Tenon diameter and form

[Dean Center]

The recent thread that included suggestions on chuck size and the advantages of smaller diameter chucks reminded me of something I learned earlier this year about desirable tenon diameter. Rather than poach the other thread, I thought I'd share the observation separately, for whatever it is worth.

While looking back on the bowls I've made over the last couple of years, I became dismayed at how many of them have a just a little too big diameter bottom. (Ironically, I just purchased a copy of Richard Raffan's "Turned Bowl Design", and he shows exactly this in a couple of pictures.) This is a common experience when looking back at our earliest work, but I should know better. And I do, so I pondered what had happened.

The light bulb went on when I realized that after a learning session on coring, I decided I would have more secure mountings with larger diameter jaws. As a result, the base of the bowl also increased a little in diameter and I'm not good at seeing the true shape/form while the wood is horizontal on the lathe.

I've been trying to reshape the base when the bowl is reversed to take off the tenon, but as a result of the experienced comments on chuck diameter in the other thread, I'm going to downsize my jaw size when possible. So, not only is chuck diameter important, the diameter of the jaws is quite important to the success of the piece.

On a related note, I've always been a fan of dovetailed jaws, thinking the shape would pull the tenon snugly into the chuck. In spite of being fanatical about making well shaped and clean tenons, I still break one off from time to time, usually at about the midpoint when turning green blanks. I'm reconsidering whether straight jaws might be more secure after all, and the tenons are certainly easier to form.

 

[hockenbery]

I do most of my hollow forms on face plates. So I turn away this large block of wood on every form.
I make a notch at the top of the tenon I screw the faceplate on so that my eye can follow the imaginary curve into the block of wood. Tenons are no different and I turn that wood away to. The foot is often much smaller than the holding device.
When I do the final reverse chuck I continue the curve of the side into the tenon wood. If there is slight misalignment movement of wood there is tiny line to blend with sanding.

You might get a different perspective from the video of the seed jar demo.
I have two tenons which are both turned away. One leaves a finished opening one leaves a foot finished on the bottom. These little forms go really quick. Turn 2 or 3 and you will have a new perspective on working the wood inside the tenon.
http://www.aawforum.org/community/index.php?threads/turning-a-seed-jar-split-hollow-form.13584/

I prefer dove tail jaws. Most tenon failures are from not having a solid fit on the top of the jaws which can let the piece rock on the top of the chuck jaws. A scraper will often make a bad tenon. It turn all my tenons with a spindle gouge. Also the strongest hold with dovetail jaws is just proud of the true circle diameter of the jaws. This is around an 1/8” gap from closed.

That said I made a bad tenon in a demo last week.
The tenon was getting a bit wobbly in the chuck
I was able to retighten it 3 times a got away with it. What I think I did was cut a vicmarc angle dovetail when I was using a ONEWAY dovetail jaw so the jaws never closed completely into the dovedail.

 

[John Jordan]

You need a secondary step on the bottom of the bowl. For a normal sized piece, make a "waste" area or block about an inch long and 3 inches in diameter. Make a 2 inch tenon about 3/8" long, which will leave a three inch diameter block about 5/8" long. The curve of the bowl meets this block, then continues unturned through this block. The remaining curve/bottom is turned when the piece is reversed. No reshaping of the bowl/vessel is needed. Sticking a tenon directly on the bottom without this step/waste area means you don't have wood you need to continue the curve of the bowl down to the desired base/foot/bottom diameter. Ignore the top picture and look at the drawing.

John

 

[Dean Center]

Thanks guys. When I watch the top turners demonstrate, my Scotch DNA gets riled up at all the wood they're wasting. It's an inclination I struggle with--wanting to get the biggest out of the blank rather than the best.

 

[hockenbery]

Thanks guys. When I watch the top turners demonstrate, my Scotch DNA gets riled up at all the wood they're wasting. It's an inclination I struggle with--wanting to get the biggest out of the blank rather than the best.

Using all the wood is a common affliction and sometimes a defense mechanism developed from the 5” diameter bowls we make as beginners from a 12” diameter blank.

