Slip Joint Spring

S

Steve Beckwith

Joined
May 29, 2004
Messages
399
So I made a slip joint folder using .093 15N20, both blade and spring. I must have assembled and disassembled this thing 100 times fitting it all up. Nice tight snap both opening and closing. I finished it. Done. Peened pins, this one is for me. I buffed up the scales, polished it, open it one more time to admire it and "ping" the spring breaks off. Faaaaaa! Aaaand gone. Needless to say, my disappointment level is extreme.

Anyway, the spring broke probably because it was too thin. I had ground down the actual spring portion to about 1/16". I came up with this dimension after disassembling a couple of old Buck knives that I have. My question is, how thick should I leave that portion of the spring?

Both blade and spring were heated to 1480, quenched in Parks 50, and the spring was tempered for an hour twice at 575. The broken end of the spring has what I would consider to be a tight small grain. Comments ans suggestions would be greatly appreciated, thanks, Steve B
 
John L said:
Spring width as compared to thickness should be at least as wide as thick and preferably 1.25-1.5 x thickness. Your heat treat is also off. At least 800F for 2 hours.
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Yeh.. what he said. your spring was way to hard.
 
I keep my springs at least .110 at its thinnest point.
 
Spring thickness really depends heavily on the length of the spring. If it snapped at a pin hole then you need to taper it more gently. If it snapped in the thin leg of the spring, then it was getting too much deflection, some combination of the spring too short the tang too thick or too much preload. I am unfamiliar with the steel you used, but I think there has to be something wrong other than the temper. As a test one time i tempered a 1095 spring 1/16" x 1/16 " x 1.5" at 450 degrees and bent it with a deflection of 1/4" at the end about 500 times and it stayed intact and true. I don't know how repeatable that is, though.
 
It broke right in the middle of the thin section. I had probably opened and closed the blade 300 times. You are probably right about too much deflection, I probably left too much metal on the pivot end of the blade, that is where the spring gets its leverage to keep the blade open and closed. Also I had preloaded the spring, I'll start over.
 
Well you got to have some preload. If it's really hard to dry fit everything together when fitting the blade, like so hard you have to use a vise to bend the spring far enough and your pin holes are starting to wallor out, that's too much preload.

Pics would help determine if it's a design problem or a temper problem.
 
I changed this post to be a bit clearer.

Another issue is having too much metal on the end of the tang. Newer makers like to make them too rectangular in the beginning ... to make sure it snaps open and closed with a big "click". These higher corners add a lot of extra bend to the spring twice each opening/closing operation, and can lead to stress failure in a couple hundred operations ... especially on a .060" thick spring.

I like to scribe a circle around the pivot hole and use this as my guide for how much the corners/heels should stick out past the arc. In a perfect match, they are the same open, half open, closed. This is where a rise and fall jig is great.If your spring sticks out past or below the scales/frame in any of the three positions, one of the sides isn't right on the tang.

As the next post said, the corners are not round, but just rounded.
 
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One more comment i have is you don't want to radius a corner in the tang. This will make the blade stick when partially opened or closed. Just knock the corner off with a pass or two of a file, and maybe lightly hit it with sandpaper, nothing more. This will keep the action nice and positive.
 
I know that I left too much metal on the tang. The spring projected quite a ways out of the back of the body of the knife when it was on the high points of the tang opening and closing. After a couple hundred opening/closings, I thought I had gotten away with it and it surely opened and closed with some authority. Also, the thinnest portion of the spring was only about 3/4" long. Now I think that should be as long as possible to spread out the flex in the spring. Another thing that may very well have caused failure of the spring was I had several deep scratches on the interior portion of the thinnest portion of the spring. They weren't (very) visible when the knife was assembled. The spring might have broken at one of those marks. I was hesitant to go after those because the spring was already pretty (too)thin. I also understand not rounding the corners of the tang, that is metal that the spring needs for leverage to hold the blade open/closed. Back to the drawing board. Thanks to all who have responded.
 
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