Chuck jaw size and blank height

[Alan Weinberg]

There are conversations about tenon or mortise, about sizing the spigot or recess between 33 to 40% of the blank diameter, but I haven’t seen a conversation about bowl depth.

What occasions this question is that I just bought a Nova chuck and was looking at accessory jaws. For most of them, even fairly large jaws, they recommend not turning anything beyond 6 inches or so in height and not using square blanks. There are other warnings in bold such as not turning above 684 rpm.

For example, the 100 mm jaws have in capital letters: THIS METHOD MUST NOT BE USED FOR ANY WORK LONGER THAN 127 MM AS THERE WOULD BE GREAT DANGER OF WOOD TEARING OUT OF CHUCK. That’s only 5 inches for pretty big jaws.

None of their chucks are “rated” for significant height blanks or rpm.

Now I know tailstock support helps but their blank size, max rpm, and avoiding square blanks sure limit turning.

I suspect the lawyers and not the turners wrote the manual but was wondering what thoughts others more experienced than I had.

This is my first new chuck and my used Oneways had no instruction booklet, so I’ve naively turned bowls up to 14 inches in height and started sometimes with square blanks.

 

[Wally Crawford]

I use Nova chucks and have an idea (purely my own thoughts) that this is legal boiler plate brought about in the interest of caution for Nova, not you. I have turned bowls of 20" with my Nova G3 chuck without a problem. I do use the tailstock as long as I can to help support the bowl. Be careful with your tenons, both shape and depth.

I generally use a face plate for the roughing work, again with tailstock support and to turn the tenon, but once I have a good tenon in good, solid wood I have had no issues. Unrelated to the topic but in the interest of safety, wear a facemask or better yet a PAPR helmet.

 

[Sam Fleisher]

I'm still a beginner but I've learned that I don't really need a very deep bowl. I've had a couple come off my chuck because I was trying to go too deep for my tools and experience. Down the road I may try some hollow forms and when I first started I thought that one of the hollowing systems would be great to turn a deeper bowl but I've since learned if you go too deep then it's probably out of proportion to the diameter and you might as well make a hollow form. Here's one example that was on the floor twice when I was making it.

I don't have a band saw so some of the stuff I've stuck on my lathe probably shouldn't have been. Turned a lot of air on some logs. Now I try to get them as balanced as possible using a faceplate and tailstock before making the tenon. And I have learned the truer your tenon is the better it will turn. I don't use a mortise much because I like to be able to remove a tenon and be at the correct thickness for the bottom. With a mortise the way I do them so far it makes a thicker bottom on the finished bowl around the mortise. I could fix that but I'm still working on my technique to get good cuts so I dont have to sand as much.

 

[William Rogers]

I think Wally is right. I have read Nova’s instruction sheets and if that was the case they wouldn’t sell many chucks if that restriction was realistic. I don’t remember reading any other makers of chuck jaws that state restrictions as Nova. I use Axminster chucks and their description is usually large or small turnings, no specific size. In the end good sense likely dominates. Using a faceplate or worm screw on large turnings and keeping the tail stock in use as long as practical is the best approach IMO. Once the majority of the mass is remove I have never worried about loosing the piece from the chuck. Hidden cracks are a bigger concern to me and of course “Stay out of the line of Fire”.

 

[Richard Coers]

It's not up the sellers of turning tools to teach us how to turn. You start simple and then work your way up. You get a feel of what is working very well, then pretty soon you learn what exceeding the limit really is. If the sellers said you can go up to 127mm, they know darned well someone is going to double that because they think they can. They had to draw the line somewhere.

 

[Alan Weinberg]

It was my impression as I noted in the original post that the size restriction was legalese. But I find it interesting that there’s a lot of talk about mortise/tenon size with bowl width and none on bowl height. Seems to me it is also weight related—the mortise or tenon doesn’t know the width of what it’s holding—or the weight—but both can pull the bowl free.

 

[Mike Peace]

I have been using various Teknatool chucks for years and they have more definitive guidance than any other chuck manufacturer I am aware of. My experience is that that their guidlines appear reasonable. But I am sure a skilled turner could push them all further. I would keep in mind that a bowl blank taller than 6" is likely to be quite an instrument of destruction if it launches. A lot has to do with how well it is mounted, that is, is the tenon or recess properly done to match the jaws.

 

[Alan Weinberg]

I have been using various Teknatool chucks

Love your videos—I’m a follower. Thanks.

 

[Bill Blasic]

The jaws are just one part of the equation. Any jaw set is capable of putting just about any hunk of wood into them. With the 2" jaws (regardless of brand as long as they are dovetailed) I seldom have my tenon bigger than an 1/8" or less. But with that in mind I don't try to take 1" of cut. You can always take too big a cut or get a dreaded catch. Along with those factors don't forget speed, just as important as any of the other factors. I'm sure Nova is just erring on the side of caution or whomever it was that wrote the warnings was just a real bad turner.

 

[Doug Freeman]

Yes the “size statements” by Nova are lawyer boiler plate, and since no other chuck mfr makes such statements, I wonder if NZ requires such instructions?

Square blanks - all my turnings start with a chainsawed blank, and typically have very irregular shape. I start all between centers, using a Nova Jumbo drive center on ~6” and larger. The smaller drive centers cant take as much force before starting to “drill” in. If I do a large blank too big to fit (>16”), I use a faceplate since I cant make a tenon or mortise. An option here is to use a router template to form a mortise - a face plate is easier.

As work dia and length go up, the tenon needs to get bigger or longer. Unfortunately I dont know of an equation, simply the rule of thumb for dia you stated. For bowls its easier and better (wall support) to go big vs long, and less blank length goes to the tenon. For hollow forms, it’s a balance of growing both dia and length, and its more experience - I havent found a rule of thumb. I use Oneway tower jaws that take up to 1-1/4” long tenon for HF long tenons, and they work fine for bowls as well, proving more tool access around the bottom (1/4” long tenon in the dovetail at the top of the jaws.

Something to keep in mind with larger pieces - start with a large tenon, as the cutting gets closer to the chuck ( dia and length) the piece can be put back between centers to turn a smaller tenon. I do this because I sand and buff most all my work on the lathe. It doesnt take much of a tenon to hold a big piece for sanding/buffing. Its necessary if I want a small dia bottom of the piece. HF can just be cut down to get a narrow bottom and leave a largish tenon that gets cut off at the end.

 

[Lou Jacobs]

Definitely, as others have said, lawyer speak. I can remember when I began turning I’d regularly get beaten up by blanks, usually because I was trying to round them from the end grain rather than side grain, and also because I had a low end Grizzly lathe that couldn’t turn slower than about 500 rpm. Broke several (many) tenons and had many catches. As I developed skills and had some instruction, learned to be much easier on the wood and myself. Sizes of pieces grew as my sense of confidence and safety expanded. While they may have been prudent for a beginning turner, Nova’s stated limitations don’t make sense to me any more. You develop a sense of when you’re pushing boundaries.

 

[robo hippy]

Well, the farther off the headstock you are, the more vibration you will get. Using the tailstock will remove most of the vibration issues. If you are turning a deep bowl, then when you remove the tailstock, the vibration will increase. This is a huge problem when you get a catch, and you can tear just about anything off of the lathe. I have done that a number of times, but did learn from my mistakes. Do ALWAYS stand out of the line of fire.

robo hippy