I try not to let the size of the wood or the holding method determine the design.
I might choose or cut a blank to near the size of the object so I don’t have to turn much away.
Often in the turning process I might raise the bottom or lower the rim to improve the curve.

It is rare that a body of work where every foot is the size of a tenon or recess has much esthetic appeal.

When I turn NE bowls from a crotch I spend more time on the layout because I can’t turn away the rim and the bottom has the pretty wood. So I set the rim with the bandsaw circle cut, turn a notion of the where the bottom will be and then turn a pleasing curve to the rim. It may not be the best curve but it will be a good one. A compromise of the curve to show the flame figure in the bottom.

 

[John Torchick]

my Scotch DNA gets riled up at all the wood they're wasting.

My wife asked why we take a piece of wood and turn off 90% of it. She watched me turn a Slimline pen; 3/4 square blank and turn it down to a fraction of that. For tenons- I read somewhere that when tightening the jaws, the jaws should be almost touching to get the best grip. The wider the space between jaws, the more likely for the piece to come loose. Something we definitely don't want! Comments from the experienced forum members?

 

[Mike Johnson]

If you are rough turning a green bowl you want to turn the tenon a little oversized so you can true it up after it is dried. When you are ready to finish turning the piece a tenon slightly larger in diameter then the closed diameter of the chuck will provide the most amount of contact with the jaws. If the tenon is oversized you are only using 8 points of contact on the jaws surface to support the piece which can easily be pulled loose with a good catch if unsupported by the tail stock. A properly sized tenon will have 360 degrees of wood tenon contact on the chuck jaws which provides a positive grip on the work piece. The wood tenon itself would need to fail for the work piece to come out of the chuck.

 

[Bernie Hrytzak]

For round objects:
The jaws form a perfect circle when they are closed. When they are open larger than this circle, the jaws contact the wood at 8 points. At max. open size, these 8 points will not provide as secure holding power(wiggle free) as a perfect circle would.
For square spindles: the above would not apply.

Similair to Mikes, I was typing as he posted.

 

[Bill Boehme]

.... I read somewhere that when tightening the jaws, the jaws should be almost touching to get the best grip. The wider the space between jaws, the more likely for the piece to come loose. Something we definitely don't want! Comments from the experienced forum members?

For Vicmarc jaws the perfect circle diameter of the jaws is when the gap between adjacent jaws is about ⅛". This is because the set of four jaws are machined as a single piece and then sawed into four parts. The jaws are numbered to keep them in the same position relationship to each other. For optimum gripping performance the mortise or tenon should be sized so that the jaws will be at the perfect circle diameter.

 

[John Torchick]

Right, Bill. My SuperNova2 is numbered. And there is always duck tape.

 

[Tom Albrecht]

Thanks guys. When I watch the top turners demonstrate, my Scotch DNA gets riled up at all the wood they're wasting. It's an inclination I struggle with--wanting to get the biggest out of the blank rather than the best.

Hey, that explains why I so often end up making lamp shades instead of bowls!
This is a good subject, thanks for bringing it up.
I now remember Ellsworth stressing that "extra section for relief", and John Jordan's illustrations above are right on.

BTW, if you want some non-dovetail Oneway jaws, I have a bunch that I'll give away if you pay the shipping

 

[Don Wattenhofer]

Hey, that explains why I so often end up making lamp shades instead of bowls!
This is a good subject, thanks for bringing it up.
I now remember Ellsworth stressing that "extra section for relief", and John Jordan's illustrations above are right on.

BTW, if you want some non-dovetail Oneway jaws, I have a bunch that I'll give away if you pay the shipping

Concerning the Oneway profile jaws and the perfect circle, First if using the 2 " standard Oneway profile jaws for small work say up to 6" diameter they hold just fine and if they don't you are not cutting you are hogging. Second I install the wood worm screw into the chuck and the outside dovetail is a perfect circle which I know because I than mount it on my engine lathe and make it a clean dovetail. Third the profile jaws are great for gripping square blanks which eliminates the need to mount between centers to make a tenon. The photos are 2 examples of mounting square pieces.

