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JacksonMaberry

Hollow grinding - 8 vs 6 inch wheels

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It seems more powerful grinders/sharpeners have larger diameter wheels, which would give less of a hollow grind. The more powerful machines are sold as professional spec.

Maybe it does not matter much in practice, but theoretically a smaller wheel may mean more honing before needing to recreate the hollow surfaces again.

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Here's the 1725 r.p.m. grinder I use. It takes an 8 inch x 1 5/16 inch soft aluminium oxide wheel. But the wheel is now at about 6.5 inches and I'll be buying a new one soon. I wouldn't want to use a smaller wheel.

I only see hollow grinding as helpful in speeding up the honing process, just as with chisels. I've found that even hollow grinding very close to the cutting edge it still takes quite a while to develop a burr with a 1000 grit water stone, at least with Addis gouges. I sometimes start honing with a medium/worn diamond plate to smooth the edge before switching to the water stone because the ground edge can sometimes dig into water stones.

 

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I think my stone is down to about five inches. If you want a hollow, why grind with a stone that's so flat that the hollow quickly goes away? Also, with the larger ones it's harder to locate the rest so you're sure to be grinding out the middle of the bevel.


I use a hand grinder and a Veritas tool rest, and hollow grind everything. Then my honing might be precise, or casual (rounded or slightly micro-beveled), depending on how I'm going to use the tool.

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I don't hollow grind chisels and gouges for any reason other than to reduce the width of the honing bevel. If I had a flat honing disc or belt sander to produce a primary bevel quickly, as long as it didn't leave deep scratches at the cutting edge, I'd probably use that, especially in the case of gouges. I like to start honing them with a fine stone because coarser stones can easily cut through the curve and distort the cutting edge.

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The belt grinder, with trizact belts down to 6 microns, then finishing with a leather belt charged with green compound, produces excellent results for me. I only wish I could slow it down - the cheap 1/4 horse AC motor runs the belt very fast and if I'm not careful I can ruin the temper of the steel. Perhaps I should look at building a variable speed belt machine instead. 

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I prefer the 8"wheels (1750RPM).  I got rid of the 6" grinder, as the 3550 RPM vibration was too annoying.

For gouges, I have been grinding them on the side of the wheel, no hollow, and the 8" wheels have more space for that.  Subsequently, I made a 6" diamond disc grinder (1750RPM) that I will now be using to grind gouges.

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Regrinding a primary bevel usually requires removing a fair bit of metal and I think doing it fast can create too much heat. I find the soft aluminium oxide wheels safe enough but I usually cool the blade in water every five seconds or so just to make sure. I think a Tormek is probably the best way to go for gouges, but I manage pretty well without it so I haven't bothered to put my hand in my pocket yet.

 

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The disc grinder has always intrigued me, likewise the horizontal wet grinder hitachi(?) makes. Veritas wants way too much for their version for me to seriously consider. I should probably just put a VFD on the belt machine I have and I bet I'd be happy.

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IMHO, hollow grinding a cutting edge gives an inferior result compared to flat or convex bevels.  :)

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Just now, Violadamore said:

IMHO, hollow grinding a cutting edge gives an inferior result compared to flat or convex bevels.  :)

I have long been of that opinion as well, but so many people I respect a lot use a hollow grind and so I thought I'd see what the fuss was about 

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45 minutes ago, JacksonMaberry said:

I have long been of that opinion as well, but so many people I respect a lot use a hollow grind and so I thought I'd see what the fuss was about 

It is my experience that cutting with a hollow-ground edge takes more effort, and is not as clean cutting.  I feel that the chip impacting the reverse curve, rather than sliding over it, increases drag.  Also, wheel-ground surfaces are not as smooth or sharp as side- or cup-ground, stoned, or lapped.  I find as well that hollow-ground edges have a greater tendency to bind in the workpiece.  Some of this may be disguised when using comparatively tiny (or narrow) chisels and plane irons, or when cutting with the bevel down (as with out-cannel gouges), but shows up in a hurry with some of the blades I use, such as drawknives, food preparation knives, and swords for cutting competitions. 

Because many luthiery tools are used bevel-down, a lot of the people here may have never had to confront the issue, but I doubt that any of them hollow-grind their bridge knives.  :)

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44 minutes ago, Mark Norfleet said:

My bridge knife has been hollow ground for decades...

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OK, cool.  Why?  Do you do it all on a shop grinder?  What wheel and grit?  :)

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My 1” belt sander/grinder has an 1800 rpm motor, but a 4” drive wheel, so the belts run at 1885 ft/min, which is 1/2 of the rim speed on an 8” wheel grinder running at 1800 rpm. I still have to be careful about overheating though. I often wish that I could slow it down more.

And I added a reversing switch which is great when using a leather strop belt, and for a surprising number of other tasks too. I had an electric motor service shop wire the reversing switch into the machine, and altogether this belt grinder was an expensive investment, but very useful.

 

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9 minutes ago, MarkBouquet said:

My 1” belt sander/grinder has an 1800 rpm motor, but a 4” drive wheel, so the belts run at 1885 ft/min, which is 1/2 of the rim speed on an 8” wheel grinder running at 1800 rpm. I still have to be careful about overheating though.

