How much pressure to apply when grinding

F

FunkCoaster

Joined
Nov 2, 2010
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I have a KMG grinder. How much pressure should I apply when grinding?

To reduce variables, let's say a flat, primary grind on 1/8" thick, 2" wide blade, using 60 grit orange blaze belt. Running at high speed.

I've seen the phrase "avoid excessive pressure", which is no help. It's hard to see in videos just how hard people are pressing. When Nick Wheeler presses gently, it's probably like me pressing hard.

Push hard enough to - Break a pencil? Anger a cat when petting it? Anger a newfoundland dog?
(The above scale is not ANSI approved... yet)

I'm also under the impression that significant force would cause ceramics to crack and expose new surfaces. Would that then shorten the life of the belt by applying constant, hard force?
 
When hogging the bevels with a structured abrasive belt, HARD is what you want. This requires a big motor, with 2-3HP being the norm. Run the belt fast and push hard. With 1/8" stock, it may cut an entire bevel in one pass, so practice on some scrap steel.

On blue Zirconium belts and gators, use a lighter, but firm pressure. As the grits get finer, decrease the pressure.

Obviously, when doing post HT grinding you dip every pass and use low pressure.
 
Not sure if the story is true, but at Ashokan once I was told that Jim ??? mounted his grinder opposite a wall so he could push against the wall for more pressure when grinding. That's the reason he had a 5HP motor on his grinder.
 
If you're talking about Jim Siska, he uses a tremendous amount of pressure when grinding. Combining that with his standard hollow grinds removes steel at a rate that really needs to be witnessed to believe. I can't approach that level of force. Jim says I'll never be a good grinder because I don't have the proper biological work rest, like he does (aka beer belly;)).

No one's going to be able to tell you how much pressure to use, though. You need to figure out what's right for you, or watch someone else do it in person so you can see the subtleties you can't see on a YouTube vid. The bad news is, trial and error is expensive. The good news is, that's how you get good at grinding!!!
 
The better you get the harder you can press without gouging the heck out of your piece. And I would say the more you are finish grinding the less you are pressing.

So the best answer from me is you push as hard as you skill allows while your experience dictates when to push hard and when not to.
 
With ceramic belts they work better with speed and high pressure, the grit fracturing doesn't shorten the life, it actually extends it. If the grit isn't fracturing, it dulls and glazes and doesn't want to cut. the grit is designed to fracture over the life of the belt so that it maintains its ability to cut by exposing new sharp edges. I use enough pressure that i would regularly stall my 1hp grinder and pop a breaker. This is why I switched to a 2hp now.
 
I would give my opinion that you should not press so hard that you diminish your tactile control of the blade but hard enough to remove some material with each pass. I run my grinder at 4500 SPM with a 60 grit ceramic and it takes me a half hour to go through all the grits before heat treat. My blades are nearly finished before heat treat and I go slower with each finer grit to keep both sides the same with a sharp grind line and a well finished hollow. It always takes me more time changing belts than grinding but this is the way I learned to grind and there are many faster knife makers out there. I have seen a video of one maker that runs grinder at 7000+ SPM and grinds the basic hollow of his hunting knives in less than a minute and then goes straight to 220 grit and calls it good. ( He heat treats the blade before grinding) I enjoy the process and take more time at a slower speed. You should do your grinding by trial and error and find out what pressure and speed you are comfortable with that gives you the results you want. After all the quality of the grind is the most important thing....isn't it? Just my opinion. Larry
 
I haven't set one up yet, but using a "hanging parallelogram" grinding setup greatly increases control on every grinding step right up to sharpening. The new shop will have one on each grinder. When not needed, just hook it up out of the way.

Nathan the Machinist is the champ on these.

Video on post #108 - http://www.bladeforums.com/forums/showthread.php/1280945-sharpening/page6
 
LARRYZ10 said:
I have seen a video of one maker that runs grinder at 7000+ SPM and grinds the basic hollow of his hunting knives in less than a minute and then goes straight to 220 grit and calls it good. ( He heat treats the blade before grinding) thing....isn't it? Just my opinion. Larry
Click to expand...

Is this the vid you're referring to, Larry, on Entrek Knives? Yea, 36 grit @ 7500 SFM! :eek:

Of course, I've never been to any other knifemaker/bladesmith shops for it to be a normal thing for me, so it's quite gratifying to watch the steel dissappear :D

He starts the bevel grinding at 5:20 in case anybody is interested, but it's also fun watchin him hog the blank down to it's profile in the beginning, using his platen nonetheless. If I had a good contact wheel I'd definitely be doing all my profiling on that! Although, I might feel differently if I had his grinding setup. lol

[video=youtube;kvlKfzW12d0]https://www.youtube.com/watch?v=kvlKfzW12d0[/video]


~Paul
My YT Channel Lsubslimed
... (It's been quite a while since I've posted any new vids)
 
Stacy E. Apelt - Bladesmith said:
I haven't set one up yet, but using a "hanging parallelogram" grinding setup greatly increases control on every grinding step right up to sharpening. The new shop will have one on each grinder. When not needed, just hook it up out of the way.
Click to expand...

Stacy, what happened to "The actual bevel angle is insignificant." and "you will need accuracy of a hundredth of a degree or more". If your going to use a parallelogram grinding setup, you will certainly need to set the angle of your platen to grind your bevels just like you do on a grinding sled.

BTW, Nathans system works great for sharpening, but you do have to set the angle.
 
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