D Bit Confusion

but for a dedicated drill I would add 5 degrees of back relief and grind it further up the shank for better chip exits.
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Would you be speaking of adding a rake angle to the top of the side edge parallel to the length the rod? (everyone has their own terms and it gets confusing)
 
Can you fill us in on angles and tell us how well it works?

I will post some pics of my dreamers tomorrow . It is a combined drill and reamer .
 
OK. so today I took delivery of thin book that answered my initial question- why the funky geometry on D Drills in general and gun drills in specific. The book is one of the old Lindsay publication called "Making Rifle Barrels". IT is a reprint of an old article showing how the Enfield Armoury made barrels for the No1 Mk 3 in 1916.

One sub article is on gun drills and how they are made and function. It is stated that the reason the front cutting edge is ground off centre is so that it forms a circular valley and ridge to the inside next to the centre line of the bore. This gives the bit added support to keep it drilling straight. It also says that those barrels were drilled at 2000 RPM!!! With a .003" feed per revolution!!! Yikes! I think I shall slow mine down a bit given that I don't have 500 PSI oil flushing the chips out.

I looked at the bit I had made previously that didn't cut true and I see that I had the geometry out of whack, It is now in whack but I don't have there pressing need to drill a long 3/8" hole. If I ever get caught up I may drill one just for funzies.

It also showed that the D Bit they used was a short cutting end with all the geometry and clearances and it was soldered onto a tube that supplied the oil. The business end also had a small oil supply hole drilled into it before being soldered on. So it takes a gun drill to make a gun drill.

P.S. The book was purchased from Your Old Time Bookstore along with a number of others. Great prices on these old gems.
 
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