Belt grinder speed, for sharpening

LyonHaert · May 9, 2005

The search function doesn't seem to work at all right now, so I'm going to ask anyway.

I'm interested in starting to use a belt grinder for sharpening. A little shopping into quality belts will yield information on what size grinder I'll be looking for, but speed is a different issue.

Some grinders I've seen with information in ft/s and in RPM, but some only with RPM. So, from the experience of the learned sharpeners here who use belt grinders, what ceiling values should I be watching for? How fast is too fast?

And if I can't find one that's within acceptable limits, how do I hack it to slow it down?

z537z · May 9, 2005

From what I've learned, you'll want to use light pressure anyway, and a high grit belt, which works better at lower speeds, but basically it's just because you don't want it overheating too fast. The rule of thumb seems to be, as soon as it gets hot enough to start hurting your fingers, dunk it in water to cool it off.

So long story short you can use any speed as long as you don't try to do too much at once and overheat the blade, screwing with the heat treatment.

For finish work like this, the consensus SFPM (surface feet per minute) rate seems to be about 800 SFPM for grits 200 and above, whereas with a rough 36 grit belt you can run 2000SFPM (or probably higher) no problem.

Hope that helps...I did a LOT of searching a while back prepping to make a new knife.

_z

HoB · May 9, 2005

There are large discrepancies as the virtue of dunking it in cold water. Some (e.g. Leonard Lee) are of the opinion that it is death on your edge.

z537z · May 9, 2005

Good point. Dunking is more for rougher grinding, I think. If you're just sharpening, chances are you won't really get it hot enough with that light grinding anyway. But if it does get hot, let it chill on a windowsill for a few

Cliff Stamp · May 9, 2005

HoB said:
There are large discrepancies as the virtue of dunking it in cold water. Some (e.g. Leonard Lee) are of the opinion that it is death on your edge.

Corrosion, or temperature fluxes and fatigue?

-Cliff

HoB · May 9, 2005

Leonard Lee argues along fatigue. He says that you rapidly cycle expansion and contraction which will lead to mirco fractures in the edge that run perpendicular to the edge. Not overly deep, but enough to greatly increase the likelyhood of chipping. I do not have any personal opinion on this, I just repeat what I read. I work mostly on benchstones. I guess I just like the manual action. I work on either a 180 or 220 waterstone or on a 220 diamond stone for the rough work.

z537z · May 10, 2005

Good point

LyonHaert, there's not much you can do with a grinder sharpening-wise that you can't do with a lot less risk with a coarse DMT stone (blue or black). Those badboys will reprofile an axe in no time.

_z

Cliff Stamp · May 10, 2005

HoB said:
He says that you rapidly cycle expansion and contraction which will lead to mirco fractures in the edge that run perpendicular to the edge.

I had a knifemaker argue the same thing, I tested it with hand and belt sharpened edges and saw no difference. However I always run them wet to keep them cool, I try not to let them get steaming hot and then cool them.

I would assume that most problems are simply from overheating. If you are just sharpening this really isn't as issue as sharpening is one pass, even removing chips only takes a couple of passes.

Has Lee mentioned that he tested this or it just theory. The thermal expansion coefficient of steel is quite small and the temperature range it is is seeing is actually quite small on the absolute scale so it would surprise me if it passed the resilience of the steel.

-Cliff

Andy_L · May 10, 2005

would it be more of a problem in damacus steel? I guess there must be a slight difference in the amount different steels expand with temp.
I just mak sure they don't get anywhere near "hot" when sharpening them. I tend to have a lot to do so just do a bit of each if it's a lot of work to keep them cool.

Cliff Stamp · May 10, 2005

Different steels will have different expansion coefficients, but the difference is so small it is insignificant, no where near the flex the edge would see in trivial use for example.

-Cliff

LyonHaert · May 10, 2005

z537z said:
LyonHaert, there's not much you can do with a grinder sharpening-wise that you can't do with a lot less risk with a coarse DMT stone (blue or black). Those badboys will reprofile an axe in no time.

I use an EdgePro right now, and it works great, just not fast. I was entertaining notions of a side-business of knife sharpening, so I thought a grinder might make things more efficient.

Nick Hyle · May 11, 2005

LyonHaert said:
I use an EdgePro right now, and it works great, just not fast. I was entertaining notions of a side-business of knife sharpening, so I thought a grinder might make things more efficient.

Much more efficient. Enormously more efficient. Insanely more efficient.

