Ok, you've forged, ground, hardened and tempered the blade. Now, especially for a big one for heavy chopping, it needs testing. You do this to find out
Does it hold an edge as it should?
Is it too hard/soft?
Is it tough enough to take hard use without breaking or chipping?
And does it handle as it should?
For the start, I clean up the surface, either grinding or using a buffing wheel with a 80 or 120-grit compound to shine it up. Look it over for warp, for any cracks or anything else that shouldn't be there. For hardness, basic test is with a sharp file. Generally, it either should not bite, or should barely bite on the edge. This depends on use; for a heavy cutting blade, the edge would not be as hard, usually. That's another thing that comes with experience and what you prefer on your blades.
All this checks out. This next I primarily do with a bigger blade. For flexibility, I clamp about 1.5-2" in a vise(padded, of course), grab the tank and flex it. I don't test to destruction unless I'm trying something that requires it, so I don't try to take it to 90 degrees; I flex it a suitable amount(yeah, that's not an exact measurement, live with it), and see if it properly returns to straight. If it cracks or breaks, either it was flawed or too hard; if it doesn't come back to straight, it was too soft where it counts. And I'll flex it both ways. This means the vise needs to be solidly mounted on something that won't move; that 12" bowie I showed once before, if I clamp it in the vise on the anvil stump, will act as a spring and dump the whole thing over.
Next comes cutting. I do the bulk of sharpening on the belt sander with a fine belt. Work both sides evenly at the proper angle(more variables, I know) until you get a burr that shows up on the side opposite the belt; that means it's as fine as you can get at that grit. Go to a finer belt or a buffing wheel, stone, whatever, and finish it. Good test for a smooth edge is to hold a piece of magazine page or newspaper up and slice it. Any roughness will snag and tear instead of cutting. If that checks, it's cutting time. But first...
This step I found in a 'Knives Illustrated' magazine, I can't remember the makers name. Take a piece of 1/4" brass rod and stick it in a vise lengthwise, so a third to half of the piece is showing above the jaws. You need good light for this: take the blade and lay it on the brass, then tilt it up enough that the edge itself, not the bevel, is on the brass, and push down. Watch the edge closely; it will flex. Don't push it too far, you just need a visible flex in the edge, then release the pressure. A properly hardened/tempered piece will flex, and the edge will return to true when you release the pressure. If it chips, it's too hard, put it back in the oven at about 25 degrees hotter for an hour and try again after it cools. If it stays bent, it may well be too soft for your purpose. If so, you'll have to go back to the forge/furnace and harden it again, then temper it at lower heat. I would suggest finishing the other tests first though, as it may turn out to be just fine for your purposes. You'll find out.
My main test for edge-holding is 1/4" manilla or sisal rope. Set a piece of wood in the vise, and start cutting. Depending on tang size/shape I may wrap it up with tape to give a better grip, and will wear gloves, and I mark a section with a marker so I keep cutting with the same area. Set the edge down about 1/4" back from the end, and push it through with your off-hand on the back. Move back a quarter inch and do repeat. Over and over, until it will no longer cut cleanly through. If you're being properly methodical about this, you'll take a knife of known cutting ability and use it to check a new coil of rope to see if it gives more/fewer cuts than the original, so you can properly guage things.
Results here will depend on the steel and intended purpose; one you need to hold an edge really well vs. one that has to regularly chop into hard stuff, for instance. Using 52100 or O1 steel, as a general rule, I should get at least 75 cuts(depending on rope) and still be able to shave hair off the arm, and it's not unusual to get over 100. With 5160 in a bigger blade, I still aim for at least 75; big blades should hold their edge well, too. At this point I'll go to a piece of copy paper and try slicing it to check for edge roughness; having dulled some should not cause roughness. If all this is good, with large blades I move to the next step.
Usually I touch the edge back up here, so as to start at the beginning, and go to heavy cutting. I try to save branches 1-3" thick when I prune just for this. Set a branch up so you can properly cut, and start chopping. Not light, delicate stuff, actually chop like you were using an axe. Chips should fly, if you have the bevels right. I'll cut through two limbs this thickness and check for sharpness(yes, when working on several I have bare patches on my arms); it should get through two and still either shave or almost, and it should still slice copy paper cleanly. Please note that if your first efforts won't shave at this point but will still slice the paper, be happy; that edge is still very sharp and very useable.
Let me touch on something here, sharpness. You sharpen different blade types differently. An axe has a wide, gradual bevel for toughness, and you wouldn't take one down to a shaving edge. A small knife for delicate cutting may need to be very thin to make curves or other cuts, and would need a thin, fine edge. Most blades are somewhere in between, that balance between thin/sharp enough for easy cutting and thick/strong enough not to break or chip under stress. You learn to balance the cutting/strength tests according to the blade type and use.
And that's the testing. If a piece needs some special quality, I'll test for that, but these generally cover toughness and edge-holding ability, which is what I'm interested in here.
By the way, I did the first part of this earlier; the rest was done just before bed with a glass of Cask & Cream on ice beside me, so if my spelling is a bit off, I really don't care right now.
Later.
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