Long boring D bit

[Christophe Mineau]

Here is another use case of my hand chuck, it is for holding a long boring D bit



Here is how the D bit is made : it's a piece of HSS round stock, ground to shape, that is silver brazed to a rod of standard steel. The tip has a D shape section, that is to say it is ground just above the diameter line. The tip is also ground at a maybe 30° angle, if you look at it from the top :



Here it is in use on the lathe, through the tail stock, hold by hand with my hand chuck.

You need a steady rest and you need to drill first a guiding hole in order to drive the D bit at the begining.
The progress is quite slow, you need to be patient. You push for maybe 5 to 10mm, sometimes less, and you remove the bit to clean up the chips that strick together on the flat section of the bit.
I frequently soak the bit in a bottle of lin seed oil in order to lubricate.



And here is the result, through a 40cm long piece of cherry. This is the output end, you can see that I am only maybe 5 mm off center, which is a quite good result.
No need to say that with a twist bit, this result is nearly impossible, you woold be way more off center.



The piece is then trued up between centers on the lathe.