Tenon Size

[Emiliano Achaval]

Professional woodturner Phil Irons contacted me after the demo that I did for the Woodturners Worldwide Online symposiums. During the demo, I mentioned, as I often did, that I had to take it easy when hollowing the calabash. First, because my lathe is 3HP, and the Milo wood is rather soft. I have had a few bowls come off when rough turning with my 3/4 in Thompson gouge. Phil told me that he is on a crusade, to educate as many turners as possible to use big jaws, make big tenons. Phil makes his tenon so that by turning less than a quarter turn the big jaws are completely closed. A sign of success is when the entire spigot is black, just like when you see only a few areas of the spigot black, where the jaws were making contact. I sort of dismissed his advice. Then I had my small accident. I have now started to use my Vicmarc VM120 with the big jaws for most of my work. I'm not quite there, I have to do about one turn of the key. I can really hog out now while turning, and the bowl does not move at all, with the small jaws, even with the perfect tenon, which I do, no bragging, just a statement, the bowl can move a bit, and I had to re-sit it in the tenon. I just ordered a Vicmarc VM150. Yesterday I did an IRD to Phil's club in England, he recommended me because he enjoyed my demo for WW. That was a very high compliment for me. Also in attendance were professional turners Andrew Hall and fellow Woodturning Magazine writer Pat Caroll.
On the other side of the spectrum, we have Stuart Batty. he spent a week with me here. Turning every day. His tenons are tiny. Stu's tenon gave me that warning in the stomach, I always moved out of the line of fire, LOL. After my 9 stitches in my arm, I will be making what my gut instinct tells me, big nice perfect tenon for the large jaws on the 120, and soon on the 150.
I used to ask during demos, who said the tenon has to be 1/3 of the size of the bowl? Now I can say, I do, or even bigger!

 

[odie]

Phil's advice about large tenons, so that the jaws fit with minimum spaces between the jaw gaps is good from my POV.......but, I question whether "completely" closed jaws are advisable. Here's my take: If you close the jaws completely, it takes away from the sense of feel for the grip you're getting. IMHO, a small gap between the jaws is preferable to completely closed.

This, in combination with 3hp creates another problem. Power to the spindle makes one tend to take bigger bites. Bigger bites will increase the disruptive force applied between chuck jaws, and the tenon.

@Emiliano Achaval .......Getting the Vicmarc VM150 is a good move for you, or anyone who turns very large and heavy bowls. Since my Woodfast swing is 16", for my purposes, the Stronghold chuck with #3 jaws will handle the biggest and heaviest blocks of wood that are within my lathe's capabilities.

-----odie-----

 

[William Rogers]

I agree with Oldie about leaving a small gap. Just yesterday I had a small spindle (making Christmas ornaments) about 3” in diameter and turned the tenon where the jaws closed. It held just fine turning the outside, but when I went to hollow it came out of the chuck. I am a believer in large jaws however. For my Axminister I have a set of gripper jaws that can accommodate a 1” long tenon. This is as big as I will ever need.

 

[Dave Landers]

@Emiliano Achaval , you mentioned this when you demoed for our club a couple weeks ago. I mostly dismissed it, too - good advice for newbies, but not for the likes of me!

Sure enough, that weekend I was trying my hand at a calabash - a decent size bowl (probably 10 - 12"). I made a perfect tenon for my VM120 standard jaws (about 2.5") - probably because that's what was on the chuck. When I got to hogging out the inside, the tenon sheared off. Tenon itself held well, and was in good, solid wood - but there just wasn't enough wood there to keep the tenon attached to the rest of the bowl.

Nothing flew across the shop, no bruises, no blood, no stitches. And I had enough wood to remount and turn a 5" tenon.

Hopefully I will remember that it's not just about how perfect my tenons are. Need enough wood to handle the stresses.

 

[Paul M. Kaplowitz]

The Tenon should be 40 Percent of the diameter of the bowl. Make sure the face of the jaws are in contact with the bottom of the bowl.