 

[Bill Boehme]

Concerning the Oneway profile jaws and the perfect circle, First if using the 2 " standard Oneway profile jaws for small work say up to 6" diameter they hold just fine and if they don't you are not cutting you are hogging. Second I install the wood worm screw into the chuck and the outside dovetail is a perfect circle which I know because I than mount it on my engine lathe and make it a clean dovetail. Third the profile jaws are great for gripping square blanks which eliminates the need to mount between centers to make a tenon. The photos are 2 examples of mounting square pieces.

I agree. I might consider getting some more Oneway dovetail jaws, but I certainly won't be getting rid of the profiled jaws. They are so versatile and aren't fussy about tenon diameter. I don't use them with a recess because I don't understand the {what appears to me to be a} strange bead near the top.

 

[Don Wattenhofer]

I agree. I might consider getting some more Oneway dovetail jaws, but I certainly won't be getting rid of the profiled jaws. They are so versatile and aren't fussy about tenon diameter. I don't use them with a recess because I don't understand the {what appears to me to be a} strange bead near the top.

That strange bead is what I machined off.

 

[Clifton C]

I didn't understand that "strange bead" either. the jaws are soft enough, I just used a scraper and scraped it off, then added a little dovetail. Used mostly for plates, this will easily hold in an 1/8 inch recess.

 

[William Rogers]

What I think I did was cut a vicmarc angle dovetail when I was using a ONEWAY dovetail jaw so the jaws never closed completely into the dovedail.

I don't have the Vicmark 0r Oneway chucks. i do have the HTC 125 chuck and have ordered another Record chuck with 75mm bowl jaws that have a dovetail. I thought all dovetails were the same, but I guess not. So how do you cut the dovetails to match the chuck?

 

[Bill Boehme]

I don't have the Vicmark 0r Oneway chucks. i do have the HTC 125 chuck and have ordered another Record chuck with 75mm bowl jaws that have a dovetail. I thought all dovetails were the same, but I guess not. So how do you cut the dovetails to match the chuck?

I do it by eyeball. And, I prefer to err so that the outer edge of the tenon or mortise makes contact with the jaws slightly before the inside corner of the wood contacts the sharp corner at the top of the jaws. My rationale is that I want to avoid concentrating stresses in a sharp corner because that would increase the risk of shearing off the tenon or mortise.

Make certain that there's a good flat surface to mate with the top of the jaws.

 

[hockenbery]

I don't have the Vicmark 0r Oneway chucks. i do have the HTC 125 chuck and have ordered another Record chuck with 75mm bowl jaws that have a dovetail. I thought all dovetails were the same, but I guess not. So how do you cut the dovetails to match the chuck?

I look at the angle of the jaws and cut the same(close) angle on the tenon.
After doing lots of them I remember the angles the ONEWAY is a bit less acute.
Except in a demo I lost focus on the angle from some distraction.

You can always take one jaw off and hold it against the tenonnasna check.

Opening the jaws wide may let you check too as you can see from the sides the dovetail fits.

 

[Don Wattenhofer]

I do it by eyeball. And, I prefer to err so that the outer edge of the tenon or mortise makes contact with the jaws slightly before the inside corner of the wood contacts the sharp corner at the top of the jaws. My rationale is that I want to avoid concentrating stresses in a sharp corner because that would increase the risk of shearing off the tenon or mortise.

Make certain that there's a good flat surface to mate with the top of the jaws.

Ditto the eyeball method but if it concerns you make an angle gauge out of some 1/8th" lexan or plexiglas or brass.

 

[Bill Boehme]

I don't have the Vicmark 0r Oneway chucks. i do have the HTC 125 chuck and have ordered another Record chuck with 75mm bowl jaws that have a dovetail. I thought all dovetails were the same, but I guess not. So how do you cut the dovetails to match the chuck?

