grinding too much causes warp which way?

samuraistuart

samuraistuart

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Dec 21, 2006
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I'm spinning wheels right now, trying to figure out what I'm doing wrong. I'm making a camp knife, a little longer than I've made in the past. Nothing out of my area, but I'm having a fit! If you lay a knife down right side up, and you grind too much on the right side, which direction will the blade warp? To the right, correct? Removing too much steel on any given side will cause it to become concave on that side, correct? I have a slight bow to the right, when looking down the edge, but I've been trying to remove steel on the left side....just isn't working. Just wanted to at least be sure that I know which direction the knife will bend when removing too much steel on one side. Thanks for the help guys.
 
I had the opposite experience. I ground one side and the blade warped away from the ground side, making it convex. I think it depends on what has happened to the blade before grinding, at least in my case.
 
When the steel heats, it expands, but then as it cools it contracts. It will convex initially, then it will concave. Auto body mechanics and bicycle frame builders use these principles to predict where the metal will end up.
 
Is the process of grinding inducing these stresses or does it depend on what the internal stresses in the material are? If it is just residual stress it can go anyway depending on how it was manufactured. In thin strips one way to remove residual stress is called stretcher leveling where the material is pulled and under goes a slight amount of elongation but removes all residual stresses.
 
Stuart, this may be a case where general rules don't apply exactly. If you start out with a piece that is flat and grind on one side it may bow that direction every time, but if you start out with a piece that was warped (stressed) in heat treat, the residual stresses may overcome anything your trying to do with a grinder.

I don't think tension, roller or stretcher leveling is an option here.
 
In my shop, steel bows away from the side being worked, if it is going to warp. Sometimes, it comes back, though... if the tides are right.;)

I think a few things are at play. I am only going on what I see in my own blades and don't claim to know the actual science behind it. So, here is my take on it....

I feel that the heat of grinding combined with the removal of steel relieves and induces stress at the same time.(if that makes sense). What you do to one side effects the other, which is why slow, even work(whether it's grinding, drilling, heating or hammering) is important. I played the game of "grind-straighten-grind-straighten"... for a bit until I realized the blade was wagging like a dog's tail(exaggerated, of course) and I was just trying to rush. I did experiments with a torch, where I heated a blade that was clamped in a vice taking note of how it moved. Then I did the same thing while placing side load on the steel and found I could encourage the steel to move how I wanted. This eventually led me to correcting warp during the temper. I noticed that hardened steel moved more dramatically(go figure) and that annealled steel was like working with clay. This is common knowledge for the metallurgical literate but at the time, it was "breakthough research" for a Canuckian redneck, holed up in an 8x8 shop, making knives from files. Now, I am familiar with how the steel moves in my shop and find I get less freaked out when things go South beacause I've screwed up just about every way possible.... oh... and that spheroidizing is a lovely bit of witchcraft.:cool::thumbup:
 
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So what are some techniques to mitigate the negative effects of this? Would it be good to alternate sides more frequently when getting close to finishing the bevels?
 
Great info!
Reminds me of a blacksmithing demo I saw in which a smith who had worked in industry made a pretty dramatic bend in a sturdy I-beam by heating it to color with a torch and then cooling with a water mist, causing it to pull toward the cooling action.
Fabricators also do this to straighten shelving and such, torching the side opposite a weld to pull it back straight.
I'd guess that on blades it has a lot to do with how much the blade is heated, whether heating/cooling is on one surface or throughout, and probably a lot of other stuffs.

PS. and as Rick said much better than me, knowing that you can staighten at a couple different points during HT takes the "stress" out of it, har har.
 
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creature7 said:
So what are some techniques to mitigate the negative effects of this? Would it be good to alternate sides more frequently when getting close to finishing the bevels?
Click to expand...

Sharp belts help. Use the right grit. Too fine will heat up more than just using a coarser belt with less pressure. Dip the blade more frequently as you get closer to finish shape.
 
Willie71 said:
Sharp belts help. Use the right grit. Too fine will heat up more than just using a coarser belt with less pressure. Dip the blade more frequently as you get closer to finish shape.
Click to expand...

Great, thanks! I really haven't had a problem with it yet but I use crisp belts most of the time. I read the thread and was like "shit, right when I think I start having things figured out. Another variable to account for."
 
Just now got back to the computer to read your replies. Thanks so much. So I had it backwards....which explains my problem. But, of course, I tried the other side as well, thinking I was wrong, but wasn't making progress. Hence the need to ask you guys. Thanks so much. I still have plenty steel to work with on both sides.

Willie....because I was worried about a bow forming by uneven grinding....that is exactly what I did when I started the bevels. (Files). I did a little on one side.... a little on the other.....trying to KEEP the darn thing straight. No such luck. I thought I REALLY was on top of it all. Wrong answer.

Rick...I thought the same thing....this really is so little of a bow...I can deal with it easily during tempering in the oven. BTW....I've got a kithen knife order coming up.....I'm going to normalize (and then spherod) that bugger BEFORE grinding this time!!!!! Thanks again for your help on all that!!! Gonna save me some time/steps...doing it the RIGHT way!!! Whooda thunk?
 
Thought I would update this with what happened yesterday....just in case someone comes across this in need of it. I had originally ground too much off of the wrong side, having my "too much grind causes bow this way", backwards. After all the effort trying to get it "out", the heat treat caused it to bend because of my excess grinding. And as Rick recommended....it is being straightened by the 3 point clamp method during tempering. Thanks again, fellas. Should be pretty darn straight when I'm done. Looks really cool...I think. Edge ended being thinner than I wanted....but that's OK.
 
Super Stu!

I forgot to mention in this thread that I snap temper at 375F before I attempt to straighten. My first straightening starts at 400F. Raising the temperature by 10-15F during subsequent cycles will give it a bit more "persuasion". I try not to temper past 440F and always end with a "clamp free" cycle to make sure things are permanent.
 
Exactly what I did. First a snap temp at 360. Then a straightening at 400. Not quite there. Another at 400. Almost perfect....it's in the oven right now at 400 on the third try, just to get the tip peeeerfeeeeeect. Once it's perfect....I'll put it back in (sans clamps) at a bit higher temp.....just to toughen it up a tad. It is a longer camp knife after all. All is going supremely well.

And, just for Rick, I started my first scandi grind knife. W2...the new stuff. I profiled the blade, and into the kiln for normalizing/grain refining. 1650F cool to black. 1500F cool to black. 1400F cool to black. 1400F cool to black again. 1400F, cooling to 1250F at 50d per hour, and a hold at 1250F for 1.5 hours. I will then shape the bevels and then harden. I believe this will give me a spheroidized structure....perfect for hardening....just requiring a bit more soak at temp. I'll go with 1470F for 15 minutes into Parks 50.
 
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