Nova jaws leaving black marks!

[Fred Oliver]

All my bowl feet show black marks from my SuperNova2 chuck jaws -50mm, 100mm and Powergrip - in both expansion and contraction mode, and in all types of wood; they're very difficult to sand out.

I cleaned the jaws with Semichrome, then mineral spirits -even tried waxing them. Still getting black marks.

A coat of shellac on the foot reduces, but does not eliminate, the black ring. Painter's tape helps a bit but also reduces the jaws' grip.

What's causing this, and what can be done to prevent it?

Thanks for your thoughts.

[Matt Hutchinson]

I don't know what your setup is, but I always turn away any material that was held in my chuck. Dents and marks are not a problem, and I like a nice turned surface. Now might be a good time to gain new experience by changing your method of work. If nothing else, I can guarantee you will have a nice finished product. :)

Hutch

[Alan Tolchinsky]

I'm with Matt here. I recently started turning away the tenons that my chuck grips to. It looks a lot nicer without them.

[Don Robert]

I'll bet you are turning green wood. If so, the sap in the wood will turn the jaws black and the jaws will then stain the wood. The sap of some wood is nastier than the sap of others. For instance, olive is more corrosive than mesquite. After you have dried the roughed-out piece and have trued the tenon, you should have no more problems with the jaws staining the piece. Fortunately, the stain is pretty much confined to the surface.

[tom martin]

I've had the same problem. When I want to keep the tenon on the finished piece, Ill put a rubber band around the tenon (or around the jaws for a recess expansion grip). I've had pretty good luck doing this, but I do try to be less aggresive in my cuts.
Most of my bowls don't require this as I generally prefer to turn the tenon off after hollowing and sanding.

[Allen Neighbors]

I also turn all chuck-wood away. If the bottom of the piece needs to be larger than the tenon, I make provisions for that when turning the outside of the piece, but I still turn away any wood that has been in the chuck... in either mode.

[Bernie Weishapl]

I turn away all signs of the chuck especially when using a tenon.

[Richard Madison]

Any chance that the iron in the jaws is reacting with tannin in the wood (in the presence of moisture) and turning black? That is ebonizing. Same as when we make ebonizing solution with acetic acid (white vinegar) and steel wool. Get iron acetate (or whatever) that reacts with tannin in the wood and turns it black.

Just a thought.

Anybody with chrome plated or nickel plated jaws have this problem?

[Fred Oliver]

Matt, Alan, Allen, Bernie: I often turn off the tenon, but sometimes want to leave a foot, and there's the problem.

Tom: The rubber band sounds like a good idea, although that won't protect the bottom of the bowl adjacent to the tenon. I'll try it. The painter's tape does help, but it reduces the jaw grip.

To remove the black marks and indentations, while leaving a foot, I reverse the bowl into the Cole jaws and gently take off 1/8" or so, as sanding into that corner is problematic. By then the green bowls have distorted enough to make the use of turning tools a dicey proposition.

Don & Richard: I think you've hit the nail on the head, as the problem seems worst in wet black cherry, the black marks do look ebonized, they're very hard to sand off, and cleaning/waxing the jaws has not helped.

So far, a washcoat or two of 1# shellac seems the best way to reduce the marks, but of course that clogs up the sandpaper when finishing.

I wonder if there's some coating/protection that could be applied to the jaws themselves.

[Reed Gray]

The black comes from metal reacting to moisture in the wood. It is easiest to remove from a recess. Simply drip in enough concentrated lemon juice to cover the stain, and it will be gone in about 5 minutes. I believe that Boric Acid will do the same thing. I would think that a tenon will do the same thing, but you will probably have to paint it on, or pour enough in a dish to cover the tenon. I use a recess, and leave it in. Some times I will add detail, but not often. I used to put a thin piece of plastic over the chuck jaws, or even a nitrile glove which is very thin. This was just too much trouble. The lemon juice is much easier.
robo hippy

[Matt Hutchinson]

I guess I am not sure what the exact situation is. I understand why and how the black marks are left, but if you are turning the tenon off, aren't you also turning a foot at the same time? Is it an issue of when you are finishing? I finish the bowl before I turn the foot. That way I don't have to rechuck it after making the foot.

Hutch

[D-Alan Grogg]

Only one of my jaw sets is black (the 25mm pin jaws). But, even that set doesn't leave black marks. Not sure what could be causing your problem.

For those that turn off the tenon, I too am not sure how you do that if you want to leave a foot???

[Chris Barnett]

If only from wet wood, you could have your jaws nickel or chrome plated which would be very resistant to acid attack. Or if you are flush, the yellow metal :D plating would permanently solve your rust problems.
I have had cast iron parts in raw river water service nickel plated, and after years of use they looked like new with no rust. I'm don't know the metal compostion of NOVA chuck jaws, but could rust just as badly as CI. Do no know if sufficient clearance exists between the jaws and chuck to not cause a binding problem though.

[Jim Becker]

In theory, you should be reversing the piece to clean up the "foot" after turning the piece. It's not generally going to work well to leave a tenon "as is" after gripping it with any chuck of any brand. Same goes for those who use a recess rather than a tenon.

[Matt Hutchinson]

As I am turning a bowl, it is chucked using a dovetail tenon. The bowl's form is created with a foot already incorporated into the design, with the tenon as a separate entity. Once it's finished, the tenon is turned off, and the bowl is complete, foot and all. The only thing left is to sand and put finish on the foot, then wax the whole bloomin' thing.

Also, here's a PDF with a photostory of this process. The pictures aren't the greatest, but if you look closely you can see the foot and tenon.

http://sarasotawoodturners.com/Basic%20Bowl.pdf

Hope this helps. :)

Hutch

[Allen Neighbors]

FWIW, using my SN2 chuck with 50mm jaws on a 2.25" dia tenon: If the foot needs to be smaller than the tenon... no sweat, just trim the tenon down to proper size.
If the foot needs to be larger than the tenon, I make that decision when I am shaping the exterior, and cut in a 3/8" wide flat-bottomed groove for the jaws to sit in, and make it as deep as necessary for the tenon length I want... usually about 1/4" or less. Then I make a pencil mark on the outside of the jaw groove to mark the depth of the groove, so I don't make a funnel out of the bowl. Then, after hollowing the interior, I turn away the tenon, and shape the foot and underside details.
Speaking of funnels, I forgot the depth mark, and made one this morning. 10.75" diameter X 3" deep funnel with a 3/4" hole. Arrgghhh!!!!!

[Paul Engle]

Wet wood/ moisture will cause 99.9% of the black marks.

[robert hainstock]

In the laundry section of some grocery stores, there is a product called variously rust stain remover or whatever. It like the twoo acid products already mentioned is an acid. Hydroflouric to be precise. It should work. If you remove the tenon compleatly a shallow recess where it used to be will serve as a foot. Good luck! :D
Bob

[Allen Neighbors]

Right on, Jim. :)

[Richard Madison]

As a couple others said, to make a foot just clean up the O.D. of the tenon, maybe straighten it a bit if it's a dovetail. I use a 3/8" tenon and usually shorten it to make 1/8" to 1/4" tall foot. Usually try to decide what kind of a foot the piece is going to have when I start, and make the tenon diameter a bit larger. Maybe decide later that the piece does not need a foot and turn the tenon off completely.