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
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