# Tenon size verses chuck holding power



## TomC (Oct 27, 2008)

I was at a turning club meeting today and the presenter said you need to make the tenon diameter so the chuck jaws are almost closed to get maximum holding power. This does not seem correct to me. The surface area of the jaw contacting the tenon is the same if the jaws are wide open or completely closed so it looks like the holding force is the same. If I am wrong can someone explain why?
Tom
PS I did not question him on his statement.


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## robert421960 (Dec 9, 2010)

TomC said:


> I was at a turning club meeting today and the presenter said you need to make the tenon diameter so the chuck jaws are almost closed to get maximum holding power. This does not seem correct to me. The surface area of the jaw contacting the tenon is the same if the jaws are wide open or completely closed so it looks like the holding force is the same. If I am wrong can someone explain why?
> Tom
> PS I did not question him on his statement.


if your tenon is larger your jaws are just clamping with its edges but with them almost closed you are getting much more contact
think about taking a 2 inch circles then cutting it in 4 pieces then spread them to 3 inch circles and look at where they connect
inly the points of the circle touch


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## robert421960 (Dec 9, 2010)

im not good with words so i hope you understand what i said


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## john lucas (Sep 18, 2007)

Robert has it correct. Althought I will add an exception. First it depends on the jaws. Oneway makes some of their jaws with an odd non circular shape. They claim it will hold with more surface area on more sizes. 
Most chucks will grip with only 8 small points when the 4 jaws are wide open. As you close the jaws more surface area is in contact with the wood so you get more holding power. 
Now you should make the tenon shorter than the jaws are tall and make the top shoulder of the tenon square so it sits firmly on the top of the jaws. This gives even more holding power because it helps keep the tenon from rocking in the chuck. 
Now the exception. The gripping power of the chuck is only one consideration. The flat shoulder sitting on top of the jaws is another. The stability of the tenon itself is another. For example if you are turning a side grain bowl and you have a 1" tenon I think you can see that the wood could easily split along the grain and rip the tenon off and bowl goes fllying. If the tenon was 4" there is a lot more wood there to keep the tenon from spliting off. So in theory it could be a toss up between a large tenon with the jaws not holding well vs a small tenon where the jaws hold well but the tenon might break. I'll leave it up to the engineers to decide on this one. If the bowl was end grain where splitting along the grain will not happen then the smaller tenon with more gripping power would be my choice


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## TomC (Oct 27, 2008)

I still does not make sense to me. If the jaws stay in a true circle from open to close the surface area of the jaws don't change, so I am contacting the same surface area of the wood if I am fully open or almost closed. Still confused.
Tom


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## duncsuss (Aug 21, 2009)

TomC said:


> I still does not make sense to me. If the jaws stay in a true circle from open to close the surface area of the jaws don't change, so I am contacting the same surface area of the wood if I am fully open or almost closed. Still confused.
> Tom


They don't.

The jaws are metal -- they don't flex and change the shape of the rim as the chuck is opened and closed.

Draw a circle on a piece of paper, about the diameter of a coffee cup. Slice the paper into 4 quarters, and move the pieces apart like opening the jaws of a chuck.

Arrange the pieces around a plate, and you'll see that only the points at each end of the jaw touch the rim of the plate -- none of the inner edge of those jaws is going to touch the rim.


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## duncsuss (Aug 21, 2009)

Here's a SketchUp drawing of the concept ... the jaws are identical, simply moved apart to accommodate the larger piece of wood in the middle.

Only the corner points touch ...


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## PhilipCollier (Jan 2, 2012)

I was told they cut the jaw pieces out of one piece of steel as a perfect circle. All things being solid, the perfect tenon is when the jaws are tight around aound it and the gap between them is the kerf of the wheel that cut them apart when made.

I have got to learn Sketchup...that really looks good.


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## Dave Paine (May 30, 2012)

PhilipCollier said:


> I was told they cut the jaw pieces out of one piece of steel as a perfect circle. All things being solid, the perfect tenon is when the jaws are tight around aound it and the gap between them is the kerf of the wheel that cut them apart when made.


