I saw your post yesterday, so I waited until this morning, while I am still tidying up my shop, to show you how I make them.
Everything what the others have said, I do, except that now I grind the half diameter using my surface grinder. I tend to attempt to get the bit exactly half the diameter, only because I have the facilities to grind it very accurately, if I was grinding by hand, I would err on the high side, as suggested by the others.
To me a D-bit is a temporary tool, only used for maybe a few holes at most, so unlike the others, I rarely temper after hardening, unless I think the tool will be used again or reshaped by grinding in the future, or if it is going to be removing a lot more than a few thou that they are usually used for. I just polish up the surface and grind away to shape.
Only ever dress the cutting edge from the top flat face, and very gently, you don't want to be removing too much metal. I tend to stick with a fine Arkansas stone with thin oil as the lubricant, only because that I find diamond laps tend to clog up too quickly when used on steel, and also they have a tendency to remove too much metal.
This is my collection that I have saved over the years, a lot of the ones I make, I throw away after they have finished the job.
This shot shows the front rake that I use.
To me, one of the most important bits is to relieve the front cutting face on the right hand side.
I relieve away just over half the width, unless it is a round nose tool, then I try to get it spot on centre.
You can just make out the cutting face on this shot.
As Dave and Stew have said, there are many ways and methods to make D-bits, and everyone has their own favourite way to grind them up and heat treat them. My way is just one of many. You just have to find what works for you and stick with it.
Making tapered D-bits require a slightly different technique as you don't know when you have ground down to half diameter. If you need to know about that, just let us know.
Bogs
