Aluminum CBN wheel catching or digging

[Joe Cornell]

I recently had a minor accident with my low speed (1750 rpm) grinder. I was using a 350 grit CBN coated aluminum wheel with my vari-grind jig. When I rolled the tool over to the left to touch up the fingernail profile, I heard a loud bang and the next thing I know my left hand was bleeding, my thumb was smashed and the jig was on the floor still holding the gouge.
When I went back to determine what happened, I found the leg angle on the vari-grind had reset from 23 degrees to zero and the gouge was no longer projecting 2 inches from the front of the jig. There was a dig in the wheel that appears to be caused by the left edge of the gouge digging into the wheel (see photo). Portions of the vari-grind that should never come in contact with the wheel also had some deep scratches.
The particular gouge I was sharpening was made of ASP 2060 steel which may be harder than most currently available gouges.
Does anybody know of a similar incident? I have never heard of anybody getting a dig in an aluminum oxide wheel. I'm interested in comments.
I have more photos and details if anybody is interested. I asked the vendor who sold me the CBN wheel if they wanted it back or more information from me but have not heard back.

 

[Dean Center]

Sharpening is a lot like turning. What you describe would be very common while turning--having an unsupported edge contact the wood. Since you were sharpening with a jig, it would be hard to roll it far enough to have that happen and there would be no tool rest anyway.

With that in mind, it's possible that the screw/clamp that holds the tool got loose and the tool rolled to the left from where it should have been. The clamp screw gets loose all the time on our varigrind at the high school, though all that happens to me is the tool slides away from the wheel. In your situation, it might be the way you hold the handle when you grind caused the tool to roll counterclockwise when it loosened.

Or possibly the leg screw was loose and that moved all of a sudden, causing a 'collapse'.

I suggest you do a 'replay', with the grinder turned off, by setting yourself up the way you normally grind, and see if it's possible to roll the tool far enough over to catch an edge on the wheel.

 

[Brian Gustin]

I'd (with grinder unplugged) reset everything back to where you normally had it and then slowly by hand rotate the wheel while mimicking the sharpening process, and pay special attention to if any portion of anything drops at or below the horizontal center line of the wheel. if by any chance anything touched below center, then much like with a scraper on your lathe, you would have force driving the gouge down, and with the gouge trapped between wheel and V arm via the jig, several things likely had to give to make space for everything to fit until the wheel could throw gouge clear. If that is the case, might have to rethink your grinds to change angle a touch, or review your setup process. I'd imagine a CBN wheel at least didn't explode (I'd expect if the same thing happened on a frangible aluminum oxide grinding wheel, you'd count yourself lucky if it did not turn into a grenade. - I have seen grinding wheels quite literally explode on a severe catch and the person operating was extremely lucky to come away unhurt.. I got hit in the face from a piece of shrapnel and I was 15 feet away from it.)

 

[Joe Cornell]

I'd (with grinder unplugged) reset everything back to where you normally had it and then slowly by hand rotate the wheel while mimicking the sharpening process, and pay special attention to if any portion of anything drops at or below the horizontal center line of the wheel. if by any chance anything touched below center, then much like with a scraper on your lathe, you would have force driving the gouge down, and with the gouge trapped between wheel and V arm via the jig, several things likely had to give to make space for everything to fit until the wheel could throw gouge clear. If that is the case, might have to rethink your grinds to change angle a touch, or review your setup process. I'd imagine a CBN wheel at least didn't explode (I'd expect if the same thing happened on a frangible aluminum oxide grinding wheel, you'd count yourself lucky if it did not turn into a grenade. - I have seen grinding wheels quite literally explode on a severe catch and the person operating was extremely lucky to come away unhurt.. I got hit in the face from a piece of shrapnel and I was 15 feet away from it.)

Brian,
Thank you for sharing and for your suggestions. I have tried the replay as you suggest. Nothing gets below center line in the replay no matter what I loosen and move. I did find that the leg angle setting on my vari-grind changes sometimes when it seems fully tightened. Something is wrong with it. I know a butterfly nut is convenient but I wonder if I didn't drop the jig in a way that struck the wing of the butterfly nut which then led to a loose nut.

Here is a scenario that may explain my incident - the left edge of the gouge gets a catch which forced the gouge to spin rapidly in the direction it was going. This results in my hand hitting the grinder table and the gouge flying loose. Once it was loose, it kept spinning until the body of the vari-grind went between the support arm and the grinder wheel.

