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Thread: Bowl out of true when remounting

  1. #16
    Join Date
    May 2009
    Location
    Boston
    Posts
    1,621
    I’ve run into this same issue and made sure the bottom around the tenon is flat so the top of the jaws sit even around. I also make the tenon with a slight angle inward say 85 percent since my jaws are dovetailed. I’ve tried to sit the bowl without the jaws hitting the bottom but it’s hard to get it to sit right.
    Don

  2. #17
    Join Date
    Feb 2008
    Location
    E TN, near Knoxville
    Posts
    5,592

    Things do consider and do

    Quote Originally Posted by Staffan Hamala View Post
    ...When I remount the bowl on my chuck, no matter how carefully I adjust it, the newly turned outside of the bowl seems out of true. I realize this always happen to some degree, but it just seems like it's too much....
    Most chucks have a small amount of runout (wobble) that you can often see. Tolerances in the jaws can add to that runout.

    Some things to consider about the chuck and mounting it on the lathe:

    - When mounting the jaws on the chuck, be certain the mating surfaces are clean with no sawdust, rust, or burrs. Tighten as recommended - barely snug the screws, close the jaws fully, then tighten the screws firmly.

    - If the chuck has a threaded insert to adapt it to the lathe (such as the Nova chucks I use) when mounting it be sure it is also clean and without burrs and seats firmly in the chuck.

    - If your chuck spindle adapter is suspect, there are several companies than sell well-made adapters. I personally don't like adapters due to the possibility of loss of precision. It is better to get a chuck that has the same threads as the lathe. That's one reason I like the Nova chucks - I can change the threaded insert if I need to use one on a different lathe.

    - When mounting the chuck on the lathe, make sure the spindle shoulder and the mating surface on the back of the chuck are clean and free of sawdust. Chris Ramsey, who turns large diameter cowboy hats to very thin wall thicknesses, stresses this repeatedly.

    Some things I do if I want to remount a piece for further turning:

    - When making the tenon, make it almost the same diameter of the jaws of the chuck when almost fully closed. This lets the curved parts of the jaws grip the tenon all the way around. The chuck manuals recommend this. If the tenon is too large, the sharp corners of each jaw will bite into the wood and distort it. Since wood has natural variations all over some jaw corners might distort deeper on one side than the other when mounted and again when remounted. (Even the smooth curves of the jaws can compress one part more than another but not as much as the corners.)

    - When I mount the piece in the chuck for the first time, whether with a tenon or recess, I make sure the wood is free of sawdust then press the piece firmly into the nearly-closed jaws and rotate it back and forth a little then continue to hold firm pressure while tightening the jaws. You can hold the piece firmly with the tailstock but if pressed with a live center with a point the point itself can keep the piece from seating properly. Use a small block of wood between the work and a live center with a point, or a different method completely.

    - *** Since the chuck may have a bit of runout AND the jaws may distort the wood, before I take the piece out of the chuck I mark the position of the piece in the jaws so it can be put back in exactly the same place. My own convention is to make a mark on the wood between jaw 1 and jaw 4. When remounting line up the mark carefully and don't over-tighten. You can often see the impressions of the jaws on the wood and try to get the jaws exactly in the same place. I do sometimes use a dial indicator to check for proper mounting to make sure it's the best it can be and adjust it a bit if needed.

    - Note that even dry wood can move significantly as internal stresses are released by turning. When more precision is wanted, for example when making a lidded box or when planning on remounting, it often helps to turn the piece nearly to the final wall thickness then let it sit for a few hours or overnight to allow stresses to relax, then make the final cuts to true it up. Distortion from internal stresses, however, might not give you the type of misalignment you reported - it usually results in a slight out-of-round oval shape. Yours sounds more like it was improperly seated in the chuck.

    As others mentioned, a clean, flat shoulder is extremely important. The tenon should never bottom out in the bottom of the jaws but always have a small gap at the bottom and bear only on the shoulder.

    BTW, most of the time I prefer using a recess instead of a tenon to hold bowls. I think it is often stronger than a tenon - a 2" recess is very strong if there is plenty of wood outside. A 2" tenon can break off if there is a defect in the wood or the wood is brittle or weak.

    And it's surprising, but you certainly can cut steel with woodturning tools! (Aluminum and brass are easier!) I turned down some small jaws to get a precise diameter. I ground a cutting edge on a Thompson scraper to a shape similar to some metal-turning cutters. The turning was slow, made some needle-sharp chips, and I had to sharpen the scraper a few times but it worked well. I've also trued up jaws but usually don't have to. When metal workers use hand tools like this on the lathe they call them "gravers."

    turning-steel_IMG_20170605_081521_124.jpg

    JKJ

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