View Full Version : d-bit problem

loose nut
09-03-2006, 08:13 PM
I have been reading here for a while and this is my first post to this board, desperation is a great motivator. HELP!!!! I have been trying to make a d-bit for a special size (.550") reaming job. First I turned a piece of drill rod (O1) down to .560" so there would be a grinding allowence, then I milled out the end to form the half dia. d bit shape. hardened the tool by putting the shank in the drill press, rotating at slow speed over a can of oil, heated it evenly with a torch and dunked it in the oil when hot enough. This should have keep it fairly straight but there was some distortion, then I drew the temper from the shank down. I have made a tool post grinder out of a Unimat SL lathe head stock mounted on a cross slide of my old 9" by 20 lathe and I ground a piece of 1" round bar. It turned out perfect better than I could have hoped for, But when I put the d-bit on to grind it, the d-bit looks like it was mauled by a mad beaver. What ever I tried changing feed rates,depth of cut nothing helps and I don't know why. I think my next try will be to turn to finished size ,mill the end and then only harden the 1/4 to 3/8" at the cutting edge. This should prevent warping, I hope.
Will this work or is there a better way to do this job. Buying expensive cutters is out of question. Any help would be appreciated

09-03-2006, 10:05 PM
loose nut,
If I recall you harden it while still round, then grind the flat on it and then stone clearance by hand before use.

But then I haven't made one yet.


09-04-2006, 01:07 AM
I would have ground it to size, ground the flat and tip shape then just heat treat the tip for about 1/2". The stone and part may be bouncing because of the interrupted grind.

09-04-2006, 02:46 AM
Hey Loose Nut. I usually turn and polish to size. Then mill the flat about .005above center line back about .250 or so. This doesn't leave a lot of room for shavings to gather but leaves less length to worry about warping after heat treating. Then heat treat only about .375 to .500 back like you did. Then I usually stone the cutting lip clearance and lightly polish the black oxidation off and then use the tool. I've had good luck with this routine and usually don't have any problems with warping. Good luck and let us know how you make out.

Norman Atkinson
09-04-2006, 02:59 AM
I went around the houses, so to speak.

.550 is a thous under 14mm!
You have a standard bit of high speed steel available.
You don't need to add a variable of tempering drillrod/silver steel.


You grind the flat to leave 0.275 plus 0.005 thous.
Your rake should be 10 degrees and your lead should be 10 degrees.

And that should be it
(Source Ian Bradley, The Grinding Machine, pub by MAP)



loose nut
09-04-2006, 09:14 AM
Thanx for the replies. I don't have the facilities to accurately grind away half of the d bit after hardening, though this would probably solve my problem. I forgot to mention that I had turned down the shank to it's finished size and that may have been a big part of the problem (to thin and allowed flexing) ,so I will use your ideas and give it another shot today (a little experimentation can be a good thing, alot will drive you to drink).
P.S. if you don't hear from me soon I will be in the shop with a case of beer!

Al Messer
09-04-2006, 09:51 AM
What japcas says---I second the motion.

Norman Atkinson
09-04-2006, 10:27 AM
I third the Lotion


09-04-2006, 12:23 PM
If you have a bench grinder you have the technology, at least thats how I grind the D-bits I have made after hardening, very carefully by hand. Then I stone the edge to finish, works great.

loose nut
09-04-2006, 04:01 PM
OK I just finished the reamer much the way Japcas said and it turned out pretty good but about .0007 under size "sgood 'nuf for this job". As for the grinder I tryed another piece of bar to see how it would turn out and the beaver invited some friends over. There is definatly something wrong with it. I'm going to dress the wheel down completly first and see what happens. I don't see how just siting there for a couple of weeks could screw it up. Gremlins maybe!

09-04-2006, 11:23 PM
Thinking about what I'd do if making that myself- probably bandsaw away the unwanted metal, then file or mill the result to get a good 'D'. I often grind a 45 degree chamfer on the cutting end, and that seems to work for me. Prior to notching out the unwanted metal, I'd turn the piece to nearly finished diameter, then after taking the divet out to create the 'D', I'd finish grind to diameter using the TP grinder. Then harden, clean, and check size again. Remount and finish grind to size. This step will be removing very little material, only that which has expanded the material due to heating and quenching while hot. Because of this, you'll have to be able to re-mount the piece with precision.

Final steps include honing the flat using a diamond stick, then possibly grinding on the chamfer somewhat to create some clearance so the resulting tool can actually enter the hole.

Don't take this as advice from a pro- I'm not- but this is what seems to work for me.