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Post Heat Treat Grinding

Discussion in 'Working the Steel' started by Griff, Feb 9, 2020.

  1. Griff

    Griff Active Member

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    Hearing a lot of makers talking about post-heat-treatment grinding lately. Mostly it’s coming from the Culinary side of things, as the stock is typical thin already and prone to warping if ground thinner pre-heat-treatment.

    The reason I ask is they are always fretting about overheating the blade, remember we are not talking just post HT clean-up here, they are grinding in full bevels.

    My question is to those who do this (or just know) is why most of these makers grind after tempering? Is it a danger to grind in bevels after the blade cools post quench but pre-temper?

    Surely there’s no grinder or ceramic belt capable of reaching and destroying the extreme HT temperatures, but they sure can easily ruin a 400F temper. Also some stainless temper at extreme temperatures too, so I would think only the very edge and tip to be at risk.

    Anyway let me know your thoughts,

    cheers,

    Griff
     
  2. FORGE

    FORGE Maker of the Year Best Knife

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    Griff that is my biggest fear, getting the blade to hot. It does not take much to see a shiny blade turn blue because you ground in one spot to long, had a dull belt or looked away for a second.
    Some steels like 52100 are tough to grind when it hasn't been hardened and after hardening you can just about forget it especially if you want to hand sand after grinding.
    I find if the edge is thinner than .020 thousands and you heat treat the blade is going to turn into a sin wave.
    Once that happens it is either going in the junk pail or you are going to turn it into a very slim knife blade.
    That is why I DO NOT grind after heat treat and finish my blades around .020-025 before they go for heat treat.
     
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  3. Griff

    Griff Active Member

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    Cheers Cal.

    I have 5 short bars of .100 AEB-L, all designed to be Kitchen cutters, and I was toying with post HT grinds. All my experience thus far is with 01, and I have always gone down to 0.10 - 0.15 thousandths at the edge before HT with it (its very forgiving in the quench). I feel comfortable taking your lead and only going to 0.20-0.25 thousandths with the AEB-L and see what happens.

    I am starting here because I have a 35 layer stainless pattern billet with a VG-10 core from Takefu special steels that I want to try my damned hardest not to screw up lol!
     
    Last edited: Feb 10, 2020
  4. dancom

    dancom Dust Maker Best Shop Tool

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    There will usually be some grinding after heat treatment. Most makers take the bevels down to a dime thick edge before heat treatment, then finish them after heat treat. I do all my bevel grinding after heat treatment. Sharp ceramic belts, bare hands and a bucket of water. I start with 36 grit until the edge is down to about 20 thousandths, then few passes at 60 grit, then 120 grit. By the time you get to 120 grit, the edge will be around 15 thou. Can be hand sanded or worked gently with conditioning belts. The secret is manage the pressure and time and not be in a hurry. At least 100 passes, dipping the steel every one or two. The last step of setting the edge I do with a sharp belt as well. This is the most risky step for potential bluing, so I have learned to go light and quick. Slowing the belt speed can help here too. Try around 1200 sf/m and about two second light pressure passes.

    Dan
     
  5. Magnus

    Magnus Member

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    I'm with Dan on this one. Lately I've been grinding all bevels post heat treat. I go through a few more belts but I find I get less warping and the warping that does occur is easier to correct.

    I use bare hands when I grind so I can tell when the blade starts to get hot. It's worth noting that I usually end up with a few heat blisters on my thumbs from doing this (200ºF is actually pretty hot on your skin) but guess that's the price of using yourself as a thermometer.

    Magnus
     
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  6. John Noon

    John Noon Well-Known Member

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    I had good luck with the Purple Ceramic belts from Phoenix Abrasives for post heat treat grinding. They are made to be used on hardened steel and get the job done pretty well.
     
  7. dancom

    dancom Dust Maker Best Shop Tool

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    Yah the thumbs and fingertips get fried sometimes. These days some ice in the bucket helps.
     
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  8. Griff

    Griff Active Member

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    Okay, but still I’d like to know if there’s a difference grinding post heat treatment but pre-temper?

    I am asking because I am on board with ruining the temper on the grinder, but can you ruin a HT on the grinder? If not the later, shouldn’t you grind in bevels before tempering?

    My thought is if I profile my knife, get it flat on the disc grinder, HT and quench the AEB-L and then grind in the bevels before tempering then I have less chance of (a) warping from HT and (b) there no risk of overshooting the temper temperature grinding in the bevels...because...well...I haven’t tempered it lol!

    Maybe I am looking at this all wrong. Let me know.
     
  9. dancom

    dancom Dust Maker Best Shop Tool

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    Hi Griff,

    I understand the question. To clarify, for me, heat treatment is a two part process: the first part is hardening and the second part is tempering. I think by HT you mean hardening.

    Personally, I wouldn't grind before tempering for several reasons. Firstly, the hardened (un-tempered) steel is quite fragile due the stresses undergone during heating and quenching. The blade is more akin to glass than steel at this stage. I dropped a blade on the concrete floor before going into the tempering oven and handily snapped in half. That wouldn't happen after tempering. The stresses in the hardened steel are somewhat relieved by tempering.

    Secondly, if you accidentally burned a hardened steel by grinding you have essentially flash tempered it. If grinding heats it past the tempering temp it will change colour at the burnt spot, say brown or blue. You've probably seen springs that are tempered into the blue colours. For most steel this bluing occurs in the 500°C to 800°C. Congratulations, you've made spring steel. Ideally the tempering process takes a few hours, but a flash temper can take only a second. Make it blue and you've lost the hardness. If you go past the desired tempering temp, you cannot undo this. It's back to the oven for a full run to critical temp and quench again.

