Making single flute D bit

Even a "center cutting" end mill doesn't plunge cut really freely. There is no relative motion at the center. Difficult for them to clear the chips. Your drill bit grind didn't have any cutting edges near the center! A D bit grind has the advantage of lots of chip clearance. It would be pretty difficult to hand grind a D bit so the split was dead center.

If your project will permit, drill a small shallow hole in the center, slightly deeper the the final flat. Then use a drill bit that you have flattened and relieved to form cutting edges.
 
Search on this site for ( Hand grinding a flat bottom drill ) by JimSehr Oct 23 2017.
 
Here are some pictures. The smaller drill is the one @Jimsehr gave me. It works great. The large drill is not center cutting. You have to have a hole in the center to clear the web. You can see the primary and secondary faces.
 

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pontiac428 said:
The flute profile of a drill bit is dependent on the point angle. I'm not surprised it didn't work without re-grinding the base and flank to something that provides better angles for cutting. You won't get any center cutting with a cut-off drill, either. My effort would be spent on making a D-bit for bottoming, like the plan you started out with. If you don't have any HSS stubs handy, get a stick of some O1 or W1, and slather it with damp borax before heating and quenching the roughed tool like @ericc said. Then clean it up on the wire wheel and do the final grind. It will work. D-bits are seriously underrated in this world of easy-bake insert carbide. They got machinists through a lot of challenges for a century and a half before computer generated tooling came along. There are some great chapters in the texts about D-bits, they shouldn't be so easily disregarded.
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That’s what I thought, but I’m no professional. The fact that people used d bits to make the first bored gun barrels I thought was a testament to the D bits reliability. I haven’t had much luck in making this angle right on the old drill bit. Turning drill rod, which I acquired tonight, then grinding two sides at a mild angle/flat seems easier than trying to get this drill bit right.

I’m determined to succeed with both methods. Being biased towards our own ideas when someone agrees with them is always good too….
 
ericc said:
Here are some pictures. The smaller drill is the one @Jimsehr gave me. It works great. The large drill is not center cutting. You have to have a hole in the center to clear the web. You can see the primary and secondary faces.
Click to expand...
Thank you ericc
 
pontiac428 said:
The flute profile of a drill bit is dependent on the point angle. I'm not surprised it didn't work without re-grinding the base and flank to something that provides better angles for cutting. You won't get any center cutting with a cut-off drill, either. My effort would be spent on making a D-bit for bottoming, like the plan you started out with. If you don't have any HSS stubs handy, get a stick of some O1 or W1, and slather it with damp borax before heating and quenching the roughed tool like @ericc said. Then clean it up on the wire wheel and do the final grind. It will work. D-bits are seriously underrated in this world of easy-bake insert carbide. They got machinists through a lot of challenges for a century and a half before computer generated tooling came along. There are some great chapters in the texts about D-bits, they shouldn't be so easily disregarded.
Click to expand...
Is the borax a must? Also, should I drill with regular bit first?
 
FTlatheworks said:
Is the borax a must? Also, should I drill with regular bit first?
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The borax just keeps the steel from oxidizing and pitting when you do your heat treat. As far as I know, it plays no other role in metallurgy. Removing stock with a regular drill and finishing the profile with a D-bit will make it easier when you are going for a flat bottom, but no pilot hole is needed for a D-bit if you grind a relief on the bottom edge from center on the non-cutting side. Just extend the 30 degree relief in the diagram below all the way to the center and it will plunge and leave a flat bottom. For more info, read the attached Deckel manual for a brief, or check some of the texts like Moltrecht if you have a copy, or the Workshop Practice series available on our H-M download library.
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If you search this site you will find a video where I drilled a steel plate with a flat bottomed drill. I drilled straight into the plate without a starting hole. The drill cut about .005 oversize. I used a new Harbor Freight drill that I cut shorter to make it more stout. Jimsehr
 
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