Convex grind??

Lerch

Well-Known Member
As of late all of the knives i have been making i have been doing what i believe to be a convex grind on. I am grinding the knife around 1-1.5" below the end of my flat platen giving the blade a nice gentle curve from the spine to the cutting edge. I usually grind the knives to where the edge is around 20-30 thousandths thick and then grind a edge like i always have. After reading more i am thinking that in actuality i am not doing a full convex grind but instead a convex grind with a secondary bevel cutting edge.

can anyone tell me the pro's and con's of this or if i would be better off to do a "full convex" grind with no secondary bevel??

thanks
steve
 
A convex grind is going to have more steel and thus more strength behind it. The down side is that it will have a steadily increasing angle as you get back from the edge which will make it harder to slip into the cut. It will have to push the two layers apart more. With a flat primary grind and a convex secondary grind you limit how far back on the edge you have a problem with that at the expense of more strength in the blade with a full convex grind. The full convex grind will make a less handy slicer than a flat grind but it will be a stronger chopper. A flat primary grind with a convex secondary grind or a convex primary grind with a flat secondary grind will be a compromise between the two.

To illustrate another issue, I made two knives from 9260 and hardened then tempered them together. Both had flat primary grinds. One had a convex secondary grind and the other a flat secondary grind. The one that had the convex secondary grind passed the wire cutting test with no more than a slight dulling of the edge. The one with the flat secondary grind indented with rolled over edges indicating too soft a temper. I at first thought that I hadn't properly hardened the blade before tempering and I went back and repeated everything but I got the same result when I tested it the second time so I went back, again, and rehardened the blade and then tempered at 25 degrees lower. It then passed the wire test. It turned out for that steel with that grind I needed a higher hardness or I needed more steel behind the edge with a slightly softer temper to prevent failure of the edge.

I could have also found that the convex edge might have resisted chipping out better than a edge with a different knife due to the added strength behind the edge. But that would have been a some expense of it's slicing ability. When it comes to knife design, everything is trade off and compromise.

Doug
 
What works on my knives is to have a flat for about the top 2/3" from the spine and then to convex to the edge on the bottom 1/3rd.

When doing this I frequently convex completely to the edge giving what I referred to as a Bullet grind.

Like a spitzer Nato Military rifle bullet. Buff or strop to get rid of the wire on the edge.

There is no third bevel for the edge. These edges will even cut circles very smoothly.

I just delivered my finished chef knives with this edge and grind. I will try to get some pics and post as soon as I finish a few more.

Laurence

www.rhinoknives.com/
 
[video=youtube;2j8V-Rcyj4s]http://www.youtube.com/watch?v=2j8V-Rcyj4s&feature=player_embedded[/video]​
 
Finally! Some one with the math skills has put into numbers and words, What I knew! but could not back up with numbers, Just my knives.

Excellent! well worth watching!

Laurence

www.rhinoknives.com/

Friction is a big part of the equation. There is a reason that raindrops, airfoils, ship hulls, igloos...are curved. Curves are strong and reduce parasite drag/friction.

432506-Royalty-Free-RF-Clipart-Illustration-Of-A-Water-Droplet-Cartoon-Character-Holding-A-Thumb-Up-Pose-1.jpg
 
Last edited by a moderator:
Thanks guys

I didnt know if what i was doing would still qualify as a "convex grind" since i did a secondary flat bevel cutting edge. The info in that vid is great, i need to watch it a few times to let it all sink in :)

thanks
steve
 
Also the part were he explains how "Tension" cause by the curve of a convex knife increases the cutting ability of the knife.

Yes, I bookmarked that vid and will watch it a few times also.

Laurence

www.rhinoknives.com/
 
Back
Top