So I've been playing with my new chuck...don't happen to own a dovetail scraper, so I used a skew chisel to cut the dovetail. Cut it at an angle all the way to the bottom of the hole I turned, which was the same distance as from the face of the chuck to the end of the chuck. I noticed that the chuck dovetails for only about half that distance, but didn't think that would matter. It apparently did matter because my piece flew off the chuck a couple of times. So I tried mounting it where the dovetail ended and the straight part started and so far that seems to have worked. I have reservations about whether I should make the hole bigger and turn it dovetail for half the distance and straight for the other half. that way the face of the chuck would make sure that my wood was straight. directions for the chuck seem a little vague (I'm dumb??). rich
The bottom of your recess or the back edge around your tenon is where your chuck's JAWS should sit AGAINST, that is what keeps the blank from wobbling, DO NOT make the recess or tenon to deep/long. The other problem with the tenon jaws is that you have to have a very close fit size wise as well as the dovetail's angle has to be close.
You can try this for your self: draw an accurate circle, let's say 3"D now draw an other accurate circle 1/8" larger or smaller that has one side against the larger/smaller circle and see how much in contact you still have ??.
You'll see that you lost at least 50% of the contact surface area, if you would take 1/4 of that circle as a dovetail jaw, so it is very important to have a very close fit size wise and also dovetail angle wise.
Now if you keep your tailstock up against your turning at all times there will be not much of a chance that your piece of wood will come out of your chuck and it will stay wherever you first centered it, it does however restrict your access to and manner in which you are able to turn your project. Taking this in consideration is why I chose a Oneway chuck with their standard jaws they do have a superior design IMO.
Don,t feel dumb, The directions are vague to say the least. Glad to hear it works for you as i,ve had the same problem. Now I know what is the best way to mount it.
apparently I did make the hole wrong... i made the hole dovetail all the way in...instead of making it straight for half the chuck distance and dovetail for the rest...hmm, let's see if i can draw this: ______ hole ______ / \ /__________\ should have been this: ________ ______ | | /_________\ that way the whole jaw contacts the wood and the very end of the jaws contact the bottom of the hole....correct? rich
You need to make the hole just like you did in the first drawing, only not as deep. You should only make it as deep as it says in the instructions for that jaw set, which in the case of the standard Nova
50mm jaws is a *maximum* of 6mm, and in any case never deeper than the flared section of the jaws.
For smallish items you won't need, or want, to make it that deep, and if I recall correctly they suggest a minimum of 3mm. What you *don't* want is for any part of the parallel section of the jaws to be inside the recess.
The jaws should bottom the piece, and the depth should not exceed the depth of the dovetail portion, just as Alun says. Leo's explanation does not take this into account. It's not "grip" that counts, it's resistance to force applied parallel to the axis of rotation. Fortunately, that's also the way the wood wishes to be cut in a standard bowl mount. The dovetail is a wedge, not a grip. I thought I included a link to a demonstration yesterday, but in case you missed it:
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and the following five pages will show it. The radius of the recess or the outside of the jaws/shoulder contact determines the length of the lever perpendicular to the axis of rotation, and thus resistance to forces applied at right angles to it. I don't know a practical way of computing it, but I can say from experience that the decreasing amount of metal in contact with wood parallel to the axis of rotation caused by larger openings does not seem as significant as the increase in leverage perpendicular to it in securing a piece. With dry, less compressible wood, it becomes less important how well the curvature of the jaws, inside or out, is matched to the wood. You do, however, sacrifice some recentering assurance if there's a great discrepancy, because you can easily compress the wood if you apply too much force to the jaws. Since wood is seldom uniform throughout, the jaws will follow the line of least resistance, resulting in centering problems _if you "grip" rather than hold_ the piece. If you apply enough pressure only to maintain contact with the bottom or shoulder, no problem. So it's not grip that counts. That's in the wrong direction, and the lever length is insignificant.
