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Forum topic by McNamar posted 12-22-2021 10:40 PM 837 views 0 times favorited 9 replies Add to Favorites Watch
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McNamar

25 posts in 1025 days


12-22-2021 10:40 PM

Topic tags/keywords: lathe question turning

I am fairly new to wood turning and I’ve been having issues with my tenons breaking when turning bowls. I am having a repeated problem in that the tenon breaks while I am turning the inside of the bowl and I now get paper weights. Does anyone have any advice as to what I may be doing wrong or how to ensure that doesn’t happen again? It doesn’t happen immediately, but, usually partway through the turning of the inside of the bowl. I thought it was because the wood I was using was a little punky, but I just did one with a bowl blank that was not punky and experienced the same thing. I’ve seen several videos on YouTube that instructed on how best to create a tenon too, but I still have the same issue. I am hoping that the many more skilled and experienced turners can provide some advice. Thanks in advance and happy holidays.

v/r
Brad


9 replies so far

View ibewjon's profile

ibewjon

3035 posts in 4291 days


#1 posted 12-22-2021 11:44 PM

What size tennon are you breaking?

View LesB's profile

LesB

3506 posts in 4941 days


#2 posted 12-23-2021 01:05 AM

As usual a picture is worth a lot of words.

You don’t say but I assume you are using a chuck to hold the tenon. In that case the problem suggest the tenon is either too small or too long. Or, the wood is to soft.
Other more subtle things could be you are “hogging” out to much at a pass, you could also be going too slow which also makes for deeper cuts and more pressure on the tenon.

Have you tried the mortise method instead?
Here is a blog I wrote on making a mortise in the blank. https://www.lumberjocks.com/LesB/blog

-- Les B, Oregon

View Lazyman's profile

Lazyman

9799 posts in 2885 days


#3 posted 12-23-2021 02:06 PM

Pictures will help but in my experience, an improperly sized or shaped tenon is usually the cause. Here are few thoughts. They may all be obvious to you but I will list them anyway.

  • Tenon diameter is too small for the size of the bowl.
  • Tenon diameter is too large for the size of jaws used. This usually just causes it to release but in the process may break the tenon off too. Try to get size the tenon so that when the jaws are closed around it, the jaws form a perfect circle. This maximizes the contact of the jaws with the tenon. If you find that you have the jaws open closer to max than minimum, you need to use a larger set of jaws. When turning green wood, you may have to start with larger jaws for rough turning and after drying and turning it back to round, resize the tenon to use a smaller set of jaws.
  • Tenon too tall for the bowl or chuck. Make sure that the tenon does not bottom out in the jaws. Only the end of the the jaws should contact the bottom of the blank —basically the flat that will become the bottom of the bowl. Even if it doesn’t bottom out, a longer tenon is also more likely to snap off if you get a catch. Err on the side of shorter if you can. I try to make the tenon about half the maximum depth the jaw can accept.
  • The angle of the dovetail tenon is not the same as the jaws. A shorter tenon makes this less of an issue.
  • Where the base of the bowl rests against the jaw, it must be perfectly flat. If there is any part of the end of the jaw that is not flat against the base, it can move which can lead to breakage or even just pulling out of the jaw. This jaw/bottom interface is really what makes the blank stable in the chuck.
  • The base of the tenon, where the tenon dovetail meets the bottom of the bowl, must make a sharp corner. If the corner is round at all or the corner of the jaw has any sort of space when closed, the jaw cannot sit flat against the base or get a good grip and the blank can move. I sometimes use a skew, held flat like a scraper, to make the final shape of the tenon, using the point to make a nice clean corner. (Note that this is about the only use of a skew that is safe in bowl turning)
  • Keeping the tail stock in place as long as possible will reduce the chances of a bowl releasing or breaking off. On particularly large blanks, I will sometimes do most of the hollowing with the tail stock in place.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View KYtoolsmith's profile

KYtoolsmith

254 posts in 1358 days


#4 posted 12-23-2021 02:28 PM

I’ve been using a glue block for a tenon. I attach the block after turning the outside and making a flat bottom. The block is glued on while the blank is still mounted on the faceplate. My tailstock provides clamping pressure while the CA glue sets up. Then I turn a tenon on the glue block before removing the work from the faceplate. Once the piece is reversed and the glue block tenon is in the chuck, I use the tailstock and live center to keep pressure on the workpiece as I start hollowing. The remaining nub at the live center is easily removed as the last step.
Regards, The Kentucky Toolsmith!

-- "Good enough" is just another way of saying "it could be better"...

View McNamar's profile

McNamar

25 posts in 1025 days


#5 posted 12-24-2021 02:43 AM

Thanks guys. I will try the advice you all provided. i am using a Nova 3 chuck with the chuck opened just enough to look like a circle, but I may not have enough of a length of the tenon. I already disposed of the wood blank as it was not too deep to begin with and even trying to redo it would make it too short. So, unfortunately, I have no pictures to show of my issue. I am now doing Christmas things with the family, but will let you all know how things go with adjusting the tenon depth. Thanks again!


