Broken Tenon's

[Rob Holcomb]

Since I began turning about a year ago on my Delts 46-460, I've noticed that whenever I try to turn a large bowl, the tenon ends up breaking off. I'm making sure the tenon's are of the correct size (about 2 1/4 inches in diameter and 1/4 to 3/8 of an inch in depth) for my Nova chuck and they are dovetailed as that's what my chuck requires. If I'm turning a bowl that's seven inches by two inches, no problems at all, but if I'm doing a larger deep bowl, say ten inches by five inches, I can't get it completed and it becomes firewood. Recently, I've been trying to do my first natural edge bowl. the first was not completed because all of the bark came off in one piece as I gently tugged at a strand of bark that was sticking out. Obviously there were issues with the log, rather than something I did wrong. Today, I tried another natural edge bowl out of Ash. It measured 8 1/2 inches in diameter and about 7 inches in depth. I had hoped to make a deep bowl for my wife do her magic with a fake floral arrangement. I turned the outside of the bowl and made the tenon and everything was looking good. I then mounted the bowl on my chuck and turned on the lathe to start hogging out the inside. In less than a minute, the tenon broke off the bowl. My progress had only reached 1/4 inch deep and a diameter of 2 inches. I had hardly touched the wood and mostly bark had been taken off at that point and I didn't have any catches. My thoughts are that maybe it's too much weight for the tenon to support. I might also think there was a defect in the wood where the tenon was made but this occurs with any large bowl I try. What are your thoughts?

[William Hutchinson]

I feel your pain, it's tough watching your work bounce across the shop. The rule of thumb I've been told is to make your tenons at least 45% of the bowl size. With larger pieces a steady rest will be needed for stabilization of the blank.

[Steve Schlumpf]

Rob - this is one of those times where a few photos would sure help us out. Only thing for me to ask at this point - are you using the tailstock for support when you start hollowing out the bowl?

[Rob Holcomb]

Steve, No I didn't use the tailstock for support. I did of course for turning the outside of the bowl but when I started roughing out the inside, I didn't use the tailstock. I just went with what I was taught by the Bill Grumbine video and chucked it up. How would I gouge out the inside of a bowl with the tailstock used for support?

As for pictures...I would have liked to snap some but my camera is in for repairs. I have a Canon digital and it stopped working. Turns out, there was a recall and they're fixing it for free and I should have it back in another week or so.

[David E Keller]

It may be too much stress on that sized tenon, or perhaps you are cursed!:eek::D

The tail stock can be used support the piece especially when working on the first 2/3 of the depth... I usually turn a bowl in stages getting the rim thickness I want before turning out the center. You can get pretty deep into the piece before the tailstock starts to get in the way. That will help you get some of the mass out of the piece before you rely on the tenon alone.

[Steve Schlumpf]

Rob - had the same problem with my Canon and they did a great job of handling everything. I've since upgraded to a Canon DSLR but still use the old Canon when out and about.

David is right in that you can use the tailstock for support for quite a while when first starting to hollow out a bowl. You just leave the center tailstock support in place - which will eventually become more of a cone shape as you remove the wood. Once most of the bulk is gone - there is a lot less stress on the tenon and you can back the tailstock off and turn down the cone area. Once that is removed - hollow out the rest of the bowl just like in the Bill Grumbine movies!

[Cathy Schaewe]

I've found that using the tailstock for support until you've got 1/2 to 2/3 of the mass removed is crucial. Just leave a cone/cylinder shape in the center, and work around it until you're comfortable removing it. When you have all the mass of the center in there, it's too much strain on the tenon without the tailstock.

[Ray Bell]

I had this problem on three concurrent bowls (ok I'm a slow learner). I not only started hollowing with this tail stock in place, I also soak the tenon/bowl bottom transition area with CA. I honestly don't know if this adds strength or not, but no more broken tenons since.

[Dennis Ford]

Consider using a faceplate, its slightly less convenient but a much stronger attachment.

[Jack Mincey]

A very good way of helping with this problem is to buy the largest set of jaws that will fit your chuck. I use #4 jaws on my strong hold chuck for bowls up to 20" and have never had an issue with the tenon. The #4 jaws take a 5" tenon which gives you about 4 times the amount of wood holding your bowl. I use these jaws when coring as well. As the others have said using the tail center for support when possible is also a good thing to do as well.
Jack

[Rob Holcomb]

I just learned something new! Thanks to all that have posted. Next time I will use the tailstock and see if I can do a larger bowl. I'll also look into a larger chuck as the $$$ permit. That's what I love about this forum. You not only get to see some incredible work done by the members here, everyone is very helpful. Here's hoping all your tomorrow's are filled with smiles! Rob

[Fred Perreault]

Without knowing what you consider large, it would be hard to know for sure. But for 2 years I have used the 3"-4" jaws on my 3 chucks... a Nova, a PSI and a Patriot. I rarely use the 2,25"-2.275" jaws unless it is a small item. It may be foolish, or lucky, but I have never had to use the tailstock as support, and never lost a project at the tenon. I turn up to 15.5"x7" bowls, from red oak, ash, and cherry, without shake, shudder or incident. I think the largest tenon you can provide is a safe way to go.