If you're doing end grain work, try to match the circle better, because hollowing forces are not parallel to the axis of rotation, but more perpendicular. Best thing to do is get a steady to resist these, but a large diameter set of jaws is quite a leg up.
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Can it really be neccessary with all this shaping and tooling? I use a Oneway talon. To make the recess I use a Forstner-bit of 2" diameter, and a depth of 3 - 5mm, depending on the size of the blank. This gives just a litte clearance when the chuck is compressed. Then I roughturn the piece, and make a new hole on the opposite side with a straight parting tool. After I turn the piece, I clean the first hole with my parting tool, so as to make sure the two holes are exactly parallell.
I have never lost a piece, except because of faulty wood.
If I had the dovetail jaws, I would still use the exact same method. I do not see why it should not work equally well.
They certainly offer you a greater grip length parallel to the axis of rotation to resist the outward forces of hollowing on an end grain piece, but they are a bear to use to rechuck a piece without losing concentricity, because they, almost of necessity, distort the wood of the spigot (tenon) with their serrations. It's almost impossible to get back to the status quo ante with them if you for some reason dismount the piece, or if turning a green piece which you leave to dry long enough for it to become oval. Some people might even get a catch that compresses the wood of the spigot!
I hedge my bets when using them (35mm) by mounting a centering cone (I have a number premade) on the Nova live center, which offers that opportunity. I use it to hold things in the proper relationship while trying to reestablish the contact between the nose of the jaws and the shoulder of the tenon.
I got the power Grip jaws with my Super Nova, and the external dovetail has done for pieces hanging out 14 inches from the chuck, but you have to start with a substantial piece of stock to make the dovetail spigot. With the steady, the 50mm jaws, gripping outside, have proven adequate for the way I cut.
Nope, you had it right the first time. Almost. You just made the dovetail recess too deep. Your chuck jaws should bottom out in the recess. The recess should be a bit shallower than the chuck jaws will reach. The recess should be dovetailed all the way with no straight-walled portion.
i'm still a beginner really and it's better to ask one of these other guys that seem to know. however, i do like the chuck i bought better than the outside metal turning chuck i have attached to a faceplate. it's useful for small things but not when you need a solid hold because you will be turning with some force or distance from the chuck. it has essentially no gripping surfaces, just compression surfaces. maybe it would work better if i put CA glue on the surface to be gripped before mounting it so it would be harder? rich
It would be nice to see all of this in person, because I get lost in the forces, axis, perpendiculars, parallels, etc. I have 2 Vicmarcs. The dovetails on the jaws go all the way down. With the big chuck, and bowl being 16 inches plus, my mortice is about 1/4 to 5/16 inch deep, and 1/16 or so wider than the jaws. This allows enough hold to safely core with the McNarghton system, and no tailstock. On smaller pieces, I go no less than 1/8 inch deep, depending on size. The top of the jaws must bottom out on a flat spot in the mortice, and the mortice can't be deeper that the jaws. Wet wood seems to compress more than dry wood, to the point that sometimes, I have to retighten the chuck once or twice while turning. You can tighten the jaws too much, which can break the mortice before you begin to turn, or if and when you catch, the added stress, can break your grip. When tightening, there are 2 places on my chuck to insert the wrench for tightening. I snug it up in one hole, then go to the next and snug it up some more. I may rotate it a couple of times, snuging it up a little until I am satisfied. Having enough shoulder on the mortice is important also. With a little shoulder, you have to be gentle when turning out the inside, with a bigger shoulder, you can be more aggresive. Because woods are so different, there are no fixed rules on how much is enough (shoulder size, mortice depth, jaw tension). Trial and error will help you learn. robo hippy
One more after thonght, I use the mortice as my finished bottom. After using a dovetailed scraper to make the hole, I use a small spindle gouge for the finish cut. One pass across the bottom and one pass down the side. The point on the spindle gouge will get all the way into the corner of the dovetail, which I can't do with a bowl gouge or skew. It is also easier than sanding it out. robo hippy
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