LyonHaert, I sharpen on my Burr King. I do a convex edge on the slack, and I do it whether I have the big contact wheel on (very fast SFPM) or the flat grinding attachment with the much smaller (hence slower) contact/drive wheel. Higher speed sharpens faster but generates more heat, but both are well within my comfort range. You're going to want a while to practice with whatever you get, but I really wouldn't worry about speed that much on any standard wood or metal working belt sander/grinder. I prefer the lower speed for control, but the results are the same.

(If you want a less convexed, flatter edge, work right next to the wheel rather than in the middle of the slack area. But stay on the loose belt, except for heavy reprofiling or relieving work.)

If you want to use it for business - so time is money - I would look at horsepower, as in 'more is better'. Even for something as light as sharpening, the difference in work time between an industrial one horsepower (15-20 amp) machine and a 'home' quarter horsepower machine is a large factor.

If you're looking at it as a sharpening machine - not a knifemaking machine - then the bang for your buck answers might be something like either a 3/4+ HP wide belt woodworking sander - like a 6"x48" - or an inexpensive but not underpowered 2"x72" machine like the one Grizzly sells. Before I got my Burr King I got good results on a 3/4 HP 4"x36" from Harbor Freight, but 2" (and split 1") belts do make recurves and hawk/hook blades much easier to sharpen. Of course you can split 4" and 6" belts narrower as well.

If you do go this way, I'll also say that in the time = money sense, good abrasives will be your friend, belts like the structured Apexes and the 3M ceramics rather than less expensive Klingspors, etc. Unless a blade needs serious relief work, I tend to use a 240/280 grit belt for a toothy edge, a ~400 grit for a push cutter, and finish by stropping in either case.

I think you might get a good range of opinions by floating this in the Knife Maker's Shop Talk forum as well.

cbwx34 · May 11, 2005

I use the 1x30 sander from Harbor Freight... works excellent. I mounted my on a piece of a cutting board so it is horizontal. I don't see the need for a high horsepower motor for sharpening... i've never slowed my down by sharpening a knife. Another guy mounted his with a bracket.. you can see it here....

HF sander

I use a 600 grit belt, followed by a leather belt charged with Chrom. Oxide, or white 'rouge'.... puts a nice edge on a knife.

p.s. Just so you know.... EdgePro also makes a professional model more related to a business.

Nick Hyle · May 11, 2005

cbwx34 said:
I use the 1x30 sander from Harbor Freight... works excellent. I mounted my on a piece of a cutting board so it is horizontal. I don't see the need for a high horsepower motor for sharpening... i've never slowed my down by sharpening a knife.

I've used a 1x30 and a 1x42, and no, I've never pushed hard enough to slow the motor down. But my nominal 3/4 HP (9 amp, I think, so more like 1/2 HP actual) machine sharpened faster, and my one horse Burr King sharpens MUCH faster. A little 1/4 HP machine will work, and it's what I recommend to people who want to make their sharpening life easier. But in a situation where your time is your livliehood, the bigger machine will eat them alive. Or at least that's my experience. If you ever get the chance, try it out.

Jeff Clark · May 11, 2005

I would suggest that you don't go towards wider belts. Theoretically you will heat the knife faster with a wider belt which may aggravate edge burning. My practical experience is that a wider belt makes it harder to hold the knives steady as you work. The wider belt grabs or drags harder and tends to move your hands and makes your work less even. When you sharpen for money you want your bevels to look precise. They don't work better that way, but it impresses the customer. For the same reason I try not to do a convex grind since the bevel blends in and looks less distinct. If you charge the customer a lot he wants to see that you've removed a lot of material.

cbwx34 · May 11, 2005

Nick Hyle said:
I've used a 1x30 and a 1x42, and no, I've never pushed hard enough to slow the motor down. But my nominal 3/4 HP (9 amp, I think, so more like 1/2 HP actual) machine sharpened faster, and my one horse Burr King sharpens MUCH faster. A little 1/4 HP machine will work, and it's what I recommend to people who want to make their sharpening life easier. But in a situation where your time is your livliehood, the bigger machine will eat them alive. Or at least that's my experience. If you ever get the chance, try it out.

Unless the higher HP machine is actually turning faster, or unless you're slowing it down with the sharpening action.... the higher HP won't make a difference. Your statement, therefore is misleading. Kinda like saying I need a 3/4 ton truck to haul one 10 pound brick. Higher HP doesn't automatically equate to a faster job. If you can explain to me how it does... with the same speed and no load on the motor (and same belt BTW)... I'd be happy to be enlightened.