Will a grinder ruin a heat treated knife?

B

Blackie Scantling

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Dec 26, 2013
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I plan on getting a G.I. Tanto and doing some mods such as removing the top guard, adding gimping, dropping the point to be more like a spearpoint and softening the radical angle of the tanto. I was planning on doing the grinding with a hand held angle grinder (no pun intended).

I just watched a You tube vid of a guy dulling and re profiling his GI on a bench grinder, turning a once fine design into a crude metal shiv. The responses posted were less then complimentary telling him he just destroyed a GI Tanto and that putting it on a bench grinder totally ruined the heat treat.

I have shaped blades before but with the hand held grinder and in short buzzes so the steel doesn't get too hot. I don't see any other way to remove and reshape that much steel on the CS G.I. if I want to finish the knife in my lifetime. Will it destroy the temper in the blade like they were accusing the video maker of doing?
 
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Can a grinder ruin heat treat? Yes.

Will it? Maybe.


You have to go slow, and maybe dip the blade in water every 10-15 seconds to keep it cold. If you just stick the grinder on the blade and grind away until the job is done, you'll heat the blade up a lot and probably ruin it. Slow and steady wins the race when modding a finished knife.


Easy way to know you're safe is to (carefully) touch bare skin to the blade. If you can't bear to touch it due to heat, you're at risk of a ruined tempering.
 
You also have to be careful with your "short buzzes". I've seen my fair share of people doing exactly that on hardened steel and the wheel explodes. This usually happens when the wheel isn't trued up and when impatience takes over. Eye protection is absolutely required.
 
So if I make a few passes, hit it with a wet rag, repeat, I should be OK? I was even concerned that the continual hot/cold temp rise and fall would do something uncanny to the steel's molecular structure, not that I intend to get it too hot.
 
RevDevil said:
You also have to be careful with your "short buzzes". I've seen my fair share of people doing exactly that on harden steel and the wheel explodes. This usually happens when the wheel isn't trued up and when impatience takes over. Eye protection is absolutely required.
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I'll be wearing protection for the baby blues, thanks, and using a four inch hand grinder.

I know belt sanders are the bomb but how do the pro's do it if they need to re shape a finished knife? Will wrapping a wet rag on the tang end help soak up heat?
 
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Pros use belt sanders, and experience to guide them. The blade gets dipped frequently. It's like you can tell after a while when the blade is getting too hot.
 
I just watched a vid of Mike Snody grinding a blade with a belt sander and every pass or two he'd dunk it in water, while he was holding it bare handed. So apparently it can stay cool enough to handle but still overheat.
 
Blackie Scantling said:
I'll be wearing protection for the baby blues, thanks, and using a four inch hand grinder.

I know belt sanders are the bomb but how do the pro's do it if they need to re shape a finished knife? Will wrapping a wet rag on the tang end help soak up heat?
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An angle grinder, like a bench grinder will create a lot of heat. Beyond that, they spin extremely fast and are hard to control. You'll have no gauge of where you're at removing metal, as your work will be below the spinning blade. Chances are you'll end up with an equally crude "shiv" as the guy in the YouTube video you saw (pretty sure I just found the one you were referring to).

If you do use it, you most definitely will not want to dip it every 10-15 seconds... even with a belt sander cranked down to a low speed, I dip every pass or two (constantly moving the length of the blade along the belt). If the blade is too hot to hold, begins to show any color, or the water from dipping boils off, you've potentially already gone too far in destroying the heat treat. You basically can't cool it "too much"... but you can easily do it "not enough." With the angle grinder, your work will be clamped down (if you cherish your eyes/fingers/life... eye protection won't protect you against a shard of abrasive disk at 400 mph to your jugular, and gloves will just make it easier for it to catch and pull your fingers off) rather than in your hand with a belt sander, so you won't even know when it is too hot.

Wrapping a wet rag around the tang will do nothing but give you a false sense of security, and make it easier to hold while you blow the temper. Steel is a poor conductor... so if the tang is hot, the blade is has likely been a few hundred degrees hotter for a while. You'll need to put the steel in direct contact with water if you want to cool it at any reasonable rate. You'll need more than a wet rag to do that... keep a bucket of water nearby.

Belt sanders are used for a reason. Pick up a cheap 1x30 from Harbor Freight and get yourself some decent belts if you really insist on continuing with the project. It'll cost you about as much as the knife you're about to destroy, and you'll actually have a use for it afterwards. Then, instead of spending $25 on the knife, spend it on a 4 foot piece of carbon steel and grind out 4+ knives in the design you desire. I'll even heat treat it for you free of charge (if you pay for shipping), clean up the grinds (if I repair my belt sander by then), and throw in some micarta for ya to use on the handle.
 
Well thank you Weebus, for both your time explaining and your more than generous offer, although I would want to pay you. As an aside, I have been eyeing HF belt sanders and have a desire to get some fixed blades out of my system. When I told my lady friend I wish I could go into making Bowie knives she said "...Well then do it."
I think you've pushed me around the curve. I'll keep you updated.
 
