How to hold this piece

[Jim Kountz]

I wanted to get some opinions on how to handle this type or workpiece for turning. I tried one today with the bottom glued to a sacrificial block held in a chuck and the more I turned it the more whip I got and it finally came off completely ruining the thing altogether. This is a segmented piece (at least the one I was doing was) about 2.5" at the bottom and just under 9.5" at the rim. Height is about 5.75". This thing came off the lathe at me today and let me tell you Im not afraid to admit that it scared the crap out of me! I had to go sit down afterwards!

[Steve Schlumpf]

Jim - are you builting it in rings, gluing a couple of rings to the base at a time and then turning that section? Or - are you building the entire thing and then turning it?

[Bernie Weishapl]

I am afraid to ask. Were you wearing your face schield?? It sounds to me like you had it a little off center. What speed were you running? I had the same thing happen with a piece Jim. I had it slightly off center and then was running around 900 rpm. When I turned it by hand it looked pretty good but when the lathe come on I could see it wasn't going to work. Before I could hit the on/off switch well you know it flew. Just a thought.

[Matt Hutchinson]

I use a dovetail style chuck, and I love it. You might consider simply making the base piece by making a dovetail tenon (or whatever tenon fits your chuck), chucking it up, then turning the face flat. Finish the segmenting, keeping it locked in the chuck the whole time. I imagine you could use a jamb chuck in the bell, and bring up the tailstock on the tenon to turn it away after it's complete. I am not a segmenter, so maybe this isn't a great option, but hope this helps.

Hutch

[Robert McGowen]

Use a glue block and a face plate on the bottom. Then turn a cone out of some hard wood and once the piece has enough layers on it so that you feel like it is going to start whipping around, take the cone and place it inside the piece. The cone only needs to be an inch or so thick and about two thirds as wide as the piece you are turning. Think more of a plate with sloping sides. Push it up tight with the tail stock. It should pretty well center itself with the two shapes matching up. You will still be able to turn the outside and down the inside to where the cone is. You should, of course, have already turned the inside up to where the cone will go. The easiest way though, is to use a bowl steady or something like that. I have two of the Oneway bowl steadies and use them in tandem with each other. Works great. With the shape you have though, they could easily pull the piece out of the chuck from the outward pressure of the steady. I always use a glue block and face plate and just part it off as the last step. I hope this helps.

[Jim Kountz]

Steve, I glued the first three rings together on the lathe and roughed them round. Then the other three I glued on as one piece. I had the thing roughed out actually, it was when I was trying to smooth the shape that it started whipping. I stopped and looked at everything. But like Bernie said there, it was ok by hand but when I powered up to about 1000rpm. the lathe started shaking and before I knew it THUNK!! It came flying off at me.

Bernie luckily I did have the face shield on, never turn without it!
Steve Im surprised you didnt hear some expletives all the way up there in MI as loud as I was shouting them out of total fear mind ya!!:D:D:D:D

[Mike Golka]

Jim I have used a disk of MDF on the tail stock to keep things together when rough turning the out side and then a steady rest to fininsh the inside.

[Richard Madison]

Jim,
Maybe too fast?

[Leo Van Der Loo]

Hi Jim
I'll show a couple of turnings I made before, all of these are turned with a recess of 2³⁄₈" or slightly larger, I hold them with my Oneway stronghold chuck in the recess after shaping the outside, no tailstock support as I have non on the outboard side.
Most of these are turned at 380 to 550 RPM, I do make sure the blank runs absolutely true, so it won't whip right from the start, and no I don't tighten the *^#@ out of it, I normally use one hand to tighten the chuck and keep going around and around till it is gripping and tight. HTH

[David Walser]

Jim,

From your description, I can't tell why your method of holding the piece failed. My best guess is that your glue block was not properly sized and shaped to fit into your chuck. The glue block should have a tenon that has a 90 degree shoulder at the top of the tenon. The diameter of the tenon should be just a little bit larger than the smallest diameter the chuck jaws will grip securely. When mounted in the jaws, the tenon's shoulder should sit flat and flush with the top of the chuck jaws. This should resist a piece's tendency to flex from side-to-side and prevent the whipping action you experienced.

