Undercut

[Seelector]

Great undercut up through my polishing stone, an old King s-3, which I’m told polishes closer to 8k. Nothing in the coti.

Sounds the same as what I see, although I’ve never pushed a finisher as far as you. Maybe mine would undercut eventually but I never got that far. 20 minutes ago I shaved with a great 12k edge that I bet wasn’t undercutting like a demon, so it can’t matter all that much.

I did a quick hone with a test razor today and this is what I saw.

Wester Bros 5/8. EBay special that had a bad frown when I got it. It’s still kinda there now that I look at it again. I know what you’re going to say @H Brad Boonshaft and I agree the heel does need correction

Maybe 20 laps on 8k shapton glass and it’s undercutting nicely

Now onto a coti with light slurry from a bout. It took 5-10 laps to get there and it undercuts, but not as strong as the 8k.

I stayed with the slurry for a while and diluted it down, and the undercut got better but was never as good as it was on the 8k. I should have take a picture then.

I washed off the stone and blade then honed on clear water. Nothing, and it never got there. It’s hard to see but the blade is pushing a wave of water in front of it.

Maybe it would have got there eventually, or I should have stayed with the slurry longer. I guess this where I think there’s room for some improvement. Or is a weak undercut just what some stones do?

 

[captaincaed]

Also consider how much softer the Shapton is than the coti. It’s DESIGNED to release grit, clearly visible in that photo. It’ll release some quicker than you might think. That’ll undercut pretty easily.

If you do a fresh wash and super light pressure, no torque, can you still get undercut? Only the first 3-5 strokes really count, then wash rinse repeat, literally. When you can undercut the whole length of the edge in those first 3-5, That’s when I feel like the 8k is maxed out or getting close to it.

 

[Christian1212]

I would avoid slurry on the coticule after the 8k to start. Chances are you probably don't need to slurry the coticule unless maybe coming from like a 5k. Check to see how the razor shaves though, that is what is important.

 

[Darth Scandalous]

I hear many folks say 8-20 passes on a finisher after a proper 8k polish is enough

I think that is entirely dependent on what the finisher is.

 

[Seelector]

If you do a fresh wash and super light pressure, no torque, can you still get undercut? Only the first 3-5 strokes really count, then wash rinse repeat, literally. When you can undercut the whole length of the edge in those first 3-5, That’s when I feel like the 8k is maxed out or getting close to it.

Yep, that's exactly what it does. I use a dollar store scrub brush to clean off the swarf and the undercut is just as strong on the first or maybe 2nd lap with clear water on a clean stone.

I forgot to mention that I checked the coti and it does have a slight dish. I didn't lap it flat for the pictures above.

 

[Eshebert]

I think that is entirely dependent on what the finisher is.

Exactly, we see that so many people want to put every finisher in a box, every Coti in a box. They want a prescribed method, make statements about slurry as absolutes. My thought and my comments have been, stay with synthetics and be happy. Naturals and especially coticules are not simple, you will need to figure out what the stone/razor combination requires. I try never to speak in absolutes. I don't care if you disagree because I do not feel the need to be right. I know what works for me, and when it don't, I reach out to like minded people and bump ideas off of them. I enjoy the process and the tinkering. If that ain't for you, stay with what you know.

 

[Stapelfeld]

I’m definitely doing more than 10 to 15 laps on the 12k. I’ll cut them back and see what my edges are like.

I agree with JPO 10-12 laps for me as well. After 5 laps you can actually see the mirror edge on a microscope.

 

[Stapelfeld]

I would avoid slurry on the coticule after the 8k to start. Chances are you probably don't need to slurry the coticule unless maybe coming from like a 5k. Check to see how the razor shaves though, that is what is important.

I agree with this, it's important to know how much slurry you need depending from where you're doing the jump. From 5k I say you need some form of slurry, you will need to find out what works for you. From 8k I probably wouldn't use any slurry just water.

 

[Eshebert]

Maybe it would have got there eventually, or I should have stayed with the slurry longer. I guess this where I think there’s room for some improvement. Or is a weak undercut just what some stones do?

I have a Coticule that has very similar characteristics, and it looks similar to yours in the picture. It can be frustrating but the finish is worth it. What works for my stone is more laps on water after Dilucot. I don't count laps on that stone, it only builds frustration. I use medium/light pressure (so I am turning the edge into the stone). Once I see undercut across the full blade length I start reducing pressure, I step down the pressure while maintaining undercut. Its a long slow process, but worth the patience. If I was a one coticule fella, this would not be it, Its a great finisher for me so I use it in my rotation.

 

[Frank Shaves]

I have noticed this behavior. It looks like the edge is making good contact with the stone and displacing all of the water in its way without the water rising up onto the blade. My guess is there is some scientific explanation having to do with surface tension that is beyond my knowledge.

As long as you are getting to sharp, this may not matter.

 

[captaincaed]

I use medium/light pressure (so I am turning the edge into the stone). Once I see undercut across the full blade length I start reducing pressure, I step down the pressure while maintaining undercut. Its a long slow process, but worth the patience.

This is very similar to what I’ve been experimenting with. It really can be a lengthy process. But some stones seem to reward the extra work.

 

[Frank Shaves]

How high the water rises on the blade may depend on the speed and acceleration of the blade, and this may vary depending on the stone and whether you are using one or two hands.

 

[bmiller3230]

Undercut is important to me as well. Given that you are not using more speed, etc it should undercut at each stage on every part of the blade, assuming a flat (lapped) honing surface. After bevel set, I don't move on from a stage until every part of the blade does. I'll do some half strokes, or circles and then test with x-strokes on that part of the blade until it passes on each side before moving on.

 

[H Brad Boonshaft]

So, keep in mind that you do not need to “pass” all test to ensure that an edge is refining or “shave ready”.

You just need to “calibrate” your test. You need to understand what the results of your test mean, for that razor, on that stone and at that level of honing.

One of the simplest tests is to look straight down on the edge with magnification and some light behind you.

The edge off a 1k looks nothing like the edge off an 8k. If you tried to make the 1k edge look like an 8k edge, you could make yourself crazy.

It is all about learning your stone, with your current technique. Your “technique” will evolve, as will your stone progression and if you use the same stones, you will get to understand their limitations and benefits better.

Personally, I do not use water undercut as a test, and never really pay attention to what the water is doing on a particular stone. I also do not use thumb nail test or hair test.

I look at the straightness of the edge, is the stria pattern going all the way to the edge and look straight down on the bevels to ensure they are meeting microchip free. I also do not use thumb nail or hair test. I do use thumb nail test for knives, though. I do use a lot of ink.

It is all about what works for you.