Reliability of mortise on bowl

[Randy McCreight]

re: Michael Mill's graphic. Is it better to let the jaws grip the piece at the bottom of the recess or against the bottom/end of the wood, as depicted?

[Michael Mills]

You are correct. I thought I had deleted that one which I drew up wrong. The wood should rest against the top of the jaws.
Sorry for the confusion.
I can't find a way to edit it out or put a correction in the post now.

[John K Jordan]

You are correct. I thought I had deleted that one which I drew up wrong. The wood should rest against the top of the jaws.
Sorry for the confusion.
I can't find a way to edit it out or put a correction in the post now.

Michael, if you post the graphic here I'll be glad replace the one in your earlier message. I'd say email it but with the latest changes I'm not sure if you can do that.

Here's one describing another issue - the wrong angle on the dovetail. Since it is very difficult to get the exact angle, it's best to err on the side of less acute. The hold on the upper diagram is weaker since the wood can deform at the sharp corner right at the surface. The lower method is much stronger since the jaws are seated well into the strongest part of the recess.

chuck-recess.jpg

Not much depth is needed. I seldom use over 1/8", sometimes 1/16" for platters. The biggest I've done with a 1/16" recess was almost 20" in diameter. The type and strength of the wood needs to be considered, as does geometry of the piece and the skill of the turner - given weak wood with a long lever (such as a deep bowl or form) and a turner who might get a catch it might be better to use a bit deeper recess but not too deep. For their 50mm jaws Nova specifies "Maximum recess depth 6mm (1/4)."

There is another thing that helps not as much with the holding strength but with the precision of the mounting, especially when reversing a piece into a recess cut on the lathe. If you get some wobble or runout after reversing the problem may be a little junk in the recess corner. This is less likely of you angle the contact surface very slightly at the outer circumference of the recess such that the surface is a little less than exactly perpendicular to the lathe axis. The idea is to have the inside circumference of the jaw faces to press against the wood instead of the outside circumference as shown in the bottom diagram. Since the very rim of the jaws are not seated exactly in the corner the chance of misalignment due to sawdust or wood fibers in the corner is eliminated. It's a little hard to describe in words and I haven't yet drawn a diagram. Since I started doing this on my little platters with wings/corners the thickness of the edges have been more consistent all the way around the pieces.

JKJ


JKJ

[Leo Van Der Loo]

Here is some good info in a previous thread, I addedsomepictures, as visuals do help often

called Chucking inside a recess

I don't seem to ba able to link to this thread, sorry

[Michael Mills]

Michael, if you post the graphic here I'll be glad replace the one in your earlier message.
JKJ

Thanks for the offer but I guess when I redrew it I deleted the correct one. I have now deleted all of them so that should solve my memory problem and keep me from posting inaccurate data.

You are correct about the 6mm depth on the 50mm jaws. Per their data, the suggested depth is between 4mm (pin jaws) and 9mm (100mm jaws). I will probably continue to use my 3/8" (9mm) myself if the length of the dovetail allows it but will refrain from suggesting it to others.

[Randy McCreight]

I tested contact of my jaws with my 13 degree recess. The fit looked good because contact was tight at the outside edge of the recess, as in John's upper graphic, but it was mainly gripping toward the thin outer edge. I applied a layer of Sharpie ink to the jaws and tightened the jaws so the bowl could rotate on the chuck: the ink transferred primarily to the outer half of the recess. I sanded the recess lightly to remove the transferred ink and tested again. After several trials, the ink transferred deeper into the recess. I have more confidence in my recess now. 11 or 12 degrees might be better.

[Ron Page]

Randy, I realize you've corrected your problem so that the mortise fits. I hesitated earlier because I didn't know if you could remove the bowl, recenter in opposite direction, and recut the mortise. Looks like you did. However, the lack of supporting wall is still critical in my opinion. Maybe I tend to over tighten?

What I would have done in this case is recut the mortise square - no taper. Then I would have turned another glue block to exactly fit into the new mortise. Glue it in and cut it as a tendon and use it that way. I've broken out more than one mortise but have never crushed a tenon beyond use.

ron

[Randy McCreight]

I improved it a bit. I have slight runout after remounting, which might could be attributed to normal movement of the wood after turning inside. It's almost done.
Glue block and faceplate sounds good, too.

[John K Jordan]

I improved it a bit. I have slight runout after remounting, which might could be attributed to normal movement of the wood after turning inside. It's almost done.
Glue block and faceplate sounds good, too.

A friend of mine turned a bunch of large bowls (up to about 20" I think) by fastening a flat surface to a faceplate with double-sided tape and no screws. He used two layers of the good woodturners tape from Woodcraft, one layer burnished onto the face plate and the other layer on the wood. Stuck them together with the rows of tape crossing and applied pressure with the tailstock. They held the heavy solid blanks well. The problem was getting the bowl off the faceplate! He would inject acetone into the tape from the side with a hypodermic needle. (It is actually easier drive a thin wood wedge into the tape which, with patience, will pull the pieces apart.

JKJ

[Randy McCreight]

I am ready to mount a bowl on a faceplate via a wood block, but I am not ready for some tape. How big was the flat surface?

[Randy McCreight]

Holey Bowl under a coat of wet tung oil. I am starting to love tung oil. I worked thru my earlier difficulty with my recess.

[John K Jordan]

I am ready to mount a bowl on a faceplate via a wood block, but I am not ready for some tape. How big was the flat surface?

I think his faceplate was about 6". The friend didn't know of any way to reverse the bowl for cutting away or removing a glue block so the tape method finishing up by hand. Sometimes called woodturner's tape or woodworker's tape. Available from Woodcraft, Highland, and elsewhere including here: https://www.rockler.com/double-sided-turners-tape

I have used the double-sided tape on a number of pieces on the wood lathe, usually oddly shaped things hard to hold. I last used it to mount an eccentric piece - it didn't really take all the much tape. We also use it quite a bit to hold metal parts for milling, especially thin things that can't be held easily in a vise.

But gluing a waste block to the bottom and holding as a tenon in a chuck or with screws on a faceplate is certainly a respectable method.

JKJ

[Randy McCreight]

I made a snap of my evaluation of contact between recess and jaws. The red marker transferred from jaws to recess. Not perfect, but better than the first recess.
It is not an issue to reverse chuck the bowl.

[John K Jordan]

I made a snap of my evaluation of contact between recess and jaws. The red marker transferred from jaws to recess. Not perfect, but better than the first recess.
It is not an issue to reverse chuck the bowl.

recess reduced.jpg

If you put marker on the jaws all over the dovetail could it be the dovetail angle is not yet right? I don't see any marker down near the deepest part of the recess in some places. This might indicate bearing primarily against the weaker upper situation in the picture posted earlier:

chuck-recess.jpg

But if you just didn't coat the jaws near the ends and you are just showing the contact around the circumference, then nevermind! That looks great.

JKJ

[Randy McCreight]

recess reduced.jpg

If you put marker on the jaws all over the dovetail could it be the dovetail angle is not yet right?
But if you just didn't coat the jaws near the ends and you are just showing the contact around the circumference, then nevermind! That looks great.

JKJ

The angle is not quite right, but it is close. I coated jaws completely.Finishing is almost finished.