Let's Talk Drill Bit Grinders!

[pontiac428]

Nice job with the pics, the views from the side of the cutting edges are very clear.

I don't think the little kick up on the outer edge of the bit is an issue. It looks like it comes from the flute profile, not from the grind. You've already proven it's not a problem with testing!

Remember, the stickout defines the radius of the cylinder swept, so less stickout means a tighter sweep and a higher relief angle. It seems like the set point in the instructions for your jig gives a good result.

In the photo of the ground bit from the front, you can see ever so slightly that the cutting lip rolls off and that the cutting edges are bowed. This indicates that the clocking/indexing of the flutes needs a slight adjustment to bring the cutting edge straight and avoid that roll off.

A little bit of fine tuning, and you'll be re-grinding factory sharpened bits because you know you can do better than mass production can!

 

[Weldo]

The steel plate not being square to the drill might have caused you to only get one chip.

Oh I hadn't considered that! This drill press has a tilting table and one time years ago I meant to check the squareness of my drill press table to the spindle but I couldn't get the darn lock screw loose. I'll have to revisit that.

Remember, the stickout defines the radius of the cylinder swept, so less stickout means a tighter sweep and a higher relief angle.

The trouble I had with this setup is, what should be the orientation of the cutting edge? Since the stick out is not what the instructions recommend you can't rest the flute against the little index tab. I tried to eye-ball the cutting edge in a vertical orientation but the grind was way off, nowhere near enough relief.

In the photo of the ground bit from the front, you can see ever so slightly that the cutting lip rolls off and that the cutting edges are bowed. This indicates that the clocking/indexing of the flutes needs a slight adjustment to bring the cutting edge straight and avoid that roll off.

I was gonna ask a question about this! When grinding the bit should I try to not sweep PAST the cutting edge? I wasn't sure if it mattered or not but I can envision when I sweep the jig from heel to cutting edge, that is the drill tip contacts the abrasive belt heel first and then sweeps TOWARD the cutting edge, if I sweep PAST the cutting edge it will probably roll it over ever so slightly. Theoretically the cutting edge should break contact right at that tangent point. I may be experiencing a tiny bit of spring in the belt because a belt riding on a platen is not as rigid as a solid grinding wheel.

 

[pontiac428]

With my jig, I start from the cutting edge and sweep to the back. The point at which the cutting edge touches the wheel face will change if a bit needs any amount of grinding to restore its profile. As you grind back into the bit, the cutting edge follows the flutes and the clocking of the bit changes, so I find I need to re-set the bit in the fixture to restore the relationship before finish grinding. So I start with the cutting edge, which means I can establish the edge and its relationship to the flutes right away. As I feed the bit in, I may need to re-clock the edge. If I feed too much, I just end up with a facet that rolls off like you have pictured. So the root cause for that result is clocking.

This diagram shows the result of being off of correct clocking with the cutting edge. There's the bow in the cutting edge. This bit needs to be rotated clockwise on infeed (as viewed from the back of the bit's shank) to correct.

 

[Weldo]

So as the bit is fed into the abrasive I have to make sure that the flute is still up against the index stop?

I haven't been paying attention to that. After the initial setting I was just merrily feeding the bit into the belt. Gotta keep an eye on this!

 

[Weldo]

Nice job with the pics, the views from the side of the cutting edges are very clear.

Thanks for the compliment by the way! I picked up a macro lens for my micro 4/3 camera a few years ago for nature photography. Who knew it would come in handy in the workshop!

 

[Weldo]

Sharped a few more drills today! These ones I keep with my work stuff. I'm working in residential construction so they mostly see wood use.

The first is a 3/8" Used to be a split point. It sharped up well.

The next few pics are smaller drills, 5/16" and 1/4" and maybe a 3/16". They went less well. Geometry is kinda wonky... The pics are of bits progressively smaller.

5/16"

1/4" I think.

3/16" or there abouts.

It seemed like the smaller the diameter of the bit, the wider the chisel gets. I followed the procedure the same for all but the smaller ones didn't come out to well. Maybe smaller bits have a thicker web for added stiffness?

 

[pontiac428]

Hmmm, I still think your clocking is off a bit. I removed the flute guide from my jig because it wasn't a one-size-fits-all solution. Instead, I eyeball flute alignment off of the wheel and make corrections based on "reading" the point. What angle are you grinding your points to? Might be worth trying some flatter angles and seeing how it goes.

 

[Weldo]

So if I understand, the exact point in the inward travel where the drill contacts the grinding surface must be perfectly vertical. Is that right?

As you feed in toward the grinding surface, with each swing of the jig, the cutting edge will migrate slightly off of vertical, so do you have to reposition it every swipe or two?

 

[pontiac428]

So if I understand, the exact point in the inward travel where the drill contacts the grinding surface must be perfectly vertical. Is that right?

I'd have to see your jig in action to say for sure. My jig is oriented differently even though it provides the same action. In my case, at least with the stickout I've chosen, is 15 degrees off vertical, but that won't have any bearing on your setup.

As you feed in toward the grinding surface, with each swing of the jig, the cutting edge will migrate slightly off of vertical, so do you have to reposition it every swipe or two?

YES, this is the case with my jig. As I grind, the ground tip loses phase with the flutes, so I re-clock after roughing. Your infeed may rotate the bit for you as the flute follows the indexing finger. I think this is how it was meant to work, but with my setup that rotation was not very precise. So I removed the indexing finger and I made a scribe line on my jig for reference. Now I just align the cutting edge to my mark and no longer feel the need for the indexing finger.

Now I've got to go grind some bits so I can explain it better. Might need to dull some first.

 

[Weldo]

Haha, dulling bits is easy! Sharpeneing is another matter!

Thanks for the insights. I'll try removing the flute index and grinding a few drills with the cutting edge set each way from vertical.