Two paring chisel questions

[Steve Voigt]

A number of threads this year have piqued my interest in the long, thin, slightly flexible paring chisels of yore, so I've decided to try and make one. I've got an old Simonds USA red tang, 5/32 thick, completely shot. I ground the teeth off today and will attack the sides tomorrow. So far it's looking good.
The file is slightly bowed; it is convex on one side, about 3 or 4 thou from end to end, and concave on the other side. I didn't introduce the bow; it was there before. I will take as much of this out as I can, but I may not be able to remove it all. My question is: which side should be the back (flat) side, and which side should have the bevel? I am leaning toward making the concave side the back, but would love to hear opinions.
Also, since files are typically brittle, I was planning to throw it in the oven at 350 for a couple hours to temper. Any opinions on that?
Thanks
Steve

[Jim Matthews]

If you put the cupped side "down" you can register two points along the length to describe a line as you cut.

If you put the curved side (belly) "down" the chisel will register at one point along its length and carve in an arc.
If taking the temper out makes it a little malleable - can you flatten it some?

All my favorite chisels have either a hollow, or dead flat back.

[george wilson]

Steve,the trouble with kitchen ovens is that they can vary in heat by 75º. Files are inherently brittle. I doubt that you'll be able to get rid of that brittleness effectively. For a paring chisel,it doesn't matter that much,but DO NOT use a file for a wood lathe tool. I had a 2" wide skew break in half and go right past my ear back in the 60's.

You can try 350 or even 500º and the file will still be brittle. Grind to well below the tooth line or you will get hard and softer spots on the cutting edge due to the deep penetration of the cyanide.

I would opt for the bottom of the chisel to be the convex side to keep the cutting edge from diving into the wood if you get it really sharp. I have seen plenty of long chisels,frequently 1/4" ones,with convex curved bottoms so the handles clear the wood when cutting.

It is possible to bend a file straight if it is REALLY HOT. I have done this many times when making long tools like reamers. When tempering them,take them fresh out of the oven and immediately twist them between 2 sturdy dowels spaced about 1/2" clearance from each other,and set into a hard wood block clamped in a sturdy vise. In fact,files are straightened this way at the factory between billets of lead. Do not let the file get cool much at all,or it will revert to brittle and snap off. It needs to be at a medium straw heat,and work fast and decisively. Make sure also that the file doesn't have a "propeller twist" to it. Those have to be straightened hot with the nose of the file in wooden jaws,twisting the other end with a Crescent wrench.

[Steve Voigt]

Jim & George, thanks for replying.
George--great suggestion on the hot bending. It took me a few tries to get the right amount of overcompensation, but I eventually got the thing really straight and flat. Will post a pic if/when it works out.

-Steve

[Ryan Mooney]

Files are inherently brittle. I doubt that you'll be able to get rid of that brittleness effectively.

It is possible if you're lucky and spend enough time at it (or good, I'm not good but sometimes lucky :D). I used some files to make some knives back when I was a kid and had access to a forge. Out of three two were really good and the other one I didn't get the temper right and it shattered when stress testing[1] (the two that worked were older smaller files the one that didn't was a newer/larger rasp type dunno if that made a difference). What I did was:

anneal them by heating to cherry and then just leaving buried in the coal forge until the next day
grind the ridges off (annealing first made this a whole lot easier)
cut and hot forged to shape
annealed again the same way to take the forging stress out
Finish ground
Heat treated at cherry then quenching in salt water, we used the quick quench, pull, hit with a file, watch the colors run and then finish quench method which my dad could do but I was never that good at it. Heat treating and then soaking in an oven would be more consistent (although Georges point about oven variation is well taken - try checking yours with a decent oven thermometer, and realize that it can still swing by quite a few degrees. You can mitigate the swing to a large extent by putting a large metal box of sand (metal box :D) in the oven and measuring the temperature of the sand as it will buffer the oven swings and just adjust the oven temp until the sand is stably where you want it).
Polish and sharpen, add handle, etc..


The one knife was the best I've ever owned, you could skin a whole cow with it without sharpening it once and it would be darn near as sharp at the end as it was at the beginning. I attribute its success largely to luck rather than skill but still loved the result (sadly it was lost when it bounced out of its sheath when I was out riding in the mountains).

[1] Stress testing involved putting a piece of 1/4" soft wire on a block of hardwood and then hammering the edge of the knife through it with another block of wood. If it chipped it was to hard, if it bent it was to soft otherwise it was just close enough.