    Lastly, grinding after hardening (but before tempering) will be that much harder on belts.

    Hope that makes sense.

    :)

    Dan
     
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  10. Griff

    Griff Active Member

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    Thanks for explaining very clearly why not to grind hardened untempered steel Dan.

    I guess the only case you would actually risk this is if you forge out blades that are extremely crude and thick, and need to grind of some meat, so it’s not something a stock-removal maker would want to risk on already thin stock.

    I totally get it now. Cheers:beer:
     
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  11. dancom

    dancom Dust Maker Best Shop Tool

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    In all fairness, you could burn and ruin fully tempered blade too, as Cal mentioned. Fully tempered blade will be a little less hard and and tougher than an un-tempered one.

    I started grinding post-heat treatment after learning Ray Rogers did it that way. I haven't burnt a knife since 2014. So the water-bucket-hot-thumbs is a viable, albeit messy method. One key point is that things heat up fast with worn belts. For a chef's knife I have a "one belt per knife" figure to work with. I know belts are like $10 a pop, but when making an expensive knife, one shouldn't skimp on consumables. :)

    Also, grinding bevels isn't a race. I see videos of guys grinding insane amounts of steel at one shot, applying huge amounts of pressure, sparks flying everywhere. Yikes! Maybe it's a machismo thing? The same reason guys want 10 hp grinders. In my opinion fabricators should make sparks like that, not knifemakers. For me, it's more about finesse. Like everything else at my age goes...slowly. LOL

    The steps: profile, harden, temper, grind & attach handle are pretty well established.
    Here you can watch the robots at Wusthoff do it. Note the flood of coolant used during grinding!


    Dan
     
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  12. Griff

    Griff Active Member

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    Dan I am the same way, slow and steady. Everybody does things differently, but to the same ends. Like my personal preference for initially grinding in bevels is only 120G belts. I only use 36G , 50G or even 80G to profile the design shape off the bandsaw. Plunges are filed in, and then I grind in bevels with a 120. Also I am constantly tweaking the vfd speed knob to a desired feel as the knife is drawn across the platen. I’m itching for this weather to break so I can clean out the shop, and get back into it!

    Also, glad to see the forum back up and running *thumbs up*
     
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  13. John Noon

    John Noon Well-Known Member

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    I learned that 36 grit was awesome to remove material first time I tried one. Also found out the scratches went much deeper than I thought they would and spent far too much time fixing things.

    After that I was less interested in lots of sparks and material removal and more about getting the job done in a proper manner. Mind you 36 grit for the outline is one heck of a time saver
     
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  14. dancom

    dancom Dust Maker Best Shop Tool

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    The worst for scratches for me were from a contaminated conditioning belt. In the final stages, everything looking great. Toss on a green conditioning belt to give is a polish and start seeing huge, ugly scratches appear. It turns out the conditioning belt must have acquired a few specs of 36 grit from a hogging belt. Now, I keep my conditioning belts in a cabinet away from the grinders.
     
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  15. Griff

    Griff Active Member

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    I do the same with my conditioning belts after hearing you say this once before, I would have never have thought of it, but makes total sense as the way the scotchbrite material is woven/ or made in such away it would catch and collect dust and abrasive material flying around. Sorry you had to find out the hard way to our benefit there Dan.
     
  16. tmr

    tmr New Member

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    after years of fighting warped fillet knife blades in the h/t i started post h/t grinding and have enjoyed nice straight thin blades since.....ceramic belts like the norton blaze have made this very easy.....if you think about it if you are grinding to a dime thickness before h/t you still have to finish grind and polish a hardened blade anyway.....even on larger blades i leave lots of material before h/t and finish them hard......it is very rare that warping is an issue now
     
  17. Scott Kozub

    Scott Kozub Member

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    I grind all my AEBL post heat treat. I do my own heat treat and AEBL warps like crazy. Another thing about AEBL is that I find it grinds cleaners when hard. Grinding AEBL pre ht almost feels gummy on the belt and a bit difficult to control. Post ht the belt skates easier making it easier to get clean grinds.

    I take my AEBL as thin as possible on the grinder. For kitchen knives they will slice paper before being sharpened. Usually once I get to 0.01" I change to a slack belt to get a slight convex and it runs way cooler.

    Bare fingers on the blade edge helps, you'll be too worried about burning your fingers to use a heavy touch. I also recently started keeping a wet shop towel in my dunk bucket. With every pass I put the wet towel on the back of the blade to act as a heat sink. It a bit messy but way cleaner than spraying the belt. You have to be careful though. You want the towel as close to the edge as possible but it can get sucked between the belt and blade. It's not dangerous it just makes you loose concentration. Simple green on the belt also help to keep the temperature down.
     
  18. dancom

    dancom Dust Maker Best Shop Tool

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    Scott,

    Nitro-V is the same. What I have found to help with warping is to clamp the cooling blade(s) to a piece of angle iron before going into the first tempering cycle; welding gloves on of course. As there are no bevels cut or tapers to the blades yet, the clamping helps keep things nice and straight. I use a few old steel 2" C clamps (Galvanized, not painted ones) that can take the heat in the tempering oven. I've had perfectly straight blades going into tempering oven come out looking like hockey sticks. Air cooled steel is really fun to straighten out.

    Dan
     
  19. John Noon

    John Noon Well-Known Member

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    I found through a little experimentation with plates flat on the bench and vertical in a bench vice.
    AEB-L reacts quickly to any asymmetrical cooling even just a few seconds.

    Vertical plates that clamp the blade at virtually the same time really helps especially if combined with clamping and a 30 min to 1 hour visit to the freezer followed by tempering while clamped
     
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