Friday, July 25, 2014

A couple of knives, beginning to end

I'd mentioned making some kitchen knives for the offspring & wife.  I got the big one done in time for them to take back with them, but the smaller, no.  So thought I'd go over how I'm making them*

I'm starting with a flat piece of O1 tool steel, 2" wide and 1/8" thick. 

 Use the hacksaw to cut an appropriate piece.
 Mark the point(a bit oversize), and the tang(where the grips go),
 and cut them, leaving the rough blank.
 Go to the grinder and clean up the profile.

Here I had taken it to the anvil to straighten, as it had a bit of curve in it.  Then I hit it with the belt sander to clean up the sides a bit.  Which could've been done on the grinder as well.
 Next step, grind** the edge area to a little over 1/16" thick. 

I work back from there, grinding material off, cutting deeper toward the edge, on both sides.  Light pressure, and keep it even.  Keep a bucket of water handy so you can cool it off when it gets uncomfortably hot.  In this case, the steel being in an annealed state, you can't mess up the hardness by getting it too hot, but you can burn the crap out of yourself.

When it looks pretty even on both sides,
 make sure the face of the grinding wheel is even, and work it lengthwise.
 This lets you see if you have any high or low spots to deal with, also if you need to change your angle a bit.  I'm grinding this so it's a straight taper from back to edge, so if I do this step and find the bevel is a bit off, I can correct that. You can make the bevel run from the back, partway down, wherever you choose for the blade shape/size/steel you're using.
 When that's done and all looks even, I go to the belt sander.  This is a 6x48" model with a 9" disc on one side(same one I used in crowning that barrel).  Keep that water handy, and start with a coarse belt(in this case 80 grit).  Work both sides, and clean up the marks from the grindstone.

You can also clean up the profile now.  When you have all the coarser marks gone, go to a finer belt. 
Note: they make a eraser-type block to clean the dust out of the grit, I highly recommend it.  I used to have a tube of a belt lubricant as well, you used it on the belt before you started cutting and it helped keep the grit from clogging.  Wish had some of it left.

Work each grit at a slightly different angle, which makes it easier to be sure you got rid of all the previous scratches.  And make sure the tang is flat as well.

A word on belts, this size or any other: locally I can find 80, 100, 120 and 180 grit.  If ordering, I can get anything from VERY coarse(as in 36) up to 400 grit.  If I were working on something I wanted a very fine finish on, I'd get a selection up to 400, then use buffing wheels for a higher polish.

But not right now.  We'll be going to the forge to harden these blades, and taking it beyond 180-200 would be a waste.  Much like polishing a gun you're going to rust-blue beyond, say, 320.

One more note here: the steel is as soft as it's ever going to be, so get the profile cleaned up and the bevels correct now.

Here's my forge.
  It's an old Champion portable, cast-iron bed and a hand-crank blower(I did have a bigger forge, different design; it gave up not long after I had to stop forging).  I used to use coal for the fuel(more heat per pound), and I still have a five-gallon bucket left, but to keep the smoke down here, since it's now available, I'm using charcoal.

NOT briquets.  They make those by grinding the charcoal to dust and mixing it with a binder, usually clay, wetting it and forming it into those nice, uniform pieces.  But the binder generates a LOT of ash, and causes it to spit badly when you start pushing air through.  Now, happily, raw charcoal is easily available, so I'm using that.

This odd-looking piece is for heating longer blades than these; piece of iron pipe cut in two with a bunch of holes drilled through.  Allows you to have a longer, narrower fire when needed.  In this case did not use it, blades not that long.  Do note that you need to either use a piece of brick or something to block the ends so the air doesn't all take the easy way out.
 I use some fire bricks to form a narrow channel(Yes, I forgot to take it out for the pic). 

A barbeque fire chimney works nicely to light it,

just let it sit until the coal is all burning, then dump it into the firebed.

This is my quenching trough.  Since these are small blades I'll prop one end up and just use the other end, that'll be plenty of oil.  As the fire is burning in, I'll stick a piece of heavy steel in- in this case a railroad spike- to heat, then I'll drop it into the oil to warm it.  Warm oil flows more easily, and gives a more even quench.  And if it's cool or cold outside, that'll also mean a bit less thermal shock to the steel.