 

[Ron Solfest]

While I don’t have the expertise of Emiliano, I also question the advice of fully closing the jaws so their are no gaps. As Odie says I believe it’s better to have a small space.

I grew up with the advice to tighten the jaws using at least all of the key holes. At first I thought this had something to do with internal binding on the chucks. I’ve come to realize what it really does is allow the wood fibers to respond/relax from the initial tightening, then they can absorb a little more tightening. Often I do a ‘last’ tightening after a few seconds. If you get the jaws to fully closed position, it’s pretty hard to know that coincided with exactly the point where the wood was fully compressed.

All of my comments apply to the Oneway serrated jaws that I use, I suspect smooth dovetail jaws have less of this compression effect, but I still would expect some.

My two cents

 

[Ricc Havens]

I guess I can see that the perfect size for dry bowls would be a good idea. But, when turning twice turned bowls shouldn't you leave the tenon a little larger. Every twice turned bowl I have done the tenon has gone slightly oval just like the bowl itself. When dry I have enough meat on the tenon to re-true the tenon. If I made it exact the first time when the bowl is wet then I wouldn't have enough meat on the tenon to re-true it for the perfect fit once the bowl is dry.

 

[Kevin Jenness]

It seems that the "perfect fit" between a dovetail tenon and the chuck jaws depends on how the jaws were manufactured. As I understand it they are turned in one piece and then cut into 4, so would form a perfect circle when spaced apart the distance of the tool used to separate them.

Ricc makes a good point in the case of twice turned bowls.

I have Oneway chucks and usually use the profiled jaws as they seem to work well throughout their range of adjustability and give me more size options on the tenon. I know many people prefer dovetail jaws and won't argue the merits, but the profile jaws have worked for me.

In any case Emiliano's initial post is a good reminder to use an adequately sized tenon. I have watched a couple of Stewart Batty demos where he turns pretty good sized bowls at high speed with what looks like rather small tenons, and wondered if that was really a good idea. I decided it probably was not- for me. It obviously works for him. On the other side of the aisle, Lyle Jamison eschews scroll chucks and uses faceplates exclusively to achieve what he feels is a safe hold of the work.

 

[hockenbery]

David Ellsworth is also a big advocate of using large diameter tenons.

Like everything else it is a trade-off
big tenon more strength small tenon more cutting room to the base.
#3 and #4 jaws give that extra strength.
Often a #2 tenon is strong enough depends on the wood and the turning style.

On NE crotch bowls I will #2 tenon up to 15” diameter. I also do a good bit of the rough hollowing with the tailstock in place. It is easier for me to follow the curve to bottom center inside and measure the wall thickness right up to the tenon with a #2. The larger jaws put me an inch more away from the center.

I almost always use faceplates for hollow forms.

 

[robo hippy]

I use a recess exclusively on my bowls. With the Vic 120, the jaws are 2 5/8 inch diameter when closed. That is plenty for bowls up to 14 inches, and getting a bit on the lean side for 16 inch bowls. I use scrapers for all of my roughing. I would never expand my Vic 100 into a recess that size for a bowl over about 6 inch diameter. You would have only 4 points of contact. With a tenon, if I cut max diameter that I could hold in that chuck, I would have 8 points of contact, which I would consider to be better than 4 points.

The depth of a recess or tenon comes into play here as well. With a big platter, you can get away with a 1/16 inch deep recess. To do it safely, you need sharp edges on your dove tail jaws, and a pretty much perfect fit for angles. I generally go about 3/16 deep, but just eyeball it. I never go deeper than 1/8 inch as it is a waste of wood and there is no benefit from going deeper.

A lot of how secure your grip is depends on the wood. Very hard woods like to break off more than softer woods. This is similar to what happens if you drop your bowl on a hard floor. Softer woods tend to bounce, harder woods tend to crack. I consider cherry or madrone to be medium hard woods. They are fine for the bounce test.