What size tennon are you breaking?

- ibewjon
a

View OSU55's profile

OSU55

3051 posts in 3487 days


#6 posted 12-24-2021 02:10 PM

Do you mean a Nova G3 chuck? Nova 3 does not exist. If a G3 you are probably using the 50mm jaws. If they are the “bird beak” jaws, use a straight tenon. These jaws hold very well.

I much prefer a tenon to a mortise. IMO keep working at getting tenons right vs moving to a mortise. A big part of the choice between the 2 is based on how someone processes a project through the entire process.

Sounds like the tenon size is ok based on your description. I dont have problems with tenons used with jaws wide open. Made a lot of bowls, platters, etc that way. I do not use dovetail jaws much, which do not grip well fully open. Its more important to have a big enough tenon vs perfect circle fit of jaws, unless one uses DT jaws.

The most probable cause is the top of the jaws are not sitting properly on the base of the tenon. As already mentioned, the tenon base must have a fairly sharp corner so the radius of the chuck jaw does not touch. The base of the tenon must be flat to slightly concave, so the top of the jaws rest flat on it, or the top OD of the jaws make contact and not the ID. I make my tenon bases slightly concave to ensure the top OD of the jaws contact the base before the ID. It is easy to think the base is flat but a few thou off is enough to pose a problem. Never lost a tenon with too much concavity, but had wobbles and breaks if the ID is just a little above the OD.

The top of the tenon cannot touch the base of the jaws. Also put a small chamfer at the top of the tenon. Sometimes the tenon is not the same length around it. The chamfer ensures an equal length.

If using dovetail jaws the tenon angle must be the same to slightly more, forcing the jaws to grip at the top just before the bottom.

All of this applies to fitting a glue block tenon to a chuck. A glue block does 2 things: 1) preserves project height if blank size is limited, 2) good for punky wood that is too soft to chuck. Wood that soft will not turn well and will need some type of reinforcement to strengthen the fibers. If the blank needs reinforcement, do the tenon are as well and a glue block is not needed. Glue blocks come with their own set of issues.

View McNamar's profile

McNamar

25 posts in 1025 days


#7 posted 12-25-2021 08:39 PM

Yes, I meant a Nova G3 chuck of 50mm. I will take a look at how I am making my tenon and see if your advice helps to correct my problem. Thanks!

v/r
Brad


Do you mean a Nova G3 chuck? Nova 3 does not exist. If a G3 you are probably using the 50mm jaws. If they are the “bird beak” jaws, use a straight tenon. These jaws hold very well.

I much prefer a tenon to a mortise. IMO keep working at getting tenons right vs moving to a mortise. A big part of the choice between the 2 is based on how someone processes a project through the entire process.

Sounds like the tenon size is ok based on your description. I dont have problems with tenons used with jaws wide open. Made a lot of bowls, platters, etc that way. I do not use dovetail jaws much, which do not grip well fully open. Its more important to have a big enough tenon vs perfect circle fit of jaws, unless one uses DT jaws.

The most probable cause is the top of the jaws are not sitting properly on the base of the tenon. As already mentioned, the tenon base must have a fairly sharp corner so the radius of the chuck jaw does not touch. The base of the tenon must be flat to slightly concave, so the top of the jaws rest flat on it, or the top OD of the jaws make contact and not the ID. I make my tenon bases slightly concave to ensure the top OD of the jaws contact the base before the ID. It is easy to think the base is flat but a few thou off is enough to pose a problem. Never lost a tenon with too much concavity, but had wobbles and breaks if the ID is just a little above the OD.

The top of the tenon cannot touch the base of the jaws. Also put a small chamfer at the top of the tenon. Sometimes the tenon is not the same length around it. The chamfer ensures an equal length.

If using dovetail jaws the tenon angle must be the same to slightly more, forcing the jaws to grip at the top just before the bottom.

All of this applies to fitting a glue block tenon to a chuck. A glue block does 2 things: 1) preserves project height if blank size is limited, 2) good for punky wood that is too soft to chuck. Wood that soft will not turn well and will need some type of reinforcement to strengthen the fibers. If the blank needs reinforcement, do the tenon are as well and a glue block is not needed. Glue blocks come with their own set of issues.

- OSU55


View Phil32's profile

Phil32

1807 posts in 1401 days


#8 posted 12-25-2021 09:51 PM

Not mentioned is that a tenon works best if the axis of the tenon is in line with the wood grain. Cross-grain tenons are much more likely to break off when hogging out the inside of the bowl.

-- You know, this site doesn't require woodworking skills, but you should know how to write.

View OSU55's profile

OSU55

3051 posts in 3487 days


#9 posted 12-26-2021 02:49 AM



Not mentioned is that a tenon works best if the axis of the tenon is in line with the wood grain. Cross-grain tenons are much more likely to break off when hogging out the inside of the bowl.

- Phil32


While an end grain bowl and tenon is stronger the majority of bowls are face or “cross” grain.

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