Bowl out of true when remounting

[Staffan Hamala]

I have a problem.. Or, I'm not sure if it's a problem or not.

I have some limited experience with bowl turning (using my previous lathe, a few years ago), which makes me wonder if I do have a problem or not. But I want to consult some experts.

I bought a new (used) lathe a little over a year ago, but haven't been turning bowls until this weekend. Now that I've tried turning a few bowls using almost dry wood (has been drying for five years, but still feels a bit heavy), I do have some doubts. I meant to rough turn them, and let them dry for a while (I have a few such bowls already on the shelf since a couple of years, that I also mean to finish now).

I first turn the outside, mounted on a screw chuck (I have both a dedicated screw chuck that came with the machine, and a screw insert in my Sorby Patriot chuck). I have tried both screw chucks with the same result. When I have finished with the outside, I make a tenon for my chuck.

This is where the problem starts. When I remount the bowl on my chuck, no matter how carefully I adjust it, the newly turned outside of the bowl seems out of true. I realize this always happen to some degree, but it just seems like it's too much.

After I turn the inside, and remove the bowl, I can see that the rim of the bowl is visibly of different thickness depending on where you look. I haven't measured, but it looks like it's several millimeters off. Say 8 mm on one side of the bowl, and 5 mm on the opposite edge. The thickness is visibly different.

Is this normal? I don't remember this happening in the past. Not to this extent at least.

If it's not normal, I'd want to find the problem.

My suspicion is that it has do do with the chuck adaptor (which I had to have specially made for the spindle thread - Whitworth 1 1/4" if I remember correctly).

I have been checking things with a dial gauge, but can't come to a conclusion. It seems like the outside of the chuck is a bit off, but then, it isn't the outside that is gripping the bowl. I've tried measuring the jaws, but they have a small amount of play when not gripping anything, so I can't come to any conclusion there. Just now, an idea struck me.. Maybe obvious to others, but I should probably measure the jaws when gripping something, or maybe the object they are gripping.

A friend that used to work at a machine shop helped me make the adapter. He's retired and moved out of town since then, so I probably need to pay someone to make a new adapter. If it's really the adaptor that is the problem... It might as well be the chuck.. And if the chuck is bad, a new adaptor would not help. I would just be throwing my money away.

I've also been thinking of ordering a new chuck. Maybe a Vicmarc. But, as my spindle thread is unusual today, I can't buy an insert for any chuck. I'd still have to have one made for me.

Or could it be something else? Maybe the lathe itself? I haven't noticed any problems when turning chisel handles and such things, but that might not mean anything.

How would you investigate this? What to measure?

I can also add that I also have a metalworking, 4-jaw scroll chuck that came with the lathe. When checking the outside rim of this chuck with a dial gauge, it is actually only moving 0.01 mm when turning the chuck around. This feels like quality to me. I would like my other chuck to be as good. :-)

But, still, I'm not sure how much this means. I'm only measuring on the outside rim of the chucks. But it feels like an indication of where the problem is.

[Bill Blasic]

One thing that might help you is to make sure that your last cut before putting the piece in your chuck is to take a finish cut on your tenon. Lots of folks cut the tenon first then shape the outside of the bowl releasing tensions in the wood and that without recutting the tenon will cause the piece to be a little out of round when chucked on the tenon.

[John Keeton]

There could be multiple factors at play, including what Bill mentioned. This situation will happen to some degree most of the time and the proper corrective action, if needed, would be to true up the outside of the bowl after chucking and before hollowing the bowl.