With the little experience I have had with tenons, I have found the 40% rule to be very safe for heavy roughing with scrapers. I can hog off as fast as the lathe will let me go. I haven't met a lathe I can't stall yet. I can go down to about 30%, but have to be way more careful on how I turn and rough. One big catch on a 30% tenon, and it will probably go flying. With light steady cuts, you can get away with it.

Emeliano, that piece of wood that came off your lathe would not have been helped by a bigger tenon. That failure was from a wood defect.

robo hippy

 

[Kevin Jenness]

Reed, I find it interesting that you are comfortable with such a small recess relative to a tenon. 2 3/4" (likely close to a circle on your VM 120 jaws when slightly opened) is less than 20% of a 14" bowl's diameter yet you like a tenon at least one and one half times that size. How big of a margin around your recess do you typically allow before starting the upward curve on your bowls?

 

[Don Wattenhofer]

The dovetail jaws on my OneWay strong hold chuck are at a perfect circle when the wood worm screw is clamped in place so that is the diameter that I measure and use that for the tenon diameter except when rough turning then you must go larger so that there is enough material to true up for finish turning. The gap between jaws is about 1/8" at perfect circle point, I have never run out of travel for a properly sized tenon and I can't understand why anyone would would want to close them all the way.

 

[Glenn Lefley]

You would never want your jaws to close all the way. Maybe in an absolutely perfect situation when you are tightening the jaws an d at the precise moment you can’t tighten any more and at that same precise moment your jaws fully close you would want that. But in reality that’s not happening. If your jaws fully close 9 times outta 10 your tenon is too small. With a very small gap and tight you are at the perfect clamp!

 

[Emiliano Achaval]

You would never want your jaws to close all the way. Maybe in an absolutely perfect situation when you are tightening the jaws an d at the precise moment you can’t tighten any more and at that same precise moment your jaws fully close you would want that. But in reality that’s not happening. If your jaws fully close 9 times outta 10 your tenon is too small. With a very small gap and tight you are at the perfect clamp!

9 out 10 times would be too small? I do own calipers and know how to use them, LOL.

 

[Dean Center]

As Al briefly mentioned, the larger jaw sets reduce the amount of room at the bottom of the blank, and they also alter the visual appearance while you're working. I suspect the visual part would get better in time, but the final depth of the bowl probably ends up being a little less, as a result of necessary changes in turning technique.

 

[Emiliano Achaval]

I have invited Phil to join us and give us his opinion. I did not expect so many answers, LOL

 

[Emiliano Achaval]

As Al briefly mentioned, the larger jaw sets reduce the amount of room at the bottom of the blank, and they also alter the visual appearance while you're working. I suspect the visual part would get better in time, but the final depth of the bowl probably ends up being a little less, as a result of necessary changes in turning technique.

That was my excuse not to use large jaws on mid-size calabashes. Not true whatsoever, I can do as good of a bottom curve with large or mid-size jaws. I did not change my technique, not sure whatever that means...

 

[Carl Burn]

Hello Emiliano, enjoyed your demo to our club on Monday evening. Just to follow up on this thread you can see Phil discussing tenon sizes and chucking as part of a video he made for Woodcut tools on their coring system, it is available on YT here:

View: https://www.youtube.com/watch?v=lJesIjGrwKc

The discussion on chucking starts around about the 5:18 point.

 

[John Torchick]

My chuck is a smaller than what most of you turners have. My rule of thumb (still have both of them!) is to leave a gap between the jaws. My feeling is if there is no gap, there might be just a smidgen of space between the jaws and the tenon. The result is flying wood!

 

[Glenn Lefley]

9 out 10 times would be too small? I do own calipers and know how to use them, LOL.

I understand what your saying, but a lot of people could take this to heart. Then turn softer wood that compresses easily. Make a perfect tenon with there calipers, chuck it up, then take it off rechuck a few times and now it’s to small as tenon compressed a 1/1000 of an inch.

ive did this before, had a perfectly tight tenon and closed chuck. Worked great. Then when I rechecked it started slipping. A lot of amateurs on the site that look up to better turners and take everything as the god given word. I’m sure your method works perfect for once chucked harder woods. But do you find it works for a majority of woods that can compress, and are rechecked numerous times, or are you talking about once turned items.

