I have 1/2″, 3/4″, and 1″ screw boxes and taps for cutting wood
threads. I have had very little success using the screw boxes because
of grain tear-out. My original boxes were commercial which never
worked. I later made my own but always used a commercial knife as the
cutter. Although I have had some success with the home made ones,
tear-out is still a problem. The bits are razor sharp but the angle is
blunt to the cut. The commercial blade is a 1/4″ square block of
tool steel ground to a V and sharpened.
Recently my wife and I visited a Shaker farm in western Massachusetts.
There they had the original screw boxes used by the Shakers. The
blade in the box was a V shaped piece of very thin spring steel (very
much like a bent razor blade) and set at a very low angle to the cut.
I know that different types of wood work better than others and
soaking them in oil helps. The problem with doing this is that
different wood types don’t match the finished project and when the
wood has been soaked in oil finishing is a problem.
My questions are: Has anyone had any success hand cutting the male
thread using a screw box in say oak or cherry? If so, what did you do
to get the —- thing to work? Does anyone know how the shaker blade
was made or can it be purchased today?
I have no problems with tapping holes because when the tap is driven in it more or less compresses the thread into the wood.
Replies
Bugsy,
The tools are basically good tools, they are just not sharp. I have had several sets and have tuned up dozens more for folks.
The link below will help.
Stephen Shepherd
http://www.ilovewood.com/alburnam10.htm#Threads
Thanks Stephen,
I thought they were sharp but I will try again. Questions:
Currently my knives are only sharpen on the outside of the V and have micro bevel. Should the inside of the V be sharpen? The V is cut into the block of steel at a 30 degree angle, is this corrent? About 3/16" back from the tip of the V a Notch is cut. From that notch to the tip of the "V" it slops up at about 10 degrees. Is this correct? When looking at the side view of the Knife, the tip of the "V" slops back from the top two edges of the "V" by about 5 degrees. Does this sound OK? Some hints here sure would help. Thanks again
Bugsy
Bugsy,
I sharpen the little cutter just like a V chisel with the two wings sweeping forward. The inside should be honed flat and the outside bevel should be very sharp. Pay particular attention to the point at the bottom of the V so that both side bevels line up.
The angle of the cutter insures that the top of the cutter is actually engaging the wood first, scoring and preventing splitting as the rest of the cutter makes the external thread. Starting the wood squarely and properly into the screw box will insure a good cut. I put a strong chamfer on the end to help get it engaged and I usually allow a little extra to cut off the ends, sometimes they are chewed up if the cutter and threads don't line up.
The tap should be dressed with a triangular file in the V's and a flat file in the slot, makes for much easier and smoother cutting. Avoid threading in endgrain, it is tricky.
Stephen Shepherd
http://www.ilovewood.com
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