[Thomas Canfield]

In addition to the CA tip and using the tailstock at start, I would add that longer tenons for larger bowls give more support. My major failures have occured when getting over confident and hogging out too much material with a tool that needed sharpening and getting even the smallest of catches. Having the tool overhanging the toolrest too far also tends to help me have a catch and a problem.

[James Combs]

I generally use the tail stock for as long as I can but I have turned a couple of heavy blanks w/o it and not had any problems with my SN-2 and 1.75" diameter tenons. You said;
"'m making sure the tenon's are of the correct size (about 2 1/4 inches in diameter and 1/4 to 3/8 of an inch in depth) for my Nova chuck and they are dovetailed as that's what my chuck requires".

That depth does not sound like the tenon bottoms out inside the chuck jaws, I suspect the the outside edge of the jaws are bottoming out against the bowl bottom. If that is the case the dovetail is trying to "pull" the tenon off of the bowl bottom as you tighten the chuck creating a lot of stress on the tenon. Try making your tenons slightly deeper so that the tenon bottoms out against the inside of the chuck instead of the jaws against the bowl bottom.

[Harvey Schneider]

No matter how strong the tenon, a catch near the rim of a large bowl can yank the bowl right out of the chuck. I know that from personal experience.
Using the tailstock for support will certainly help. Gentle cuts with very sharp tools will help.
Ultimately you need to develop judgement and be responsive to what the wood and tool are doing. The sounds and sights of the wood and tool convey important information about what is going on.
I have only been turning for about four years, and I remember the sight of my precious wood flying through the air. It can be disheartening. I was trying to emulate what I had seen. Experienced turners can take aggressive cuts because they have developed the judgement to know what the wood will tolerate. I had to start off gently and work my way up to more aggressive cutting, but even now I consider myself a gentle woodturner.
Patience and persistence will get you there.

[Mike Peace]

...they are dovetailed as that's what my chuck requires.

I guess I take the position that I should follow the manufacturers advice until I have enought experience to think I know better.
Here is what the G3 manual says about the 50MM jaws: The 50mm (2 inches) standard jaw has a thin lip (or shoulder at the front face) which is designed to bite into the timber as the jaws are tightened. DO NOT CUT A RECESS FOR THE LIP TO FIT INTO, AS THIS WILL REDUCE GRIPPING POWER

I won't argue with folks that encourage the use of a tail stock as much as possible. I think the length sticking out from the chuck is a major issue with the 50MM jaws once you get past 4". There can be a lot of leverage. I think the comments about larger jaws are right on for deep bowls.

I would also reduce the size of the tenon to about 2" if you are pushing the limits.

I have turned a number of 10 to 14" bowls with the SN2 chuck with 50 MM jaws with no problems (and no dovetail).

[Leo Van Der Loo]

I don't have a tailstock on my large lathe when turning outboard, and that's where the largest pieces are turned, I hardly ever use a tenon, just for the reason you give.
I usually start off with the blank screwed to a faceplate, and after shaping the outside, I make a recess, you do need enough wood around the recess of course, but I can't remember the last time a turning came out of chuck, I don't like using a tenon as the wood can split off easily the recess is much stronger with side-grain turning, though not everyone agrees on that :rolleyes:, the other reason I use a recess is there's less wood on the bottom and it helps the drying without splitting that can occur because of the big lump of extra wood that sticks out the bottom.

Here's an album that shows me turning a large bowl with a recess, and also a bunch of rough turned bowls waiting to be returned one day ;)

http://homepage.mac.com/l.vanderloo/PhotoAlbum26.html

[John Keeton]

You are getting a variety of comments, so I will add mine. I disagree on letting the tenon bottom out in the chuck. The chuck should actually shoulder up to the piece as that is the only way it will run true. Seems I recall the instructions stating that one is not to let the tenon bottom out in the floor of the chuck, but my memory may not be correct. In either event, I don't think a long tenon gains much. Most failures occur as a result of breakage not slippage, so tenon length would not appear to be a factor.

I rarely have a 1/4" tenon - more in the range of 3/16".

On harder woods, I do a very slight dovetail on my tenons, but if the wood is a soft species, I will have no dovetail and let the jaws bite the wood.

I think it is possible to overtighten the chuck and crush the fibers in the wood depending on species, thereby weakening the tenon.

