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dian
01-22-2013, 02:44 PM
so, i have been thinking:

according to conventional wisdom, the chisel angle on conventional drills is supposed to be 130°. indeed, on commercial drills it seems to be so. however i reckon an angle close to 100° will result in the shortest chisel edge (which seems desirable). ???

what benefit is there to the cuting edges being straight? again on commercial drills they are straight (althoung i have carbide drills with a concave edge). does the straightnes imply some other desirable geometrical feature?

last: what is this guy doing? grinding on the inside edge of the cup wheel? is he going to get a conical grind? how so?

http://i973.photobucket.com/albums/ae218/romandian/LrgDrillSharp13.jpg

Paul Alciatore
01-22-2013, 03:12 PM
If you look at his fixture, it appears to allow the drill to slide across the face of the wheel, not toward the inside. The photo was taken with it slid to the rear and it would be slid toward the camera to sharpen the rear most edge as seen in the photo. This will produce a FLAT facet on the end of the drill.

Caution: Sharpening something on the inside of a cup wheel will add additional forces that tend to tear the wheel apart. This is dangerous and it is not recommended under any circumstances.

As for the chisel angle, different angles will be better for different materials. Hardware store drills are probably a compromise for general work. Better drill geometry can apply to many specific materials and this includes other angles like the rake angle and the relief angle. Whole books can and have been written on this subject.

As for a straight cutting edge vs a curved one, I suspect that the mostly straight edge on most drills is more a factor of coincidence and convenience than one of design. An edge that is curved, within reason, would also cut just fine. But it would make controlling the geometry over the length of that curve more difficult. One possible advantage of a concave cutting edge would be to concentrate the chip near the center of that edge which would help keep it away from the OD of the hole. This would, PERHAPS, provide a better finish on the inside of the hole.

SGW
01-22-2013, 05:54 PM
This looks slightly like the setup used by the SRD/TDR drill grinders, which grind drills on the inside curve of a small cup wheel.

Jaakko Fagerlund
01-22-2013, 11:53 PM
The angle doesn't matter much when drilling, but on some materials/setups certain agnles work better than others, like 90 degrees on acrylic sheet and about 130-135 degrees on stainless.

One good reason for having bigger angle (like the 130 for stainless) is reduced lip length, which also means less power needed and less heat generated.

What affects the most when drilling different materials is the relief angle(s)

The setup in that picture is of a 4-facet grinder, looks like he is grinding the secondary facet on the rear side of the drill.

dian
01-23-2013, 04:13 AM
This looks slightly like the setup used by the SRD/TDR drill grinders, which grind drills on the inside curve of a small cup wheel.

are you really sure? the drills must be getting a hell of a relief.

"One good reason for having bigger angle (like the 130 for stainless) is reduced lip length, which also means less power needed and less heat generated."

i thought the chisel generates heat and eats power.

Mcgyver
01-23-2013, 07:43 AM
i bet that rig doesn't grind on the inside but traverses the face. Where did it come from, anymore context or pics?

to your Q, this is required reading for anyone who makes holes.

http://www.mmsonline.com/articles/choose-the-best-drill-point-geometry

I think this is the same thing but gives some background on Mazoff

http://www.icscuttingtools.com/the-champ/drill_point_geometry.pdf

sasquatch
01-23-2013, 10:07 AM
Mcgyver, Great posting, good info to keep.

Jaakko Fagerlund
01-23-2013, 11:24 AM
are you really sure? the drills must be getting a hell of a relief.

"One good reason for having bigger angle (like the 130 for stainless) is reduced lip length, which also means less power needed and less heat generated."

i thought the chisel generates heat and eats power.
That drill is getting its second relief angle ground and for a 4-facet grind that angle is perfectly well. The primary facet angle is the one that does the cutting and is in the 8-20 degrees (smaller angle for bigger drills).

Sure, the chisel edge generates heat and requires power, but it is not the only thing in the drill tip. One can easily reduce the cutting force (and thus heat generation) dramatically by splitting the point or grinding a cutting angle under the chisel edge so it really cuts instead of pushes.

becksmachine
01-23-2013, 11:41 AM
This looks slightly like the setup used by the SRD/TDR drill grinders, which grind drills on the inside curve of a small cup wheel.

are you really sure? the drills must be getting a hell of a relief.