Blackie Scantling said:
Well thank you Weebus, for both your time explaining and your offer. I've been eyeing HF belt sanders and have a desire to get some fixed blades out of my system. When I told my lady friend I wish I could go into making Bowie knives she said "...Well then do it."
I think you've pushed me around the curve. I'll keep you updated.
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Happy to help. Feel free to PM me whenever, as I don't really hang out in this section of the forum much. Replace "Bowie" with "kitchen" and I was in the same position as you 10 months ago... only my lady friend was less than thrilled about my decision :P

I really do hope you take me up on my offer though :) I'd offer to send my old HF sander on top of it, but the shipping would probably be more than it's worth, haha.
 
Blackie Scantling said:
So apparently it can stay cool enough to handle but still overheat.
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It certainly can.

A hand-held grinder is absolutely the last thing I would use to re-grind a finished blade, for several reasons. A regular bench grinder is probably second-to-last. I strongly advise not doing it.

Go ahead and get a belt grinder, even a very basic one. You'll be much better off :)
 
I have ground off the top guard on my GI Tanto.

I have reshaped my fair share of edges on a HF 1x30. Steady passes, dip every pass. Bare fingers on the knife for feel, and no lingering on any spot. Take special care of the point, where the heat WILL focus and easily burn the steel.
 
weebus said:
An angle grinder, like a bench grinder will create a lot of heat. Beyond that, they spin extremely fast and are hard to control. You'll have no gauge of where you're at removing metal, as your work will be below the spinning blade. Chances are you'll end up with an equally crude "shiv" as the guy in the YouTube video you saw (pretty sure I just found the one you were referring to).

If you do use it, you most definitely will not want to dip it every 10-15 seconds... even with a belt sander cranked down to a low speed, I dip every pass or two (constantly moving the length of the blade along the belt). If the blade is too hot to hold, begins to show any color, or the water from dipping boils off, you've potentially already gone too far in destroying the heat treat. You basically can't cool it "too much"... but you can easily do it "not enough." With the angle grinder, your work will be clamped down (if you cherish your eyes/fingers/life... eye protection won't protect you against a shard of abrasive disk at 400 mph to your jugular, and gloves will just make it easier for it to catch and pull your fingers off) rather than in your hand with a belt sander, so you won't even know when it is too hot.

Wrapping a wet rag around the tang will do nothing but give you a false sense of security, and make it easier to hold while you blow the temper. Steel is a poor conductor... so if the tang is hot, the blade is has likely been a few hundred degrees hotter for a while. You'll need to put the steel in direct contact with water if you want to cool it at any reasonable rate. You'll need more than a wet rag to do that... keep a bucket of water nearby.

Belt sanders are used for a reason. Pick up a cheap 1x30 from Harbor Freight and get yourself some decent belts if you really insist on continuing with the project. It'll cost you about as much as the knife you're about to destroy, and you'll actually have a use for it afterwards. Then, instead of spending $25 on the knife, spend it on a 4 foot piece of carbon steel and grind out 4+ knives in the design you desire. I'll even heat treat it for you free of charge (if you pay for shipping), clean up the grinds (if I repair my belt sander by then), and throw in some micarta for ya to use on the handle.
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Bold my emphasis. So if you were doing a regrind and had one or more of those happen, how would you know if the heat treat was ruined?
 
Squashfan said:
Bold my emphasis. So if you were doing a regrind and had one or more of those happen, how would you know if the heat treat was ruined?
Click to expand...

Altered heat treat would change edge retention; usually, if not always, for the worse. The steel is weakened by overheating it, so that could be manifested in an edge that won't hold it's sharpness at all, or a tip that becomes way too fragile/brittle and breaks off. Probably other ways too.


David
 
The over-heated/excess-tempered edge section roll more/easily than other sections. Basically you soften + toughen the harden steel with heat. Exception, most tool steels tempering chart have 2 hardness peaks (around 400F & 1000F range) - i.e. more complicated to figure out whether soft or actually increase hardness.
 
You got some great advice, and a generous offer.

I'd just reinforce the comment about getting to the tip of your knife on a belt grinder. Only draw the tip halfway across the belt until you really have a feel for things. Fastest way I've found to make a nice sharp point more child friendly. :)
 
Probably yes! Here is a citation from a previous thread, more about the final sharpening:

"Sharpening on the belt and on paper wheels is ruining the heat treat of the most important part of your knife, namely the first 5-10 microns that comprise the edge. According to Roman Landes' presentations at Ashokan that first couple of microns will get to as high as 2000 degrees from dry abrasion. You will not notice because that heat energy quickly dissipates into the larger mass of the blade.

After Roman's first Ashokan presentation a couple of years ago I started doing my final edge bevel by hand on DMT diamond stones with a water/dish soap coolant/lubricant and found noticeable improvement in sharpness and edge retention"

Here is the link to the thread:
http://www.bladeforums.com/forums/showthread.php/1117568-How-does-everyone-do-their-final-sharpening

You likely need a slow running belt sander with water cooling (mist) to be on the safe side.
 
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