Convex Grinding Time

M

me2

Joined
Oct 11, 2003
Messages
5,106
For my next home made project, I'd like to try a fully convex blade. I started on one, but it seems to take forever. I was using a slack belt, 40 grit Zirconia. The steel was 1018 bar stock from Home Depot. This was basically just a practice run for later.
 
Break the bevels with the contact wheel, very short flat grinds (1/4") then go to the slack belt. Slack belt with the cutting edge trailing unlike the cutting edge up for normal grinding. Slack belts like this better and don't break as easily.
 
Depends on what grinder your using but on my Coote there is a 2" space between the bottom of the platen and the contact wheel. After breaking the edges and doing a halfway up flat grind this is my sweet spot for convex grinding.
Do mine edge up but only because it is safe to do so there. Plenty of belt tension but loose enough give for a great convex edge.
Usually wind up like George says and do the final grind edge down.
Experiment and "USE NEW BELTS" or it will take twice as long.
 
yep...good advice all around. New belts are a must.

I'm not sure what "break the bevels" means....probably the same thing I do, which is to "shape" the bevel by grinding several different-angled bevels. Then "smoothing" them over on the slack area.
 
I do the entire grind on the slack belt. I break the edge and walk the grind up the toward the spine. As has been stated, fresh belts are a must. As you work the grind up, more pressure on the belt will be necessary. If you don't have a strong motor you can't "lean" into the belt because of bogging the motor down.
Scott
 
I was basically doing it the way you describe Razorback. The edge is just a little thicker than I want before moving on to finer belts. I guess now I need to start moving the grind back up the blade and thinning it out. I'm using a 1"x30" belt sander, and can bog down the motor using a piece of wood. I may have to try the other methods mentioned, or I'll be at it for 20 hours. I use the belts to polish 1/2" plate to etch welding coupons, and the 40 grit took out the rough torch marks pretty quickly. That was against the platen though.

A quick question about heat treating convex grinds, if I may. Can you take them down to the final shape, so you just need to polish and sharpen after HT, or should the be left with a little flat on the edge to help prevent warping and such. I plan on making the real knife w/ A-2, so there won't be an oil or water quench. After the time its taken to grind, I'll be making it out of 1/8" stock instead of 3/16" as well.
 
I've found you can pretty much take them to the final edge, especially if it's a pretty hefty convex as opposed to a nearly flat one. You'd want to keep careful control of your atmosphere though since you'll have less room to grind off decarb and scale.
 
grind roughly convex bevels on the contact wheel, then super-glue a piece of mousepad to your contact platen an even your bevels out..
you will get a perfect convex
 
me2 said:
I was basically doing it the way you describe Razorback. The edge is just a little thicker than I want before moving on to finer belts. I guess now I need to start moving the grind back up the blade and thinning it out. I'm using a 1"x30" belt sander, and can bog down the motor using a piece of wood. I may have to try the other methods mentioned, or I'll be at it for 20 hours. I use the belts to polish 1/2" plate to etch welding coupons, and the 40 grit took out the rough torch marks pretty quickly. That was against the platen though.

A quick question about heat treating convex grinds, if I may. Can you take them down to the final shape, so you just need to polish and sharpen after HT, or should the be left with a little flat on the edge to help prevent warping and such. I plan on making the real knife w/ A-2, so there won't be an oil or water quench. After the time its taken to grind, I'll be making it out of 1/8" stock instead of 3/16" as well.
Click to expand...
What you need to do is as the grind comes up toward the spine, lessen the angle at which you're grinding. This will thin the blade from the edge up toward the spine. You may have to put another fresh belt on. A new belt is sharp and tight. It will thin the grind from the edge to midway up the blade and blend the entire grind from edge to spine.
I grind mine all the way to the edge before heat treat. If you're grinding 1/8" stock it will be quick. I can convex grind 1/8" stock in about 15 to 20 minutes depending on blade length. Good luck.;)
BTW, how much dead space do you have to slack belt grind? That can make a difference in how much you can thin the blade. Every grinder is different for slack belt grinding.
Scott
 
The little grinder I'm using has a lot of room from the top pulley to the bottom. There is no contact wheel. If I add the platen, I have about 3 inches, maybe less, up to the top wheel. I'm trying this one without the platen, so I have maybe 8-10 inches. The old belt is quite loose. I blew it off the wheels with a leaf blower while cleaning the wood dust out today. I'm basically looking to make a copy of the CRKT Hissatsu, but thinner (1/8") and without the break at the point/straight transition. Won't the mouse pad get torn up? The black paint on my platen has already been worn/polished off.
 
me2 said:
The little grinder I'm using has a lot of room from the top pulley to the bottom. There is no contact wheel. If I add the platen, I have about 3 inches, maybe less, up to the top wheel. I'm trying this one without the platen, so I have maybe 8-10 inches. The old belt is quite loose. I blew it off the wheels with a leaf blower while cleaning the wood dust out today. I'm basically looking to make a copy of the CRKT Hissatsu, but thinner (1/8") and without the break at the point/straight transition. Won't the mouse pad get torn up? The black paint on my platen has already been worn/polished off.
Click to expand...
If your belts are loose, that is why you can't thin the grind down as much. Tight belts are necessary for slack belt grinding.
Scott
 
This is in response to "me2" and his search for a quick way to make a convex blade. I have a way to do this but want to make it clear that I'm new to this forum and knife making so probably doing it all wrong.

So far I have made about a dozen blades by stock removal from old car springs and structural tubes. I tend to rework them as I get more experienced, and I expect eventually I will build a forge and try making some steel, but so far I am still just reading up. Since most of my steel is about a quarter inch thick I tend to have a lot of stock to remove.

I don't have a machine shop so I cut the blades using a skillsaw with an abrasive metal cutting blade. The spring steel I have is curved rather than flat so you can't work it unless you hold the tools or the steel in your hand.

So far I haven't rigged a knife grinder so I grind them using beltsanders with successive grits. I clean them up with files and sandpaper and put handles on them by routing a dado and gluing the wood together with gorilla glue.

I haven't worked with leather much so instead of a sheath I make a wood case for the blade that matches up to the handle.

Hopefully that makes it clear that I am speaking from ignorance of the right way to do things, but I did find I could free hand a convex grind quickly by dragging the abrasive metal cutting blade in the skillsaw along the blade
with the plate set low.

I was able to improve my skillsaw technique a little when my brother gave me a bench grinder. I put the same abrasive wheels in it and then just slide the blade along the platen at an angle. I then rigged a jig on the skill saw so I can use it to cut a little more accurately.

My first knife was made from a rusty old wickedly curved carriage spring. Because of its curve I got the idea to make one side come out convex and the other concave like a shard of glass. I polished half the blade to 1500 and left the upper half pitted because I read someplace that the pits help reduce friction.
 
You must log in or register to reply here.
Back
Top