Effects of plasma/laser cutting

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Will Estes

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Apr 25, 2022
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This has probably been asked a million times, but how does plasma/laser cutting affect steel? I was thinking about cutting out 10-20 knives with one of those, but I have heard that it can affect the steel around the cuts. I would go ahead and cut about 1/10” from the line and then grind away the excess, but is that enough room? Also, would the heat actually make any difference if I cut it before heat treat? I would think that the composition you want is achieved with the heat treat, so it wouldn’t matter if you used a laser cutter or not. I’ve also thought about a waterjet cutter, but that is more expensive and less common, so I would want to avoid that if possible.
 
I ran a laser department for a couple of years. Laser cutting does have a heat affected zone but it depends on the type of laser and the thickness of the material. Since you would be heat treating your knives anyways it really shouldn't be much of an issue.
 
Machinist for years machining lasered parts and working next to the laser area.
Stainless will work harden around a laser cut and be harder'n woodpecker lips, so I'm sure knife steel would be affected too... somehow.

Stainless got a fairly thin "skin" which was hard. If I was drilling and tapping an edge, we would plan things for me to skim .03" off, possibly .06" tops.
So .100" (1/10") might be more than enough.

However, if you are annealing and then re-hardening, if shouldn't make a difference either way.

Also, I'm not sure how knife steel would be affected. If it cooled slowly it could anneal it? Dunno.
 
Great! I think I may go into the 1/16”-1/20” range, then. That seems to be a good margin. I bet I could just anneal the blanks after cutting and it would be fine. How thick of steel were you working with? I’ll probably be using 0.1-0.125, so maybe the heat would travel farther in the thinner stock. I guess it wouldn’t matter if I annealed it, though.
 
Go water. No heat, no worries.

It's also cheaper than a wire
 
What do you mean cheaper than a wire? Do you know of any good waterjet cutting services? I have looked around a bit but I didn’t really see much that would be better than a laser cutting service like knifeprint or something like that. I have not used either of them, though, so maybe if I find a waterjet service I can compare the two and see what I like better.
 
Will Estes said:
What do you mean cheaper than a wire? Do you know of any good waterjet cutting services? I have looked around a bit but I didn’t really see much that would be better than a laser cutting service like knifeprint or something like that. I have not used either of them, though, so maybe if I find a waterjet service I can compare the two and see what I like better.
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Curtiss Knives also owns Great Lakes Waterjet & Laser. Could give them a call
 
Will Estes said:
Great! I think I may go into the 1/16”-1/20” range, then. That seems to be a good margin. I bet I could just anneal the blanks after cutting and it would be fine. How thick of steel were you working with? I’ll probably be using 0.1-0.125, so maybe the heat would travel farther in the thinner stock. I guess it wouldn’t matter if I annealed it, though.
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Up to 1/2". However, it was stainless, and the 1/2" did work harden more.
If you anneal it, it will surely be fine.

You could always run a test piece and hit it on a grinding wheel looking at the spark.
 
In my experience, the HAZ on knife steel was minor. Since we are not drilling and tapping, it is just ground away in profiling. An extra .100" past the desired profile is plenty.

The HAZ area in knife steel has two things to deal with. First is hardening of the steel on the side of the laser cut. This isn't much of a problem, as the belt grinder will remove it easily. Second is a skin of decarb along the cut. This is very shallow, and not really an issue at all.

Holes for screws and pins should be drilled instead of laser cut. Larger holes, like handle skeleton cuts, can be lasered and cleaned up with carbide burrs or left as-is if they don't show.
 
I have a friend that cuts deer hooves into my “skeleton “ blade handles. I used to profile the blades and give them to him to cut the hooves in. He later told me it would be much easier if he cut the whole thing. After that, it became noticeably more difficult for me to sharpen the blades to a razor edge with my Wicked Edge. I asked this very same question here a few years ago and the answers I got were mostly the same…probably won’t effect it. It effected the blades in my case. He was using plasma cutters.
C14531-E4-3784-48-F7-82-A5-F75319-BBD1-C9.jpg
 
godogs57 said:
I have a friend that cuts deer hooves into my “skeleton “ blade handles. I used to profile the blades and give them to him to cut the hooves in. He later told me it would be much easier if he cut the whole thing. After that, it became noticeably more difficult for me to sharpen the blades to a razor edge with my Wicked Edge. I asked this very same question here a few years ago and the answers I got were mostly the same…probably won’t effect it. It effected the blades in my case. He was using plasma cutters.
C14531-E4-3784-48-F7-82-A5-F75319-BBD1-C9.jpg
Click to expand...
Interesting. Did you heat treat after he cut them out? I wonder how much of a difference there is between a plasma cutter and a laser cutter.
 
Will Estes said:
Interesting. Did you heat treat after he cut them out? I wonder how much of a difference there is between a plasma cutter and a laser cutter.
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Yes. I’d get the blades back from him, hollow grind, heat treat. Finish up and bead blast. Then sharpen.
 
That’s a great question. At first I didn’t clean any at all…..the profile was as perfect as if it came from my template, so to speak. That’s when my problems became apparent and I reached out on this forum way back when. Afterwards I took off a smidge. I can’t say how many thousandths….just a bit, not enough to tell the profile changed from the original pattern. It helped some, but not as well as if I had profiled it on my grinders.

The answer for me would be to make a new template just for this purpose, a little proud on the cutting edge so I can grind it back some when I get em back.
 
A plasma cutter even if dialed in will leave a bit more HAZ than a laser, I’ve had profiles laser cut and only had to make a light pass with a 60-120 grit belt along the edge to remove the haz and it did not effect cutting performance on the knives. I know one of the members here uses a cnc plasma table and goes over the profile after cutting to remove haz and it doesn’t seem to be an issue for anyone, just requires a bit of clean up. Honestly when I’ve had profiles laser cut I only clean the edge up pre heat treat the spine and handle both get cleaned up as a by product of putting a handle on the blade so for me it really didn’t take any time to clean up haz and laser cutting atleast in my experience seems to be cheaper than waterjet.
 
Sending perfectly good steel to a shop is a gamble - there's always a chance the job will go south, and the odds are better on waterjet, as explained below.

A poor quality cut on a waterjet isn't a big deal because the cut surface and the burr (if any) are the same hardness as the parent steel. A waterjet is more reliable in terms of producing a good quality cut.

On the other hand, a poor quality cut on a laser means a striated cut surface that is 70RC and a gnarly burr ("dross") that is also 70RC. Lasers can be unpredictable in terms of producing a good quality cut, especially on an uncommon grade of steel. Also laser is likely to warp the blanks.
 
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