[JohnC Lucas]

2 thoughts. Bill is correct about the wood moving. Also make your tenon with a shoulder. The wood should sit on this shoulder which helps keep it aligned and also helps keep it from rocking and coming loose in the chuck later. Then tenon should also be cut so the V at the bottom is I guess I'll say out of the way. In other words the jaws should sit against the shoulder and the tenon and never really hit the bottom of the V where the 2 meet. Watched a great Stuart Batty video on this. If the V was not cut perfectly which most of us can't do the jaws can try to grip the bottom of that V and not the 2 aligning reference point of the flat and sides of the tenon. The second thing is to mount your wood in the chuck so that you have equal side grain and end grain on each jaw. I call this 45 degrees to the grain direction. When you just randomly put the tenon in the jaws you may have 2 jaws on side grain which crushes easily and 2 jaws in end grain which doesn't and it can pull the bowl out of alignment.
That being said some bowls just dont want to go along and you either have to live with it or retrue the outside. If the bowl is thick I don't worry about it. If it's going the be thin or have beads around the lip that will show the irregularity then I re true the bowl.

[Mike Goetzke]

2 thoughts. Bill is correct about the wood moving. Also make your tenon with a shoulder. The wood should sit on this shoulder which helps keep it aligned and also helps keep it from rocking and coming loose in the chuck later. Then tenon should also be cut so the V at the bottom is I guess I'll say out of the way. In other words the jaws should sit against the shoulder and the tenon and never really hit the bottom of the V where the 2 meet. Watched a great Stuart Batty video on this. If the V was not cut perfectly which most of us can't do the jaws can try to grip the bottom of that V and not the 2 aligning reference point of the flat and sides of the tenon. The second thing is to mount your wood in the chuck so that you have equal side grain and end grain on each jaw. I call this 45 degrees to the grain direction. When you just randomly put the tenon in the jaws you may have 2 jaws on side grain which crushes easily and 2 jaws in end grain which doesn't and it can pull the bowl out of alignment.
That being said some bowls just dont want to go along and you either have to live with it or retrue the outside. If the bowl is thick I don't worry about it. If it's going the be thin or have beads around the lip that will show the irregularity then I re true the bowl.

I'm not a terribly experienced turner but did have the opportunity to attend a class at the local club taught by Stuart Batty. I was going to say what JCL said about the shoulder. Stuart showed us how this is so important but he also showed one other thing. He said he has found the face of the jaws usually are not square. I couldn't believe it when he took a wood turning tool to true one up.

Mike

[Staffan Hamala]

Thanks! I had never thought about the end grain vs side grain issue. I'll think about this next time.

I'll make sure to check the shoulders and that the V is cut as you describe. I'll check the Stuart Batty video as well!
(I guess it's the one called "Recess tenons and preparing recesses in bowl blanks").

[Michael Mills]

I'll check the Stuart Batty video as well!
(I guess it's the one called "Recess tenons and preparing recesses in bowl blanks").

If you are looking on Vimeo also watch his "Jaws and Chucks" and "Tenons Part 2". The three as a group cover about everything.

[Prashun Patel]

Usually when this happens to me - and I turn a good deal of really green wood - it's because the tenon is not properly formed. I've rarely had wood move so quickly after turning.

First, what size tenon are you turning compared to what size bowl?
Second, how deep is your tenon? The tenon shouldn't bottom out in the chuck; instead, the tips of the chuck jaws should touch the bottom of the bowl, and sit flush.
Third, are you turning a flat on the bowl above the tenon? This will help the jaws sit flush and in the same plane relative to the rim.


One thing that really helped me is this:

After you've turned your tenon, with your bowl still mounted on the worm screw on the lathe, attach the chuck to the tenon, lightly (this assumes you have a second chuck...). Then bring the tail stock up into the threads of the chuck. This will center it relative to the headstock. Last, tighten the chuck. Now you can remove the bowl and thread the attached chuck onto the spindle of the headstock. It will be about as centered as it ever will be.

FWIW, I prefer using a spur center now for starting bowls. That point is easier to line up than the larger hole from the worm screw or face plate when you flip the bowl around.

[Rob Price]

I'm still a beginner compared to folks around here, I get a little wobble after turning the blank around, the ones that are really bad are almost always due to the tenon not having a good square shoulder to sit on. The other is not having the blank flat against the face of the jaws- often when I'm trying to mount a big heavy blank with one hand on a chuck already on the lathe. It's better to take the chuck off, turn the bowl upside down and use the weight of the chuck to seat it flat on the bowl as you tighten the jaws.