Here are the rest of my photos: https://photos.app.goo.gl/8NXsNtZvsy9kHJh56

I am replacing the vari-grind and plan to replace the butterfly nut with a hex nut. I never purposely change the angle anyway. The wheel may still be usable but I'm replacing that as well.

Joe

 

[Alan Weinberg]

this reminds me always to wear my face shield when sharpening. I’ve been too casual.

 

[Joe Cornell]

Sharpening is a lot like turning. What you describe would be very common while turning--having an unsupported edge contact the wood. Since you were sharpening with a jig, it would be hard to roll it far enough to have that happen and there would be no tool rest anyway.

With that in mind, it's possible that the screw/clamp that holds the tool got loose and the tool rolled to the left from where it should have been. The clamp screw gets loose all the time on our varigrind at the high school, though all that happens to me is the tool slides away from the wheel. In your situation, it might be the way you hold the handle when you grind caused the tool to roll counterclockwise when it loosened.

Or possibly the leg screw was loose and that moved all of a sudden, causing a 'collapse'.

I suggest you do a 'replay', with the grinder turned off, by setting yourself up the way you normally grind, and see if it's possible to roll the tool far enough over to catch an edge on the wheel.

Thank you Dean.

One of the stranger things I noticed after the event is that the gouge protruded further in front of the varigrind after the event than before. If something hit the front of the gouge, I would have expected the gouge to get pushed deeper into the jig (i.e. protrude less). In fact, the clamp screw was so tight after the event that the knurled knob came off when I tried to unscrew the jig clamp. The gouge was so tight in the jig that I needed to knock the jig off the gouge with a mallet.
This is consistent with the entire jig spinning until it caught under the wheel. I posted a link to more photos that show the marks on the jig as well as the wheel in my reply to Brian.
Let me know if you have more questions.
I won't be comfortable sharpening until I'm sure I know what happened.

Joe

 

[Joe Cornell]

this reminds me always to wear my face shield when sharpening. I’ve been too casual.

The physician that stitched up my hand commented that I was lucky to have been wearing leather gloves at the time of the incident. And yes, I always wear my powercap mask and respirator when sharpening.

How I managed to get a 1 inch long laceration while wearing gloves still puzzles me. The gloves didn't even have a hole in them.

 

[hockenbery]

The particular gouge I was sharpening was made of ASP 2060 steel which may be harder than most currently available gouges.
Does anybody know of a similar incident? I have never heard of anybody getting a dig in an aluminum oxide wheel. I'm interested in comments.

I do know of a similar accident that required hand surgery.
Glad you weren’t hurt seriously.

This was with a matrix wheel.
An instructor teaching in my shop was grinding a students tool that had a steep front bevel using the varigrind.
This put the gouge lower than usual and close to the center of the wheel
The gouge went to the center of the wheel, then the wheel pulled it below the center. and pulled the hand into the wheel.

Best guess is that the pocket was not locked tightly and the pocket Moved out letting the gouge move down the wheel.
The steep front bevel put the tool close to the center of the wheel so there was more downward pull from the wheel and no margin to recover from the pocket moving.

I have never had a problem using a jig in the pocket with a 60 degree or less front bevel.
I now, never sharpen an 80 degree front bevel gouge with a jig. Steep angles I do on a platform.

I would guess your situation came from tool getting pulled below center.
Something came loose or wasn’t tightened properly and this allowed the geometry to change to where the gouge was at center then got pulled below. Pocket rod moved, angle lock on the varigrind moved, protrusion lock on varigrind let the gouge move backwards.

 

[Joe Cornell]

I do know of a similar accident that required hand surgery.
Glad you weren’t hurt seriously.

This was with a matrix wheel.
An instructor teaching in my shop was grinding a students tool that had a steep front bevel using the varigrind.
This put the gouge lower than usual and close to the center of the wheel
The gouge went to the center of the wheel, then the wheel pulled it below the center. and pulled the hand into the wheel.

Best guess is that the pocket was not locked tightly and the pocket Moved out letting the gouge move down the wheel.
The steep front bevel put the tool close to the center of the wheel so there was more downward pull from the wheel and no margin to recover from the pocket moving.

I have never had a problem using a jig in the pocket with a 60 degree or less front bevel.
I now, never sharpen an 80 degree front bevel gouge with a jig. Steep angles I do on a platform.

