The puzzling geometry of drill sharpening

[Nelson]

While on vacation in Joisey, I'm reading Tool and Cutter Sharpening, Workshop Practice Series #38 by Harold Hall. Never any good at geometry, I am puzzled by his explanation of the angles required in drill bit grinding, to wit, projection angle, chisel edge angle, clearance angle, I am confused as to the how- to of grinding a drill bit. He uses his grinding rest to illustrate, whereas, I have a grinder with a table.

Can anyone simplify this for me- I am confused.

Thanks!

Nelson

 

[Tenn]

I don't have that particular book but try this http://www.projectsinmetal.com/video-sharpening-twist-drills-by-hand-a-2-part-video-tutorial/ it explains or shows hand sharpening fairly well. What it doesn't show is various angles/geometry for various metals but that comes better if you first grasp what they do show.

~Chris

 

[brucer]

While on vacation in Joisey, I'm reading Tool and Cutter Sharpening, Workshop Practice Series #38 by Harold Hall. Never any good at geometry, I am puzzled by his explanation of the angles required in drill bit grinding, to wit, projection angle, chisel edge angle, clearance angle, I am confused as to the how- to of grinding a drill bit. He uses his grinding rest to illustrate, whereas, I have a grinder with a table.

Can anyone simplify this for me- I am confused.

Nelson,

as a tool&die maker i found this link as one of the better simpler analogies of drill sharpening i have come across..
http://users.bart.nl/users/summer/16mmngm/Articles_htms/ColinBinnie/DrillSharpening.htm


i also agree with bill above, any and everyone that has and uses machines at home should get the machinists bible, the latest edition of "the Machinery's Handbook" is an absolute necessity and great wealth of knowledge..

 

[george wilson]

Can't you just look at an unmessed with drill bit and copy it by eye with your grinder? The most important thing(other than having the cutting edge of the drill bear on the metal to be cut,rather than the TRAILING edge) is to get the center of the drill's lips to be in the center of the resharpened bit. If you do not,the drill will swing wide on the longest edge,and drill an enlarged hole.

Old timers,and I,used this little dodge to custom drill special size holes that they needed,but had no drill on hand for.

Once you have mastered getting your drill to drill correct size,decent holes,you can work on perfecting your cutting angles further.

When I was 17,I watched a friend's truck driver father grind a drill backwards in an attempt to sharpen it to mount a license plate. I warned him he had done it backwards. Then,I had to listen to a little lecture on how he had been a maintenance man in a factory,WHILE he resharpened the drill,which would not cut!

 

[Nelson]

Thanks for the links and advice guys. The drill sharpening book by Hall had me very confused with the angles, drawings, jigs. I am also reading his dividing book, and it is much clearer. I will need to check out the MG and links right away.

Yes, the education has begun!

Nelson

 

[Nelson]

George, I'm too stupid and inexperienced to reverse engineer grinding drill bits. At the same time, I don't want to use a Drill Doctor because I learn nothing that way.

Nelson

 

[Pontiac Freak]

Thats the only way I know to sharpen a bit, copy a new one! Sometimes it takes a time or two to get it right but beats buying a new or a drill Dr. ;0

 

[george wilson]

Pontiac,that was exactly my suggestion. I forgot,Nelson,you could buy one of those drill grinding guides. Rules with an angular end protruding out on one end that you can rest your drill lips against to check that you are making the same angle on each side. I can't remember what they are called,because I have been just doing it by eye since high school.

Here is a USEFUL tip to sharpening messed up LARGE drills: Mount your toolpost grinder on the lathe. Stick the large drill out of the spindle taper adapter,chuck,or whatever you have that makes the drill RUN TRUE. No run true,no good!

Set the compound at the angle you want to grind the end of your drill to. Proceed to rotate the drill in the lathe under power,and grind the end of the drill until the cutting edges on BOTH sides have been kissed by the grinding wheel all the way across their cutting edges. Then,CAREFULLY,and by degrees,grind relief on the lips of the drill on both sides,gradually creeping up to almost touching the cutting edges. I like to get them to about 1/100" of the cutting edge. You can see a remaining bright line where the toolpost grinder ground the cutting edges. After you have gotten VERY close to the cutting edges,take a small India hand stone,and hand hone that last little bright line away on each side.

When you have done this perfectly,the drill will throw equal chips out of each flute,and drill a true to size hole(providing you got the drill well centered in the lathe before grinding.

I have rehabbed 3" drills like this,that had really been totally messed up by bad grinding by others. You can get such drills cheap.IF the seller has the sense to see that they are messed up.