Thread: Drill grinding weirdness

Getting some very odd results with grinding a drill bit.

Any drill sharpening gurus on here ?

Got a 1/2 jobber with a recent tooling order from the states.

I use a Craftsman Grinding Jig.

When I sharpen my 12 or 13mm metric drills, all good

With this 1/2 one, the chisel point ends up being very wide. Something very strange is going on, and the drill is getting pretty short

Lip angle is 118 for all the grinding. I've read the instructions for the jig again, and made sure stickout is correct etc.

Any ideas (1/2 inch on left, 13mm on right for comparison) ?

The tongue / web sometimes gets thicker as the drill is shortened, use a bigger pilot hole ?

Originally Posted by berg243

is the 1/2 inch one moving around while it is being sharpened?

Don't think so.

The facet you see is just me cleaning up the previous grind.

the cutting faces need to be 180 degrees from each other (parrallel), they also need to be the same length diagonally and the same angle

use the corner of your grinding wheel to take some of the back out of the flute that will help with reducing the nose size and start better

hard to describe over the net its easier to show you lol

As above. You need to thin down or sharpen the web using the corner of your grinding wheel. Takes a bit of practice but once you have mastered it you can drill some pretty big holes with no pilot.

The flutes on the drill are not as deep as the others. Relieve the flutes as Bobba has shown. Grinding jigs are OK to start out but with practice you will grind drills much faster with good hand and eye co-ordination, and easily alter the angle to suit the material better (flatter angle and minimal back relief for harder metals).

That drill has a thicker center section and a different flute profile ground into it, the center section generally gets thicker as a drill gets shorter or some drills are just designed stronger in that center part (most cobalt for example)
If you are going to sharpen it in your jig you will have to rotate your start point on both sides so that the non cutting center line diverges further away from 90deg to the cutting faces.....or as above other posts...some jig type grinders will do a center point tho.

Or simplest of all whack a 3-6mm pilot threw first then the center doesnt matter

@Bobba, when you are near your grinder again, can you pls take a photo of how you hold the drill to do that, and also try to explain what the movement is - are you pressing side to side into the wheel, or front front to back ? Do you rotate the drill in any way ?

I've tried watching some youtube clips on thinning the web, but have not come across one yet that explains it in a way that I understand it.

Originally Posted by ebf

@Bobba, when you are near your grinder again, can you pls take a photo of how you hold the drill to do that, and also try to explain what the movement is - are you pressing side to side into the wheel, or front front to back ? Do you rotate the drill in any way ?

I've tried watching some youtube clips on thinning the web, but have not come across one yet that explains it in a way that I understand it.

Thin out the web by using the edge of the grinding wheel.
Works best when the edge of the grinding wheels are not rounded over.


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Originally Posted by ebf

@Bobba, when you are near your grinder again, can you pls take a photo of how you hold the drill to do that, and also try to explain what the movement is - are you pressing side to side into the wheel, or front front to back ? Do you rotate the drill in any way ?

I've tried watching some youtube clips on thinning the web, but have not come across one yet that explains it in a way that I understand it.

Yet mate will have a look tomorrow.

Originally Posted by Bobba

Yet mate will have a look tomorrow.

Cheers, appreciate it

Hey @veitnamcam, so I ground it "normally" the thing looks bloody weird, but did some test cuts like you suggested.

Works fine Drilled a 5/16th pilot hole, and then used the now stubby 1/2

Cuts well, equal chips on both sides. Miked the hole, and came out at .5015, so happy with that...

Originally Posted by ebf

Hey @veitnamcam, so I ground it "normally" the thing looks bloody weird, but did some test cuts like you suggested.

Works fine Drilled a 5/16th pilot hole, and then used the now stubby 1/2

Cuts well, equal chips on both sides. Miked the hole, and came out at .5015, so happy with that...

Most of the time I am drilling holes with clearance for bolting stuff etc so I am generally not concerned with hole size accuracy. But when you want a hole bang on size here is a few wee tips.

Pilot drill but not too big,just big enough to clear the center of the final size drill.
Excessive rake and or too big a pilot while it will cut like hell can cause a hexagaonal over size hole.

And the pro tip, radius the corners of your drill....where the cutting edge meets the flute edge.It essentially turns your drill into a reamer and done right it will cut bang on size untill the flutes get worn and it starts cutting undersize.

Originally Posted by veitnamcam

That drill has a thicker center section and a different flute profile ground into it, the center section generally gets thicker as a drill gets shorter or some drills are just designed stronger in that center part (most cobalt for example)
If you are going to sharpen it in your jig you will have to rotate your start point on both sides so that the non cutting center line diverges further away from 90deg to the cutting faces.....or as above other posts...some jig type grinders will do a center point tho.

Or simplest of all whack a 3-6mm pilot threw first then the center doesnt matter

what he and gundoc said.
First test at trade school was sharpen a drill to cut nor more than 0.1 of its size. as such I look with disdain on drill sharpening gadgets.
I consider myself relatively proficient at sharpening drills without guides at least up to maybe 5/8". Once they get bigger and the hole size is important then the guide and ruler come out
A big drill can cut way oversize if you get it wrong

OK , in simple terms , start at the cutting edge , go light quick pressure & as you turn the drill go slower & harder , as you want the from the cutting edge back relieved , get some chaep drills to practice on , I used to hand sharpen drills by hand back in the day .