"D" Bits.

[BRIAN]

How do you make a D bit, and can you use them as reamers.
It all sounds easy. but are there any wrinkles to know?

 

[DMS]

I have made 2 "D" bits, and they both worked pretty well. If I get time, I will take some pictures of them later and post them.

The thing about D bits is that they cut very slowly, and they need an accurate starting hole. Run them at low speed, with plenty of oil (at least in steel, where I was using them). You need good lubrication. Also, clear chips frequently; there are no flutes to pull the chips out for you, so you have to retract the tool fully. When cutting a 0.375" hole, I was clearing chips every 0.060" of depth cut. Failure to clear chops can lead to the tool seizing, which is no fun.

They do make a hole with a very fine finish, perhaps as good as a reamer, maybe slightly less, I have not compared the two. The most common case I have seen them used for in the home shop is for drilling long straight holes, where twist drills tend to wander.

To make them, turn a piece of drill rod (I use W-1, but many others use O-1, I believe this is called "silver steel" in most of europe) to the desired diameter. At the cutting end, cut away half the diameter (this dimension is important, ideally you want it exact, but going over a few thousandths is better than going under a few thousandths. Harden, temper to light straw, then sharpen the flat surface.

Here are some pages that may be useful.

http://www.machineconcepts.co.uk/smallpipes/tools.htm
http://www.homemodelenginemachinist.com/index.php?topic=5251.0

DMS

 

[DMS]

Finally got some pictures. This one is 0.375" in diameter and cut the best of the bits I have made.

 

[Tony Wells]

It helps if a little material is polished off the shank, leaving the first 1/2-3/4" full diameter. It's asking quite a bit to have the full size drill rod not seize in the hole if you are going very deep. Reamers and twist drills (and most other rotary hole-making tools) have a little back taper to make sure there is a little clearance. After all, you wouldn't want a press fit for the shank, right?

 

[george wilson]

I have used and made D bits. The most difficult case was when I made a D bit about 9/16" diameter,and drilled through the length of a 16" propeller shaft of VERY tough bronze with a 10" Jet lathe. I was making a flintlock pistol barrel. The hole was off center only .002" where it emerged at the far end.

I made it from 01 drill rod. You mill down the last 1 1/2"(for a drill this large) of the bit,leaving 1/2 the diameter + .005" high. The tricky part is hardening the bit without it warping,which it WILL due to the difference in surface area on either side. I hardened only the tip of mine,and did not submerge in the quench except the tip of the drill. Let the drill cool before withdrawing from the quench,or the heat from the rest of the bit will creep to the tip and soften it too much.Draw to straw.

Grind a slight angle on the end of the drill. The cutting side must be the foremost edge,and you need to relieve the tip like a lathe tool so the cutting edge can reach the work FIRST,or it won't cut.

To use the bit,you must drill a hole at least as deep as the diameter of the D bit. Use a regular drill. For critical accuracy,it is best to single point bore the hole so that the D bit goes in without shake. You could ream the hole if you have the correct reamer,but reamers may have a little taper at their cutting ends,so be sure the D bit goes into the finished hole at least to 1/2 diameter. Single point boring is best as a reamer held in the lathe may be very slightly out of parallel. If you don't start the D bit absolutely true,it will follow the error,and drill off center progressively getting worse as it goes deeper.

You can make an effective reamer by hardening a piece of round drill rod,and grinding the cutting end off at a long angle,say 30º. They work quite well. If the hole is not going clear through,withdraw like the D bit to clear chips. This is an 18th.or 19th.C. type reamer,but it still works now just like it worked then.

 

[BRIAN]

A late thanks to all for the info,
i made a reamer and it worked just dandy. I am now going to make a series of D bits for the tool box.



Brian.