 

[robo hippy]

Kevin, I do discuss all this in my video about mounting things on the lathe. For the bigger bowls, I generally have at least an inch of wood shoulder outside of the recess. On the smaller 6 inch bowls, at least 1/4 inch when using the Vic 120, and a little more when using the Vic 100. I have done a lot of trial and error. I seldom turn try wood into bowls, I just like the warped shapes... With dry wood, I can use a more shallow recess. With wet wood, I go a little deeper. My big chucks all have nicks in them from when I was learning to use my McNaughton....

robo hippy

 

[Emiliano Achaval]

I understand what your saying, but a lot of people could take this to heart. Then turn softer wood that compresses easily. Make a perfect tenon with there calipers, chuck it up, then take it off rechuck a few times and now it’s to small as tenon compressed a 1/1000 of an inch.

ive did this before, had a perfectly tight tenon and closed chuck. Worked great. Then when I rechecked it started slipping. A lot of amateurs on the site that look up to better turners and take everything as the god given word. I’m sure your method works perfect for once chucked harder woods. But do you find it works for a majority of woods that can compress, and are rechecked numerous times, or are you talking about once turned items.

Nn

Hello Emiliano, enjoyed your demo to our club on Monday evening. Just to follow up on this thread you can see Phil discussing tenon sizes and chucking as part of a video he made for Woodcut tools on their coring system, it is available on YT here:

View: https://www.youtube.com/watch?v=lJesIjGrwKc

The discussion on chucking starts around about the 5:18 point.

Thank you Carl. I’m sure Phil can explain it better than me.

 

[Emiliano Achaval]

Phil's advice about large tenons, so that the jaws fit with minimum spaces between the jaw gaps is good from my POV.......but, I question whether "completely" closed jaws are advisable. Here's my take: If you close the jaws completely, it takes away from the sense of feel for the grip you're getting. IMHO, a small gap between the jaws is preferable to completely closed.

This, in combination with 3hp creates another problem. Power to the spindle makes one tend to take bigger bites. Bigger bites will increase the disruptive force applied between chuck jaws, and the tenon.

@Emiliano Achaval .......Getting the Vicmarc VM150 is a good move for you, or anyone who turns very large and heavy bowls. Since my Woodfast swing is 16", for my purposes, the Stronghold chuck with #3 jaws will handle the biggest and heaviest blocks of wood that are within my lathe's capabilities.

-----odie-----

Odie, Phil told me it was my pidgin English that caused some confusion here with this post. Can you watch Phil's video, posted below. I would like to hear your comments after that.

 

[odie]

Odie, Phil told me it was my pidgin English that caused some confusion here with this post. Can you watch Phil's video, posted below. I would like to hear your comments after that.

Hi Emiliano......

I don't see a link to a video in your post here, Are you speaking of the video posted in post #18?

-----odie-----

 

[John Torchick]

I have a woodturning book by Phil Irons that I got at a used book store.

 

[Dennis Weiner]

We tend to oversimplify in order to create general guidelines. Specifying a percentage of the diameter alone as a guideline for tenon size may be misleading in certain situations. It’s not the whole picture. Rhetorically: Would a 1/4” deep tenon hold better than 1/2 tenon? Would your 40% tenon sizing choice apply to a 10 diameter by 4” deep blank vs a 10 inch x10 deep blank? If I were turning a bigger blank say 20 inches in diameter would I want a 1/4 tenon or a 1/2 deep tenon or perhaps a faceplate? Yes, I deliberately didn’t specify whether the blank was 2” inches deep or 20 inches deep. I would suspect that you would do what I do and use your experience and common sense(your gut) in choosing a holding methodology.

