opinions on flat grind vs convex

valcas1

valcas1

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Feb 25, 2007
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I have a design for a machete/bolo knife floating around in my head. It is a fairly large blade of about 12" and I am going to use 5160 or 1095 for the steel.

My thought is to use 3/16" and go with a convex grind to a zero edge. My grinding skills are not up to the task of flat grinding such a wide blade yet. This blade will be a user not a show piece.

Here is my actual question. Can I get decent edge geometry for a hard use chopper without having to do a full flat grind on 3/16" stock? I picture a sabre like grind using slackbelt grinding until I get to the desired edge geometry. I understand that there will be a large portion of the blade that will remain untouched.

It is my thinking that many of the knives I see that are flatground .25" would have a similar edge and it would save me the frustration of trying to flatgrind 3" wide stock.

Please let me know your thoughts on the matter. Let me know if I am completely out of my mind for thinking this way. Thank you once again for all your collective knowledge and your willingness to share it.

Paul

Side not: 1095 or 5160 and what RC would you try to obtain?
 
You can do either. Why don't you use this opportunity to learn how to flat grind? You're going to learn eventually, right?
I'd go with 5160:thumbup: Makes a real nice chopper. As far as the HRc, it depends on who's doing the HT.;)
 
convex grind is good for edge strength though there are convex grinds that cannot cut that well. A zero ground full convex would be my selection. I did some convex edges before, some were good but if you press hard on slack belt the convex angle rises and the result for me is not that good, so if you want it a good performer you need to apply light pressure, that means more time spent on the grinder. A better convex technique is to grind a thicker full flat grind than put a convex to it, anyways if you dont like the convex performance you can always flatten the convex bevel on the platen...
 
Interesting. Bolos must be in season. I have 5 similar desings on my bench right now 12" blades, lol. You can also flat grind up to a point and THEN slack belt the sucker. It doesn't have to be pretty just make it as even as possible and then go over it with another belt on the slack to blend everything in. If you do it this way leave the edge a little thicker. If you flat grind it and then put a convex edge on it touching up the edge may be easier than touching up a true full convex zero edge. Also the weight would be reduced with the flat grind and since it is 3" wide, 12" long, and 3/16" thick that may not be such a bad thing.... Either of those steels would work but I would recommend sending it out to someone to HT it and don't be surprised if it comes back looking like a boomerang if you don't have much experience.....
 
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Oh, I am not heat treating. It is going out to Peters Heat Treat. Is going to 57-58 RC will be pushing the limits for 5160 for a chopper?

For those who have convexed. Is it possible to slack belt with two belts stacked together to give it a firmer backing? I actually thought about getting a 2x 42 rubber fan belt custom made to back up my belts and give it a firmer backing. Any input? has anyone tried something like this.

I am using a Wilton square wheel grinder so all of those fancy KMG attachments wont work for me.
 
You can do either. Why don't you use this opportunity to learn how to flat grind? You're going to learn eventually, right?
I'd go with 5160 Makes a real nice chopper. As far as the HRc, it depends on who's doing the HT.
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Troop,

I plan on learning how to flat grind, yes! however this design has been gnawing at me and I have to let it out. This is the best way I can think of to complete this project and come out with a knife the will be satisfactory to me.
 
valcas1 said:
Oh, I am not heat treating. It is going out to Peters Heat Treat. Is going to 57-58 RC will be pushing the limits for 5160 for a chopper?

For those who have convexed. Is it possible to slack belt with two belts stacked together to give it a firmer backing? I actually thought about getting a 2x 42 rubber fan belt custom made to back up my belts and give it a firmer backing. Any input? has anyone tried something like this.

I am using a Wilton square wheel grinder so all of those fancy KMG attachments wont work for me.
Click to expand...

57-58 HRc is fine for 5160; it's a tough steel. I draw mine at 350F for three 2-Hour cycles; it leaves the HRc at around 59-60, and it does not chip.
I tried the 'ol stacked two-belt trick, and one of the belts smacked me in the face. But I've heard that people do it with good success.
 
So, you really don't want the "slack belt" too slack, eh? Go with the flat grind, with the convex on top of that. This way, you can hide your "imperfect" flat grind with the convex.
 
If you do it all on the slack belt, you'll wind up with too long (too gradual) of a plunge.
 
Troop,

Thanks for all your input. "Note to self" where hockey goulie mask when attempting 2 belt slack belt trick.

I think I am going to try a flat sabre grind pseudo scandi esque grind about 1" up the blade and then finish that with a slack belt. We will see how it works out.

Troop,

you feel it is ok to bring the RC up to 59-60 with out any real chipping issues. I have heard 5160 is a tough steel and I am looking forward to beating this prototype up to see if the steel lives up to its reputation.

Paul
 
The edges on the larger blades like this are always more robust and thick. Assuming it is sharpened correctly with the right angle you should be good to go.
 
valcas1 said:
Troop,

Thanks for all your input. "Note to self" where hockey goulie mask when attempting 2 belt slack belt trick.

I think I am going to try a flat sabre grind pseudo scandi esque grind about 1" up the blade and then finish that with a slack belt. We will see how it works out.

Troop,

you feel it is ok to bring the RC up to 59-60 with out any real chipping issues. I have heard 5160 is a tough steel and I am looking forward to beating this prototype up to see if the steel lives up to its reputation.

Paul
Click to expand...

Paul, Joe Szilaski taught me the way that he "thermal cycles" his steel when he forges it. What he basically does is sub-critically quench his blade between heats, between 20 and 25 seconds in the ATF. After doing a lot of contemplation, and experimentation on this, I personally feel that this is too long of a time in the quench, given the reason why he does this. (Hey, but what do I know):)
Anyway, when I "thermal cycle" while forging, I find that the blade becomes tougher than would otherwise. So, while 59-60 HRc works for my 5160, it might not work for someone else that works their steel differently.
 
valcas1 said:
Here is my actual question. Can I get decent edge geometry for a hard use chopper without having to do a full flat grind on 3/16" stock? I picture a sabre like grind using slackbelt grinding until I get to the desired edge geometry. I understand that there will be a large portion of the blade that will remain untouched.
Click to expand...



I recently finished a chopper with a 3" wide blade and about 1/8" thick, the convex grind only goes up about 1/2" and it seems to be a great working edge; cut through a 6x6 pretty easily.

with 3/16 thick material, convex it back to 0.75" - 1.0" and you should be good.

Sorry if my terminology isn't correct or my understanding of the question if incorrect... still a newb. :)
 
For a blade 3" wide (are you serious?!?) of 3/16" stock, I wouldn't think a full flat grind would be necessary... A convex or flat 1" wide, give or take a half-inch, should be sufficient for slicing but strong enough for chopping wood.
 
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