I would guess your situation came from tool gettin pulled below center.
Something came loose or wasn’t tightened properly and this allowed the geometry to change to where the gouge was at center then got pulled below. Pocket rod moved, angle lock on the varigrind moved, protrusion lock on varigrind let the gouge move backwards.

Thank you.

Sorry to hear about your instructor. The physician who stitched me up showed me how close I came to needing surgery. I Missed the tendon by under 1 mm.

I think you nailed it. When I talked to my mentor in the Seattle chapter about this incident, as soon as I said grinder accident he asked "any loose screws?"

I totally agree about steep angles on a platform. It seems intuitive to me.

Joe

 

[Dave Landers]

I think your analysis about what happened is probably pretty close to right.

A couple things in the "going forward" category:

Since you never change the varigrind arm angle, and don't trust the nut to hold it... (Finger-tight doesn't do it for me - I used a pair of pliers on that wing nut and cranked it down - mine isn't coming loose on its on). ... Anyway, drill a hole between the two parts (while set at the right angle) and put in a bolt/nut. Doesn't have to be very big - a #6 would be about right. This will eliminate the dependence on the wing nut friction and keep the thing where you want it.

Watch that plastic clamp piece that sits across the flutes. One of mine has some grooves worn in it that lets the gouge sit crooked - so I have to spin it 90º to get it to clamp flat across the flutes. Worn or soft spots may also clamp less securely than it should.

But the biggest problem with the plastic clamp nut is tightening it when it's sitting on the back end of the flute (where the flats transition to the round tool bar). A couple of your photos look like it's in this trouble spot - but this may be where it jammed after the incident. In any case, when the gouge gets short, it's worth some time with an angle grinder or belt sander to create a flat for that plastic nut to sit on.

 

[Jerry Bridges]

I recently had a minor accident with my low speed (1750 rpm) grinder. I was using a 350 grit CBN coated aluminum wheel with my vari-grind jig. When I rolled the tool over to the left to touch up the fingernail profile, I heard a loud bang and the next thing I know my left hand was bleeding, my thumb was smashed and the jig was on the floor still holding the gouge.
When I went back to determine what happened, I found the leg angle on the vari-grind had reset from 23 degrees to zero and the gouge was no longer projecting 2 inches from the front of the jig. There was a dig in the wheel that appears to be caused by the left edge of the gouge digging into the wheel (see photo). Portions of the vari-grind that should never come in contact with the wheel also had some deep scratches.
The particular gouge I was sharpening was made of ASP 2060 steel which may be harder than most currently available gouges.
Does anybody know of a similar incident? I have never heard of anybody getting a dig in an aluminum oxide wheel. I'm interested in comments.
View attachment 65723I have more photos and details if anybody is interested. I asked the vendor who sold me the CBN wheel if they wanted it back or more information from me but have not heard back.

I had a similar issue with my CBN wheel. I had changed from aluminum oxide to CBN. I was using the vari-grind jig set to the proper angle and secured. When I presented the bevel of the bowl gouge to the wheel, the jig came loose and backed out of the receiver. The tip of the gouge went under the wheel causing damage similar to your picture. I was not injured but my wheel and the gouge were damaged. What I found was a build up of grit from the aluminum oxide wheel had prevented the jig from locking in place. A costly accident but no injury except to my pocket book.

 

[robo hippy]

There have been a number of similar incidents with sharpening spindle roughing gouges on the long arm set up with the Wolverine set up where the arm is extended a bit too far and the grinding edge gets below center. This does cause a big dig in. The pictures you show suggest a pretty big dig in. Only way I know of for that to happen is from an unsupported edge or getting below center while the cutting edge and/or tool are pointing up. Hope you are okay. Oh, for bleeding with no holes in the glove, my skin is so thin now days that if I just look at some thing wrong, my skin tears and I am bleeding, and most of the time I never know it. Also, I do have a lot of loose screws, but they are all in my head.....

robo hippy

 

[Kent Reisdorph]

Oh, for bleeding with no holes in the glove, my skin is so thin now days that if I just look at some thing wrong, my skin tears and I am bleeding, and most of the time I never know it.

robo hippy

Ain't that the truth. I think my skin is a little thinner than it should be at 64 but thems the breaks.

 

[Joe Cornell]

I think your analysis about what happened is probably pretty close to right.

