Vertical gouges in my recurve grinds

bquinlan

bquinlan

Joined
Jan 5, 2001
Messages
2,726
First off, let me say that I'm an extreme newbie to knifemaking. I have only ground three blades so far, and Phil Tham did a lot of the work on the first one. So it is quite possible that the answer to my problem is to just keep practicing until I get it right. :grumpy: But I am hoping that I can figure it out faster with some advice from people who actually know what they're doing...

My problem is that whenever I try to grind an inward curve I end up with a series of narrow gouges that run perpendicular to the edge. They are always worst near the back edge of the grind, but I can get them just about anywhere.

There's no mystery about where they come from. I'm using the edge of the belt to grind inside the curve and the gouges must come from the belt digging in deeper in some places than others. I try hard to keep the blade moving steadily along the edge of the belt, but I just can't seem to keep this from happening. There is also enough vibration to make it harder, although I don't think that's too unusual.

I do reasonably well at grinding clean on straights or outward curves. Anything where I can work against the body of the belt seems to work okay. Unfortunately, recurves are what I really want to be turning out, so that's what I need to learn. :(

Can anyone offer me any suggestions on how to minimize this problem? Or any ideas for correcting it after the fact? I could try hand sanding out the grooves, but I'm not sure how well that would work and it certainly would be a long process.

Help!

--Bob Q
 
What weight belts are you using? You'll want to use thin, flexible belts for recurves. Sorry, that's all I got... :)
 
Sounds like you are already keeping your elbows tight to your body and moving the blade with that exotic sway knifemakers develop... How fast is your wheel running? It might help if it's running at a slow speed, while you learn at least. I've never ground a recurve blade so I don't know the difficulties; but there is a practical reason I haven't tried it yet! :D

One thing you might try is grinding some wood for practice (practice, practice). I think the thin backed belts is a good idea, I wouldn't have thought of that myself. Also, since you said you really want to specialize in recurves, would it make sense to invest in a narrow wheel? I don't know whether that would help but it comes to mind. When I've had similar issues with grinding radical curves I ended up hand sanding. You can always take it to mirror afterwards very quickly too.

Maybe some of these folks like Dan Koster who've done recurves will chime in with some ideas of actual value. You can hope. :D Good luck and hang in there.
 
J-flex belts,Finer grit,Smaller diameter contact wheel,slow speed,PRACTICE,PRACTICE,AND MORE PRACTICE.If you are grinding on the platten,make sure you have put a slight round edge on the sides of the platen plate.
 
Also if you are grinding on a 2" contact wheel. Get youself a 1" wheel with a health crown and split your 2" belts. definately go with the j-flex belts.
 
Till you get use to grinding. Just hand sand the grooves out with 120 grit then move on to finer grits.....
 
Hard to say.....really depends on what you're using to grind with. Please post the details and we can take it from there.
 
I have been using a variety of belts, but the lower grit ones I've been starting with are not particularly flexible. I feel foolish... Time to get a few J-Flex belts! And I will try starting with higher-grit number in the first place.

Good pointers on body position. I'm already doing some of that, but paying more attention to it will probably help.

I am running my grinder as slow as I can, which is not really as slow as I'd like... Someday I will upgrade to a real speed -controlled motor. For now, I'm using the smallest pulley on my motor and the largest pulley on my grinder.

The comments about using a smaller wheel confused me there for a moment, but I think I've sorted it out. This is not about a smaller diameter, but about a narrower width, right? Hadn't heard of splitting belts either, but it makes good sense. I'll have to see if I can get a 1"-wide wheel for my grinder. I'm new to the idea of crowing wheels or the platen, but that also makes perfect sense.

I suspect that hand sanding is my best solution for the blades I've already messed up this way. I'll give that a try.

As for the details on what I'm using... My grinder is a 2"x72" Coote equipped with a 10" contact wheel (although I also have secondary contact wheels in 2-1/2", 1-1/2", 3/4", and 5/8"). It is powered by a 1.5 hp single-phase 110V motor with a base speed of 1750 rpm, routed through pulleys that reduce the speed by a factor of three. My current efforts are all hollow-grinds done on the 10" wheel using the optional knife rest as a support.

Thanks for all the information and advice! I'm on the road right now, but I will start trying it out as soon as I get back home.

--Bob Q
 
How about killing the edges of the belts, too? Run a piece of steel or an old sharpening ston on the edges of the belt to remove some of the edge grit before starting to use them. This will keep the belt edges from grinding deep, but allow the rest of the belt to do its work.

Howard
 
I had the same problem a few months back. The advice I got was to split my belts down to 1" wide. It has helped enormously. Also my own personal advice is do not use very course belts. I have been grinding the initial bevels with a fairly worn 80 grit belt. It has really helped. The finer grit belts let you ride the hollow of the grind a little better without as much chance for a major screw up. Hope this helps.
Kyle Fuglesten
 
Grind the back edge in on the wheel by grinding lengthwise with the wheel not across the wheel as you would when grinding the main bevels in.that is were the different sized wheels come in,so you can do smaller blades.
If you are going to run the back edge part way up the spine of the blade then grind that part in just like a regular bevel running the grind straiht off the blade as if the clip wasn't there,then grind in the clip lengthwise to match the edge of the other part of the gring,this way you can keep a nice transition in the grind.
Hope this makes sense.
Bruce
 
I think I had a breakthrough today! I have not yet tried going down to 1" belts and wheels, but I've been working on everything else that's been mentioned here. A lighter-backed finer-grit belt, plus some improved technique, has finally allowed me to grind on the contact wheel without using the rest. For some reason that seems to make all the difference.

I am now getting smooth, if not perfectly regular, grinds along the recurve I've been working on. The vertical gouges seem to be gone entirely.

Thanks to everyone for help me out with this!

--Bob Q
 
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