I own a supernova2 and a verschuck with O'Donnel jaws. When the bowl blank was reverse chucked I found that no matter how hard I tried I could not get the blank to run true which resulted in thick and thin sections on the rim I used to finsh the outside of the bowl including polishing, but no longer do it that way. Now I leave about one third down from the rim unfinished and then finsh and sand the outside after trueing up the rim. I then hollow the inside and this gives me fairly accurate results.
Can anyone suggest what I might be doing wrong,or a better way to do it, or maybe even a chuck which is more accurate. I live in the U.K. so if a new chuck is suggested it would need to be obtainable here.
Couple things come to mind. Make sure you register against the shoulders of the tenon on the bowl, not the bottom of the chuck.
One thing I do a lot when I rough out the outside of a bowl is to bring the tailstock up and make an indent in the tenon. That gives me a reference point for remounting it.
I usually will true up the outside, finish turn the inside, then final turn the outside, taking really small clean up cuts.
I used to turn green bowls - haven't turned any thing in a few years - lack of time.
I made them about 1/2" in the lip if I was to take it off. The drying if slow - would bow a little. I could make a thin shell with 1/2" left - I'd have margin on one side and less on the other.
Mart> I have never, ever successfully rechucked a bowl except when I made one
Tom - a couple assumptions about your question - you mentioned that you used to finish the outside before rechucking, so that leads me to believe the wood is fairly dry that you are turning? Also that you are going from the outside to the inside with out significant lapse of time - you want to get it done! Now I don't have the chuck/jaws you mentioned, but there are 2 points where things can go wrong - First - how the chuck fits on the spindle: make sure the threads are clean and the chuck fits tight against the spindle shoulder with nothing in between (no plastic washers!). Does the chuck run true when you turn it on without a bowl on it? Second - how the bowl fits on the chuck. Make sure the tenon/mortise is thinner (shallower?) than the height of the jaws so that no wood comes in contact with the bottom of the jaws or the chuck body. Make sure the bottom of the tenon/mortise as well as the sides are cut clean - I touch up with a parting tool or skew to get a clean surface. If the jaws have a dovetail, the sides need to be undercut to match. Then when you mount the blank, ensure there is no loose material in the way, and just as the jaws become snug on the bowl, rotate the bowl back and forth a quarter - half a turn while applying pressure to the tailstock side of the bowl, to help seat it snugly. You may need to bring up the tailstock, particularly on large bowls, to help this. Then turn it on and see whats happening. If its not running true rest a pencil on the tool rest and mark the high spot(s) on the outside rim. Turn off the lathe and determine if you have one or two marks on the bowl. - If there are 2 - then the bowl has elongated as you were finishing it - could have been caused by heat buildup from sanding - you 'll have to return the outside if you want it true. - If there is only one mark on the bowl, then its still not seated properly. Loosen the chuck, compensate for the high spot by shifting the bowl in the chuck a wee bit, then tighten see whats happening. Hopefully you got it - if not try again! Hope this helps - Hought
Normally I mount the blank by a screw held in the jaws
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with the screw in what will be the top of the bowl . Then turn the outside and create a tennon. Take the screw out of the chuck and reverse the blank, now gripping the tennon in the chuck, making sure to reference as suggested in previous posts. Turn the inside, making sure to true the rim, and finally reverse the bowl gripping it by the rim (I use bowl jaws, button jaws, bowl reversing jaws - depends on brand what they call them -
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1612.htm) to finish turn the tennon, or to remove it altogether.. I have never had a problem reverse chucking or re-chucking a dry blank. The blank always runs true, even after a week or more out in the barn and being taken off the chuck altogether.. I think it's in part because of the chucks I use (Axminster - a UK brand). These are very high precision, very reliable chucks.
Before doing anything make sure that your spindle is running true. If not, you're screwed till you fix that.
There are several other issues.
First, It is essential the the right jaw be in the right chuck slot. Both should be numbered. Then, when mounting the jaws to the chuck only put the bolts in tight enough so that the jaws don't flop. Then tightly close the jaws. This registers the jaws to each other. Once closed, tighten the bolts and your set to go. If you just put the jaws on in an open position and tighten there is a fair chance that they will be slightly off center. A tiny bit off center at the jaw is magnified dramatically by the time you get to the rim.
The second thing I find helpful is that when I grip tennon the first time I shade/color the tennon between jaw #1 and jaw #2. That way I can get it back to the exact position it was in initially.
Third, O'Donnel jaws
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can only grip a fairly small tennon. It might be that you need to go to somewhat bigger jaws. It depends on the size of the bowl you are trying to turn. Hope that some of this helps. To see what I do take a look at
I have to disagree, unless you are working very green wood. Even then you can cover the blank in plastic to prevent moisture loss. I've never had a problem rechucking.
If you take some thing off the lathe and remount it, it never runs totally true. With bowls, how much it is off depends mostly on how wet the wood is. With sloppy wet green wood, it can move a lot. If you turn thick, dry, then remount to return, 10% thickness is the rule: 10 inch diameter bowl leave 1 inch thick. Even totally dry wood will move some. If you are using a tenon, then do leave a center mark in it for remounting. For me, I like green wood to final thickness, let it dry and warp, then sand and finish. You will almost have to power sand, and at very slow speed (10 rpm) which is possible on some of the variable speed lathes.
That has not been my experience. I've checked with a machinist's gauge and my bowls made from dry wood run true after a number of remountings and also even after several years. I've had customers come back after having dropped a piece and broken the rim. I mounted it and re-turned the rim to get rid of the broken edge. The remounts ran perfectly. Just because you can't do it don't assume no one can.
On Sat, 22 Aug 2009 12:37:40 -0500, Tom Dougall wrote (in message ):
There is a possibility that you may be using too much tension or pressure on the bowl when you clamp it into the chuck. The base tenon (or spigot as it might be called) is fairly solid and will take a lot of clamping pressure. The bowl rim, on the other hand is not so sturdy and will be deformed if too much pressure is applied to it by the chuck jaws. Since the grain in the rim is likely of varying degrees of firmness, the bowl will respond by deflecting irregularly, when it is clamped.
My own practice works well for me, as I am a small-time amateur and high production is no concern. I turn wet blanks which are glued at the bowl's bottom to a piece of waste wood. I screw a 4" faceplate to this waste wood, and turn every part of the bowl I can reach, inside and out, leaving the bowl rough and oversize to dry. I can remove the faceplate from the roughed out blank and attach to another blank as I wish. When I am satisfied that the rough bowl can be finish-turned, I re-attach it to the faceplate and finish the bowl to whatever it tells me it can be. I sand it thoroughly on the lathe, inside and out, to its final finish. I saw the waste off of the bottom of the bowl and then chuck the rim of the bowl, to turn and sand the bottom or foot of the bowl. These bowls are small - 8 inches or less in diameter and generally not over 5 inches in height. The wall of the bowl is generally about 3/8" thick. The wood is usually either Aspen or Birch, as that is what I can get locally.
The largest bowls I have made are about 11 inches in outside diameter and turned in the same way - except that my chuck won't reach this size. I just saw off the waste, and sand the foot smooth and flat with a disc sander mounted on my lathe.
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