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Parting tool marks

John Torchick

John Torchick

Joined
Jan 24, 2010
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Location
Cleveland, Tennessee
Finishing my project. Decided I could take my time and part it off. Jam chuck and Cole jaws won't work as the rim is tapered. Here is a photo of the "almost done" turning. I would like to eliminate the tool marks and concave the bottom a bit. Thanks.
20200219_101622.jpg
 
Hi John,

A couple of thoughts. Depending on the thickness of the (tapered) rim, you could still do a jam chuck from the inside of the turning. (Just be careful not to split the work from the inside!) If you don't get a tight fit, you could support it with the tailstock and finish as much as you can with the tailstock in place, then sand and finish the little nub that remains by hand.

The other thing I noticed in your picture is that it looks like there is a gap between the chuck jaws and the bottom of the turning. Unless there is an optical illusion in the picture, it looks like your tenon is too long and the work isn't well seated in the jaws.

Good luck,

Dan
 
There is a slight gap. I checked and the piece is flat inside the chuck. Chuck was tightened and checked periodically. Will take more care to measure the tenon. Filed this for future reference. Thanks.
Not real sure that a jam chuck would hold as the inside of the turning is tapered so there wouldn't be much of the JC inside the piece. Might part it off the lathe and hit it with sandpaper. Sandpaper is the least expensive part of turning.
Live and learn. I cut some more wood this morning and try for some more bowls, platters, firewood kindling, etc.
 
A jam.chuck doesnt have to actually hold the piece. It can be used simply to align it and make it spin true. Use the tailstock to actually hold it in place. If necessary tape the bowl to the jam chuck. Then turn all.of the bottom but the last little area held by the tailstock. Carve that off. You can also.go to our club website and look up mynarticle on methods and jigs to reverse turn bowls. You should find a method you can use. Www.cumberlandwoodturners.com
 
John Torchick said:
Finishing my project. Decided I could take my time and part it off. Jam chuck and Cole jaws won't work as the rim is tapered. Here is a photo of the "almost done" turning. I would like to eliminate the tool marks and concave the bottom a bit. Thanks.]
Click to expand...

when I part off pieces (goblets etc) I sharpen the parting tool at a slight angle so that the point will ride along the surface I’m keeping. This gives a better surface off the cut than the straight across sharpening.

when I teach bowl turning I use 8-10” diameter blanks 4-5” thick.
Smaller and larger bowls are more difficult to turn. The smaller because of limited working room
The larger because they take more time and getting a big curve to look nice take years of practice.

If the wood inside the parting cut is a 1/2” or thicker you can safely jamb Chuck it over a block of wood turned to fit inside. Use a paper towel between the jamb chuck and the wood.
 
Hock, I saw an interesting video this morning. The diamond PT was modified by grinding off the side, grinding the tool at an angle and the tip was also ground at an angle. The PT acted as a shear scraper at the same time; used a regular PT to turn down the wood and the modified version to part a small amount and finish the side of the piece. Now I can't find it! :mad:
 
John, (assuming your piece is a bowl and you have chuck jaws large enough) I sometimes wrap a rubberband around the outside of the jaws (for a cushion) and then gently expand the chuck on the inside of the rim. You can also use Cole jaws or a Longworth in a similar expansion mode.
 
John, I use my vacuum chuck a lot. Easy to make different vacuum chucks from PVC. I used Cole jaws a lot before the vacuum chuck.

Another observation: Your tenon is too long. Your work should be setting against the face of the jaws. And there is some gap at the end of jaws. Those look like Nova 50mm jaws. It appears you put a notch for the end. I never put the notch for the Nova jaws, just let them bite in. I know some don’t agree and I’m not trying to stir that pot, but your notch looks below the face of the work.
20200219_101622.jpg
 
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Actually if you use a flat parting tool (vs diamond) A freshly sharpened parting tool actually has 5 cutting surfaces. If you think of a V the top of course but also the slope of the V on each side. What I often do on smaller projects is to make a parting cut down close to the line I want. The I make another very shallow pass and do it so the the tip is cutting but so is the slope of the V. It's almost a shear scrape and leaves a fairly clean surface.
 
William Rogers said:
John, I use my vacuum chuck a lot. Easy to make different vacuum chucks from PVC. I used Cole jaws a lot before the vacuum chuck.
Another observation: Your tenon is too long. Your work should be setting against the face of the jaws. And there is some gap at the end of jaws. Those look like Nova 50mm jaws. It appears you put a notch for the end. I never put the notch for the Nova jaws, just let them bite in. I know some don’t agree and I’m not trying to stir that pot, but your notch looks below the face of the work.
View attachment 32114
Click to expand...
Not stirring the pot as far as I'm concerned. I appreciate all the input and sharp eyes. The notch might be where it was chucked the first time- will leave a piece on the chuck until completely turned. Yes, tenon is too long as was mentioned earlier. This kind of turning is something new to me. Much different than pen turning!
 
john lucas said:
Actually if you use a flat parting tool (vs diamond) A freshly sharpened parting tool actually has 5 cutting surfaces. If you think of a V the top of course but also the slope of the V on each side. What I often do on smaller projects is to make a parting cut down close to the line I want. The I make another very shallow pass and do it so the the tip is cutting but so is the slope of the V. It's almost a shear scrape and leaves a fairly clean surface.
Click to expand...
Will sharpen again and rub the piece with the PT. May hit the side of the PT with 400 or 600 to "polish" it a bit. Give it the old college try!
 
As mentioned above the tenon should not bottom out against the inside of the bowl chuck interior, it should bottom out against the fingers of the chuck jaws and your tenon should be slightly smaller in diameter so the jaws close most of the way. These two practices account for many bowls coming out of the chuck jaws during a tool catch. When the tenon bottoms out on the inside surface of the chuck jaws it provides a fulcrum leverage point which quickly and easily pops the piece from the chuck when there is a tool catch. When the chuck jaws have too much gap between the jaw fingers your work piece is only making contact on 8-points, when the jaws are almost closed and the tenon is slightly larger then the closed jaws diameter you are making a continuous 360 degree contact around the entire tenon, it is very difficult to pry the work piece loose even with a tool catch. The tenon only needs to be about 3/8" long, any length shorter then the chuck jaws is fine.
 
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