But also, the person who taught me how to turn put a really big emphasis on being able to turn right or left handed. Spindle and bowls I had to learn each cut with my hands reversed. I'm by no means proficient but I do feel comfortable swapping hands when the cut calls for it, avoiding awkward body positions. That has been a great help. For me I save my finishing cuts until the bowl is reversed and chucked up and then I finish the outside shape of the bowl before I hollow. Any time you take the blank out of the chuck or off the lathe you risk introducing some out of round to it so I do as much as I can from one chuck position.

[Reed Gray]

When you reverse the bowl to turn the inside, is it running true before you turn the lathe on? This is some thing I always check before turning the lathe up to speed. If it is running fairly true, then the inside should run true as well. If the outside runs true, and the inside does not turn out true, then that sounds like an alignment problem of some sort. For how far out of round your bowl is, one other possibility is that it is moving as you turn, which should be visible. Having a flat shoulder really helps keep the bowl steady.

Here is a video clip I did a while back for mounting things on the lathe.

https://www.youtube.com/watch?v=5KHkkws9lWA

robo hippy

[Richard Dooling]

Glad I saw this thread because Stuart Batty's videos are really good. I'd seen him before forgot how well he explains and demonstrates while teaching. Plus, all the videos at the link below are free.

Here's the link:
https://vimeo.com/woodturning

Scroll down that page for the link to load more videos.

BTW, these are studio made videos and very different than some of the videos shot in turning demos where he is not in control.

[Rob Price]

Glad I saw this thread because Stuart Batty's videos are really good. I'd seen him before forgot how well he explains and demonstrates while teaching. Plus, all the videos at the link below are free.

Here's the link:
https://vimeo.com/woodturning

Scroll down that page for the link to load move videos.

BTW, these are studio made videos and very different than some of the videos shot in turning demos where he is not in control.

Great link! Thanks.

[Michael Mills]

Here's the link:
https://vimeo.com/woodturning
Scroll down that page for the link to load more videos.


Here is a similar link I use in alphabetical order.
Three pages, just go to the next page. Just a little quicker to me than the "load more".
But I am pretty lazy sometimes.
https://vimeo.com/woodturning/videos/sort:alphabetical/format:thumbnail

[Richard Dooling]

Here is a similar link I use in alphabetical order.
Three pages, just go to the next page. Just a little quicker to me than the "load more".
But I am pretty lazy sometimes.
https://vimeo.com/woodturning/videos/sort:alphabetical/format:thumbnail

Lazy?
Efficient!

Good link, thanks.

[Staffan Hamala]

I have now watched the Stuart Batty videos, and had a quick check of the tenons I cut the other day. I can see at least one error. The corner of the tenons were not cut as cleanly as they should have been, according to Stuart's demonstration. I never realized that corner was so important. It's obvious now, but it didn't occur to me before.

I'll try to be more careful when making the next tenon, when I get back to the lathe in the coming weekend.

[Don Jarvie]

I’ve run into this same issue and made sure the bottom around the tenon is flat so the top of the jaws sit even around. I also make the tenon with a slight angle inward say 85 percent since my jaws are dovetailed. I’ve tried to sit the bowl without the jaws hitting the bottom but it’s hard to get it to sit right.

[John K Jordan]

...When I remount the bowl on my chuck, no matter how carefully I adjust it, the newly turned outside of the bowl seems out of true. I realize this always happen to some degree, but it just seems like it's too much....


Most chucks have a small amount of runout (wobble) that you can often see. Tolerances in the jaws can add to that runout.

Some things to consider about the chuck and mounting it on the lathe:

- When mounting the jaws on the chuck, be certain the mating surfaces are clean with no sawdust, rust, or burrs. Tighten as recommended - barely snug the screws, close the jaws fully, then tighten the screws firmly.

