Bowl Not Turning Perfectly True After Reverse Mounting

[Jeff Paxton]

Question: Why do some bowls not rotate exactly true after reverse chucking?

Here is what happens. I have been rough turning some cherry bowls approximately 12-16 inches. The wood is green and I use a face plate. After doing the outside of the bowl, I turn a tenon and reverse chuck using a Oneway Talon or my Oneway Stronghold chuck depending on size in order to do the inside of bowl. On some of the bowls I can see and feel that it is not turning true when I remount. Not too bad, some worse than others, but I guess I thought it should run true. Could it be the chuck? I typically trying to make sure that the tenon is not too wide so the jaws contact the bottom of bowl. Should the tenon be wider so it contacts the inside of chuck rather than the bottom of the bowl?

Could it be green wood?
Could it be when the tenon is made with sap wood?

Also, when I check the headstock with the tailstock, the points are not perfectly aligned, but pretty close. I guess I thought that would not matter as far as turning a bowl since the mounting and remounting is done from the headstock.

Any thoughts? Not that it matters but my lathe is a Powermatic 3520b. Thanks for any help.

Jeff

[Paul Williams]

Aligned and "perfectly aligned" are two very different animals. It takes extremely little movement at the chuck end to see a noticeable movement at the rim of a bowl. It will never be perfectly aligned. However there are other things that can cause it to appear not centered. All wood relieves internal stress as you turn it, and may not stay perfectly round when reversed. Especially if you turn the outside one evening and go back to the inside in the morning. Green wood loses a lot of moisture during the turning and will change shape. So the question is how out of alignment is it, or is the bowl no longer round.

I have never felt the need to own one, but there is a devise to hold your chuck in your tail stock while you connect it to the tenon. That might help keep the tenon aligned along the centerline of your lathe while reversing it.

[Bill Hensley]

Jeff you need to have a flat area at the base of the tenon for the chuck to reference against instead of the bottom of the bowl. The flat shoulder doesn't need to be but a 1/8th or 3/16th wide. If that doesn't solve your problem then you may indeed have an alignment problem.

FYI, any time I've had a wobble issue when I flip the bowl around is because I forgot to square up the tenon and shoulder.

[Bill Bukovec]

I think the only one that notices the bowl is not perfect is the one who turns it.

I discovered a problem (after I was done) with a bowl that was about 3/16 out of concentricity (inside to outside). This was a bowl I made for a neighbor from a tree that went down in their yard. I gave it to them anyway. They loved it and show it off to all of their friends.

I gave up a long time ago trying to make something "perfect". As a result, I enjoy woodworking much more.

[Thomas Canfield]

Jeff,

You did not say what size tenon you are using, but for a 12-16" bowl it should be fairly large and not the minimum size for a #2 jaw. You can expect the tenon and bowl to warp during the drying process, and cherry has some severe warp. The tenon needs to be clean cut and seat against the base of the bowl and not have tenon bottom out in the chuck. I have used my #2 Stronghold jaws with 3"+ diameter tenon for bowls up to 18" D (using a faceplate for the outside) but did use my livecenter in the tailstock to start removing the inside. I now have a set o #4 jaws for my chuck and like to go with larger tenon. Remember, the tenon will need to be turned at least 1/4" oversize to allow or truing up when you do the returning. When returning, I use the open jaws of the chuck as a jam with the livecenter in a dimple left in the rough turned bowl and true up the tenon and enough of the bowl to clear the chuck when I reverse and then true up the outside from the headstock side and pressure from the live center (with pad or point removed) for stability. Some of the larger bowls turned to the 10% wall thickness, 2" for 20" D bowl, will be over 1/2" oval and have additional warpage requiring cutting more like 1" off some places to get concentric. More than you asked for, but good luck.

[Jake Helmboldt]

Jeff, another issue is that the wood in the tenon will compress unevenly around its circumference, so that will induce runout. Personally my experience has been that the Oneway profiled jaws exacerbate this as they tend to crush the fibers of the tenon more than traditional serrated or dovetailed jaws. You may be compounding this if you really torque down on the chuck key and over tighten.

[robert baccus]

All the above is good advice. Also I always use the tailstock and cone to align the bowl before tightening the chuck. The small cone on a oneway live center just fits most faceplate female threads--or just make a cone. Hold everything tight while removing your faceplate. Retighten the chuck ----again. Another way is to flatten the bowl base and CA a single screw or faceplate glueblock.

[Reed Gray]

Jeff, I have some You Tube clips on bowl turning if you type in robo hippy. I do use a recess. Having the angles match is important, and a very light cut for the final pass. Then wiggle it around till you can feel it sitting flat. Some times chuck jaws can have a screw loose and that is part of the problem.

robo hippy

[Matthew Little]

Everyone has there own technique and approach when turning a bowl. You should do some research into improving your tenon. Make sure you are making it to fit your chuck properly; the right size, a good flat shoulder, the right shape to fit the profile of your jaws.

Beyond that you can always re-true the bowl after you have reverse chucked it. In fact I do most of my shaping after I have reverse chucked, then I do a final shear scraping with the bowl gouge. I cut using the Ellsworth technique and this works good for me. I do all this will the tail stock in place after I have reverse chucked, then I move it out of the way when I go to hollow the bowl.

Now you have to keep in mind that you are working with wood and that there is always going to be a possibility of movement in any of the steps when turning a bowl.

[Dan Forman]

One problem with Oneway chucks is that the jaws are not very consistent, and the face of the jaws are not machined flat on the chuck. The jaws are not marked either, so that if you remove them, you have to mark them so that you return them to the same position.

To correct this, I mount my Oneway chucks empty on the lathe, tighten the jaws closed, then face off the jaws with a carbide or HSS scraper, until they are all even. This makes a big difference in how accurately things can be remounted. My Vicmark chucks don't have this problem, but Oneway with profiled jaws are still my favorites.

Dan

[robert baccus]

Could you expand on "facing off the jaws"--lost me there.

[Reed Gray]

You can fine tune your chuck jaws, especially if you have some marks from going through the bottom of the bowl with a McNaughton coring blade. Of course, I know nothing about that. You tighten the set screws on the jaws, then tighten the jaws down with about 2 business cards, or one old credit card in between where the jaws close (to make up for the saw kerf for when they cut them into 1/4 sections). Then you can take a standard scraper, and very lightly cut the tops down flat, and even tune up the dove tails. The HSS will cut the soft steel of the jaws without much effort, but take very light finish cuts. No gouges.

robo hippy

[Marc Himes]

I find that it is almost always the case that a bowl will need a little truing up when remounting using a tenon. I think this nay be partially due to crushing the wood in the tenon. I do not notice it nearly as much when I use a recess, but I still like tenons, even though I have to true up the bowl.