Grinding a blade before or after heat treating?

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logan5

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Jun 26, 2001
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I have a question. In Bob Tersuola's book, Anatomy of a folding Tactical Knife he talks about how he grinds the blades after heat treating. Now, I have heard the arguments on stock removal vs. forging, and the arguments on carbon steel vs. stainless, but I haven’t heard anything on grinding before or after heat treating.
It seems to me that it would be easier and less time consuming to grind before getting the blade good and hard. And isn’t the heat generated by grinding generally avoided as much as possible?

So what is it? Are there that many blades that develop problems in the heat treating that it's easier to just grind off the skin after?


Thanks for any comments.
 
Usually the only blades that are completely ground after heat treat, are thin blades, (like folder blades). These have a tendency to warp during heat treat. As far as grinding after heat treat, I think the general consensus is as long as you don't get it hot enough to change the color of the steel, you wont have any negative affects on it. I could be wrong, but that is my understanding.
Kyle Fuglesten
 
Also, most of the blades ground after HT are high alloy steels which temper up in the 600-950F range, so they are generally impervious to level of heat generated with coarse belt grinding. Variable speed grinders, running at lower speeds, also allow you to grind with less heat being generated. One significant advantage of post HT grinding is that all dimensions end up where you put them, which is important with folders especially. HT will change dimensions slightly in addition to the warpage concerns.

You won't find many people making BIG fixed blade knives in some of the really tough steels like CPM's opting for this approach though. It's bad enough just grinding out the scratches.
 
Well, here's an answer from a forger's perspective. I always pre-grind my blades before heat treating for several reasons. Mainly it's to get rid of any cold shunts or heat risers that can cause the blade to crack during the quench. I believe that an even, smooth surface will take a better heat treat. I actually hand finish each blade down to 360 grit before heat treat because it means less clean-up after the quench. And it's easier on your files, grinding belts and sandpaper. I try to forge each blade as close to specs as possible on the anvil so I don't have as much grinding to do. But I always leave plenty of thickness to work with so I can grind away the de-carbed steel after heat treat. Especially on the edge. That's the long and short of it for me.
 
This is weird! I do it half and half, doesn't anyone else? I found that if I grind about 50-60% prior to heat treat and then regrind the rest afterwards, I seldom get warpage my grinds come out alot better. I guess since it takes longer to mess up a hardened blade, it's much easier to make the grinds match and to get the ricasso the same on both sides. I rough grind with a 60 grit on small blades and a 36 on larger blades. Then take it thru a 120 grit prior to heat treat. After heat treat I regrind with a 120 grit till my grind is almost done, within 1/32" or so of my guide lines. Then I take it to finished size with a 220 grit and clean it up with the finer grits.
 
Heat treating first and then grinding is mostly done on folder blades, that way if they do warp you can surface grind them flat again. Usually the maker leaves .030 for grinding to make sure everything is flat.
Chuck
 
I make fixed blade knives with 5/32 D2. I heat treat before grinding. I like to think the blade gets the same "level" of hardness throughout the blade that way. I think of it like baking cookies. In order for the center, thicker part of the cookie to get done, the thinner edges get overdone. The cookie isn't baked evenly throughout because of the varying thickness. I'm not a metalurgist so I don't know if that's even a correct analogy but it makes sense to me. I use two 36 grit belts, one per side, to get the initial grind on a blade. I make one pass on the grinder and dip the blade in water always being consious of not heating up the blade. And as someone else pointed out, it takes forever to make a mistake.
 
Bill: While excellent for cookies your analogy doesn't hold for knives. Temperature is temperature-the edges see the same as the center, and, provided you are heat treating at the correct temperature, the edges do not "overcook". Cookie dough has ingredients that burn at the baking temperature, steel does not (assuming HT in a protective atmosphere or foil). Carbon loss through oxidation is possible in unprotected heating, but, that's not the question here.
Jerry: Both S30V, S60V and S90V temper in the range of 400F-It is EASY to reach this temperature while regrinding, perhaps moreso when the whole blade has to be ground. The metal at the interface of the belt/blade is getting HOT-look at the sparks from a new, coarse belt-they're red. I believe Terzuola uses some sort of system to apply water to the belt with a sponge-that will solve the problem. Without that system, or an eqivalent, you've got to go easy and dunk the blade after every pass.

Heat treating a full thickness blank will yield very flat blades-particularly with press quenching. It also gives the maker the opportunuty to overheat and ruin the temper. If adequate precautions are taken during post HT grinding, the hardness can be preserved.

