What's a Burr Look Like?

Re: What's a Burr Look Like? *LINK*

Dick Coers

only the curved side would contact the surface of the wood. Huh? Only the tip of the burr contacts the wood, not the side. Doc Green is the go to on the subject


https://www.docgreenwoodturner.com/scrapers1.html

Re: What's a Burr Look Like?

Don Stephan

Apologies for the poor wording. When raising a burr on a card scraper by burnishing, I expect the burr will be like an extension of the edge, perpendicular to the surface of the scraper.

When a burr is formed on a grinder, I don't know if the burr is like that on a burnished card scraper, or if the grinder burr rolls back slightly from the cutting edge.

Re: What's a Burr Look Like?

John K Jordan

I have several microscopes and I'm not afraid to use them. The burr from a grinder looks like a jagged mountain range of solidified extruded rock, some in waves and most with uneven, jagged peaks. The burr looks very weak, probably why it doesn't last very long.

A burnished burr, done correctly, is entirely different: a smooth, sharp edge smoothly folded over to some angle, depending somewhat on the angle the burnisher is held, the pressure used, and probably the radius of the burnisher. (I haven't yet looked at the edge formed from an Arno triangular burnisher I recently bought.) Lubricating the edge can make an even smoother cutting edge and those who sharpen hand scrapers recommend it.

I'm sure the angle would be much different on a hand scraper ground at 90 degrees and a NRS ground with a 30-deg bevel. The ground burrs appear to be mostly in line with the plane of the ground bevel but I can't tell for sure. The burr on a burnished NRS would logically be bent over. My stereo microscope provides depth perception so I can see the bend but I don't know a good way to measure.

Another thing I'd like to do when I get the time is use the Sharp Check sharpness tester on the two types of burrs and attempt to quantify the difference. Looking at the burrs before and after a few seconds of use is also on my list of things to do.

I don't have a camera set up for the microscopes. If you want to try some photos yourself we've had good luck with an inexpensive digital microscope. I've bought a couple of these but I don't have one set up at the moment.

JKJ

Re: What's a Burr Look Like?

Don Stephan

Thanks for taking time to look closely. I'm going with pretty much in line with the surface of the bevel, which seems pretty consistent with how the bevel is presented to the wood when the burr is working. Not what I expected from a grinder generated burr but you were able to take a much closer look than I.

I'm not surprised that the burr looks like a jagged mountain range. My guess would be the "valleys" between the peaks are due in part to the grit of the grinding wheel.

Re: What's a Burr Look Like?

John K Jordan

>>>I'm not surprised that the burr looks like a jagged mountain range. My guess would be the "valleys" between the peaks are due in part to the grit of the grinding wheel.

Burr formation on the upper side of the bevel, away from the direction of rotation, is a curious thing. I used to think the burr was from hot particles being carried around the wheel where they build up on the edge. An alternate uneducated guess is the intense heat from friction at the surface softens the metal to a plastic state and the pressure forces a thin layer up and away from the edge. I wondered if the furrows I've seen in burrs were related to the individual grains. A visual comparison of the burrs between two different grits would be useful. Or search for publications on the subject - I'm sure all of this has been done examined in a real lab by real researchers.

As for practical scraper preparation, I typically use an extra-fine diamond hone to remove the grinding burr, then burnish a smooth burr. In some cases, I'll first use the grinder burr until it's almost gone then hone and burnish. (In fine-grained wood I can see the scratches from the jagged grinder burr.)

I'll replenish the burnished burr several times before going back to the grinder - first honing to remove the existing worn burr then burnishing.

JKJ

Re: What's a Burr Look Like?

Reed Gray

I heard Eric Loffstrom talking at a demo and he commented about how much difference there was between the upside down sharpened NRS burr and the right side up sharpened NRS burr, and these were on the 20/70 or so NRS. The upside down one is more of an edge, and the right side up one is kind of curled over. I didn't see the pictures. Basically the upside down one is supposed to be sharper. I have only played with it a little bit, and can't tell much difference. I have attempted to burnish a burr on the 30/30 NRS, and I can hear the edge fracturing as I do it, even with very gentle pressure. Eric does prefer the burnished burr on his NRSs...

robo hippy

Re: What's a Burr Look Like? *LINK*

Mike Stafford

Here is a link to a video showing how Eric sharpens his tools. This is also how Boxmaster Tools are sharpened. I have started sharpening my Negative Rake Scrapers in the same manner.

I have to say I like sharp scrapers and this upside down method seems to put a razor edge on my tools. I did have to modify my grinder platforms to accommodate the jigs that are shown in the video but those jigs take all the work out of sharpening negative rake scrapers.


Using and sharpening NRS

upside down grinding

John K Jordan

It would seem helpful to have a reversible grinder motor.

JKJ

True! Does such a thing exist?

Re: True! Does such a thing exist?

John K Jordan

Could do like the Tormek and rotate the grinder 180 deg and provide for a tool rest on the back side.

Or get a grinder with w 3-phase motor and power it with a VFD - would give variable speed and reverse. I'd love to have a variable speed grinder.

JKJ

Re: What's a Burr Look Like?

Joe Fleming

I have been turning a large mesquite platter and have been using a round scraper. The grinder burr has lasted for about one pass on the surface (approximately 4" of travel) before needing to be resharpened. I got out my burnisher and hit the edge hard. The burnished edge had worked for at least 30 to 40" of travel and is still sharp. No comparison. The ragged grinder burr rips off very quickly.

Joe

Re: True! Does such a thing exist?

Joe Fleming

I have a cheap grinder on which I removed the base (along with the on/off switch) and rotated it 180 degrees. Now, the front face of the wheels rotate "up". I have installed a RazorSharp honing system for my carving tools. The only problem is that the 3450 rpm rpm is too fast. Joe Fleming

Re: What's a Burr Look Like?

Reed Gray

At the recent Oregon Woodturning Symposium, Cathleen Duncan commented that she uses her robo rest, pulls the table past the holes in the yoke, and then slips the pin through the 90 degree hole and that rests on the yoke instead of in the tubes. This gives about a -70 degree angle. I have been playing with that some, and still think the burnished burr is better. Eric does think way outside the box.

robo hippy

Re: What's a Burr Look Like?

Mike Stafford

I also prefer a burnished burr. I still have my Lee Valley scraper burnisher in use.

The Boxmaster tools with a burnished edge cut like razor blades.

Re: What's a Burr Look Like?

Neil Strong

A few years back I ran some trials against the stop watch and took some microscope images of the burr created by various methods.

My summary (reported elsewhere) was....

The hook is best, duration and finish wise, but requires more preparation time. Whether that extra prep time is worth it over a quick hand hone with the diamond credit card (the next best option IMO) may depend on whether finish or time is more important. But whatever method, I'll definitely be polishing the top of my scrapers to at least #4000 in future.

That last point is supported by Lacer and Wright's observations about the polish on the face of scrapers, viz.

https://studylib.net/doc/18747993/scraper-sharpness---alan-lacer-woodturning

I now find that raising a hook with a fine diamond on a heavy plate/rod works just as well for me as a burnishing tool. After all, diamond is harder than any burnisher, but the grit does need to be fine. As a result, my need to leave the lathe and return to the grinder is very infrequent.

I may still have a few of the microscope (x200) images that I took stored somewhere, but without a long accompanying narrative they are probably meaningless and of minimal interest, anyway.