Finish Turning bowls - how to recenter?

[Prashun Patel]

I turn most of my bowls green.

After drying, I mount the open end of the bowl in a Cole Jaws or jam chuck and true up the tenon and most of the outside. Here's the problem:

When I flip the bowl and mount the tenon, it almost always has some wobble. I have a devil of a time getting it centered perfectly; most of the time I just deal with the wobble and turn it away. However, this gives me probs at the apex of the rim where the inside meets the outside.

So, how do you guys do this? I'm thinking I need another chuck that I can mount in the tailstock that can hold my cole jaws while I secure the tenon at the headstock chuck. But is there an easier way? (especially since some of my turnings are too wide for my jaws; I have to jam chuck them).

[Dan Hintz]

Prashun,

If I understand you correctly, you should expect some wobble in the rim after truing up the tenon. The bowl warps as it dries, so even though you're only taking a bit off of the tenon to get it true, the rim has moved quite a bit more. Just make sure you leave enough thickness in the rim when green to turn out the wobble... for example, a 1" thick rim on a 12" bowl is reasonable, and for some of the warp I've seen certain woods allow, it may even be too little.

[Sean Hughto]

When I rough out my bowls I make a divot at the center of the tenon either with the point of my tailstock or with a tool (e.g., spear point).

When I re-turn the rough, I use the tail stock lodged in the divot to pin the blank to a flat plywood disk covered with that thin foam shelf liner and mounted to a faceplate on the headstock. I then retrue the tenon and turn only the bottom inch or two of the outside.

I then turn things around and put it in a scroll chuck. I then return everything else. If there is any slight discrepency where the uppermost parts of the outside meet the bottom inch or two, it is well blended and not noticeable. The rim will be true.

One thing that helps me a lot in turning the outside when it is facing the headstock is a curved rest that can be mounted in the banjo so as to make the passes up the bowl pretty straightforward.

Works for me at least.

[Prashun Patel]

Sean, I too leave a divot in the center of the tenon; it happens naturally because when I'm roughing out the outside in initially, I usually do so with the live center engaged at the tail.

Perhaps my problem is truing up the tenon AND turning the outside while the bowl is first remounted. You guys seem to only true up the tenon before flipping to the scroll chuck. Then you turn the inside AND the outside with the opening facing the tail, correct?

[Scott Hackler]

All my tenons have a very distinct divot from the live center. In fact I actually press the live center in really hard before I flip the bowl around during the rough out. Then after drying, I can use my jam chuck and the tail stock will be exactly in the same position as before. True up the tenon AND flatten the shoulder for the jaws to sit against. I might do like Sean and return the bottom 2-3 inches, at least until round, but generally I just rework the tenon and flip it around.

I wouldn't ever use cole jaws on a dried green rough out. The rim is likely NOT going to be perpendicular to the tenon shoulder because of distortion during the drying process. The majority of my rough outs have wobble, because they have ovaled during drying. Thats is the main purpose for leaving 10% wall thickness. There have been very few wood varieties that dont oval much on me. Hedge, mango, and most of my black walnut dont move much. Hackberry and oak move A LOT on me and I have begun leaving 15% of so thickness for them.

[Jake Helmboldt]

Prashun, sometimes I will create a recess in the inside of the bowl or a long spigot to use as a means of mounting it. Then I clean up the tennon/recess on the bottom, flip it around and clean up the inside and reverse it again to turn the outside to final dimension. Sometimes I skip the re-flipping if it hasn't warped badly.

One problem with a recess on the inside is accessing the chuck to tighten it. It only works with shallow bowls or platters. On deep bowls I use the long spigot (or sometime just do a friction mount as others have mentioned. Play around with the different techniques and see what works for you.

[Sean Hughto]

I don't know what anyone else does.