I'm not sure in your case if the annealing and re-tempering would add any value? You should be able to reduce the temper in the oven, but it will take a long soak to get to the center of the file (i.e. it may be pretty hard after a few sharpenings if you don't soak it long enough). I think the sand trick should probably suffice for sufficient temperature control for your purposes.

[george wilson]

I don't see the need for annealing and re hardening. The file is already fully hard for all practical purposes. Just temper it. Do leave it in a while as suggested. A sand bath would help.

I have made dozens of rifflers. I harden them,and just anneal them enough to make water I dip them in sizzle off. Then,wait 2 seconds and quench. Old timers calked that "taking the snap out of it". Just giving it enough temper to remove the glass hardness that has no mechanical strength,but still plenty hard.

[Steve Voigt]

Ryan, thanks for the detailed information. What kind of forge did you have? Was your Pops a blacksmith? Sounds like he had some skillz!
I particularly like the idea of the sand box; I imagine one could also use it for annealing? I'm thinking heat it up in the oven, bury whatever you're annealing in the sand, then close the oven and let it cool overnight.
I didn't want to anneal this piece though; I just wanted to use it as-is. I did soak it for a long time, then ran it through three more tempering cycles trying to get the bend right. I'm pretty sure it's good. And yes, I have a decent oven thermometer, so I'm not worried about that.
You are right--taking the teeth off was not easy with a hardened file!! But, it's done. Tomorrow I'll start working on the lands.

[Ryan Mooney]

Ryan, thanks for the detailed information. What kind of forge did you have? Was your Pops a blacksmith? Sounds like he had some skillz!

Dad is more of an all around old school ranch hand :D We actually had a couple of forges, one was a small portable one dad built for taking horse shoeing (about 2'x2' a bit bigger and deeper than your normal portable which made it at least moderately useful for other things), the other was an old log crib forge built sometime around the turn of the century (basically a 4'x4' - or thereabouts log crib frame filled with dirt with an open iron bar fire pit in the middle of it you could heat a big chunk of steel in it). I wasn't allowed to use much coal because of the cost (could get a bit of playing around in at the end of the day sometimes if he was doing something else and had fire leftover), but it turns out that thick douglas fir bark works pretty well for many things. Doesn't get quite hot enough to weld with very well (although I did manage to do some rake teeth once, but that was about the far edge of it) but fine for general forging and free :D. I never got very good, just handy enough to make a few googaws and things; the knives were about the peak of my forging skills and those were very hit and miss.



I particularly like the idea of the sand box; I imagine one could also use it for annealing? I'm thinking heat it up in the oven, bury whatever you're annealing in the sand, then close the oven and let it cool overnight.


People do use sand for annealing but its not optimal because it melts and leaves silica slag on the iron (not really an issue at tempering temperatures). Also you won't get it hot enough in a normal oven to do much annealing. Lime is perhaps more widely used or ashes or vermiculite, we mostly used ashes because they were ready made by the fire and just annealed at the end of the day. Ideally the whole mess would be at temperature but in practice most of the time it seems you can get by with just heating the workpiece and having the rest as hot as you can get it.



I didn't want to anneal this piece though; I just wanted to use it as-is. I did soak it for a long time, then ran it through three more tempering cycles trying to get the bend right. I'm pretty sure it's good. And yes, I have a decent oven thermometer, so I'm not worried about that.
You are right--taking the teeth off was not easy with a hardened file!! But, it's done. Tomorrow I'll start working on the lands.

Yeah there likely isn't a lot of value add to anneal it for this use. The reason i annealed it first was to make the grinding easier (and it was about no cost since the forge was already hot for something else) and to prep it for forging (just heating past magnetic doesn't remove the temper somehow, the slow cooling changes the structure further - don't ask me to explain this one, all I know is they can still break easier :D). The second annealing was to reduce forging stress before tempering; since you're not forging it there is no value there either. In theory annealing and rehardening can allow you to change the grain structure some more to a tougher and less brittle state than just reducing the temper by heat treating, but getting it to actually be "better" is fraught with oops along the way (based on my limited experience more likely to be worse than better until you practice/learn a lot).

Because I was annealing in open air (buried in ashes, but not sealed in any way), I'm pretty sure I also decarburized the metal somewhat although I have no idea how much or how deep and had no understanding of the process at the time (and re-forging it likely added some carbon back so much hand waving). If true that may have made it somewhat less brittle as well. In an uncontrolled environment like that though its all sort of hand waving :D