Threaded Tang Broke!!!

T

That Other Guy

Joined
Nov 24, 2005
Messages
77
Hello Everyone,

I didn't want my first post to be about something like this, but that's what I get for reading and not typing, I guess.

I ground out my own version of a Puma Seahunter in 1.5"x 3/16" 154CM. That went well enough. I threaded the tang for 12x24, slotted a guard out of 304 stainless, made a butt cap from the same stuff and made a take down nut out of stainless rod from the hardware store to hold the whole thing together. I'm using a block of black linen micarta for the handle-block was drilled out for the tang. I got all that done and sent the blade off for heat treating- double drawn and cryo treated.

I got the blade back a few days ago and put the thing together, fitting the guard better, etc... I must have torqued it together too hard because after about 6 screw togethers, I left it assembled and walked away. A few minutes later, I heard a sound like glass breaking and the knife came apart. The tang snapped at the last thread on the shoulder of the tang, taking about an inch and a half of tang away. I assumed that at least the threaded section of the tang would be softened by the heat treater in anticipation of being assembled. I guess I should have softened it myself. I'm just glad that this happened before I gave the knife to the person I'm making it for. Did I mention that I just picked out the most appropriate seeming size die from my tap/die set for 3/16" diameter and managed to pick an obsolete automotive thread? Things always seem to work out better when I'm making the knife for myself...

I'm thinking that I should nickel silver braze a 1/4x20 or 1/4x28 bolt (no one has 12X24) onto the tang and drillout/thread another take down nut...of course I'll have heat stop paste, wet rags, etc. to keep the blade cool during the brazing. I did try to thread further up the tang without any luck. Keeping in mind that the knife has to be readily disassembled, are there any other options for threading or any pointers for the type of steel I'm brazing?

Happy New Year, by the way!
Thanks,

Tony
 
Practice on some scrap until you are sure your joints will be good and strong.For a brazed joint, use a long angle lap joint (stronger). I just weld the threaded rod on.
 
As long as you are careful to not let the heat get the blade, welding the threaded rod on is what I do too. you will need to soak the weld with heat after the weld is done and don't let it cool to quickly or it will harden on you.
 
Unless you have a very sophisticated oven you are not really going to be able to anneal a tang of 154CM. It is a stainless and requires a very specific ramp and temp drop per hour to get soft. It is not like heating carbon steel to a cherry red and letting it cool.
 
154 Cm is very difficult to harden and to anneal afterwards.only real professionals can do it.ask for differentially tempering.
 
Can you show us a picture of the break? From the way I read your description it sounds like the threaded bit broke off at the transition point from threaded to non-threaded tang. I'm wondering if there was a sharp corner that might have caused a stress riser that caused the break.

-d
 
deker,

I don't have the camera right now...Its a slightly jagged but diagonal break if you were looking at a side view of the knife. The break was also diagonal if you were looking down at the top side of the knife (one side broke further back than the other). The sides of the thread were ground off to provide a thinner guard slot, maybe that contributed to the break...but the corners of the tang were rounded. I did check the threads with a strong light and magnifying glass for any stress cracks before assembly and found none. The appearance of the broken steel is very fine.

If I sent this blade back out for a professional heat treat again after brazing the threaded rod on, the brazing would melt- just going by Texas Knifemakers Supply's warning not to have anything that melts under 2500 degrees attached to the blade. Who does differential heat treating? Is there a shade tree expedient I can do with access to oxy/mapp or oxy/acetylene?

I'm going to start typing gibberish if I keep going, the kids are back...

Thanks again,

Tony
 
Here's another thought...

Because the sides of the thread were ground away, I couldn't get a grip with the die to try and continue the same thread. How about grinding down an inch or so of the tang to about the right diameter for a 10x32 or 8x32 thread and trying that? I'd just have to have an extra long take down rod/nut to make up for the piece of tang that already broke off. From reading a little more into the hidden tang take downs, I feel I may have over tightened way too much the last time it was assembled- some edge buckling on the micarta block.

The kids are still here and howling, so forgive me if this isn't well thought out...

Thanks,
Tony
 
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