So, guys who grind after HT are working with a different set of issues.

Stay Sharp,

RJ Martin
 
Hmmmm.., wow.., very informative thread! I've actually wondered about the pre/post heat-treat grinding issue before, but never really heard it discussed.

Thanks guys..., I'd love to hear more responses...



"Hunters seek what they [WANT].., Seekers hunt what they [NEED]"
 
I guess I'm at the other end of the spectrum because I do all of my grinding before heat treatment.

Grinding after heat treatment will yield finer finishes-per-grit, but it also increases the possibility of overheating thin sections of the blade and at the tip.

I avoid overheating the steel before sending it out for heat treatment, and Paul Bos does such a fine job that I've never had a warped blade come back.
 
I grind before heat treat all the way down to 220 grit. Any scratches heavier than 220 can and many times do show up in the steel later. In other words, when a deep scratch, (over 220 grit) is in the steel when hardened, you can grind below the scratch later, but when stressed they will show up again. I have seen this on several journeyman and master test blades. Smooth polished surface before the flex, then a wierd pattern appears in the stressed steel. This pattern matches the scratches they left in the blade before heat treating.

I leave the blade oversize because I want a heat cushion (for the lack of a better term) to make over heatin sopts on the blade accidentially. This also softens cooling rates.

There was a time when forged blades were thought to need a breaking in period. It wasn't a breaking in period but removing the scarf to get down to the best cutting edge.

You can also influence the matrix inside of the blade by hardening one geometry, then grinding to a differing geometry. These methods only work on differentially hardened blades. This gets kind of specialized, but once you understand what happens you can realize some performance variables that are very exciting. This is one of the variables that allowed the flexing performance of our recent test blade.
 
Sorry, the second paragraph should read - thermal cushion to prevent overheating spots on the blade accidently, or at least reduce the probability of hot spots.
 
As a follow up to RJ Martin's comments. Tersuola does mention something about a sponge and water being used, but wouldn't that be very dirty in a shop with slurry being flung across the room or at least all over the ceiling and floor around the grinder?
 
Originally posted by logan5
...wouldn't that be very dirty in a shop with slurry being flung across the room or at least all over the ceiling and floor around the grinder?
Click to expand...

It's going there anyway. The slurry would just make it easier to see. CLEAN is not a word used by a lot of knifemakers. :)

The bigger problem with that slurry is that it will coalesce dust from finer belts into particles that will scratch the steel as much as it sands it. With S30V I much prefer to run the belt very slow so as to generate little heat and avoid the need for water as much as possible. In any case I use my organic thermostats (thumbs) to avoid overheating. My pain threshold is well below 400F.

Sharpening a very fine edge on a belt is a greater risk to the temper than grinding is anyway IMO.

Ed, I'm not greatly surprised by your findings. Stress risers (those scratches) likely extend some during hardening/quenching.
 
Thanks Jerry: I love your concept of "organic thermostats". It doesn't get any better or precise than that
 
Ed and Jerry: When you grind, do you have your thumbs against the knife directly where it contacts the wheel, or off to each side of the wheel?

RJ Martin
 
RJ, my point end thumb is always over the most vulnerable spot on the blade for overheating, and I rarely take more than one pass at a slow speed before cooling. I think most of us who have been doing this for a lot of years are aware of temper vulnerability.
 
Jerry: I have never seen anyone grind a blade with their thumb against the wheel-that was why I asked.
When I started making knives 25 years ago, most makers ran their grinders fast, and, generally didn't get too concerned about heat build-up. Your original comment was about steels tempering in the 600-900F range and being impervious to the level of heat generated during coarse belt grinding. Actually, many steels are affected at substantially lower temperatures, which I pointed out in my post.

There are members of this forum who read the posts of experienced knifemakers and take them literally-something we must all recognize when we post. I have no issue with how you make your knives, not am I implying that you're not doing it correctly. What I am trying to do is be precise, so that new makers understand how important some of the issues discussed on these forums are to the quality of their work.

I hope you will agree-it is quite easy to overheat a heat treated blade unless the maker is paying considerable attention to belt speed, belt condition, lubrication, cooling and pressure. We have all done it. It is an easy way to ruin a blade, and, something every maker should work to avoid. Agreed?

RJ
 
I grind small blades in the hardened state anymore. Pass dip, pass dip. Using sharp abrasive the heat is minimul and I too grind barehanded. I find it easier to get a good finish and no warping during heat treat.

Big blades I grind soft.
 
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