Epic knife fail. Check your work!

J

Jason Fry

Joined
Jun 5, 2008
Messages
3,220
Argg. Usually the FUF visits are obvious and bloody. This time she snuck in on me. I am working on learning slipjoints. I had three together, with geometry problems so that they wouldn't snap shut. Re-made the blades and got them snapping nicely. Heat treated them according to the recipe I remembered. Put them together, made nice knives.

Sold one at a knife show.

It returned to me in an envelope today.

Turns out the edge rolled cutting a zip tie. The customer was rich and friendly, much better than I expected. Expressed no concern whether he got the knife or his money back, just wanted me to know it wasn't right.

Took it apart and RC tested the blade. Turns out it's RC 56. No wonder the edge didn't hold.

Moral of the story? Check your stuff x1000. I apparently overshot my tempering temperature by working from memory. Check your notes! I RC tested at 62 out of the quench. Didn't test after tempering. Stupid mistakes, and now I get to redo the blades I've already re-done. Still haven't figured out the best way to make it right for the customer.
 
Good post and a reminder to us all. That's one of the reasons I always do the brass rod test along with a little rope cutting, don't have a RC tester unfortunately.
 
Jason, were these the 154 CM slipjoints from your emails?

If you have an RC tester I might have to stop by after I use my new HT oven a few times and see how I did.
 
They are the CM 154 ones you've seen. Stop by any time.

Don,
They're usable, I suppose, but not what I want out there with my name on it. You can get them shaving sharp.
 
Brett Mathews said:
Thats what I was thinking.
Click to expand...

Really? I'm new both to slipjoints and to RC testing, but I have built quite a few smaller (less than 8" oal) fixed blades at 60-61 RC. Stay sharp a long time, and don't chip or roll. Is 56 and a rolling edge really acceptable on a custom CM 154 slipjoint?
 
Most heavy choppers RC in about 56. They hold an edge quite well. Not meant for fine cutting but still hold the edge and do not roll. It sounds like there might have been something else going on.
 
jkf96a said:
Really? I'm new both to slipjoints and to RC testing, but I have built quite a few smaller (less than 8" oal) fixed blades at 60-61 RC. Stay sharp a long time, and don't chip or roll. Is 56 and a rolling edge really acceptable on a custom CM 154 slipjoint?
Click to expand...
No, an edge rolling over is not acceptable on a custom slipjoint. Maybe it's the 154cm, but I have many older factory slipjoints with carbon steel blades that rockwell in the low to mid 50s' (some in the 40s') and they hold an edge pretty well. These high Rc numbers we are doing today are relatively new in the history of knives. But so is 154cm, which is a steel I don't use, so my rambling may mean nothing. :)
 
I've owned several factory stainless slip joint knives that would roll the edge, I won't mention any names though. That's why I like carbon steel slip joint's. Never had a factory carbon steel slip joint roll the edge. Never owned a custom slip joint though I want to start making my own some day when I learn enough about grinding and HT.

Does the edge geometry and thickness factor into whether or not the edge will roll at HRC 56?
 
I would bet the edge is softer than 56. There are a lot of possible variables here, including a bad cutting angle vs. the zip tie which put lateral pressure on the edge instead of driving it through the plastic. Even at 56 the edge should have held unless there was something else at play.
 
woodwrkr221 said:
I've owned several factory stainless slip joint knives that would roll the edge, I won't mention any names though. That's why I like carbon steel slip joint's. Never had a factory carbon steel slip joint roll the edge. Never owned a custom slip joint though I want to start making my own some day when I learn enough about grinding and HT.

Does the edge geometry and thickness factor into whether or not the edge will roll at HRC 56?
Click to expand...

I've also seen a few stainless knives roll an edge, but were of the cheap variety.

Edge geometry does make a difference. But the edge on a folder blade should be very thin, to cut well and for ease of sharpening. It also needs to be hard enough not to roll over. Very fine grain is also very important for a fine, thin edge.
 
I agree.... 56 shouldn't roll on plastic. My large beater blades hover around 56-58 HRC.... but I also agree that I don't like to go less than 60HRC on anything smaller and thin. Slicers should be higher hardness. Could your tempering oven have overshot the mark... perhaps, radiant heat from the elements overhaeted the edge. Do you check HRC before you temper? Maybe the problem was the in the quench?

Rick
 
The edge is fairly thin. I did some testing on a mild steel nail. Hard pressure with the blade perpendicular resulted in no deformation. Hard pressure with the blade at a 45 degree angle resulted in deformation.

Could be several issues with the heat treat. As I said before, I was working from memory and google, not from my notes. I also don't remember exactly what process I went through on these blades, as it was about 6 weeks ago. Expect to see a "heat treating CM 154" thread from me in a little while.
 
My initial thought was 56RC shouldn't roll like that. Where on the blade did you test the Hardness? Could you have overheated in the Post HT grinding or sharpening process? I find it very easy to heat the thin edge when profiling the secondary bevel. which can easily result in work tempering only the edge well below 50RC. Just a thought.

Thanks for the Reminder and sharing.
 
Could be that too, Ryan. I tested on the ricasso. I water dipped often when grinding the bevel, but I'm new to that too. I always grind my bevels before HT on my fixed blades, and I use alot of 1080 which will show you colors when you screw it up. Not sure what the 154 will do when it gets too hot for a second while grinding bevels.
 
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