I do agree on reducing the diameter of the tenon to the point where the chuck is nearly closed when tight. To me, that is the best metal/wood contact and results in less stress on the tenon.

And, I agree with Leo on using a recess vs. a tenon, though with the HFs that I do mostly, the tenon is better for me.

But, I have turned less than 20 bowls, so my experience is pretty limited in that area. However, the last bowl I did was 11" in cherry, and was turned on a 3/16" tenon in a G3 with 50mm jaws with no difficulty.

[Michael Mills]

1+ on the Teknatool quote from Mike Peace above.
You can download the entire manual. I believe the 50mm is about page 20. http://www.teknatool.com/products/Chuck_Accessories/50mm.htm
Teknatool (as far as I could find) only references dovetails in expansion mode, contracting is always referred to a spigot.
The first picture is of the inside of a standard Nova 50mm jaws, the next is from the outside of the same jaws which has a true dovetail.
As you can see, the inside is straight from the bottom 2/3 of the way up. About where I drew a line across the jaw. Only about the top 1/3 has a dovetail. The total protrusion of the dovetail section from the straight section is only about 1/32 inch at the very top. This would be the "thin lip which is allowed to bite into the timber".
My hand sketch shows what will happen with a dovetail cut into a spigot. The total depth of the jaws are about 5/8" so you could cut a spigot about ½" long. If the spigot is cut with a dovetail, then point (B) of the wood must be compressed before the lip of the jaws (A) ever make contact with the wood. Probably not a problem with wet wood but could be a problem with dry hard wood.
From what I have read from Teknatool, you are greatly exceeding capacity in a spigot mode but would be fine in a dovetail (recess) mode.
Many may disagree but the above may be food for thought anyway. I’m wrong about a lot of things so don’t take my word (this is my disclaimer :o).
Hope you get your problem resolved soon.

[Allen Neighbors]

Ditto, what JK said. For my Nova 50mm jaws, which I use most of the time, I turn the tenon down to just about 2.1". (I broke a lot of tenons when I made them 2-1/4" dia.) With 2.1", (which is just slightly less than 2 1/8"), when I tighten the jaws, they have only about a 1/16" gap between them (when tight on the tenon). Any more than that, and you lose some of the gripping power, by moving the gripped area from the entire jaw pattern, toward just the points of the jaws, which will break much easier.
On hard woods only, I use a 1/16" parting tool, and make a groove 1/16" deep against the shoulder of the tenon. This allows the dovetail of the jaws to "get a grip", without having to turn a dovetail. Since I've been making my tenons like this, I've never had one break off.

[Reed Gray]

Having a tenon that is too long actually makes it weaker. I guess it is a leverage thing, but after having been told that, I noticed it was true. That being said, I prefer a recess. Most of the time I am around 3/16 deep. For proper diameter, I figure minimum 1/4 to 1/3 the diameter, so for a 10 inch bowl, I would want 3 inch minimum.

The over tightening bit can also make your grip less secure. Just get it snug. Tighten in one key first, then rotate to the next key, and go around a couple of times, especially on larger pieces. It evens out the force on the jaws where if you just tighten from one key, the jaw pressure can be lopsided.

The closer the fit of your jaws and tenon or recess is the better the grip is. Fit means not only diameter, but angles. On dry wood, you will get little to no compression, on green wood, you will get more. Also, when you mount the bowl, you do not want 2 of the jaws on the end grain, and 2 on the side grain. Rotate it 45 degrees so that the jaws are pushing more into the side grain. The end grain is a point of weakness (as in when we split wood for the fire place, we do it down the grain, not across it).

You didn't mention if you were having catches. A combination of over tightening, jaw orientation, fit and size can put the tenon/recess under a lot of pressure without actually breaking it. One little catch can be just enough to over stress it and cause failure.

I don't use the tailstock when bowl turning. Learned to do without it. When you are getting farther away from the headstock with deeper bowls/vessels, then it can be handy, as can a steady rest. More so when you are starting out, and are still developing tool skills.

robo hippy

[Wally Dickerman]

Without actually watching you while you are turning I can only make a guess on solving your problem just as everyone else is.

In over 20 years of teaching turning I have encountered just about every possible turning problem there could be. Yours is not an uncommon one. I have seen beginning turners rip a 7 inch bowl off a screw chuck. The problem is usually leverage.

It has been said that you could lift the world with leverage. A 22 inch long bowl gouge has a tremendous amount of leverage if you push the tool against the rim as you are making a cut, especially in a deep bowl. Try to avoid touching the rim with the gouge as you hollow and you may possibly solve your problem.

I do most of my turning on the outboard side of my lathe, so I almost never use a tailstock when hollowing. It shouldn't be necessary.

