Question Regarding Grinding of Large/Thick Blades

Matt Mann USA

Matt Mann USA

Outside the Box
Knifemaker / Craftsman / Service Provider
Joined
Jun 3, 2012
Messages
12
Hi, I'm Matt. I've made a couple of knives using what I believe is 5160 steel (leaf springs) and I am wondering how I can flat grind them faster. I'm using a Coote 2x72" belt grinder with a 1hp 220v motor and I have a variety of belts for it. I can grind small knives fairly quickly, but I'm looking for suggestions on grinding blades around 12"x2"x1/4". I was working on one knife this evening using a very coarse belt and it seemed like the belt dulled rather quickly. I've seen video clips where it looked like guys were grinding large 1/4"-thick bowies by making only about ten passes on each side. Is that possible? If so, what equipment is required? And if not, how do you recommend I grind large/thick blades? Thank you very much for any advice!
 
I heated them in my gas forge to a bright orange and flattened them between an anvil and the flat side of a section of railroad track. I then let them air cool for about 30 minutes until they were cool enough to pick up without gloves. Since I didn't quench the steel, shouldn't it now be in it's softest state?
 
Doesn't sound like they fully annealed. Get it above austenizing temp and then burry it in a bucket of sand or vermiculite. It should take several hours to cool. Even just putting it against a cold metal object (your anvil in this case) can cause it to harden slightly. Thick or thin, annealed steel is easy to grind.

- Chris

ETA: an even lower tech option is to heat it in your forge and then turn off the burner and let the steel cool in there. Fire brick (or whatever your forge is made of) should hold heat pretty well and cool slowly. This isn't the best option but it will do in a pinch.
 
MM60 said:
Since I didn't quench the steel, shouldn't it now be in it's softest state?
Click to expand...

No. It is certainly not in its softest, most machineable state. Buying virgin, fully annealed steel from a reputable source is worth every penny.

What kind of belts are you using? Cheap AO belts are next to worthless for serious hogging; good ceramics are well worth their initial cost.

What surface behind the belt are you grinding against? Slack belts remove metal very slowly, flat platens are much faster, and contact wheels really peel it off.

MM60 said:
...I'm looking for suggestions on grinding blades around 12"x2"x1/4".
Click to expand...

Honestly, my best suggestion is "don't bother"... if you do, be willing to take your time and expenses into account. I'm not saying it can't or shouldn't be done, but stock-removal on bars that big is terribly inefficient. There's a good reason why most folks who make large, heavy blades forge them to shape. Grinding away 30 or 40% (perhaps more, depending on sabre vs. flat vs. hollow grind) of a long, thick bar literally leaves a whole lot of money on the floor and burns up a good deal of time and belts in the process.
 
Thanks for the suggestions, guys! When I had flattened the leaf spring between the anvil and railroad track - they were already almost too hot to touch because I had been using them together on hot steel for around an hour, so I don't think they cooled the blade off much, but it sounds like allowing it to air cool in 40-degree air probably cooled the steel too fast. I went out and bought three bags of virmiculite today. I emptied two of them into a cardboard box slightly longer than my blade, heated the blade to a dark orange, rested it in the virmiculite and poured the third bag over top of it - then closed up the box. This was several hours ago and I haven't checked on it yet. Do you think this will soften the steel enough to be easily grindable? Or is there a way that I should have pre-heated the virmiculite?

I've been grinding with the blue Norton Zirconia grinding belts from USA Knifemaker.com - which I was under the impression were the next best thing after ceramic belts. I've been grinding against the flat platen on my Coote grinder, but I bought some more carbide cutting wheels today for my angle grinder which I'm planning on using to do a quick rough grind prior to flat grinding with my Coote grinder. Does that sound like a good idea? Or - if not - what would be a better way to go about this? The blade I'm currently working on is about 15" long and the entire knife is about 22" long. In the near future I'm going to be working on a batch of knives with blades ranging from 4.5" to 12". I'd like to find a method of fully flat-grinding a 12" blade in under an hour. Thanks again for the suggestions given and for any more to come!
 
What grit belts you using? a 24 grit belt a good speed will really remove some serious metal in a hurry. The only "carbide" cutting wheels I have ever seen for an angle grinder where metal with chips of carbide stuck to the surface and where designed to rough shape WOOD. I have a stack of angle grinders from 4 1/2 to 9 inch and I would never chose one of those over my belt grinder to the surface of a knife. Profile yes.

Best way is a heavy grit quality ceramic belt. 36 will surprise you and 24 scare you. Get a good magnet to hold the cutting portion of the blade you will be grinding and get with it. I am sure I could have a 12" 1/4" thick rough profiled blank flat ground to 24 grit in about 15 minutes if I got on it. Then start working up through the grits. A little more HP on the motor would help. You should be almost stalling that 1 HP.
 
I grind a lot of thick knives and they jut take time. Expect to go through a more belts and spend exponentially more time.

The cool thing is that once you go back to smaller knives they will seem easy to grind! This week I spent one whole afternoon rough grinding one chopper and the next afternoon was spent on rough grinding 10 small knives.
 
I used to use AO belts, thinking the much higher price of ceramic couldn't be outweighed by increased performance. Such are the follies of youth. For blades big and small, ceramic is the way to go.

- Chris
 
I went through one whole AO belt on just one knife, but with a BLAZE PLUS NORTON (I am way too cheap) I have profiled and ground the bevels on nearly 25 blades, along with handle work and random stuff that never came to be... Highly recommended.
 
I've been using a blue Norton Zirconia belt in 36 grit, and I also have some in 60 and 120 grit. I've been ordering from USA Knifemaker and the coarsest ceramic belts I see there are 36 grit - I'll buy some of those with my next order.

It has been nearly stalling my 1hp motor when I push hard and I've been looking at getting a more powerful one. Would 5hp be too much? What strength motor do you guys use? Whatever I get, it has to be able to run from a standard dryer outlet. My current motor apparently came from an industrial air conditioning unit (the type installed on the roofs of commercial buildings) and turns at around 3400rpm, but with the 3-step pulleys I'm using the drive wheel is turning at only about half that speed - 1700rpm - which is what the Coote grinder is rated at.

Any input regarding my annealing process? I heated the blade to dark orange and buried it in the middle of a box of virmiculite last night hoping that it would cool very slowly. Should the steel be fairly soft now? Or is there more to it than that? Thanks!
 
MM60 said:
Any input regarding my annealing process? I heated the blade to dark orange and buried it in the middle of a box of virmiculite last night hoping that it would cool very slowly. Should the steel be fairly soft now? Or is there more to it than that? Thanks!
Click to expand...
I don't know about the other stuff, but in my experience some steels stay hard if they are slowly cooled, 1045 for instance. Some steels need to be taken to cherry and stashed in vermiculite and others need to be taken to black and quenched in brine. Just one more benefit of using known steel.
 
I currently have and use a Coote with a 1.5HP motor from grizzley. When I flat grind larger blades I can nearly stall the motor....this is one reason why Im getting a 2KMG. Also I found that while grinding with 36 grit belts there is just not enough speed to allow it to work like they are supposed to. I use the Norton Blaze and 3m Cubitron. 50 grit is the lowest grit that will work on the Coote.....At least with ny set up.
 
You must log in or register to reply here.
Back
Top