Turning hollow forms

[Nick Fournier]

I've tried to turn a few hollow forms and taller forms, but I keep running into problems with the form popping out of the chuck.

I have a Nova G3 chuck with #2 jaws. I'm starting to wonder whether the jaws just aren't large enough for the forms I'm turning. The forms tend to be about 5" in diameter and 4'5" tall. I don't have similar problems with bowls that are larger in diameter -- but only about 3" deep.

Any suggestions? Should I get larger jaws or is it a problem with the way I cut my tenon. I'll try and post pictures of the last tenon later tonight -- I'm off to a wedding now.

Thanks.

[John Keeton]

Nick, I have used a G3 with 50mm jaws for every turning I have done without problem. I do not cut a dovetail tenon, but use a square tenon that is 3/16" - 1/4" deep. Most folks talk about cutting the tenon with a dovetail, but the Teknatool chuck manual is pretty clear on this - "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."

[David E Keller]

Nick, I have used a G3 with 50mm jaws for every turning I have done without problem. I do not cut a dovetail tenon, but use a square tenon that is 3/16" - 1/4" deep. Most folks talk about cutting the tenon with a dovetail, but the Teknatool chuck manual is pretty clear on this - "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've not used that particular chuck, but I differ with you on the interpretation there... To me, a dovetail and a recess for the lip are two different things. It's working for you, but I believe that the dovetail jaws are intended to work with a dovetail tenon... Just don't cut an extra recess for the little lip. Again, I haven't used the chuck in question, so I'm not sure you haven't just wasted you time reading what I typed.

[Jack Mincey]

Some of my students have trouble like you are having, even when I cut the tenon for them. Presenting the tool to the wood is where they have their problem. If one gets a catch from not presenting the tool to the wood correctly it can cause the object to be jerked out of the chuck no matter how the tenon is cut. I can only tell you what I tell my students and that is to pay close attention to what you are doing and if something bad like a catch happens try and not repeat the same tool presentation again since the same thing will most likely occur again. Try different tools as well, since not everyone starts out hollowing with a gouge. Some of my students do wonderful work with nothing but a round nose scraper. I turn vases up to 12" deep on a regular basis with the #2 jaws that came on my strong hold chuck. The only time I've ever had trouble is with one redwood blank I got that had a flaw in the wood that caused the tenon to break off. This was taken care of with the use of a home made steady rest I have.
Good Luck,
Jack

[Wally Dickerman]

If your tenons on bowls work for you then the problem is probably elsewhere. There are many ways to knock the piece off the chuck when hollowing. Just about all have to do with what you are doing with the tool. One of the most common bowl busters in hollowing is allowing the tool to press against the rim. When you get a catch, the tool just levers the piece enough to break the rim if it's thin, or otherwise levers the piece off the chuck. I've also seen students do that when turning a deep open bowl.

[John Keeton]

I've not used that particular chuck, but I differ with you on the interpretation there... To me, a dovetail and a recess for the lip are two different things. It's working for you, but I believe that the dovetail jaws are intended to work with a dovetail tenon... Just don't cut an extra recess for the little lip. Again, I haven't used the chuck in question, so I'm not sure you haven't just wasted you time reading what I typed.

David, actually, the manual has two separate sections on the "recess" method, and the spigot method. The quote is from the spigot method. Here is the entire paragraph - note that they recommend a 5/8" tenon/spigot, but I do not make mine that long.

FORMING SPIGOT:


When selecting wood make sure it is sound without splits or weakness -

especially around the area where the spigot is to be formed. REMEMBER WITH FREE END

TURNING THIS IS THE ONLY AREA OF GRIP. IF ANY WEAKNESS IS FOUND, DO NOT PROCEED.

Mount wood between centres and turn the spigot area. Make the spigot as parallel as possible to

maximise the efficiency of the clamping action. Only approximate sizing of the spigot is necessary

as the jaws will accommodate a wide range of spigot diameters within the spigot limits stated above.

The length of the spigot area for the standard jaw must not be less than 16mm (5/8 inch). The 50mm

standard jaw has a thin lip or shoulder at the front face. This 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.

[Steve Schlumpf]

Nick - I have to agree with the advice so far - that your problem probably has more to do with tool presentation than a tenon or recess problem. What tool are you using for hollowing? Are you cutting at center-line or slightly above? Where on the form and in what direction are you moving the tool when the catch happens? Lots of questions because there can be lots of possible causes.