I don't know anything about those chucks, but it reminded me of the gadget pictured below that I won as a door prize. It's sold by Technatool and they call it a 10 in 1 tool. Among a host of other things it has dovetail gauges for the following sizes of Nova chuck jaws: 25 mm, 45 mm, 50 mm, and 100 mm. It occurred to me that I could probably modify the dovetail gauges to match some of my more frequently used dovetail jaws.

 

[John Jordan]

Looks like it gets a lot of use.

John

 

[Zach LaPerriere]

I have Oneway chucks, and the dovetail is 7°. So I took an old scraper and sharpened it to 7° with the help of an angle finder. Works like a champ.

 

[Gerald Lawrence]

I don't know anything about those chucks, but it reminded me of the gadget pictured below that I won as a doorprise. It's sold by Technatool and they call it a 10 in 1 tool. Among a host of other things it has dovetail gauges for the following sizes of Nova chuck jaws: 25 mm, 45 mm, 50 mm, and 100 mm. It occurred to me that I could probably modify the dovetail gauges to match some of my more frequently used dovetail jaws.

View attachment 28449

Bill all Novas have a outside dovetail but only the 100 and 130 have inside dovetail. In the Nova video the bump on the inside can be cut as a slight recess on the tenon or not if you so desire, but I think the hold is better with it

 

[William Rogers]

Novas have a outside dovetail but only the 100 and 130 have inside dovetail.

Gerald

The Nova 75mm heavy bowl jaws also have a inside dovetail. I have never cut the recess for the "bird beak" bump. I think it will hold better without the recess for the bird beak. I have never had one come out. Just opinion, but If over cut I think it will not hold as well.

 

[hockenbery]

In beginner classes I inspect the first tenon before the student can put it in the chuck but in most classes the instructor and assistants don’t routinely inspect the tenons because. The students know how to make a good one. But even good students make a poor tenon now and then.

90 degree tenons
What I have seen in classes and workshops is that tenon failures are nearly always the result of a poorly shaped tenon. The Nova jaws with the bird beak In my experience do not hold a poorly shaped tenon very often.
The ONEWAY profile jaws have the best chance on a poorly shaped tenon.
Both the nova and ONEWAY profile probably hold a tenon cut at 87 degrees. A tenon cut at 93 the novas are likely to fail and the ONEWAY will probably work.

A poorly made 90 deg tenon with a rounded corner or torn grain will not hold well in the nova.
If the round hits the edge of the first profile tooth the ONEWAY jaws may not hold well.
This rounded corner will usually push the tenon up on one side a tiny bit as the chuck closes and then the tenon can walk out of the chuck.

A long time back I started having students using the nova jaws cut a tiny recess for the “beak”.
With the recess cut the the nova jaws will hold the 93 degree tenon and the recess takes out a rounded corner. This little recess virtually eliminates tenon failures with nova jaws.

Dove tail tenons
Vicmarc jaws are 14 degrees the inside angle. The tenon needs to be 76 degrees.
ONEWAY jaws are 7 degrees ( using @Zach LaPerriere above) tenon needs to be 83 degrees.

A 14 degree tenon in a ONEWAY dovetail does not hold well at all.
A 7 degree tenon in a vicmarc will often hold well enough
Both jaws work with tenons slightly off

When we get more experience we make better tenons and don’t put as much pressure on the tenons from turning.

 

[Gerald Lawrence]

Gerald

The Nova 75mm heavy bowl jaws also have a inside dovetail. I have never cut the recess for the "bird beak" bump. I think it will hold better without the recess for the bird beak. I have never had one come out. Just opinion, but If over cut I think it will not hold as well.

My 70 is the type with breaks so there are two posts on each jaw. I think the one you have is about $25 more than mine. My guess is it is more expensive milling and inside dovetail.

 

[William Rogers]

Gerald,
I have both, the "normal" do not have a inside dovetail where the heavy duty do.

Al, I do pay attention to my tenon profile where some may not. For the non-dovetail NovaI make them as close to 90 as possible with a sharp cornor, no recess. I think this is the best method is without the recess where the beak will push the wood out instead of relying on the side only to hold the tenon. If the recess is cut too deep, to me that would be less holding force.