Equipment for Cutting Knife Steel

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Cutting with plasma versus grinding wheels. January 16, 2004

If I use a plasma cutter with nitrogen to rough out profiled cutter knives, will it have any effect on the hardness of the steel? Looks like it would be a good way to save time and $38 grinding wheels.

Forum Responses
(Solid Wood Machining Forum)
From contributor A:
There has been quite a bit of debate on the subject of what damages knife steel and what doesn't. The general point made by all steel manufacturers is that a temperature of about 1200 degrees must be maintained for several minutes before the steel loses its hardened properties.

In that respect, one would think that a plasma cutter would be okay. However...

Since this debate also covers the damage done by abrasive cut-off wheels, grinding without coolant, and a few other questionable actions, I would think that the last thing a person would want to do is apply a plasma cutter (which is basically reverse welding) to a hardened product and expect it to last.

Next, there are the rough edges left by the plasma cutter that have to be dealt with. Even more so if you are using this to cut it to length. All those burrs need to be cleaned off before you can even mount it in the head.

Also, if you are spending $38 for a roughing wheel that is not aggressive enough and doesn't last that long, why not just look for a better, cheaper wheel?

Also, check your grinder's RPMs. You can find a nice balance between wheel life and aggressive steel removal if you play with that a little.

From Dave Rankin, forum technical advisor:

I agree about the use of plasma on a hardened tool steel. There are many good roughing wheels available for fast removal of steel while profiling. I use a ceramic wheel most of the time for roughing and can rough out a 3" wide crown in less than 2 minutes with the correct wheel and speed.

From contributor B:
First off, the wheels we offer for your Foley 75-12 grinder are less than $38 each. We offer a grinding wheel that is more aggressive than anything Foley United stocks. It's a part number WA462-10 raised hub wheel that has been used industry-wide for the past twenty years for roughing of pattern knives. There are other companies that may also stock rough wheels for your Foley.

Now on to the issue of using a plasma cutter to rough out your knives. While a plasma cutter works well to rough out mild steel, I feel that the end results on hardened steel would adversely affect the heat treat of the knife. You would also be able to accomplish, at best, a very rough cut. We do have customers and grinding services that make use of EDM and water jet technology to rough out pattern knives. The EDM would be the most feasible, as water jet technology is quite expensive.

From the original questioner:
Rob, most people use a dry grinding wheel to cut the knife length. Doesn't this procedure produce enough heat to effect the temper? A plasma cutter doesn't leave slag like a cutting torch. From my observation, a plasma cutter cuts very smooth with extreme accuracy... The use of nitrogen cools the steel instantly, therefore the heat would never penetrate to the actual cutting edge of the steel, since we would only be roughing out the pattern. I'd bet the heat from the abrasive cutoff wheel has a more negative effect than a plasma cutter, but I could be wrong. You can hold a piece of steel in your hands without getting burnt immediately after using a plasma cutter with nitrogen.

I may very well end up going with a more aggressive (cheaper) grinding wheel, but I would first like to know logically why this wouldn't work.

From contributor A:
The use of the nitrogen in the plasma cutter as an inert gas only prevents oxidation and burning. It has little or no effect on actual cooling. It could if you applied copious amounts of the gas, but it is only creating an atmosphere to cut in, not cool in.

If I could give scientific facts on what I am trying to say, I would, but I will leave it at this:

1) Myself and a lot of these fellas have been doing this type of thing for a long time. I think we can all agree that we have seen some of the best and worst ways to damage knife steel before we even have the chance to use it. If we all agree that cutting knife steel with a plasma cutter is an exceptionally bad idea, then it might be worthwhile advice.

2) However... If you find that it is working for you, it is not degrading your finish, it is not taking away from the life of the knife, and is not causing any other production flaws, then by all means use it.

In regard to the abrasive cut-off wheel, it takes less time to get through knife steel with a cut-off wheel than it does with a plasma cutter. I know it sounds funny, but it's true.

From the original questioner:
As for cutting the steel to length, we have a Porter Cable metal cutting chop saw but I'm not real pleased with the cutting speed and I don't get real straight cuts with the type of wheels that we're using. Is there a particular type of cutoff wheel that we can purchase at the hardware store that might work better? The ones we're using now are made by a company called ABMAST and the code is F431R. They appear to have a reinforced mesh but I wouldn't say they're triple reinforced.