And I added a reversing switch which is great when using a leather strop belt, and for a surprising number of other tasks too. I had an electric motor service shop wire the reversing switch into the machine, and altogether this belt grinder was an expensive investment, but very useful.

 

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Think I've seen you post this before. Very nice! My setup is quite similar, but not quite as powerful I'd guess. I bought the Viel belt grinder frame from LV and did the rest myself. I should try a smaller drive wheel than I'm using, perhaps. Thanks for sharing your machine!

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I just don't understand you guys fascination with things that spin fast.  All I use my bench grinder for are drill bits, metalworking tools, and stock removal.  Everything else goes to stones and plates.  :)

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3 hours ago, JacksonMaberry said:

The disc grinder has always intrigued me, likewise the horizontal wet grinder hitachi(?) makes. Veritas wants way too much for their version for me to seriously consider. I should probably just put a VFD on the belt machine I have and I bet I'd be happy.

I read this and thought “what’s a VFD?” So I looked it up, and I’ve realized that a variable frequency drive could be extremely useful to me for slowing down my belt grinder, and my wheel grinder too. I’m seeing that there’s a huge range of prices, some very expensive and some pretty cheap. Does anyone have any experience with these to share? And thank you for the tip, Jackson.

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7 hours ago, MarkBouquet said:

I read this and thought “what’s a VFD?” So I looked it up, and I’ve realized that a variable frequency drive could be extremely useful to me for slowing down my belt grinder, and my wheel grinder too. I’m seeing that there’s a huge range of prices, some very expensive and some pretty cheap. Does anyone have any experience with these to share? And thank you for the tip, Jackson.

I was going to mention VFDs when Jackson first mentioned his plan for a DC variable drive, but then he brought them up!  :)  I don't have any personal experience with VFDs except that I own one that I'll probably be using to run a small three phase motor on a machine I'm restoring.  I do however hang out in a few different machine forums and have the impression that many are satisfied with the results they get from the less expensive VFDs and use them for years in situations that are a lot more demanding than controlling the speed on a grinder.  The more expensive ones have more features and control options such as braking and ramp up speed, but those capabilities are probably not needed on small tool grinding machines.  The important thing is to do all of the motor control with the VFD and to not use any switching between the VFD and the motor.

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9 hours ago, Violadamore said:

OK, cool.  Why?  Do you do it all on a shop grinder?  What wheel and grit?  :)

Why?  David told me to and it has worked ok for me for a while.

Having a hollow ground surface greatly reduces the amount of material that needs to be removed during honing (on fine grit water stones) and gives me a reliable consistent place to start.  I don't know the grit of the grinding wheel that's on my (non belt) grinder.  It's been there for years and I've forgotten.  I do remember that it's aluminum oxide and "K" bond, which results in a somewhat friable wheel surface that breaks down fairly quickly leaving a sharper surface and less tendency to glazing and burning than more common silicon carbide wheels.

Like Jackson, I've had the idea to make a slow speed grinder for such things and have it be water cooled (or possibly high velocity air) but haven't gotten to that project yet.  Maybe next week... ;) 

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A good friend of mine who makes furniture made his own massive belt machine powered by a 3/4 horse AC motor controlled with a VFD. The only thing I don't love about it is that at it's very lowest speed, the torque drops off hard. DC is better in that respect, but DC motors are significantly more expensive. Hence the $2k price tag on a fully loaded Tradesman grinder from Canada. Those are especially cool because you can have a CBN wheel on one side and a 2x42" belt rig on the other. The quality of the machining is Alberti level. 

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2 hours ago, Mark Norfleet said:

Why?  David told me to and it has worked ok for me for a while.

Having a hollow ground surface greatly reduces the amount of material that needs to be removed during honing (on fine grit water stones) and gives me a reliable consistent place to start.  I don't know the grit of the grinding wheel that's on my (non belt) grinder.  It's been there for years and I've forgotten.  I do remember that it's aluminum oxide and "K" bond, which results in a somewhat friable wheel surface that breaks down fairly quickly leaving a sharper surface and less tendency to glazing and burning than more common silicon carbide wheels.

Like Jackson, I've had the idea to make a slow speed grinder for such things and have it be water cooled (or possibly high velocity air) but haven't gotten to that project yet.  Maybe next week... ;) 

Yup, about what I figured, actually.  You're using a grinder wheel for stock removal, but then shaping and sharpening with a succession of stones to arrive at a flat or convex edge bevel profile..  When I refer to a "hollow ground blade", that's not what I mean at all, nor do most other people who say nasty things about them.  I'm talking about blades which are primarily shaped or entirely finished on a grinder, leaving a concave "holiday" between the cutting edge and the body of the finished tool.  That concavity will interfere with efficient cutting.  To see exactly what I'm saying to avoid, compare a cheap hollow-ground chef or butcher knife with a classic convex-ground (teardrop cross-section) Marttini fish filet knife. 

Note well, in most of my dealings with blades, I'm cutting deeply into or entirely through something, rather than merely shaving the surface, which affects my perspective on shaping/sharpening.

BTW, overall, I'm a vocal opponent of producing blade profiles by stock removal rather than forging to shape, so with most blades I handle, this never comes up,  :)

 

 

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