99.9% of the time our internal “ gut meter” works unless there is a rare catastrophic event like a catch which causes a piece go go airborne. That’s what we want to avoid at all costs and that is what preaching safety should be all about. Prevention against the rare stress event comes with Knowing the physical limits of our jaws and chucks. No manufacturer publishes this although the forces from our gouges applied to the chuck jaws do follow the laws of physics. It really isn’t about your gut. It would be nice if stress studies were done and tables were published. I know there are so many variables to consider. Perhaps, this Could this be an AAW project?

 

[John Torchick]

Good points, Dennis. So many variables to consider when turning a piece, whether a platter or bowl.
Saw the NYC location. Lived in Queens and spent three weeks in training in the North Tower of the WTC in 1998. Miss all the ethnic foods!

 

[David Shumaker]

I agree with a tenon sized appropriately to the piece being turned. I also agree that the jaws in theory have maximum contact and grip in the nearly closed position. That being said, I like to leave a little gap to allow for "snugging" up while turning green wood. That slightly oversized tenon also allows me to true up the tenon before the second turning after drying.

 

[robo hippy]

Probably no actual benefit on a 20 inch bowl with using a 1/4 or 1/2 inch tenon. Diameter is far more important. Probably little difference if you go down to 1/8 inch, but in that range, it depends more on the wood. If the tenon bottoms out in your chuck, as in sitting on the bottom of the jaws, you can lose grip power. Mostly because you want the jaws to sit flush on the bowl blank, on a shoulder, and if you try to bottom out, you end up either a bit shallow, which isn't a bad thing, or you end up too deep and then the bowl is not sitting on the face of the chuck jaws, so you lose strength in your grip.

robo hippy

 

[Emiliano Achaval]

Hi Emiliano......

I don't see a link to a video in your post here, Are you speaking of the video posted in post #18?

-----odie-----

The video is on post #18. Posted by Carl Burn.

 

[odie]

The video is on post #18. Posted by Carl Burn.

Odie, Phil told me it was my pidgin English that caused some confusion here with this post. Can you watch Phil's video, posted below. I would like to hear your comments after that.

Emiliano......

From what I could tell, Phil's chuck didn't close all the way, so that the jaws touched each other. That's in line with what I and a few other people who posted here felt was best. IMHO, this doesn't mean more space between jaws won't work......it does. Only that theoretically, almost closed would be a better overall grip.

Was there anything else of interest, you particularly wanted me to comment on?

I am one who has gotten used to straight walled tenons, because I'm using Oneway serrated jaws. I don't claim the dovetailed walls of a tenon is less advantageous, as long as the jaw position is marked so as to re-chuck with the jaws in the exact same position.

-----odie-----

 

[Emiliano Achaval]

Emiliano......

From what I could tell, Phil's chuck didn't close all the way, so that the jaws touched each other. That's in line with what I and a few other people who posted here felt was best. IMHO, this doesn't mean more space between jaws won't work......it does. Only that theoretically, almost closed would be a better overall grip.

Was there anything else of interest, you particularly wanted me to comment on?

I am one who has gotten used to straight walled tenons, because I'm using Oneway serrated jaws. I don't claim the dovetailed walls of a tenon is less advantageous, as long as the jaw position is marked so as to re-chuck with the jaws in the exact same position.

-----odie-----

Thank you Odie. I guess I could have worded it or explain it better. But, to me, leaving the jaws just 1/8 in I would call that closed jaws. I did not bother to answer the posts where people where saying if you rough turn you need to reshape the tenon so you need a bigger one. That is so obvious to me that I did not think I need it to expand on that. I have a Oneway with serrated jaws, I do not like it at all, I let students use it, LOL I ordered another set of calipers. Now I will have a dedicated set so I don't have to measure it every time since I also use other jaws... I should tell Phil that for the engineers out there he needs to give exact measurements, to the decimal point how much to close it.
He has now moved to the mainland, but we had an engineer at the club, he would bring calipers to the meeting, LOL Old habits die hard.