Chucks, jaws, and tenons

Hello,

I recently upgraded from a midi lathe with a 12" swing to a full size lathe with an 18" swing. I am thinking about the chucks and jawsets I will need to turn some bigger bowls. The rule of thumb for tenon size that I am familiar with is to make the tenon 30-40% of the diameter of the bowl. To simplify the math, I am just going to say that a tenon of about 1/3rd (33.33%) the diameter of the bowl diameter is appropriate. So if you are turning a 15" diameter bowl, you would want to have a tenon with a diameter of about 5".

So taking a look at the Vicmarc chucks and and jaws, the VM120 (one of their big chucks), with 5" (128mm) jaws, its specs say it can grip a tenon of 4-1/8" to 5-7/8". Converting the fractions to decimals its 4.125" to 5.875". Multiplying that range by 3 (the reciprocal of 1/3rd) you get: 12.375" to 17.626". So this chuck with those jaws can "handle" bowl blanks with a diameter in that range.

Now that sounds "okay" since, theoretically, I can safely chuck up a bowl blank that at 17.626" is very close to the maximum swing of my lathe (18"). But woodturners also know that the more you open up the chuck, the less gripping power it has -- meaning that the chuck grips best when it is mostly closed. So, using one of the bigger chucks with one of the bigger jawsets, the only way I can grip a bowl blank that is at the upper end of my lathe's swing is if I am using the chuck at its limits. And given that a 17+" bowl blank is fairly big, pushing the chuck to its limit with a big piece of wood seems unsafe.

So I could go with an even bigger jawset, for example the Vicmarc 148mm jaws have a range of 5" to 6 1/8", so that would allow me to do 15" diameter bowls safely, but again, as I get close to 18" I'm still at the limits of the chuck. So I could go bigger again, with the 173mm jaws, which go from 6" to 7 1/8". Meaning that its tightest (most closed) compression "setting" would cover an 18" diameter bowl.

An 18" swing lathe is not huge. There are plenty of 20 and 24" swing lathes out there (and beyond). So I guess I'm surprised that to mount a blank that is at the upper end of my lathe's swing I need a really big set of jaws. Am I missing something? Is the 30-40% rule super conservative?

To make the point another way, if you look at Nova's literature, they say their Super Nova chucks are suitable for lathes with a 20" swing. But the biggest Nova jawset the Super Nova chuck can handle is Nova's 130mm jaws. Those only go from 4.2" to 5.04", meaning that, multiplying by 3 you could only do a 15" diameter bowl blank with the Super Nova. So how is the Super Nova chuck suitable for lathes with a 20" swing when it can't handle anything over a 15" swing based on tenon size?

Thanks for any insights.

--dan

There are lots of opinions on this but I'd say as a guess that most turners use smaller tenons. What is required is an examination of the wood quality. My tenons are closer to 20% than 40% using mostly Oneway profiled jaws - dovetail jaws, nearly closed when tight, are seen as a better grip but I've never had anything come out of my profiled jaws. Tenon shearing can be a factor and this is more often in very straight-grained woods.
On the rare times I use my coring system I get out the #4 jaws.

All of it is a best guess scenario. Wood, being a natural product varies widely, so there can be no definitive sizes or ratio when it comes to mounting. A 3" tenon might hold a 15" bowl, a 6" tenon can break off on a 10" bowl if the wood has an inclusion or weak area.

All the ratios, "rules" and proportions are just rough guidelines.

Thanks for the replies. Bill that makes sense. With my smaller lathe my tenons are closer to 20% too. I figured it was okay with smaller pieces of wood. And it seemed better to use a smaller tenon than to have the chuck opened too wide. I didn’t worry about it too much because the wood blanks were generally smaller so less risky.

I'm sure you know about keeping the tailstock in place as long as possible for extra safety. Once the bulk of the bowl interior is cut away it is a lot lighter with perhaps just a little bit more of a torqueing due to cutting from the outside of a larger diameter.

The VM120 with the standard and a 4" jaw set is a good choice. I've turned quite a few 14"+ bowls on the standard 2" jaws. With solid wood and a non-aggressive cut, it's plenty safe. The issue is that the piece will flex more with a smaller tenon, especially as it gets thin. This makes it hard to get a clean cut as it gets thinner.

I would look at the shark jaws. Hard to beat a Vicmarc chuck.

I also have a Nova Titan with 5" deep jaws. This is also a nice chuck.

I have a couple of the Vicmark 120s for my bowls. I use the standard jaws for bowls up to 14 inches, in a recess. Some will argue that tenon is better than a recess, but to me, as long as they are made correctly, no problems. I do prefer the dove tail jaws to the straight jaws. Take a look at my 'Mounting things on the lathe' video. I agree with a tenon only needing to be in the 1/4 the diameter range. I do use tenons on spindles.

robo hippy

For my biggest bowls (20-22") that I've ever done, I use the Nova Titan chuck with the deep 5" jaws. I wish I had a little bigger so that I could make the tenon my permanent base as a 5" base looks to small for a bowl that size, but using the 5" has been fine for turning and then I just create a base around the tenon and remove the tenon at the end. As others have said, using tailstock as long as possible and somewhat careful cutting has never resulted in a problem for me, though I've only turned maybe 5-6 bowls in the 19-22" range (but currently have six 23-24" blanks of cottonwood ready to be turned so I'll let you know after the holidays if anything different occurs).

Tom

I think you may be being too generous with your sizing. My lathe will swing up to 25”. I typically harvest woods that yield blanks up to 23 - 24”. I core nearly all the bigger blanks I rough with the Macnaughton. Everything over 10 or 12” is roughed and cored using 4” smooth dovetail jaws. Tenons can range from 4-1/8” to 4-3/4”. With a well shaped tenon, a step that matches up with the jaw faces, the hold is very secure. The only time I can recall that a tenon failed was when roughing some wood that had either a crack or ring shake that wasn’t obvious. A 4” tenon is 20% of a 20” blank. I find that more than adequate.

I tend to use much smaller tenons than that, as does most every turner I've ever had occasion to watch work. Clearly the wood species and condition is a factor, but in sound wood I'be turned plenty of 15-20" bowls, including coring, on the standard VM120 jaws, though now that I have a 4" set I use them more often. I do use the tailstock support when truing a half log of that size. I also don't worry about the "perfect circle" fit-- grabbing at 8 points hasn't failed me yet, so I use the jaws set much bigger than their minimal capacity. What has failed me, frequently, is the non-dovetail jaws we have on our club lathes, where the piece slips out. I only recall one instance of a tenon shearing off, that was due to a pre-existing flaw in the wood. In the maple/walnut/cherry I turn most often smaller tenons are just not a problem.

Thank you all for the replies. This is exactly why I asked the question. There are rules of thumb and manufacturer’s recommendations but there is no substitute for the actual experience of the many knowledgeable folks on this site. Very helpful indeed!