Hope the wedding went well! Yours?

[Harry Robinette]

Nick
I've been using nova chucks for about 11 years and the trouble your having I believe is from both the tenon and your tool control.
John is right the Nova has a DOVETAIL ON THE RECESS the outside of the jaws.BUT the grip onto a tenon is straight and the tenon must never touch the bottom of the chuck.
The jaws should only make contact with the tenon and the very top of the jaws were the piece sets on the jaws.
Your cutting is probably what Wally said you have to make the initial contact easy and in the correct direction.
As always this is JMHO.

[David E Keller]

I didn't realize the inside surface of the jaws was a straight surface with the outside being for a dovetail... In other words, straight spigot but dovetailed recess.

If not already done, please ignore my previous post. You can ignore this one too if you'd like.

[Nick Fournier]

Thanks for the advice.

I guess I'll go and re-cut this tenon (it was slightly damaged the last time it popped out of the chuck) and try again. The problems tend to happen when I try and face off the top surface. Typically I use an EasyWoodTools style tool, so it is possible I'm just slightly above center which could easily cause a catch.

Steve, it wasn't my wedding, but it was a friend from high school which is always kind of fun.

[John Keeton]

Nick, I would think your chances of a catch are greater below center than above it, but I have not used the carbide tools - they may react differently.

[Reed Gray]

Nick, on top, you said your form is 4'5" tall. Does that mean 4 to 5 inches tall, or 4 ft. 5 inches tall? I would expect the smaller measure. I would expect a 3 inch tenon should be plenty of support for your hollow form.

Are you having catches, and that is what is ripping the forms loose from your chuck? Tool technique issue.

Are the pieces slowly working themselves loose and coming off? Tightening issue.

Most hollow form cutters are 1/8 to 1/4 max wide cutting surfaces. I think most of this is because a lot of the time, people are reaching pretty far off the tool rest, and you don't want a big cutter way out there. With scrapers, and that is what almost all hollow form cutters are, you always want to be slightly above center on the inside of a form. If you have a nub/cone in the center, the back side of your tool can catch on it, be pulled down and under the nub, then slammed back onto the tool rest. Getting the nub out is an art, and very touchy/feely type of thing where you gently brush from side to side, as well as up and down. Don't let a big nub build up.

One thing Stewart Batty pointed out to me, during a workshop I took with him, was that a too long tenon, even if you don't bottom out in your jaws, makes the tenon weaker. It is a leverage thing, where most of the pressure is where the tenon meets the bowl bottom. He had me round over the bottom of some that I made that were maybe 3/8 inch long. I would thing also, that if the tenon tapers out at all towards the bottom, then all of the pressure would be there, rather than at the bowl bottom. Again, it makes a longer lever. He and Teknatool disagree on that point. For me, it is a waste of wood to make it more than 1/4 inch deep. I do believe that the tenon shape should match the shape of the chuck jaws as closely as possible. Straight/square sided jaws, then straight/square sided tenon. The shoulder that they jaws rest on should be square to the tenon sides.

As small as the 'dove tail lip' is on the Nova chuck, I would doubt that cutting a recess for it would make any difference in gripping power, but if you do make one, make it as minimal as possible. Dove tailed jaws, like on the Vicmark chucks, and I think Oneway makes them now as well, will give a better grip than straight sided jaws. You have a locking wedged dove tailed joint, which has been used in flat work forever, which gives a mechanical advantage that you don't have on straight sided jaws.

robo hippy

[Chip Sutherland]

Lots of good reading in this thread. I think the problem is both technical/mechanical and technique. At this point, I would proffer is tapping into the local woodturning club members in the Austin area. You've got a great network there to tap into for some hands-on advice.

[robert baccus]

Many turners have good luck with tenons. i don't. i purchased the long nova jaws and they are a great improvement. i have the best luck with a glueblock screwed to a faceplate. i recently mounted a 110 # sweet gum blank 12"x17" on a 6" glue block and turned and hollowed it completely. finished it on the block. long pieces which are frequently hollowed as vases exert a tremendous strain on the mounting compared to a bowl. of course a dry blank could be just screwed to the faceplate. i sometimes use both when i am chicken-----------old forester