Let's Talk Drill Bit Grinders!

[Weldo]

This is all great information! Thanks to everyone for sharing!

Also if you first grind a full flatter clearance angle, then partially grind a tighter clearance angle (it helps to use a stop) you can create a 4 facet grind. I have done it and it works, but I did have to use a stop for accuracy of the secondary facet.

So if I understand the way this jig works, you'd grind a full sweep on each cutting edge with the minimum (recommended) drill bit extension, then you would reposition the jig a bit further AWAY from the grinding wheel and INCREASE the bit extension to meet the wheel. Then you would grind about 80% of the rear cutting edge. And you would end up with something like this...

Is that right?

 

[pontiac428]

That is a beautiful picture of a 4-facet grind. This grind is great for spot drills. If you have a high relief angle (from having short stickout on your Blue Point/Craftsman swing fixture), then you could grind just the primary reliefs similar to what you describe and be done. It would be my preference to do this on some sort of fixture grinder with an indexing head. Then you can dial in 5 degrees for the primary relief and whatever you want for the secondary relief angle, never needing the swing fixture. What I do is grind the secondary relief first at around 12 degrees, then make a chip clearing relief that removes the tail of the flute as viewed from the front, and lastly grind in the primary relief that makes the cutting edge.

I'd like to point out that what I call swing fixtures do a really good job for your general purpose bits, and are very fast to set up and use. When you start grinding facets, precision becomes more important, so a cutter grinder is the better tool for that. Either one will make sharp bits, so at the end of the day the guy who spent $35 on a swing fixture will be drilling holes just as happily as the guy who spent $1200 on a cutter grinder.

 

[Shootymacshootface]

This is all great information! Thanks to everyone for sharing!



So if I understand the way this jig works, you'd grind a full sweep on each cutting edge with the minimum (recommended) drill bit extension, then you would reposition the jig a bit further AWAY from the grinding wheel and INCREASE the bit extension to meet the wheel. Then you would grind about 80% of the rear cutting edge. And you would end up with something like this...

View attachment 313357

Is that right?

Wow! I am going to do some experimenting with my grinding fixture.

 

[Weldo]

I think I get what you're saying. So first you would grind a higher relief angle then the cutting edge. If you do your secondary relief at 12* what is the cutting edge? Something like 5* or 8*?

I believe you are suggesting to use the pivot jig ONLY for the secondary relief angle and then use a much more closely controlled method to establish the actual cutting edge. Is that correct?

 

[pontiac428]

I think I get what you're saying. So first you would grind a higher relief angle then the cutting edge. If you do your secondary relief at 12* what is the cutting edge? Something like 5* or 8*?

Yep, I use the same geometries as an end mill on a 4-facet grind. The primary relief forms the cutting edge along with the flute, and 5 degrees is the most common angle I see in the books. The secondary relief can be anything, but 12 degrees seems to work for most materials. Drill feed rate over RPM is what really determines what the angle needs to be. It's not critical, I've actually made the secondary relief very steep experimentally, it drills the same and clears chips better, but if you remove too much material from the secondary face the drill's rigidity and cutting edge support is compromised. So I stick with 12 degrees. The third relief, what I call the "back cut", can be totally random since all it is there to do is allow chips to clear.

I believe you are suggesting to use the pivot jig ONLY for the secondary relief angle and then use a much more closely controlled method to establish the actual cutting edge. Is that correct?

Yes, basically. The swing fixture actually faces the bit with a curved sweep from cutting edge to back cut, starting out shallow at the cutting edge and sweeping steeply to the back cut. That's what makes them fast and effective to use. To grind a primary relief facet to a bit sharpened with a swing fixture, you would be best served to use an indexing head for the second operation. Of course, you could do it freehand or in any number of shop-made grinding fixtures. I usually don't work this way, either I use my Drill Doctor for small bits and split-points, or a swing fixture for large bits, or I use the cutter grinder and grind a true 4-facet pattern with a back cut. There are many viable ways to arrive at the same result, so I wouldn't say one way is more correct than the other.

 

[Shootymacshootface]

I have about 3" from the face of the wheel to the hold down bolt. When I move the fixture to the side of the wheel it will be much closer for the small drill bits.

 

[Weldo]

Thanks for the explanation @pontiac428!

With my equipment I'll probably have to stick standard sweeping geometry but I expect that to work out just fine. The drills I'm working with are from a cheap Harbor Freight set and they are so abysmally bad from the factory. Almost every single one in the set of 115pcs had the heel end proud of the cutting edge. I don't know if that would be positive or negative rake but it's the exact opposite of what you need!

Even just a little touching up by hand and they cut pretty decent so I can't wait to set up a proper jig.

 

[rwm]

I had a Drill Doctor. I did not like it at all. The plastic was very flexible and trying to get a very precise split point was impossible. I am confused by some of the terms and claims. The 4 point facet grind shown above looks beautiful. I don't see these machines doing this. For example, here is the grind of the Cuttermaster:

This is a similar grind to the Drill Doctor as would be expected since they use basically the same technique. This pic differs from the 4 point grind:

I have found that if the secondary relief does not meet exactly at the center, the drill will not penetrate without a pilot hole and it will tend to skate. That was my problem with the Drill Doctor. Am I missing something here?
Robert

 

[Weldo]

Whoa! I had to google cuttermaster. Man that’s some nice stuff! I’ll lodge that away in my memory in case I can ever afford one of their machines. Cool stuff, and lots of different machines from end mill grinders to tap sharpeners. I didn’t even know that was a thing!

In talking about relief angles.

What’s really the benefit of a 4 facet over a standard curved type grind. Do they not both achieve the same end?

 

[pontiac428]

Here is a short but well-written primer from Joseph Mazoff, a subject matter expert on drill point geometry, that should help cast more light on the subject. Tubal Cain and Harold Hall both do a good job in their respective chapters of the Workshop Practice series, which are in the downloads section of this forum. I also like to read the various machine shop textbook chapters on tool and cutter grinding, some go deep while others cover the subject in brief, but the varying perspectives and illustrations help cement the understanding for me. Total disclosure; I like to read.