View Full Version : Flat grind question


David Peterson
05-19-2003, 12:25 PM
I've been flat grinding on my new Coote grinder, and I'm running into some small defects that I can't seem to pinpoint. I will grind up to 120 grit just fine, then when I go to a new 220 belt I get these vertical scars that look like the belt dug in too deep in spots. I tried grinding without too much pressure, making sure I'm not tilting the blade, using a different belt, etc.. Same problem. They are a bear to remove by hand, so I was hoping you all could help me figure this out. Am I supposed to slow the belt down the higher the grit?


Another question: Is it possible to flat grind up to 400 or 600 grit without doing it all by hand? I used to just go up to 120 grit and then do the rest by hand sanding, but that's getting old. I bought all these 2x72 belt up to 800 grit, but as I move up the grits, the blade starts looking like crap. Thanks.

-Dave

Ray Rogers
05-19-2003, 12:36 PM
You can use the finer grits but it really helps to be able to slow the belt speed down. Also, you must be careful to remove all the scratches from a previous grit before moving on to the next grit.

One thing that helps me is to alternate directions on each successive belt. In other words, maybe the 60 grit was used across the blade as when the flats are being ground. Next, maybe a 120 grit will be used lengthwise on the blade, then across with 220, then lengthwise with 400....etc.

I find it easier to do the legthwise sanding on a contact wheel rather than the platen. Like everything else, it takes practice so that you don't dig into the flats. Of course, it gets easier as you move to finer grits because they can't dig in very much anyway....

Don Cowles
05-19-2003, 12:47 PM
Dave, the vertical marks you are experiencing are part of the past of most hollow-grinders. They are referred to as the "Dread 2-inch Marks." :(

Practice helps more than anything else, but there are a couple of other things you can do. As Ray said, slowing the machine down for the finer grits can make a huge difference.

Also, consider filing a radius on the edges of your contact wheel. It is that square corner that is digging into your blades. This will also give you nicer plunge cuts.

Finally, going to a softer (70 durometer) rubber on the contact wheel can make a significant improvement.

Don Cowles
05-19-2003, 12:51 PM
:eek:
Shame on me- I should have paid attention to the name of the thread. You were asking about flat grinding, and I am yammering about hollow grinding. Sorry. Some of the same principles apply, I guess- speed, radiusing your platen - but, obviously, you can't get a softer one of those.

David Peterson
05-19-2003, 10:13 PM
Don,
You saved me some time in the future by talking about hollow grinding. I probably would have asked this question later anyway, since I'm looking at the contact wheel trying to figure out how the heck to hollow grind. But that's for a later discussion.

I slowed the speed down with my step pulleys and it seems to help a little. I tried a wider push stick too. I think the belt is just moving too much side to side and digging in. I must not notice it when the belt is going so fast because it only takes a microsecond for it to cause a problem. Any way to tighten the tension on the Coote to keep the belt from going side to side?

One thing that bugs me is when the blade is being pulled back and forth across the platten, it will duck down between the rest and the platten when I get to the tip. There's not much room between the rest and the belt, but it's just enough to let the blade go down there. That really screws up the grind, especially when the belt is flying by. How do you stop that from happening? Am I OK by using a push stick or do you guys use your bare hands. I tried bare hands but it gets too hot after only one or two passes. Thanks.

-Dave

SVanderkolff
05-20-2003, 05:53 PM
Dave
A lot of the problem you are having will disappear with practice. I use a very short push stick and very little pressure. This will help with the grind divots. As for the blade ducking in between I found that if I adjust the distance between the work rest and the belt at each successive grit and make sure I REALLY concentrate when I get towards the end, I stopped with that probem. Mostly it was a split second of not concentrating or pushing too hard with the stick that caused the blade to catch the belt and dive between the rest and the belt.
Hope this helps.
Steve

Ray Rogers
05-20-2003, 06:22 PM
I use bare hands and I think a lot of other guys do too. Part of the idea behind using bare hands is that you can feel the steel getting hot. Then, you can dip it in water to cool it before it gets hot enough to ruin the temper.