A couple things in the "going forward" category:

Since you never change the varigrind arm angle, and don't trust the nut to hold it... (Finger-tight doesn't do it for me - I used a pair of pliers on that wing nut and cranked it down - mine isn't coming loose on its on). ... Anyway, drill a hole between the two parts (while set at the right angle) and put in a bolt/nut. Doesn't have to be very big - a #6 would be about right. This will eliminate the dependence on the wing nut friction and keep the thing where you want it.

Watch that plastic clamp piece that sits across the flutes. One of mine has some grooves worn in it that lets the gouge sit crooked - so I have to spin it 90º to get it to clamp flat across the flutes. Worn or soft spots may also clamp less securely than it should.

But the biggest problem with the plastic clamp nut is tightening it when it's sitting on the back end of the flute (where the flats transition to the round tool bar). A couple of your photos look like it's in this trouble spot - but this may be where it jammed after the incident. In any case, when the gouge gets short, it's worth some time with an angle grinder or belt sander to create a flat for that plastic nut to sit on.

Dave, thank you for your analysis. The plastic clamp piece did not have any grooves that I noticed before the incident but it definitely has a pair now. I like your idea about the extra bolt. I will give it some thought.

 

[Joe Cornell]

I had a similar issue with my CBN wheel. I had changed from aluminum oxide to CBN. I was using the vari-grind jig set to the proper angle and secured. When I presented the bevel of the bowl gouge to the wheel, the jig came loose and backed out of the receiver. The tip of the gouge went under the wheel causing damage similar to your picture. I was not injured but my wheel and the gouge were damaged. What I found was a build up of grit from the aluminum oxide wheel had prevented the jig from locking in place. A costly accident but no injury except to my pocket book.

Thanks for sharing, Jerry.

 

[Joe Cornell]

There have been a number of similar incidents with sharpening spindle roughing gouges on the long arm set up with the Wolverine set up where the arm is extended a bit too far and the grinding edge gets below center. This does cause a big dig in. The pictures you show suggest a pretty big dig in. Only way I know of for that to happen is from an unsupported edge or getting below center while the cutting edge and/or tool are pointing up. Hope you are okay. Oh, for bleeding with no holes in the glove, my skin is so thin now days that if I just look at some thing wrong, my skin tears and I am bleeding, and most of the time I never know it. Also, I do have a lot of loose screws, but they are all in my head.....

robo hippy

Robo,

Thanks. I needed a good laugh.

As a side project before this incident, I was looking into collecting statistics and case studies of woodturning related injuries. Maybe I didn't use the right search terms but I was dismayed by how little there is on the U.S. Consumer Product Safety website.

I haven't gotten around to searching these sites:

https://www.nsc.org/
https://www.cdc.gov/nchs/fastats/accidental-injury.htm

Can you think of any other publicly accessible databases I may want to check?

Joe

 

[Dean Center]

Folks have secured their varigrind tail/foot in a variety of ways. John Lucas, one of our really good contributors, has posted he drills a hole and uses a rivet IIRC. Dave's suggestion of a small bolt is the same concept. I seem to recall someone welding theirs in place, but I think mine is aluminum and maybe not easily welded. In any case, fixing it solidly is a good idea.

 

[Mark Jundanian]

@Joe Cornell , check to make sure the axel of your grinder is the correct height above your Wolverine. If it's too high, that might cause the gouge to lie closer to the wheel's center.

Since you were working the left wing on the right wheel, you may have brought the knob on top of the Varigrind in contact with the grinder motor housing. Conceivably this could have forced the gouge to the corner of the wheel (but the scar on the wheel is in the center).

Also, take a minute to look at the Vector system/jig from Hannes Tools. The tool holder leg is already fixed. I think the Vector has a lot of advantages over the Varigrind & Wolverine system (even if you're not interested in the hatmaker's grind). The screw that holds the tool is on the bottom of the jig, so it doesn't hit the grinder motor housing--much better for doing the wings. Because that screw is on the bottom, you're clamping down on the flute side, hence no grooves such as Dave Landers mentioned, and you can secure the gouge closer to the handle as it wears down. The Vector set up device is slick and works well. It's probably more repeatable, too. The Vector fits into the Wolverine's base. The plate has 5 divots in it. The two side divots are for the hat grind. The center divot furthest from the grinder is the one to use for "regular" grinds. The two divots closer to the grinder are for taking down the gouge's heel. It occurs to me, too, that in a similar event as befell you, the leg would likely have pop out of the divot, where the Wolverine V pocket holds it firm. I don't do much spindle work, but I don't see why the Vector wouldn't work to sharpen a spindle gouge. I can't remember if I've done that or not.