Another possibility is that your glue block was made out of too soft a wood and that it's fibers were crushed by your chuck, allowing it to move within the chuck. After a few cuts, the crushed fibers may have allowed you to tighten you chuck a little more. Again, a properly shaped tenon (with the shoulder flush against the top of the jaws) should have resisted the whipping action -- but your piece gave a lot of leverage against the tenon and soft wood would be more likely to fail under that stress.

The last possibility is that your glue joint failed. With today's glues, that's unlikely -- unless you didn't allow the glue time to cure or you didn't prepare the joint properly.

What to do next time? Try using a face plate instead of a chuck. A face plate eliminates two of the possible causes of failure described above. (Those risks are replaced with the far less likely risk that the screws will fail. Don't use drywall screws, and that shouldn't be a problem.) You might also want to use a steady rest, which will help prevent the turning pressure at the rim from levering the piece off the lathe.

One last suggestion: As soon as you start feeling a piece whip around, turn off the lathe and examine things. If you're using a chuck, retighten the jaws and make sure it is turning true. If you're using a face plate, retighten the screws. Examine the glue joint by trying to flex it. I've had things fly off the lathe. I'm sure all of us have. Unless the the blow up was caused by a severe catch, I've always had some warning signs -- such as the whipping action you described -- before my turning took its scenic tour into outerspace.

Good luck and hope this helped.

[Jim Becker]

Leo, using a recess on pieces with this form is not necessarily the best idea simply because there isn't enough wood to provide support for the tension force of the jaws. A tenon with a small shoulder is safer as you are exerting compression. Recesses are best for platters and other forms with wider flat bottoms that surround the recess with a lot of wood for support.

To the OP, no matter how you mount it, you're going to be best served by keeping it between centers as much as possible and/or using a steady to stabilize.

[scott schmidt grasshopper]

I liked some of the ideas others have had on why it came off. bad glue.. soft wasteblock.. one question I have . what sort of cutting method were you using? I have used scrapers and gotten tons more forces generated than when i use a bowl gouge in a peeling cut. . ps if the thing starts to wobble definently time to examine until you figure out whats going wrong. I get real serious in my wiggling the bowl back and forth before i turn on the lathe after stopping it for wobble. after all if I didnt find it/ fix it I can plan on it only getting worse.

[Leo Van Der Loo]

Hi Jim
Yes I know the common belief that a tenon is stronger than a recess to hold a turning :), it isn't.
That's WHY I added the pieces I turned, these are all much bigger than the OP was trying to turn, in my experience (and that's been now close to half a century of turning wood as a hobby) a recess is stronger than a tenon.
Can you split a piece of wood, you darn right, especially if you start with a piece that has a split in it already or if you crank the living daylight out of the chuck :D.
Just Like I said, I tighten the chuck with ONE hand on the key, going from side to side, so that any slight give or uneven chuck pressure is made even and tight, granted I don't have many catches anymore.
Anyway that's the way I do my turning, which by the way I had to teach myself without someone telling me this is the only way to do it, I say if it works for you it's the right way :).
Did you have a look at this photo album ?.
http://homepage.mac.com/l.vanderloo/PhotoAlbum26.html

[Kim Ford]

Jim;

Cool project. . . I read through the thread and didn't see suggested what I do, so here goes. Because my pieces go on and off the lathe so much, I prettly much gave up on the chuck except roughing out the first time. I use glue blocks which are tapped to match my inboard spindle. If you use a good tight hardwood for the glue block and adequate glue surface area your base is pretty much "stuck" to the lathe. The good part is that you can work on the piece, take it off, do something else, glue more stock on or whatever however, when you put it back on the lathe you have center.

Now, that doesn't mean that a lot of things cannot go wrong. If you have enough force to split the glue block, something else in way wrong. A couple of other things;

Exact center each time is important;
RPM is critical, let me say that again "RPM IS CRITICAL" (hope you have a VFD),
What type of cut you are presenting
Additional tail stock or lathe bed support
These have been addressed quite well by others.

Just my thoughts; Good luck and be safe, this is one of those projects that can challange you quickly.