Friday, October 28, 2016

Reverse Chucking, What!

 This is a process in which you can turn the bottom of the bowl after the inside and outside are finished.  This will add a nice appearance to your work and make the bowl stand out.  I am making the assumption that most readers will have some experience turning wood, if you have question please email me a at:

First step is to attach a face plate to front of the bowl; these natural edge bowls will always have an uneven surface to one degree or another.  Slip some shims under the face plate to level the face plate to bottom of the turning block.  The big trick here is to use long screws that will go through the shims and well into the wood, picture shows 3 inch wood screws and shims.  As an added safety measure support the turning with the tail stock using a live center, 60 degree cone works well. 
During the rough turning phase create a spigot for the scroll chuck; I use a recessed dove tail of about 3/8 inch deep.  Before you remove the face plate make sure that the scroll chuck will tighten within the dove tail.
Remove the face plate, turn the bowl around and attach to the scroll chuck; again supporting with the tail stock and live center.

Turn out the inside of the bowl leaving about 1 inch of thickness all around sides and bottom.  The center post can be removed with a Forstner Bit. Clean up the bottom and set aside for the wood to dry.

 Next step after the wood has dried to moisture content of about 12%.  Finish turning the bowl to the final shape and thickness of your choice, sand up to 250 grit. Put one coat of finish on the bowl to protect the wood from stains (fingerprints) and discoloring.



Ready to turn the bottom, in the picture (right) you see an air rotary valve attached to a long lamp nipple with a fitting to seal the head stock and a extension with a 6 inch vacuum cup.  Do not over tighten finger tight only and make sure the vacuum line is securely attached to the air rotary valve.   See above vacuum generator, gauge and value to control the amount of vacuum.  In round numbers (at sea level) 1 inch of vacuum equals .5 lbs. of pressure.  6 inch vacuum cup has about 28 square inches of surface area; 26 inches of vacuum X .5 = 13 lbs X 28” = about 364 lbs of holding force on the bowl; this works well unless you go to fast or snag a chisel; this will move the bowl off center.

Reattach the chuck to the bottom of the bowl and attach a Reverse Chucking Alignment Adapter to chuck (left) and place in the tailstock.  Advance bowl to within about a 1” of the vacuum cup and lock the tailstock to the lathe bed, Turn on the vacuum up to about 10 inches and advance the quill to contact the vacuum cup watch the vacuum gauge, the inches of vacuum will increase when the bowl is in contact with the vacuum cup.  Loosen the chuck and withdraw the tail stock, adjust the vacuum to what you want, to much vacuum can mark the bowl.

The bowl will always be slightly of center maybe 1/8”, this is ok; it won’t be noticeable.  Be sure and don’t touch your finished surface with a tool. Turn the bottom, sand and apply finish. 

Need tools check me out at we sell handcrafted Crown Tools.

No comments:

Post a Comment