beyond whatever reveal you've decided on. That way after the bowl warps into final shape, you'll be able to sand the bottom without having fatter section on the ends and thin on the sides. Just looks better.

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Although you don't say how much inaccuracy there is in the alignment (and even the best machines will have some tolerance) I think you maybe expecting too much.

First of all, perfect realignment (in my experience) when you remount a work piece is just not going to happen. In the scenario above, just think about all the interfaces between the block of wood and tip of the drill bit. Small innacuracies are bound to creep in so many places and compound to make a noticeable problem. Secondly, making a lathe with a sliding tailstock and moveable quill and getting the two axes to be in perfect alignment is an engineering feat beyond my comprehension. Add to that a rotating headstock and the engineering compromises that must be made to manufacture a cheap lathe, and you are gonna be a long way from perfection.

What can you do? Other than check the basic alignment of the headstock, your best bet is to be sure that the tenon is is the bext possible natch for the jaws. Choose the opitmum diameter for the jaws, where they form a circle. If the jaws have a dovetail, make sure you cut a matching shape in the tenon. You want the jaws to grip but not compress the wood. All the cuts should be nice and clean. Close grained hardwood like maple will give best results.

I have actually changed the jaws, so that is even more of a possibility :) However, the error is always in the same direction; this is not eccentricity, it is simply pointing wrong. I did check them over again as well, just in case...

PK

THis is good reasoning and quite informative. However, in this case we have the following:

- I was using dry maple. Trust me, it doesn't compress much :)

- I don't crank on the chuck. This will happen with the lightest of pressure.

- different pieces in different orientation always result in missing the exact same way, i.e the error is always in the same direction.

Considering that, I would suspect it cannot be a wood compression issue.

Yup. But, of course, *in theory*, they could both be 30 degrees to each other and still meet at a point; in practice, only a small error could happen and the ends still meet. Bah. Must revisit the headstock.

LOL, I was telling a friend off in the shop just they other day; "Stop being an engineer!" I said. Thanks for the reality check; I too need it some days.

Unfortunately, in this case, I think I might not be expecting too much; For example, if I was to drill out something thin walled like a pepper mill, the drill would exit the side within 3". This is not error measured with a vernier caliper; it is serious enough to cause trouble when you need a reasonably parallel centered hole.

PK

Double ended morse taper?? I never knew such a beast existed; now that would be just the ticket. I have been trying to figure out how one would get the two parallel with each other. I will have to track one down.

Ahh, also a great idea. Never thought of that. Some things to try.

Many thanks for the help on this; it is a great help!

PK

Ah yes, I was also going to suggest this "Biological impossibility" device. Those with rotating headstocks shouldn't be without them.

How about loosening the jaws on the chuck, sliding the nose of the tail center into gripping position, and tightening both grip and jaw alignment with it as a reference?

You can get one at Woodcraft.com.

Peter Teubel Milford, MA