By the way, I wasn't challenging anyone's expertise. I just couldn't figure out why nobody used a plasma cutter to cut the knife length since the heat only penetrates 1/8" from the cut. Of course, getting a good square accurate cut may not be such a big deal if my cutoff wheel was appropriate.

From contributor B:
We use a 14" triple reinforced wheel on our 14" Makita. The part number is A30-1418. This same type wheel is available in other sizes as well. It is not an item we are able to find at a local hardware store; it's one we have the factory special make just for the purpose of cutting corrugated steel to length.

From contributor C:
Consider base metal cracking at the edges of the plasma cut. HSS doesn't like rapid temperature changes. If you ground away the distressed HSS, I'd think there'd be no big problem.

On the other hand, if you plasma cut the knife to near net shape and bevel, I'd be concerned about sub-surface cracking adjacent to the HAZ. The instantaneous temperature rise in the plasma cut is about a jillion degrees a second and the drop after the passage of the jet isn't much less - especially if a CNC cutter in used.

I'd think there'd be at least 1/32" to 1/16" distressed metal to remove before you get to safe, sound base metal. Try it and see. Let us know. It's your money and time.

I kerfed the very few profiled knives I've made to rough shape from electric pencil layout with the chop saw. If you space the blank out some distance from the back jaw, you can get the bevel pretty close.

From contributor D:
I have tempered different metals over the years and as a rule of thumb, if you change the color from shiny to something else, you have changed the rockwell hardness. Not knowing what steel is used (0-1, D2, etc.), it is hard to say how much hardness is lost. You would need to grind all color left from heating out of the finished knife to be sure that the knife will retain its edge as long as possible. Flooding the knife with coolant during grinding is the life of a knife steel.

As far as grinding wheels go, they must break down fairly quick to cut fast. You are ahead of the game to run coolant when roughing knives. The coolant washes the metal particles out of the wheel and does not allow them to weld or stick to the wheel material. When the metal particles are pulled back through the cut, they create more heat and thus burning.

Surely there is a suitable wheel for less than $38.

From contributor E:
Has anyone actually tried this? I could see where cutting away some of steel on deep profiles would save wheel life. I only would consider this on deep patterns where you are removing large amounts of steel.

From Dave Rankin, forum technical advisor:
As a grinderman, I have used many wheels. The one that I use for the Foley grinds very fast and lasts a long time. The price of the grinding wheel and the cost are different. If the wheel costs $25 and will grind only three knives then it costs more than a $50 grinding wheel that will grind 10 knives. For example, yesterday I ground 40 inches of profile knives, average depth 1/2". It took less than 4 hours and I used just a little over 1/4" of the grinding wheel. With most grinding wheels, I would have used at least two complete wheels. Price is not the concern, but cost is.

A top quality wheel also reduces the grinding time and the dressing time.

From the original questioner:
Just so no-one tries this at home.. The plasma cutter does weaken the steel. We have found better ways of cutting our knife stock to length using some information found on this website.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor S:
Some tool steels, such as D2, cannot be cut by a plasma torch - I've tried roughing out some blocks for a stamping die. A big Miller Plasma would not cut into it. I have heard it works worse the higher the chrome content. D2 is 10-12% chrome and stainless steels are 16-18% (with some exceptions). I have not tried it on stainless steel.

Comment from contributor H:
Plasma cutting should be fine for knife steel. While the plasma beam itself reaches 45,000 degrees or higher, this all hits the metal that is removed rather quickly with little thermal transfer to remaining metal.

A properly cut kerf with decent equipment should not have any burrs at all. You should be able to run your finger along a smooth surface and the cut should be 87-90 degrees. Keep in mind that there are two sides to a kerf, due to the right hand swirl of the plasma interacting with the cutting direction, so one side of the cut will have a rounded cut edge, and the other side will be sharp and either 90 degrees or very close. You want this edge to be on the side of the production part. Definitely use nitrogen, unless you have a high amp cutter and are using solid silver electrodes. You can use pure oxygen with these - the solid silver electrode offers the best possible cooling, and thus highest possible cutting speeds while maintaining perfect cuts and very narrow kerfs and extended life to the hafnium tip.

I would highly recommend plasma over cutting wheels. Cutting wheels are too slow, and speed is the important factor with loss of hardening. If you are not stamping the parts out of sheet, the best options are plasma or waterjet.