Convex grind

[Drac]

Hi,

How do you do a convex grind? All my books cover hollow and flat grinds (I prefer flat myself), but while there's a lot of talk about convex grinds I haven't come across any information on how their done. Maybe I missing something so simple that I'm going to get laughed out of the forums but I lost.

Thanks,
Jim

[VSMBlades]

I am fairly new myself and posted on your question about steel in the other forum. I usually end up with a convex grind whenever I try to grind anything. I have read though of using a "Slack Belt" and trying to keep the belt from running over the edge and rounding it. I personally dont have a belt grinder. Using files I end up with a somewhat convex grind but that is due to my lack of control.

[SteveS]

Jim,

A convex grind is the opposite of a hollow grind, it bows out (you knew that, but just to say it).

You could do it on a flat platen by making a series of flat grinds at various angles and then blending it all together while hand sanding.

Another way is to use a slack belt and do pretty much the same thing. The third method is to use a squishy platen, one that gives while you grind.

In all cases you work a steeper angle towards the edge than at the spine, blending as you go.

For me I flat grind to the 220 belt. Then I switch to a mouse-pad platen and work again with the 220 and up to whatever the final finish is supposed to be.

The BEST way to do it is using the Rotary Platen from Beaumont Metalworks (KMG). http://beaumontmetalworks.com/rotaryplaten.html

Does that help?

Steve

[Drac]

I think I'm following that. So about how thin do you take the edge before you take it to the slack belt/soft platen? And about how thin do you take that to before heat treat? I send mine out to Paul and on flat grinds he says "just so I don't get cut." I do have slack belt attachment for my KMG as well as the platen, I couldn't survive without it.


Thanks,
Jim

[nwknight]

My first knife turned out to have a convex grind, I had used an angle grinder to do it, and that is what I got.

[SteveS]

Jim,

It depends (don't you hate that?). If you want a very convex grind, then you start right after the initial flat grind is done. If you want a slight grind (like I do), you can flat grind and heat treat like you always do, then start the convex stuff.

I will tell you one thing, the hardest part is making the plunge area look nice when convex grinding.

One method that might work out is finishing the blade completely (hardened, finish sanding, everything) with the edge about .020" thick. Then put on a 120 grit belt on the slack attachment and work it so you're grinding the last half the bevel to the edge. Take the edge down to sharp or .002-.005". Then work the belts up the grit scale to match the rest of the blade. Believe me, that method will solve more problems than you know.

Steve

[AwP]

Convex grinds have a funny way of sharpening themselves when forming on the rough grits that other grinds don't seem to do. I've cut myself on more then one 80 grit formed convex edge. You'll probably want to just run the edge over a stone or very quick over the belt to purposely dull it before sending for HT. The slack belt way mentioned already is definately the easiest way to do it, and soft mousepad like backing a close second, but for people without either it's completely possible to do it with a flat stone or sandpaper. You just need to use a rocking/stropping type movement.

[Drac]

Thanks for both of your inputs,

I getting an idea on how it goes. Another question on finishing, I'm assuming that you use something like hard rubber to back the sand paper when hand sanding out the belt marks? I like normally like to at least remove the grind marks and most of the time finish to about 400-600 grit even on "workers."

Thanks,
Jim

[SKIVIE]

Jim, Ill scribe a centerline down both the spine and blade edge then start my Convex with my Slack. Rarely will I hi the spine near the Ricasso but will gradually grind the spine as I come to the tip to create a nice even point. On the blade side, I will grind with an 80 gt belt to within 1/32 of each side of my scribe line then move to 120 then 220 and end up with a 1/32 edge for Heat Treating. Then Ill go back with my 220 gt and clean up the blace and work it to a around a 1/64 edge or thinner then to a 400 gt. at that point Ill usually do a little sanding with a 600 gt by hand with either just my fingers and no backing so that I can feel the contour of the covex grind and not create any flat spots. Im usually ready for a nice satin finish at that point or Bead Blast.

Hope that helps.

Shane

[Jason Cutter]

I've tried all the above methods for convex grinding and I can attest that they work. Just a matter of finding your favourite and most controllable method for you. I am not a fan of the slack belt method because, without any pressure from behind, it is a lot slower and you could get too extreme a convexity by pressing too hard. My favourite method is to grind a series of flats and blend them in by hand sanding.

My way of getting the plunge cut even is by grinding the plunge cut with a flat grind that goes 1/2way up to the spine (sabre grind). It then goes a bit higher because of the hand sanding that comes after

The medium coarse 3M Scotchbrite belts are very good at blending in contours, and have worked very well for me to convex things in. However, I'd be careful by grinding with the edge DOWN. With the edge up, while you're pressing into the belt - it could start grinding down to a sharp edge. When that catches into the Scotchbrite belt - BANG !! Check how many fingers and toes you have left after that.

I personally find I can get a flat grind to look a lot cleaner. Thats just me. Jason.

[Drac]

Flat grinds are my preferred grind as well, but that's the grind the first knife maker's shop I worked in preference too. Funny how when you get lucky enough to work with someone early on how your preferences match theirs . The reasons I was thinking of a convex grind were that one of my co-workers wants a "use and abuse knife." About a 10" blade bowie that he can use to chop, slice, split pelvic bones, ECT. I know that the convex grind is the strongest edge.

Also I want to make myself some kitchen knives and the chef's/slicing knife should be convex so the items you cut (such as carrots) fall off between cuts so you can see were your next cut should be. Flat grinds cause the food to "walk up" the sides.

Thanks for the warning. As I grind edge up on half of my blade it's a major danger. I had a flat edge do the same thing to me on a 400 grit belt and ouch!! I talked to another local knife maker the other day about convex grinds and he said that it is one of the most dangerous grinds to do because of the chance of the belts being cut.

Thanks,
Jim

[SteveS]

Jim,

FWIW, here's a convex grind kitchen blade I did:



And the platen 'tool' I used:



Actuall I love that thing and finish most blades and all my handles with it.

Steve

[Drac]

That's the ticket!! Thanks for the photo, I was thinking I had to buy a new platen and couldn't figure out how to keep the mouse pad from being chewed up by the belt.

Jim

[SKIVIE]

Steve, Thats a great little idea. Is the graphite belt, mouse pad and sheet metal...are they glued together ? Whats holding it to the Platen ? Is it just the sheet metal hanging over the platen ? What did you use for graphite belting ?

Sorry for all the questions but I just might have to build me one of them.

Thanks for sharing.

Shane

[SteveS]

Shane,

The sheet metal (old computer case) is bend so it just hangs over the platen - slips on and off as needed.

The mouse pad is held on with a few strips of double sided tape from the department store. The graphite is held on with one strip of tape across the top of the sheet metal. (It doesn't take much to hold it all together. In use the sanding belt keeps it tight.)

I've been using this one for several months and haven't had to replace anything yet.

What suprises me is how often I grab that thing and put it on. It is so nice not to have any bump with doing handle stuff. When working in finger grooves you can use the left or right 1/2" of the belt as a firm slack belt --- oh I can't describe it, but you'll know when to use it.

It blends stuff together - no flat spots. I wouldn't even consider finishing the front edge of bolsters without it. You can go all the way from shaping to a 9 mircon belt and have a very smooth result. The buffing wheel time is very short.

Steve