Wally

[David Woodruff]

Hello Rob, Been there with a chuck and broken tenons. I routinely use the face plate, waste block, CA glued to bottom of work piece configuration. This is SOP for anything over 6"-7". Take a look in my gallery under White Oak HF, first two photos. This vase is about 14" tall, 6" dia. The process takes a little more time than the chuck/tenon dovetail system but it stays with the piece for at least half dozen coats into the finishing process. I am one of the finishing fanatics that finish inside and out. Then I will part off the waste block and finish the bottom, flat to .001" concave. Never had one to separate, use a larger dia. waste block for larger HF's. Occasionally I go to a 6" face plate but normally 4" and use increased surface area of waste block to achieve a stronger glue joint when required. Please ask any questions.

[Dan Kralemann]

A very good way of helping with this problem is to buy the largest set of jaws that will fit your chuck. I use #4 jaws on my strong hold chuck for bowls up to 20" and have never had an issue with the tenon. The #4 jaws take a 5" tenon which gives you about 4 times the amount of wood holding your bowl. I use these jaws when coring as well. As the others have said using the tail center for support when possible is also a good thing to do as well.
Jack


You are getting a variety of comments, so I will add mine. I disagree on letting the tenon bottom out in the chuck. The chuck should actually shoulder up to the piece as that is the only way it will run true. Seems I recall the instructions stating that one is not to let the tenon bottom out in the floor of the chuck, but my memory may not be correct. In either event, I don't think a long tenon gains much. Most failures occur as a result of breakage not slippage, so tenon length would not appear to be a factor.

I rarely have a 1/4" tenon - more in the range of 3/16".

On harder woods, I do a very slight dovetail on my tenons, but if the wood is a soft species, I will have no dovetail and let the jaws bite the wood.

I think it is possible to over tighten the chuck and crush the fibers in the wood depending on species, thereby weakening the tenon.

I do agree on reducing the diameter of the tenon to the point where the chuck is nearly closed when tight. To me, that is the best metal/wood contact and results in less stress on the tenon.



Info on a couple of DVDs that I have watched stated that the minimum tenon should be 40% of the diameter of the bowl. The great thing about having a large tenon is that the material of the tenon can become the bottom portion of the bowl when the bowl is turned around to finish the bottom.

[Dave Ogren]

Here we go again...the old story. I'll have to agree with Leo and Reed. I start all of my turnings with a faceplate when the outside is done or almost done I cut a recess, just over 2 5/8" in dia. with a dovetail. Have not lost a bowl and have turned up to (finish size) 20" dia by 8" deep being quite agressive with a 5/8" gouge. (the only size that I have)

I have never used a tailstock, the reason is I don't have one.

Am I lucky ?? I don't think so. After close to 40 years in the machine shop business a small tenon only up to 3/8" long just doesn't feel right to me.

Good Luck,

Dave

[Rob Holcomb]

I have to say that there is a lot of variation on how things get done. I'm sure whether it's a tenon, a recess, a faceplate, a glued block etc., all will work. Because I don't have years of experience, my thoughts are to try each of your suggestions until I get one that works for me and I feel comfortable with it. The one thing I have to remember is that compared to most of you, I'm very inexperienced and along with how I attach a piece of wood to the lathe, I also have to pay attention to how I use the bowl gouge, the rest and my positioning. I wish I had more time to devote to turning than I do. My progress would move along much faster but even though I have slipped into the vortex, I am still very fond of flat work. Maybe because I'm good at that aspect of woodworking and don't have to think about how to get a task done or because that's what my requests are for. I like to tease my wife and tell her my table saw, jointer, planer, compound miter saw, dust collector, etc. are my buddies. We hang out together but my lathe is my mistress. She replied with the following. "Honey, that's all fine and good, but if I hear you whispering sweet nothings to your lathe, I'm taking half of your retirement benefits" HAHA

[Dick Strauss]

Rob,
One thing I didn't notice mentioned...make sure your tenon angle is the same as your jaw angle. If your tenon is too close to perpendicular to the shoulder, the jaws will act like a wedge on a small surface area and split the tenon off the bowl. If the tenon has too acute of an angle to the shoulder, it will only grab at the bottom of the tenon. Neither option gives the best holding power.

As others have mentioned, the tenon should be shorter than the chuck jaws are deep. The shoulder to the tenon should be perpendicualr to the lathe bed length. The whole tenon/shoulder should match the chuck. The shoulder gives much more holding power than just the tenon.

If your chuck has a minimum 2" opening in compression mode, you want to be as close to 2" as possible for maximum grip on the tenon (so that the tenon and jaws have a maximum contact area).

[Rich Aldrich]

Sharp tools make a big difference in how much force a person has to apply to cut. As a beginner, I try to pay attention to how much I put into the cut so I know when the tool needs to be sharpened. The tennon has to take the force applied that does not go into the cut.