The daily woodturning blog.
Monday, October 17, 2011
Yea or Nay?
I talked to a few club members on Saturday about my latest hollow form. Consensus was that an Ebony or African Blackwood collar would probably highlight the black line spalting better than the Desert Ironwood collar I originally had in place. With that in mind I headed out to the shop last night to give it a go. Here a blank approximately 2" square and 3/8" thick is secured between the live center and the front face of the spindle. This is a trick I picked up from Mark St. Leger back in 2009. There's plenty of friction to drive the blank and the flat face of the spindle ensures that the blank is square to the lathe.
The blank is turned round and a tenon is formed that matches the diameter of the opening that it is to fit. Using this method the blank can usually be removed from the lathe for test fittings and be placed back with a high degree of accuracy. I also faced off the bottom of the tenon and under cut the shoulder on the tenon so the area near the tenon doesn't bottom out before the outer rim does. I use a 1/2" bowl gouge to turn it round, and a 3/8" diamond shaped parting tool and 3/8" spindle gouge to shape the tenon.
I also took the time to shape part of the upper profile and to start to contour the under side of the tenon. I won't have access to it any other time and when people try to feel the inside of the vessel (and they will, other turners in particular) I don't want them to feel a sharp corner. I also prefer the depth of the tenon to match the thickness of the vessel at the opening.
A quick sketch illustrating the cross section of the finished collar. The tenon matches the thickness of the wall and the under side of the tenon is contoured so the transition is as seamless as possible. I like to get the cross section as close to round as possible and there should not be a gap between the vessel and the collar at any point. The collar should look like it belongs on the piece and was not an after thought.
To turn the top and inside of the collar I use a shop made jig based on a jig used by Mark St. Leger. It's scrap end grain maple. A groove is cut on one side to allow the wood to expand and contract as the jaws of the chuck are opened and closed. The face of the jig is turned flat and a recess is turned in the face so that the tenon of the collar fits inside snugly. The recess should be deeper than the length of the tenon so to outside lip of the collar rests on the face of the jig. The jaws of the chuck are then snugged tight and the collar is held securely by the jig, not the jaws of the chuck.
The collar is placed in the jig so the top and inside of the collar can be turned. You can see ring where the end of the spindle compressed the wax sealant on the outside of the blank. If you look carefully and compare with the photo above you can see how the gap in the is narrower in this photo, the wood is flexible enough to move and compress as the jaws are tightened.
And now the face and inside of the collar have been turned and sanded. The back side of the ring is relieved and the contour matched to give me the cross section I want. The collar can be removed, test fit, and then placed back in the chuck for any additional turning with very good accuracy. All of this shaping work is done with a 3/8" spindle gouge with a fingernail grind.
The finished rings side by side. Desert Ironwood on the left and Ebony on the right. The light ring on the Ebony is light reflecting off the highly polished surface. The color of the collar is almost pure jet black, unless you look closely for the grain some might assume it's plastic, not wood. One strike against the Ebony.
The Ebony collar set in place. I think the black color brings out the black line spalting in the piece.
The original Desert Ironwood collar. I think the Desert Ironwood looks more natural (less like plastic) but it doesn't work as well with the spalt lines. I do prefer the shape of this collar better, the one above is a little flat so if I decide to go with the Ebony I will probably turn a new one anyway.
Should I go with the Ebony or with the Desert Ironwood?