That is correct for most jaws. I have the One-Way Talon chucks, and as John Lucas mentioned, these have an odd pattern in the jaw intended to provide the same holding grip at all settings.

http://www.oneway.ca/chucks/talon.htm

I am happy with these chucks and jaws.


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## Alchymist (Jan 2, 2011)

Kind of a rhetorical question - the jaws will contact fully at only one point in their travel if pressure is ignored. That point is when the diameter of the tenon matches (or very slightly larger) the diameter of the circle of the jaw faces. I say very slightly larger, because the jaws will tend to crush the wood. So diameters slightly larger and smaller could have full contact under the crush. Other sizes will have somewhat more than point contact due to surface crush. Amount of force due to tightening determines the crush. A larger tenon tightened with force in a softer wood could obtain 100% contact, or even penetrate farther than surface contact. One reason to stop at least once in the turning and check tightness; the crushed wood tends to relax slightly and loosen the grip somewhat.


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## TomC (Oct 27, 2008)

duncsuss said:


> Here's a SketchUp drawing of the concept ... the jaws are identical, simply moved apart to accommodate the larger piece of wood in the middle.
> 
> Only the corner points touch ...


OK, now I'm convinced.
Tom


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## duncsuss (Aug 21, 2009)

TomC said:


> OK, now I'm convinced.
> Tom


sometimes it takes a picture :yes:


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## john lucas (Sep 18, 2007)

I doubt the jaws are cut from a circle since you can close the jaws to form a complete circle and there isn't any gap or Kerf where even a water jet could go. I don't know. I don't manufacture them but I'm guessing a CNC machine Mills each piece seperately. 
I asked the Vicmarc rep one time why the jaws were numbered. I had assumed it was because they were milled or cut from a circle and needed to be in a certain order to maintain the accuracy. He said they were numbered simply because customers demanded it in surveys. He said each part was made to a few ten thousandths of dead on and for all practical purposes are identical so there's no need for a specific order. 
One other thing. As mentioned above wood compresses. Especially since we tend to make the tenons larger than needed so your only crushing 8 points. If you have a side grain bowl you have the grain running straight across the tenon. If you want the best grip and future alignment orient the grain so it runs diagonally through the gaps in the chuck jaws. This way you equal pressure on the long grain and side grain portions of the tenon with each jaw. This tends to keep the bowl straight in the chuck. To make that clearer imagine the grain runs horizontally across your computer monitor screen. If you orient the jaws so it graps the corners there will be a little side grain and a little long grain on each jaw. If you grab the monitor by the square sides you will have 2 jaws with long grain and 2 jaws with short grain. These will compress unevenly and can pull the bowl out of alignment.


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## duncsuss (Aug 21, 2009)

john lucas said:


> I doubt the jaws are cut from a circle since you can close the jaws to form a complete circle and there isn't any gap or Kerf where even a water jet could go


My chuck (it's a PennState "utility chuck") is not like this.

When you close it all the way down, the outer rim is not a perfect circle -- it has to be opened up maybe 1/4" before the rim lies on a circular line. (Which is why I drew it that way in the diagram above.)

Since it's the only chuck I have, it's the only one I can describe.


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## john lucas (Sep 18, 2007)

Interesting. All the chucks I have owned would close down to zero between the jaws. Making a cirlce and cutting it apart would be the ways I would do it but then I'm a brand new machinist and to me that would be the most obvious way. However when all you have to do is send a machine a drawing and it builds a part then things change a lot.


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## NCPaladin (Aug 7, 2010)

john lucas said:


