Reliability of mortise on bowl

[Randy McCreight]

I am turning the inside of the "holey bowl" discussed several weeks ago. To reduce waste, I glued a piece of birch to the bottom and turned a mortise into it while it was on the screw chuck mounted on the spindle. (I don't like to leave a mortise in the bottom of a finished bowl.) I was getting some chatter when I was only an inch into the inside. When I looked at the contact between mortise and the 4" jaws on my Nova2, I noticed that the jaws are contacting the bowl at the midpoint of the arc of each jaw. See photo.

My question is; Is that contat sufficient to hold the bowl securely?
I guess that I need to determine the perfect diameter of the mortise for gripping strength.

[Roger Chandler]

I would reshape that recess, very lightly, fitting the chuck jaws until I got almost perfect contact. You are only holding at 4 points, and an event like a big catch or too much speed could cause it to fly off.....not a pretty thing to see happen, especially if it hurts you or someone else, and it could ruin that pretty piece. It looks like it might take another 1/8" to bring your contact to a good point, but be careful you do not get that recess too thin, as the strength of the wood may not support your jaw pressure....you have a fine balance to reach with this one! Safety matters more than anything else!

[Barry McFadden]

When I glue a piece on the bottom I always make it a tenon and the chuck grips it all the way around....

[John Keeton]

Your recess is too large. For 50mm jaws a recess 2 1/16" should fit perfectly. But, you do not have enough material on the outside of the recess to be safe. A minimum of 1" is best.

[Perry Hilbert Jr]

What Mr. Keeton said. a mortise should be very close to the arc of the chuck jaws for maximum friction over the maximum area. I am constantly guilty of violating this concept and had a nice sycamore platter go frisbee golf on me a week ago. So I turned a round piece just a millimeter or two larger than the closed chuck jaws to use for a template when turning the mortise.

[Roger Chandler]

He is using 4” jaws according to his post, but the holding principles are the same. Now that I’ve considered this a bit more, perhaps reverse turn the added glue block completely off, truing up the bottom, using a jam chuck, and put a new one on, turn a new recess that is just a mm or two larger than the jaws are when completely closed.

[Reed Gray]

1/8 to 3/16 max depth of recess is needed, and like the others have said, that recess is too big. If your jaws measure 2 inches across, then you need 2 1/8 inch max diameter. Also, you want the side angles to match, so if you have 7 degree dovetail on the jaws, then you want 7 degree bevel on the recess walls. Some times you get wobble from not having the jaws seated in the bottom of the recess properly as in dead flat or even one tiny shaving under one of the jaws. Some times just one loose screw in the chuck jaws can do the same thing.

robo hippy

[Randy McCreight]

This might be a real soup sandwich. 4" jaws. I test fit the mortise at 13 degrees; it was perfect. I didn't envision a gap between jaws and too-large mortise. I left an island on the top where I can reinstall the screw chuck. I bought the 4" jaws specifically for large bowl. I think I will first reduce the depth of the mortise, as Reed suggests. If that doesn't improve my soup, I will install a new block with smaller diameter recess, as Roger suggests.
I am glad I asked. Thanks.

[Randy McCreight]

Better. Because my glue block was 15/16", I turned off the mortise and recut it about an eighth inch diameter larger than the closed jaws, 3/16" deep.

[Roger Chandler]

Better. Because my glue block was 15/16", I turned off the mortise and recut it about an eighth inch diameter larger than the closed jaws, 3/16" deep.cool deal. Sometimes its better to start fresh!

[Randy McCreight]

I spun it briefly on its new mortise; I didn't detect any wobble.

[Roger Davis TN]

Face plates and glue blocks are like bread and butter. I use this combination over and over. Just wack the space between work and glue block with a chisel and they will separate, turn the face of block and glue another day. Lyle Jamieson uses this all the time. He uses a ring of superglue I sometimes use wood glue. They both can not stand a swift hammer blow to the side but will hold on like a snapping turtle resisting twisting. God bless, Roger.

[Randy McCreight]

Thankyou, everyone.

[Michael Mills]

Just bit of a different perspective maybe...

Recess depth. I always go about 3/8" deep. Maybe excessive or maybe not. The last time a bowl was almost orbited my daughter was turning and got a catch.
One side was pulled from the jaws about 1/4". At 1/4" recess it may have been orbited, at 1/8" it surely would have.
Also, I have never had anyone remark "that bowl would look a lot better if it were 1/8" taller".
That said, I do use a shallower recess with platters where there is little wood to start with but the wood is normally very dry.