Yes, it is indeed how the SRD/TDR grinders operate. It is mainly for smaller drills (< 1/2" ??) I think you can get an optional set up for up to 3/4" that uses a larger wheel. I couldn't quickly find an image to attach but you can google the website.

Dave

Rich Carlstedt
01-23-2013, 11:49 AM
Look closely fellows, the drill bit is locked down and the plate is only allowed to run accross the FACE of the cup wheel.

Years ago, I saw a drill grinder (Blackstone ?) that used the inside of a very small cup, but the cup wheel had a shell around it to absorb the force. using that method not only handles to point geometry, but the relief as well

Rich

01-23-2013, 03:38 PM
The chisel angle is typically about 120 degrees; this is seen by looking at the end of the drill with the cutting lips horizontal. The chisel line at the center of the drill is generally between 1 and 2 o'clock, roughly 120 degrees. This angle is related to the lip relief angle where 90 degrees is zero relief (won't cut), 110 degrees is normal relief, and 130+ degrees is high relief (for soft material).

The drill's point angle is measured with a protractor placed on the cutting lips (across the point). The most common point angle for general use is 118 degrees. For more difficult material a point angle of 135 degrees is often used because the chip width is less so it needs less power.

Curved drill lips, either convex or concave, result when a drill originally supplied with a 118 or 135 degree point angle is sharpened at another point angle. This is not commonly done because the increased lip length takes more power. The curve inside the flutes is what produces a straight lip when sharpened at a specific angle - a subtle bit of magic included in even cheap drills.
(End pedantry ;-)

The picture shows a facet type sharpener where one could set it at the primary and secondary facet angles successively to produce a 4 facet point. This design requires changing the infeed when changing the angle, then incrementally adjusting infeed to cause the facets to meet at the center. I'd be interested in a reference to the original article covering this design.

dian
01-23-2013, 03:42 PM
http://www.drill-grinder.com/manuals/Operating_Instructions_Model_80M.pdf

looking at step 6 and 8, i reckon the drill is moved across the face.

dian
01-23-2013, 04:05 PM
surprise, surprise (i knew i saw this before):

doesnt this yield a cylindrical grind as opposed to the (supposedly) desirable conical grind?

becksmachine
01-23-2013, 04:52 PM
And here is the aforementioned TDR version.

http://youtu.be/wGeC7qxl84o

Dave

oldtiffie
01-24-2013, 01:18 AM
The OP:

[QUOTE=dian;825073]so, i have been thinking:

according to conventional wisdom, the chisel angle on conventional drills is supposed to be 130°. indeed, on commercial drills it seems to be so. however i reckon an angle close to 100° will result in the shortest chisel edge (which seems desirable). ???

what benefit is there to the cuting edges being straight? again on commercial drills they are straight (althoung i have carbide drills with a concave edge). does the straightnes imply some other desirable geometrical feature?

last: what is this guy doing? grinding on the inside edge of the cup wheel? is he going to get a conical grind? how so?

http://i973.photobucket.com/albums/ae218/romandian/LrgDrillSharp13.jpg

Try these:

http://i200.photobucket.com/albums/aa294/oldtiffie/Black_book/Black_book2_P40-41_1.jpg

http://i200.photobucket.com/albums/aa294/oldtiffie/Black_book/Black_book2_P42-43_1.jpg

dian
01-25-2013, 04:51 PM
"The chisel line at the center of the drill is generally between 1 and 2 o'clock, roughly 120 degrees. This angle is related to the lip relief angle where 90 degrees is zero relief (won't cut), 110 degrees is normal relief, and 130+ degrees is high relief (for soft material)."

so simple, why do i realise it only now: by putting a relief on the edge, the chisel is rotated from 90°. as I have often read, that ist desirable to have the shortest chisel, i reckon not everybody really understands this.

btw, this is a carbide drill with concave edges (no, i didnt grind it, its new):

http://i973.photobucket.com/albums/ae218/romandian/2.jpg