> I doubt the jaws are cut from a circle since you can close the jaws to form a complete circle and there isn't any gap or Kerf where even a water jet could go. I don't know. I don't manufacture them but I'm guessing a CNC machine Mills each piece seperately.
> I asked the Vicmarc rep one time why the jaws were numbered. I had assumed it was because they were milled or cut from a circle and needed to be in a certain order to maintain the accuracy. He said they were numbered simply because customers demanded it in surveys. He said each part was made to a few ten thousandths of dead on and for all practical purposes are identical so there's no need for a specific order.
> One other thing. As mentioned above wood compresses. Especially since we tend to make the tenons larger than needed so your only crushing 8 points. If you have a side grain bowl you have the grain running straight across the tenon. If you want the best grip and future alignment orient the grain so it runs diagonally through the gaps in the chuck jaws. This way you equal pressure on the long grain and side grain portions of the tenon with each jaw. This tends to keep the bowl straight in the chuck. To make that clearer imagine the grain runs horizontally across your computer monitor screen. If you orient the jaws so it graps the corners there will be a little side grain and a little long grain on each jaw. If you grab the monitor by the square sides you will have 2 jaws with long grain and 2 jaws with short grain. These will compress unevenly and can pull the bowl out of alignment.


I guess I have been ill informed. I was told the jaws were milled from a single billet of steel and then cut apart. On my Nova jaws the inside edges where the cuts are, are a little rough. Looks like they would mill the sides smooth also if on a CNC machine.
On larger jaws I can not see a difference but with my pin jaws I can see it is not a circle when scrolled all the way in. All of my chucks close down to zero between the jaws also, but then the diameter is not a circle anymore. It appears the kerf on my Novas may be 1.5-2 mm but certainally not 1/4" on the PSI chuck as duncsuss has.

I don’t know about the numbering. My _guess_ would be that IF they are sliced apart then they need to go back in the same order. If not any .001 error in the slicing would be magnified when installed in a different order. By numbering even a .03+ error would not matter since they go back to the same circle.

On the side/end grain…..I understand that some wood will compress more than other.
The only way I see for them _compress unevenly_ is if the jaws can move unevenly.
What I don’t grasp is how (with the jaws moving in the same amount when scrolling in) one jaw can move more than the others. I agree they can bite into the soft wood easier than the hard wood but not deeper. If I scroll in 2mm then it seems it would be 2mm whether soft or hard wood.


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## Alchymist (Jan 2, 2011)

NCPaladin said:


> [
> On the side/end grain…..I understand that some wood will compress more than other.
> The only way I see for them _compress unevenly_ is if the jaws can move unevenly.
> What I don’t grasp is how (with the jaws moving in the same amount when scrolling in) one jaw can move more than the others. I agree they can bite into the soft wood easier than the hard wood but not deeper. If I scroll in 2mm then it seems it would be 2mm whether soft or hard wood.


Look at it this way - take a tenon with a knot in one spot. Put the knot under one of the jaws and tighten the chuck. There will be very little penetration of the jaw over the knot, but more under the remaining jaws. The tenon will move off-center away from the jaw with the knot. The deeper the other jaws go, the further off center the tenon will move. 

Even without a knot, the wood density will vary slightly, so the tenon will move off center however slightly.


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## duncsuss (Aug 21, 2009)

john lucas said:


> All the chucks I have owned would close down to zero between the jaws.


John, mine close down to zero too ... but the gripping jaws don't form a circle when it's "fully shut".

Though it's very close to a circle, it's actually a "square with radiused sides".

If I had to create this shape, I'd make a circle, cut two kerfs through it (about 1/4" thick), then push the 4 parts together.


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## pvechart (Dec 14, 2011)

I have a Nova G3 chuck and if i don't match my 50mm jaw numbers with the chuck numbers they will not close down correctly. I also read where the Nova jaws are cut from a solid piece of material. This leads me to the conclusion that the Nova manufacturing process is different than others like Vicmarc... as John discribed earlier.


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## Dave Paine (May 30, 2012)

The thread has me interested, so I decided to take a look at my Oneway Talon #3 jaws, since the other two are mounted on chucks.

The Oneway jaws are not numbered. The blocks onto which they mount in the chuck are numbered. The chuck numbering is needed to install the blocks. They have unique spirals and the spiral starts with No1 then install No 2, etc.

The Oneway jaws have a pin to prevent opening the jaws too far. The instruction mention this jaw can be mounted in the No2 or No4 position, since these two positions have a slot for the pin.

I put the jaws on a piece of paper and then scribed around the outside. I then got a compass and to my eye I was able to scribe a circle which matched the outline.


