Dovetail Chuck

[John Ponder]

When using a dovetail chuck how do you decide the diameter of the spigot and how much shoulder is needed to fit against the jaws? Are tools sold for cutting dovetails worth the money?
Thanks,
John

[Jake Helmboldt]

John it should be almost the same diameter as the jaws when almost fully closed so that they contact all the way around the circumference, otherwise only the corners will engage. Just a bit larger so there will be enough wood to compress slightly when tightened. You don't have to crank the bejeebers out of it to get a good hold.

As for the specific tools, IMHO I think it would be a waste of money. I use a skew as a scraper (or a spear point scraper) to refine the shape of the tennon and have not had a problem.

[Ryan Baker]

The ideal size for the tenon is just slightly larger than the diameter of the jaws when fully closed, because that will allow full contact of the jaws all the way around the tenon. The depth depends somewhat on what you are turning and how you are doing it. You need it deep enough to get a good grip, but not so deep that the tenon bottoms out in the jaws. The shoulder needs a good fit to the face of the jaws, but doesn't need to extend further out than the outer edge of the jaws (any more doesn't help).

There are a lot of tools and techniques for making the taper on the tenons. You may be able to do it with a bowl or detail gouge, depending on the grind you use. You can do it with a skew, scraper, bedan, etc. The commercial tools work OK, but make sure you get one that is cut the way you want. Some of them are really only suitable for forming the inside of a recess, not a tenon. Some have tapers on both left and right sides, but even those may not be what you really want. You should cut into the face grain not the end grain -- so on a normal face grain orientation bowl blank, you would be cutting in from the tailstick end, parallel to the ways, not perpendicular to the ways. I normally use a bedan that I have ground to the angle of my jaws. Skews work, but it will dull the heck out of your skew really fast to use it that way.

[Richard Madison]

You could roughly shape the tenon with a bowl gouge and trim it up with a parting tool. With a little practice you can eyeball the dovetail angle very closely. Depth of the tenon simply needs to be less than depth of the jaws, as mentioned, so the shoulder of the workpiece bottoms on the ends of the jaws, not the insides. If you are going to remove the tenon later, the diameter is much less critical than many think. The corners of the jaws will indent into the wood, giving a solid mechanical interlock. If the tenon is to remain part of the finished piece, then size is quite important, as mentioned, since in this case you want the minimum indention of jaw corners into the work.

[John Ponder]

Thanks Guys,

I may need to clarify my question. Does the shoulder on the tennon need to be equal to the thickness of the jaw? By saying the diameter of the tennon should be slightly larger than the closed jaws you mean the inside diameter of the closed jaws? Thanks for the reminder not to bottom out inside the chuck.

John

[Bernie Weishapl]

I size my tenon by closing the jaws of the chuck and measuring the outside of the jaws. I make my tenon that size. I took a 3/16" parting tool and ground it to the shape I needed for a dovetail. It cuts the shoulder and the tenon at a dovetail angle in one cut. Works great. As far as the shoulder goes I make sure the shoulder portion is as least the width of the jaw thickness so it seats solid. Also make sure your tenon doesn't bottom out in the chuck.

[John Ponder]

Thanks Bernie,
That is just what I needed.

John

[Michael Mills]

I agree with the above. Also, just make a dovetail to fit your chuck’s angle with any old tool or piece of metal. An old chisel, large screwdriver,..almost anything will do. You are only removing a shaving 1/16 –1/8 wide and maybe 3/8 long maximum. I do cut the depth with a parting tool so all I have to do is use my dovetail chisel to cut (scrape) the indent.
Mike

[Reed Gray]

I use a recess, but the process is almost the same. For a 12 inch bowl, you want a tenon about 3 inch diameter, which is 1/4 the diameter of the bowl. Ideal fit would be jaws that close to 2 3/4 inches, but with a tenon there is more room for expansion if needed. If your jaws close to 2 inches, you still get a pretty good grip on a 3 inch tenon, and the ribbed jaws like on the Oneway chucks grip really well. If your jaws are dove tailed, then you want to match that angle as closely as possible. For me, having a dedicated tool is easier, with no guessing or fiddling around to get it just right. I prefer an in line one which is available in most of the catalogs. To get the angle perfect, just line the shaft of the tool up with the ways on your lathe bed, and make a plunge cut. As for depth, 1/4 inch max. A longer tenon doesn't give you a better grip, and actually isn't as good, some thing I learned from Stewart Batty. I did one a bit long at a work shop, and he had me round over the bottom edges of it, saying that it was more likely to break off if it was too long. You do not want it to bottom out in your chuck.

robo hippy

[John Ponder]

Thanks again guys. It appears I was making the spigot a bit too small, sizing it to the outside of the closed jaws works much better. Grinding a scraper into my own tool sounds like my next project although my bedan works well now that I have right size spigot.

John