But yes, I would say that there will often be some minute difference in center when remounting in the scroll chuck which will translate into a rim that has thicker and thinner areas. And yes, I avoid this by turning as much as I can reach with the scroll chuck engaged. The bit I do at the bottom of the outside is merely a nod to the fact that it is difficult to reach that bit of the outside when the bowl is mounted in the scroll chuck.

[Jon Nuckles]

I have not been truing up my tenons before remounting, but it sounds like that may not be good practice. Because they are relatively small, at least with my #2 jaws, they are only slightly oval and the chuck seems to get a very firm grip. I have never had a problem with one coming loose. I just mount and return the bowl, then reverse in a jam chuck or donut chuck to remove the tenon and finish the bottom. Have I just been lucky so far?

[Sean Hughto]

For small bowls in relatively softer hardwoods or woods that don't oval dratically, you can probably get away with this. Large bowls have more distortion and harder woods are more difficult for the scroll chuck to bite.

[Dan Hintz]

But yes, I would say that there will often be some minute difference in center when remounting in the scroll chuck which will translate into a rim that has thicker and thinner areas. And yes, I avoid this by turning as much as I can reach with the scroll chuck engaged. The bit I do at the bottom of the outside is merely a nod to the fact that it is difficult to reach that bit of the outside when the bowl is mounted in the scroll chuck.
What Sean said :)

[Scott Hackler]

Not only that but without a true round tenon, you can't really have the blank as centered as possible in the jaws of the chuck and so the ovaling/wobble could be even more noticable. Besides the plus of trueing and flattening the shoulder for the jaws to bottom out on. (Thats almost more important to me). Its been a long time since I have had a bowl come off the chuck but for the 10 seconds it takes to true the tenon up and trueing the shoulder area, I will continue to do so.

On a side note, the fact that I reestablish the divot when doing this helps to center the bowl again when I put the jam chuck in and turn off the tenon and finish the bottom. (I don't have a donut chuck so I finish all my bowls with a jam chuck)

[Dick Wilson]

Prashun, I too turn green and allow the bowl to dry. I make sure of two things. I leave a thick enough wall to turn round on the second turning. I also leave a divet (exact center) in the tennon. After bowl dries I use a jam chuck and bring tail stock up and secure in the divet. I return the tennon to true. I make sure there is a shoulder that runs true to force against the top of the jaws. I remount using my chuck and turn the bowl. I have never had a problem with wobble. Just make sure to leave the turned green wall thick enough to allow for wood removal on the outside as well as the inside. Good luck.

[Bernie Weishapl]

I also make sure to leave the tailstock divot in the tenon. When the bowl is returned to the lathe I put the livecenter back in the divot and just bring it up against the chuck. I don't worry about the inside if it scratches it because it will be turned away anyway. When I have the bowl done and sanded I remove the bowl with the chuck still attached. I mount the chuck still holding the bowl in my tailstock. If I use a donut chuck or my vaccum chuck when I bring the tailstock up the bowl will be centered in either one and then I finish the bottom.

[Richard Jones]

Ditto what Bernie said.

Rich

[Prashun Patel]

Thanks, Everyone. The Turners' Forum is always so helpful and warm.

I'll try all the things mentioned.

Bernie, thanks for the advice on finishing off the bottom. Usually I use a jam chuck and a live center to turn away the tenon. Since it gets completely turned away, I'm not so persnickety about keeping it centered. But I will try yr method; I have a reverse chuck adapter.

[Scott Hackler]

I been thinking about your "wobble" comment. Just wanted to check on something. Are you mating the shoulder on the top of the chuck jaws or bottoming the tenon inside the chuck?? The first is the proper method and the latter WILL cause problems. Just checking.

[Prashun Patel]

Yes, I snug up the shoulder. On my next bowl, I'll double check the shoulder.