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## NCPaladin (Aug 7, 2010)

I agree if there is one knot or other hard area that does not extend across the tenon that it may push the tenon off center.
In normal soft/hard wood if the grain is aligned with the joint in the jaws then each jaw contacts hardwood at one end and softwood at the other. Unless the jaws somehow twisted to allow the softwood end to compress more it seems to me they would all compress the same. 
If the alignment was with hardwood in the center of 1 & 3 and softwood in the center of 2 & 4 they can still only compress the amount of the greatest resistance.
Either way there is one half the diameter in hardwood and one half in softwood.

My head hurts. :laughing:

That’s interesting Dave about the jaws and Oneway. I assume the edges of the jaws are machined?
Maybe that is why they are so much more expensive also.


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## Dave Paine (May 30, 2012)

NCPaladin said:


> That’s interesting Dave about the jaws and Oneway. I assume the edges of the jaws are machined?
> Maybe that is why they are so much more expensive also.


Oneway have two styles, one with several serations in a wavy pattern on the inside, which is the style I purchased, and one without serations.


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## WillemJM (Aug 18, 2011)

The funny thing about turning is there are so many different ways to do the same thing.

I was turning this afternoon, the hold from a Oneway Talon chuck almost all the way closed, perfect grip and without a catch, the Padauk tenon sheared right of, the gripped piece remaining in the chuck. My vessel went airborne, but I fortunately saved it.

So one could debate, if the jaws were wide open, I would have a lot thicker tenon (larger diameter) and probably not launched the missile.


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## pvechart (Dec 14, 2011)

Willem...your comment is exactly what John Lucas talked about earlier about tenon size...there are always trade offs.


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## NCPaladin (Aug 7, 2010)

I agree with pvechart and John.
That is why I try to use a recess rather than a tenon.
With my 50mm jaws and one inch of wood surrounding the recess, there is three times the amount of wood that has to break away compared to a tenon.


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## firsty (Jan 14, 2013)

recently a friend of mine and I turned a rather large rough green bowl blank [ 28 in. by 7 in. deep ] slightly outboard, [ at about 22 degrees ] using a Nova 2024 with a 4 inch jaw. didn't have any problem with the jaw holding that big a blank at maximum opening. just cranked the jaws tight and started turning slowly. sometimes we turn with a foot, other times we use a mortice hold. the key appears to be cutting the mortice or tenon to the proper depth so the wood bottoms out fully against the body of the chuck to increase it's holding power. so far we have had no problems holding very big pieces of green bowl blanks or coring large blanks, even in frozen wood. I use a Nova DVR XP and regularly turn large [up to 16in.] frozen birch blanks with a 4 in. Nova chuck, and have had no problems holding these pieces with chucks ranging from 2 to 4 in. Firsty


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## duncsuss (Aug 21, 2009)

firsty said:


> ... the key appears to be cutting the mortice or tenon to the proper depth so the wood bottoms out fully against the body of the chuck to increase it's holding power ...


Well, that's the exact opposite of what I was taught and everything I've read.

As it was told to me, the tenon *should not* bottom out in the jaws of the chuck. Maximum stability comes from having a square shoulder for the front edge of the jaws to butt up against, and a long -- but not bottoming out -- tenon. If it bottoms out, the shoulder of the blank will not be tight against the front edge of the jaws.


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## Dave Paine (May 30, 2012)

duncsuss said:


> Well, that's the exact opposite of what I was taught and everything I've read.
> 
> As it was told to me, the tenon *should not* bottom out in the jaws of the chuck. Maximum stability comes from having a square shoulder for the front edge of the jaws to butt up against, and a long -- but not bottoming out -- tenon. If it bottoms out, the shoulder of the blank will not be tight against the front edge of the jaws.


+1 with Duncan, we have both read similar information to have the shoulder of the tenon tight with the top of the jaws and not bottoming out.


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## robert421960 (Dec 9, 2010)

Dave Paine said:


> +1 with Duncan, we have both read similar information to have the shoulder of the tenon tight with the top of the jaws and not bottoming out.


+2 :yes::yes:


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## sawdustfactory (Jan 30, 2011)

+3. Tenon should not bottom out.


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