Using bare hands means it's easier to keep the blade in smooth motion which will reduce your divots. It also means you won't need the tool rest and that solves the problem of getting the blade's tip caught in it...

Don Cowles
05-20-2003, 08:05 PM
Amen to bare hands, Ray! I have tried a push stick on a number of occasions, and know some very fine makers who use them - but they are definitely not for me.

Danbo
05-21-2003, 07:35 AM
I agree with the bare hands method. Flat grinding is all about feeling the grind. Using a push stick doesnt allow you to feel what is going on, both in grind accuracy and heat build up(possibly ruining blades). The bad side is, you are going to burn your fingers occasionally. The good thing is, pretty soon you will be able to take stuff out of the oven without mitts! :D

SVanderkolff
05-21-2003, 07:39 AM
A couple of replies have mentioned ruining the blades through heat build up. Does this only apply if you are grinding the blades after heat treat? I don't do any grinding after heat treat so all my grinding is on annealled blades. I have never worried about heat build up, should I?
Thanks
Steve

Ray Rogers
05-21-2003, 08:01 AM
The concern about heat build up applies mostly to grinding after heat treat. It is possible to damage annealed steel but you'd have to get it so hot it turns blue or black.

Most makers have to do some grinding after heat treat even if it's just to clean up the blade. If you do all your grinding before heat treat, making your blade thin enough to be a good cutter, you increase the chances of warpage during heat treat. If your blades aren't thin or if they are thin and you don't have much warpage problem then you are ahead of most of us.

Even finish sanding and buffing can cause enough heat to ruin a temper and you must surely do that after heat treating....

cactusforge
05-21-2003, 08:09 AM
Bare hand definitely works best , also take the work rest off and grind free hand, use sharp belts. This will solve a lot of problems.
Gib

SVanderkolff
05-21-2003, 08:10 AM
Ray
I hand sand to 1500 and then buff before heat treat. Then go back and hand sand 1200 and 1500 and buff after heat treat. The after heat treat hand sanding and buffing are quite easy and light so I rarely have much heat build up. I haven't had a problem with warpage for a while so I must be doing something right. My maple birds are certainly very thin at the edge and taper from the ricasso all the way to the point.
Thanks
Steve

KandS_KNIVES
05-25-2003, 04:24 PM
Since I do alot of flat grinding, maybe I have some ideas that might help. Does your Coote have a steel platen or have you covered it with pyruceran glass? Are you putting bevels on your edges, before you start grinding the flats? Are you using new sharp belts? Also, you might need to slow the grinder down some, till you get a feel for the machine. These 4 things really helped me, when I got my first big grinder.
Ken

tmickley
05-26-2003, 10:31 AM
You mentioned your belt was moving side to side. They will all do that a bit but you can help the belt track better by putting a crown on your tracking wheel. It's simply one layer of tape around the middle of the wheel. This gives the belt a small hump to hang on to. Also some belts track better than others. If you get one that wobbles bad flat grinding, save it for the wheel. Belts wobble more flat grinding.

On using a push stick vs. hand grinding, I can't make a push stick work grinding 'regular'. I grind one or possibly two passes and dip in water. Grind, dip, grind, dip. It gets hypnotic after a bit.
I do use a push stick with a rare earth magnet on the end when I flat grind vertically to flatten a tang or above a grind line.

David Peterson
05-26-2003, 06:20 PM
I think I found out what one of the problems was with my flat grinds. I noticed that whenever I was using the Klingspor(sp?) belts, they had a seam with wavy lines that made these little projections that matched the lines I was seeing on the grind. When I switched to another style of belt with a straight junction, the lines didn't show up. The Klingspor belt had a very rough junction and would make these thunking sounds every time the seam came around. After slowing the belt down, using straight seamed belts, and a wider push stick, my grinds are getting alot better. I was able to grind up to 400 grit this weekend without too many problem. I still need to practice, but it's getting better. I also noticed that my bench was way too high and I wasn't getting any leverage because my arms were up too high. I tried not using the rest and free handing it, but I just can't make it work. The blade kept jumping and falling out of my hand. Thanks for all the help guys.

-Dave