[Faust M. Ruggiero]

This may sound complicated but here goes. I make a center dent on the inside of my rough outs as well as the outside. Here's why. Some bowls, especially larger bowls, shrink across the grain a lot. I mount my rough outs with the inside against a jam chuck (actually, one of my vacuum chucks without vacuum) and jam it with the tail center. Since the bowl is way out of round, the inside of the rough out will only contact the jam chuck in two opposing spots in long grain. The jam chuck can act as a wedge and the force of the tail stock against it could crack the rough out. Sooooo, to correct this situation, I mount the bowl with the tenon inside the jam chuck and mount a morse taper extension between the tail stock and the live center and let the tail stock make contact with the inside of the bowl on the indent I left. The extension gives me room to reach inside the bowl. Then I true the inside of the bowl just in the area it will contact the jam chuck, dismount it and turn it around and true the tenon. In this manner, the bowl will fit so securely against the jam chuck you will be able to apply enough pressure with the tail stock to completely turn the outside of the bowl. When you are done, mount it in the chuck and finish the inside. This wasn't something I imagined up, a friend of mine with a lot of experience was kind enough to teach me.
faust

[Mark Levitski]

Prashun, just keep it simple as does Bernie--just jamb it up against the chuck and return the tenon to round (be sure to leave enough diameter when you green turn to allow for the piece to still fit in the jaws after re-turning), flip the tenon into the chuck, and finish. When you rough (green) turn the tenon, sure, I would definitely turn the outside too but expect some adjustment when you reverse it to go into the chuck. It's all a gradual process turning to dry and round. To finally finish the bottom is not critical, but centering helps to minimize the transition from turning to sanding in order to blend the finished base into the finished top.

Mark

[Thomas Canfield]

Also ditto what Bernie said. I open my chuck jaws up wide to get the largest contact, and then jam with the live center in a divot or center left intentionally in the tenon when turned green. I mounted a 16+ D piece this way over the weekend. Make sure you maintain pressure on the live center since some wood may give some during the returning.

[skott nielsen]

I thought it would be best to finish turn the outside at the same time one trues the tenon as this way you are turning towards the headstock and can use a push cut almost the entire way. I must admit I frequently have issues when I reverse to do the inside. Yet I find it difficult to get the cleanest of cuts with a pull cut.

[Faust M. Ruggiero]

Skott,
Practice working left handed. Then your push cuts will be pull cuts. It often pays to be a bit ambidextrous at the lathe.
faust

[Steve Harder]

You guys seem to only true up the tenon before flipping to the scroll chuck. Then you turn the inside AND the outside with the opening facing the tail, correct?

Yes. I true just the tenon and small area surrounding it. Then reverse into chuck and turn inside and outside. Then reverse onto vac chuck, or faceplate with duct tape and turn off tenon. Or to remove tenon, pinch between jam chuck in chuck and live center in tailstock, turn away as much tenon as possible, remove to workbench and remove final bit of tenon with Japanese saw and sand away residual.

I like the reverse onto vac and also have very good luck reversing onto very large foam padded faceplate and secure with duct tape or fiber tape.

[Ken Whitney]

I'm thinking I need another chuck that I can mount in the tailstock

I do occasionally use a tailstock adapter (1 1/4 x 8 on a #2 MT) to reverse-mount the scroll chuck (with the bowl attached) to align it with the vacuum chuck for turning the foot, etc.

But for the initial returning, I do what Bernie does.

[Mark Levitski]

Definitely what Steve H. said: After truing the tenon and at the same time re-turning the outside, you will still need to "re-re-turn" the outside after putting the newly trued tenon into the chuck. One is then using the gouge from the headstock side (tight sometimes, yes). However, when you learn some pull cuts it is no problem at all, and of course as suggested some lefty turning for push cuts can happen at a certain distance from the headstock. I use a pull shear cut with the gouge almost vertical. This leaves the cutting edge on a gouge with wings at a 45-50 degree angle. With light cuts one can shape and take out any ridges. You can rotate the axis from a scraper cut to a peeling cut. Because of the cutting edge angle, catches are rare and not so scary.