- If the chuck has a threaded insert to adapt it to the lathe (such as the Nova chucks I use) when mounting it be sure it is also clean and without burrs and seats firmly in the chuck.

- If your chuck spindle adapter is suspect, there are several companies than sell well-made adapters. I personally don't like adapters due to the possibility of loss of precision. It is better to get a chuck that has the same threads as the lathe. That's one reason I like the Nova chucks - I can change the threaded insert if I need to use one on a different lathe.

- When mounting the chuck on the lathe, make sure the spindle shoulder and the mating surface on the back of the chuck are clean and free of sawdust. Chris Ramsey, who turns large diameter cowboy hats to very thin wall thicknesses, stresses this repeatedly.

Some things I do if I want to remount a piece for further turning:

- When making the tenon, make it almost the same diameter of the jaws of the chuck when almost fully closed. This lets the curved parts of the jaws grip the tenon all the way around. The chuck manuals recommend this. If the tenon is too large, the sharp corners of each jaw will bite into the wood and distort it. Since wood has natural variations all over some jaw corners might distort deeper on one side than the other when mounted and again when remounted. (Even the smooth curves of the jaws can compress one part more than another but not as much as the corners.)

- When I mount the piece in the chuck for the first time, whether with a tenon or recess, I make sure the wood is free of sawdust then press the piece firmly into the nearly-closed jaws and rotate it back and forth a little then continue to hold firm pressure while tightening the jaws. You can hold the piece firmly with the tailstock but if pressed with a live center with a point the point itself can keep the piece from seating properly. Use a small block of wood between the work and a live center with a point, or a different method completely.

- *** Since the chuck may have a bit of runout AND the jaws may distort the wood, before I take the piece out of the chuck I mark the position of the piece in the jaws so it can be put back in exactly the same place. My own convention is to make a mark on the wood between jaw 1 and jaw 4. When remounting line up the mark carefully and don't over-tighten. You can often see the impressions of the jaws on the wood and try to get the jaws exactly in the same place. I do sometimes use a dial indicator to check for proper mounting to make sure it's the best it can be and adjust it a bit if needed.

- Note that even dry wood can move significantly as internal stresses are released by turning. When more precision is wanted, for example when making a lidded box or when planning on remounting, it often helps to turn the piece nearly to the final wall thickness then let it sit for a few hours or overnight to allow stresses to relax, then make the final cuts to true it up. Distortion from internal stresses, however, might not give you the type of misalignment you reported - it usually results in a slight out-of-round oval shape. Yours sounds more like it was improperly seated in the chuck.

As others mentioned, a clean, flat shoulder is extremely important. The tenon should never bottom out in the bottom of the jaws but always have a small gap at the bottom and bear only on the shoulder.

BTW, most of the time I prefer using a recess instead of a tenon to hold bowls. I think it is often stronger than a tenon - a 2" recess is very strong if there is plenty of wood outside. A 2" tenon can break off if there is a defect in the wood or the wood is brittle or weak.

And it's surprising, but you certainly can cut steel with woodturning tools! (Aluminum and brass are easier!) I turned down some small jaws to get a precise diameter. I ground a cutting edge on a Thompson scraper to a shape similar to some metal-turning cutters. The turning was slow, made some needle-sharp chips, and I had to sharpen the scraper a few times but it worked well. I've also trued up jaws but usually don't have to. When metal workers use hand tools like this on the lathe they call them "gravers."

385694

JKJ

[Staffan Hamala]

I just want to add that the problem was the uncleanly cut shoulders. I don't have this problem anymore when I take care to closely examine the recess or tenon before I remount.

[Brice Rogers]

Thanks Staffan for the conclusion to your original post.

[John K Jordan]

I just want to add that the problem was the uncleanly cut shoulders. I don't have this problem anymore when I take care to closely examine the recess or tenon before I remount.

It's fantastic you found and fixed that! That's one of the great things about having other turners close is someone might take one look and see the problem.

JKJ