How do I avoid tool burnishing/bruising of inside end grain in bowls?

[Ric Williams]

The title pretty much says it all. I keep getting the lighter streaks that show up across the end grain areas near the lip of bowls once I start sanding. I try to be careful about bevel pressure when cutting, to float the bevel rather than "riding" it, but the problem persists. I'd like to be able to blame the wood, impurities in the steel of my tools, increased sun spot activity, or my misspent youth, but I suspect it boils down to operator error on my part.
Any suggestions or advice?
Trying a different hobby is out, so don't bother with that one. I'm hopelessly addicted to woodturning and you're all complicit in it.

 

[hockenbery]

This more likely is some minor tear out related to not riding the bevel well on the entry cut.
this is a cool trouble spot.

try a 1/4“ bowl gouge(3/8 diameter)for the finish cuts on the first 1-2” of the inside wall.
i use a Michelson grind but a traditional grind works fine.
the small gouge works great here - a technique I picked up from Jimmy Clewes

I show this in demos where I return a dried bowl.
the second video in the thread below is returning the dried bowl.
fast forward to 24:15 you see a light cut with a 1/2 bowl gouge(5/8 dia bar) - this is a nice sandable surface.
at 24:30 I switch to the small tool - this is surface that 220 will rough up a little.

Working with green wood

The August 2015 AAW JOURNAL has great article by David Ellsworth on working with green wood. This is followed by 11 marvelous ways turners turned cracks into features. One of David's themes is that the deck is stacked against newer woodturners successfully working with wet wood. Might be a nice...

www.aawforum.org

 

[Ric Williams]

Thanks! That was very helpful. I'd watched it once before a while back, but didn't remember the tip with the smaller bowl gouge for the finish cut of that inner area below the rim. I'll try working with that for a while. I was wondering if that might be fine tear out, since it's in the end grain. Thanks for the help!

 

[john lucas]

Are you grinding the heel off your tools. This will burnish a groove if it has a sharp edge. Most of the time it happens near the bottom. And brief hesitation as your going down a bowl and you can get that burnish mark.

 

[Randy Anderson]

As John says, grinding the heel off the gouges you use for inside cuts helps. I have U flute gouges I use for all my inside cuts after I get close to shape and grind most of the heel off of them. If I don't there is a perceptible cup in the bevel from tip to heel that can drag and makes riding the bevel around the inside curve harder to do smoothly. And yes, they can be very tough to sand out.

 

[Odie]

Assuming identical grinds, sometimes simply changing to a smaller shaft diameter gouge can be all that's needed to prevent the burnishing. It all depends on the inside curvature of the bowl, the individual characteristics of that particular piece of wood, and how much pressure is required to stabilize the cut within minimal harmonic vibrations. Through your hands, the tool almost always tells you exactly what it's doing. Also, sometimes adjusting rpm can be a controllable variable that can effect the heat generated between metal and wood.

Of course, all of this is dependent on the level of sharpness in the tool edge. Sharp-ER tools are always more controllable with less effort because they are less resistant to the cut, and usually on a distinct perceptible scale.

-----odie-----

 

[Ric Williams]

Are you grinding the heel off your tools. This will burnish a groove if it has a sharp edge. Most of the time it happens near the bottom. And brief hesitation as your going down a bowl and you can get that burnish mark.

Yes, I generally grind the heels off. I made my own Wolverine compatible jig to be able to put up to 3 microbevels on in a fairly consistent manner to round the heels off.

 

[Donna Banfield]

I would take a lot more off those three bevels. I use the Michelsen grind, and I also free hand sharpen. Both those grinds have a lot more taken off the heels. My gouge grinds look like the reverse of yours. My cutting edge bevel looks more like your very bottom bevel.

 

[Ric Williams]

I would take a lot more off those three bevels. I use the Michelsen grind, and I also free hand sharpen. Both those grinds have a lot more taken off the heels. My gouge grinds look like the reverse of yours. My cutting edge bevel looks more like your very bottom bevel.

I'll give that a try. Thanks! When you refer to your grinds, I get the impression that you use a fairly narrow cutting edge bevel. Is that correct?
I figured that until I got more information and experience, I'd start with taking the bottom half of the bevel back a limited amount, to avoid making more trouble for myself than I normally do.

 

[robo hippy]

If I was doing an end grain bowl, I would use a NRS. You can get glass smooth cuts in end grain with the NRS. The bruise marks are from the heal of the gouges, as others have said. I free hand grind mine off, and make them round rather than different bevel sets. When they get to the point of half the bevel being there, I grind them down again. I want 1/4 of the cutting edge bevel on the tool, or less. I did see Christian Burshard demo a couple of times. His tool/gouge had no bevel at all on it, which Johannes said he taught Christian that grind.

If you are turning standard side grain bowls and having problems with the end grain in it, and bruising there, that is the heel bevel edge. There are some who keep the flutes more straight up and say that you don't need to grind the heel off. I have played with that some, but still, just from habit, grind the heel off.

Never understood the concept of using a smaller tool to get a cleaner cut. I tried it and didn't notice any difference. Maybe it is just that bigger tools feel more comfortable to me for cutting. To me, it reminds me of a Yogi Berra quote, "it is 90% mental and 10% in your head."

robo hippy

 

[Donna Banfield]

I'll give that a try. Thanks! When you refer to your grinds, I get the impression that you use a fairly narrow cutting edge bevel. Is that correct?
I figured that until I got more information and experience, I'd start with taking the bottom half of the bevel back a limited amount, to avoid making more trouble for myself than I normally do.

Exactly. The bevel on my cutting edge is typically 1/4" wide, even with using the Vector Fixture, Hannes' 'jig' for putting the 40/40 grind on bowl gouges; and free-hand sharpening, it's closer to 1/8 or 3/16th wide.

To remove all that heel free-hand sharpening, the tool is held vertical, flute facing me. I place the back of the gouge onto the wheel, and 'walk the gouge' straight up. I'm holding the bottom of the handle with my right hand and twisting or twirling it as the gouge is walking up the wheel. This method is really only possible if your grinder is mounted on a platform about chest high. If mounted on a standard height bench, you would need to get down on your knees to accomplish that same type of movement.

But if you are using the Vector (or your version) to remove the heel and replace it with 3 bevels, sharpen your cutting edge first, then shape your three bevels next, spending more time removing the heel. It will take more time on the heel in the beginning because you have a lot of steel to remove. Once that's done, your 'heels' will take no longer than the time spent to freshen up your cutting edge.

 

[Marc Banka]

For this difficult cut, I do these things.
Get rid of that heel. I use a Wolverine jig for a 40/40ish profile. Using my shaping wheel, I first grind the second bevel nearly, or even completely removing my cutting bevel. Then the bottom bevel. Then I put my cutting bevel with the sharpening wheel. It’s faster as you’re using the coarse wheel for the real work.
When cutting the interior I hollow to just below the lower bark, leaving plenty of stability to the wings. I always resharpen right before the ultimate cut. Use the same 5/8 gouge, usually, or use the 1/2“ with the same grind. I like the stability I get from the heavier tool. Then I calm myself, use my index finger under the tool rest for control, and commit to the cut.
This isn’t to say that I do this cut perfectly, but it’s getting there.