As to the oil, more years ago than I care to remember I dug around and found a industrial oils company in Tulsa(they're still there) and called them.  They put me through to a salesman who listened to what I was doing and suggested a light quench oil, and sold me a five-gallon bucket.  I've still got about half, and if it hadn't been for some spills there'd be more; it doesn't wear out, just cap it tightly when not using it to keep moisture out.

When the fire is ready, what I do is put the blade in back down for a couple of minutes,
with low air blast, to warm the piece slowly.  That does two things:
I can pull it out and see if the blade warped any as it heated(and can straighten it now), and
I think it acts as a stress-relief heat.

Then it'll go in edge down. 
Low air blast at first and move it back & forth to heat it evenly, then stronger air when you're ready to bring it up to critical temperature.

You don't have to get the whole blade up to critical temperature, I like to get the area from the edge back to the line
up to full heat.  Keep it moving to keep the heat even, get it to that shadowless red I wrote about before, then into the oil, edge down.
Why only that part of the blade?  It's called differential hardening: the part that does the cutting, both now and as it wears down over time with sharpening***, is fully hardened, and the area behind that, since it's below critical temperature, will harden some, but not completely; more of a spring hardness.  I started doing that quite a while back, and especially for big blades and need to be able to take the shock of heavy cutting and chopping, it works beautifully.

You can get some of the same effect from getting the whole blade up to heat, but only lowering the 2/3 to 3/4 of the blade from the edge back into the oil; the rest will cool much more slowly and not fully harden.

I do two quenches.  While the first blade is cooling, I heat and harden the second.  By the time that's done I can look the first over, see if it warped and needs straightening.  If all's well, back to the fire with the same procedure as the first time, then same with the second blade.

This is out of the oil and cooled. 

No, no pictures of the heating and hardening, I'm not going to try to do those things and juggle a camera. 

Once cooled, or if you're impatient after it's been in the oil at least 30 seconds(preferably a minute), check the edge area with a sharp, fine file; set one side of the blade on something to steady it and slide the file across.  In the case of good medium and high-carbon steels, if they fully hardened the file will slide right off, doing nothing more than cleaning the surface a bit.  With a medium-carbon steel it might be more of a sensation of the file almost biting.  If the file actually bites, you didn't get it quite up to full critical temperature and it'll have to be heated and quenched again.

Wash the pieces thoroughly to get rid of the oil, then into the oven at 400F**** with a wire rack to keep them edge up.  They're also sitting on either a cookie sheet or baking stone(another reason to do a good job washing) to even out the heat.   At this point, remember: those hardened blades are hard.  Brittle hard.  Drop them on a hard surface and they may well crack.   So handle them carefully.

Leave them in for an hour, then pull out and let air cool.  At this point I'll take them to the belt sander and shine them up, then back into the oven.  Yes, I could do this before the first tempering heat, but remember that part about brittle?  I'm more comfortable doing this after the first heat.

Another hour in the oven, this time at 425F, then let them cool and look at the color.  With this steel and for this purpose, I like a medium-bronze color, which corresponds to a tempering heat of 425, if you cleaned all the oil off and have a shiny surface to start with.  Hard enough to hold an edge quite well, not so hard that you'll chip the edge hitting a bone or something.  After it's cool, back in the oven for one more hour, at the end of which turn the oven off and leave them to cool inside.  In this weather, and tired as I am, that means I'll start the finish grinding, drilling for pins and polishing tomorrow.  Which will be another post.

Just to cover everything, while the blades were cooling I scooped the remaining charcoal out and into the water bucket, poured the oil back into its bucket, and generally cleaned-up and put-away.

You DO have a water bucket around while doing stuff like this, don't you?  Uses are wide, from putting out the charcoal afterward, to 'Guess I better put that grass out', to "OW!  HOTHOTHOT!"

*Because I didn't think to do this on the big one.
**You can do the whole thing on a belt grinder.  I like to do the heavy work on the wheel, and save the belts for flat-grinding and polishing; they're not cheap.
***Use good steel and heat-treat it right, it'll last longer than you'll live before it gets past the hardened area.
****I know this steel, the second temper will be at a higher heat but I like to start here with O1.


mr c said...

Why two separate quenches?

Anonymous said...

Meant to say temperings.

Anonymous said...

I'm not by any means a religious man, but when someone asks about the existence of god I say 'steel'. Almost magical in its uses.