Convex Grind Vs Hardness

[peter nap]

I've been playing with convex grinds on field knives the last few weeks. It seems to me that I have been able to leave the blade a little harder than either a flat grind or a hollow grind and not get any chipping.
Has anyone else had the same experience?

 

[James Lechago]

Peter I am a novice knifemaker. The convex grind is the only grind that I know how to make but after the few knives that I've made I have no interest in learning to make any other grind. There are several old threads that discuss grinds and Cliff Stamp has written some very informative work on the subject.
Personally I agree that one can create an all around, stronger blade using a convex grind.
The first knife I made was a short matchete with CPM440V. One of the tests that I put it through was to chop through quarter inch steel cable that was stretched across a piece of hardwood. It managed to cleave the cable after I got it tight enough. The only damage to the blade was a very small chip that I easily ground out on the belt sander. That was the only time that that I have ever been able to damage that blade.
Unfortunately I cannot make any comparisons for you but I can vouch for the strength of the convex grind. I hope this has helped.

 

[RHINOKNIVES]

peter, the convex was the first grind i taught myself, and is still my favorite exspecially for hard use field knives. all of my stainless is done by paul bos so i have a constant on the differnt steels. convex is always the last to chip or roll due to the sectional densetity (sp?) of the steel behind the edge.i feel you are correct is stating that you could go to a higher rc without rick of chipping.the next one of my bushwacher model i do will be in convex instead of a full vee grind. i will let you know how it goes throught the paces.

 

[Guy Thomas]

How interesting! The type of grind (convex) and the level of hardness has been a recent concern of mine. I've decided to work with 1084 mostly in forging and I have four knives ready to heat treat. One small neck knife with an absolute flat grind, a small fixed blade with only a slight convex grind, a Scandinavian style stick tang with a thin slightly convex grind and a four inch blade with a strong convex grind.

I've been trying to figure out a good all around temperature to use to temper 1084 at and have been a little stumped. Both Bruce Bump and Greg Covington both have said in this forum that they get terrific performance at 375 degrees for tempering but the first small flat ground neck knife I made from 1084 and tempered at 375 degrees chipped out when I was sharpening it. Retempering at 400 degrees fixed this. At the moment there is a similar discussion on tempering 1084/15N20 damascus at CKD in Ed Caffrey's forum and the general concensus from several makers including Ed was 425 degrees was optimal. One maker answered my question that he uses 425 degrees for straight 1084 also.

Fifty degrees is quite a range here. Greg Covington makes combat style knives from 1084 and I've been wondering if he uses the convex grind on them thus enabling them to perform without chipping at the lower 375 degree temper.

I was trying to save oven time by heat treating a batch of 1084 knives all at the same time but now I'm wondering if I shouldn't experiment with the range of grinds I have in the four knives I have ready to heat treat today and use different temps. Since I use triple tempering cycles that could run into some time! My thoughts are though, that the two convex grinds that are close to flat grinds, due to the thinness of the blade should be treated as flat grinds and tempered at the higher levels.

Sorry for the rambling, I'm just speculating on effects here. I've only just realized that I should be taking grinds and the size of the blade and it's intended use into account when figuring out tempering cycles on the same steel, in this case 1084.

 

[peter nap]

I've been using 5160 for these (big suprise) I normally temper at 325 to 350 and with this last one I tempered at 300.. 0n one before that I used 310 and have had no chipping. These are a rather full convex using my slack belt grinder and the pivot rollers set at 1 foot.

I don't expect it to replace flat grinding in my knifemaker but I do think it enables me to make a wider variety of knives. I have always been interested in Laurences cooking knives. They have a very good reputation for both sharpness and edge holding. To be honest, I'm a lot harder on my kitchen knives than I am on my camp knives!

 

[Greg Covington]

Originally posted by Silent

Greg Covington makes combat style knives from 1084 and I've been wondering if he uses the convex grind on them thus enabling them to perform without chipping at the lower 375 degree temper.

I use a convex grind on all my knives these days. I have tried flat, became pretty good at hollow, and did some chisel too. Convex works best for me and gives the type of combat knife that I like. I feel that it leaves more mass in the blade for a better cleaving action. Some, but not all, flat grind blades get too thin out near the edge, and the blade get pretty light. Edges can chip and the knife, with its lightweight, can have a tendency to deflect off an object when it is chopped. Kinda the same effect that happens when chopping hard wood with a machete.

I fully forge the blade, give it a finish heat/hammer, and leave it a little thick. Then I grind the blade to profile, again leaving it a bit thick, and go to a 220 grit before the HT. I stamp the blade at heat and return it to the forge and let it cool down in there. Then it is brought up again to nonmagnetic and then placed in vermiculite to cool.

For the heat treat I place the tang/ricasso area into the forge and let it turn good and blue, then I reverse the blade into the forge and bring her up to nonmagnetic, and then quench in my Goddards Goop quenchant which has been heated to 140F (i bring a steel rod up to orange and dip it in the solid goop and it almost always hits 140F).
Blade is edge quenched about 2/3rd's up the blade. When color disappears from the spine, I dip the blade the rest of the way into the oil. Test the edge with a file and then I clean off the oil and then it goes into the oven at 375F for one hour, then to the deep freezer, then two more 1 hr cycles. Later, after the final grind, I torch the tip of the blade a bit to draw it back a little softer.

This is the method that I use and it works great for me. I learned that the convex edge and this HT gives me a knife that with withstand serious abuse....because I have severly abused alot of these blades. Havent had any chips, flexes across the brass rod, and resharpens easily.

 

[Greg Covington]

I also might add that when I convex grind, the blade is slightly between convex and flat. I dont grind fully into the slack of the belt. I have a platen mounted to my grinder and a wood face on it. I grind in the slack just below the platen where the belt it pretty stiff and I can control the amount of convex -vs- flat better witht his method. If I want to thin the edge out more I go closer to the platen and then I can roll it up to the spine from there by moving into the belt a little further down between the platen and contact wheel. (Was that confusing?)

 

[Guy Thomas]

Greg, thanks for sharing your grinding/heat treating regimen for your knives! I just put my knives in the oven for the second tempering cycle. I decided to just try them all at 415 degrees and see how they perform compared to each other. One of the blades is intended to be sacrificed by tested and then broken to examine how well I am heat treating my knives.

Your description of how you make your convex grind was perfectly clear. If I remember right I noticed from your web site that I have the same grinder you have (a Pro-Cut, is that right?). By necessity I have been grinding pretty much as you described simply because the graphite backing on my platen is worn out. I start by flat grinding on the platen and then move to the slack area between the contact wheel and the platen to finish up. I just ordered a piece of pyroceram for my platen so I hope my flat grinding will improve by leaps and bounds soon. I am very interested in the convex grinding now though and I can't wait to try some larger blades soon.

Peter, thanks for starting this thread, and for also confirming more of the convex/hardness theory with your own forging experience.