Diameter of the recess. I cut mine where I can just get the jaws in when closed fully; my jaws are dovetail. My chucks are Nova so the dovetail is 15°. Other brands may be 7° or other angles; you should try to match your jaws as much as possible.
Nova states there is 2mm of material removed when cutting the jaws so they only have to expand 2mm from fully closed to come to full circle. There is plenty of room inside the recess for the jaws to expand that much. Other brands or straight jaws may vary. Making the recess larger only means less support imo.

This is a pic I made years ago showing the jaws in the recess.

[robert baccus]

All the forms of holding pieces work if you adjust your techniques to the situation--observe wood strength, wood greenness, weight, hole or tenon dia. (fit), length of piece ect. Leave plenty of wood around a mortise, use strong wood for glueblocks and single screws and observe all the basics. Ps--thick CA will hold anything in my experience and works on wet wood and requires no clamping like most glues

[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.

402787

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
(https://sawmillcreek.org/showthread.php?257001-Chucking-inside-a-recess&p=2717335#post2717335)
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.

404096

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:

404095

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]

404096

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.

[Alex Zeller]

How tight are you going with your chuck? I found my biggest mistake wast I wasn't going tight enough. For some reason I was thinking that the wood would crack like an egg. I'm sure my thought process was that you are trying to compress a tenon while you are trying to open up a mortise. I started out making my mortises a 1/2" deep (any more and the blank would hit the base of the jaws). As I get more and more experience I've cut back on the depth. I usually go about 1/4" deep now. It's easy for me to see the angle. I'm sure in time I'll try 1/8" or less but on rare occasions I still get a catch.

[Randy McCreight]

Akex asked: "How tight are you going with your chuck?" Very tight, I think. My angle needs a sight adjustment. 1/4" to 5/16" might provide more material for holding.
I like the strength of faceplate with glue block, but how can I ensure that it is very centered?

[Alex Zeller]

When I use a glue block, not that I do very often. I turn the glue block first. When I turn a flat surface on the blank for the block I either use my diamond point carbide tool to make a ring or I'll make a shallow mortise that will hold the glue block snugly.

[Reed Gray]

I have always preferred a fit where the angles of chuck jaws and recess match as perfectly as possible. Having the top of the bowl recess being used to snug/push the jaws down to the bottom doesn't work for me as it puts more pressure on one point than the other, which is uneven stress, and that can cause failure if you get a good catch, well, no catch is good..... Having a dedicated tool for forming both recess and tenon really cuts the guess work out. Doing it with a detail gouge or a skew is not nearly as accurate.

You can over tighten, depending on how much wood there is on the foot of the bowl. I wouldn't want less than a 1/2 inch collar before the outside walls start for a 12 inch bowl. If you have less than that, you can expand the chuck till the collar breaks. This could still make for a failure if you are turning at high speed and have a huge catch. With a 1 inch collar, that would probably be almost impossible to break for bowls up to 16 inch diameter. Over that, I haven't really played around with enough to really know.

I have found a glue block to be a waste of time for my style of turning. This is in part because 99% of the bowls I turn are wet wood. Yes, the CA glue will hold, but drilling a recess with a proper sized forstner bit is far more efficient. If you take some time to plan things out when cutting, it takes very little adjusting to get grain centered on the blank.

robo hippy

[Randy McCreight]

Please to pardon my inexperience, but how do you make a dovetail/chamfered recess with a Forstner bit?

It's a bit amusing to me that the directions for the Nova say to just use the angle of the pointy side of my skew tool.

edit question" how did this post float to the top?

[Reed Gray]

If they had a forstner bit that would cut a dove tail, I would use it, but since no one has figured that one out yet, I just drill straight in. I do drill down deeper than the recess I use for the bottom of the bowl, some times deep enough so that the blank will sit on the face of the chuck jaws. For small blanks, up to 8 or so inches, I can use my small chuck which takes a 1 5/8 bit, or my big chuck which uses 2 3/8 inch bit. Especially with the tailstock engaged, this is more than enough to spin any solid piece of wood. Not sure if I posted the link to my video about 'Mounting things on the lathe' here or not, but it is up on You Tube.

You can get a fair degree of accuracy with a skew chisel, but that depends on the nose angle as much as anything, well, a steady hand as well. Again, that is why I have a dedicated dove tail tool. Just easier to use and more accurate....

robo hippy

[Randy McCreight]

Reed said: " . . but drilling a recess with a proper sized forstner bit is far more efficient."
OK. I get it now: you drill to depth then cut the chamfer with your special dovetail tool. Could ou share a photo? Thanks.

[Reed Gray]

This is how I do it:

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

Woke up to 10 inches of snow and it is still coming down.... People get stuck in the snow here on flat ground....

robo hippy

[Randy McCreight]

We had about 18 inches on Thursday and Friday. We stayed home for two days.

Anyway, I am almost done, after working thru challenges with the recess.