Diagnose this grinding problem, please

Chiro75

Chiro75

Joined
Feb 4, 1999
Messages
5,786
I'd like to blame this problem on my platen, but before I do so I'd like some opinions. The pics below are somewhat exagerrated (actually, the one of the blade from the side is pretty accurate). Anyway, when I flat grind, if I were to set the blade flat on the platen and grind without moving the blade, I get something like you see below. In my mind that would mean that my platen has a "dip" in the middle and less material is being removed from the middle of the blade than on the edges of the platen. But, even when I move the blade, I still get this sort of shape as I come close to finishing it. Granted, my knives usually have blades in the 2"-4" range, so I'm never moving the blade a ton, but what do you think? Is it more of a platen problem, am I scooping the blade somehow, both, neither? It's a real PITA because I have to do a lot of work to even the whole thing out. I'm hoping it's a platen problem because then I can just get my nifty pyroceram from Darren and call it a day! :D

grindproblem.gif
 
put a straight edge across the platen and shine a light up through and see if it's dipped..
if not it's you... :D I think your rock-en-rollen it..
if it were easy everyone would be doing it :D
 
Goood idea, Dan! Why do I always forget the obvious? I would tend to agree that the problem is probably me, not the platen, but I'm suspicious of the platen for two reasons:
1) This dishing occurs if I lay the steel into the belt and don't move it at all and
2) It's gotten worse since I first got the machine.

Hopefully this doesn't mean my Grind Fu is getting worse as I practice it more! :eek: Will check it out and report in later on.
 
I wasn't placing the blame on you first..as said..
if it's not dipped ,,then it's you. :) .. has to be one or the other or both.:D

I'd say if it's getting worse then I'd be checking it..
I'm not familiar with the coote,,
can you just remove it and re-surface it?
 
you're definitely pushing too hard, or spending too much time "at each end". That's what causes that.

I bet if you could watch yourself with a 90 degree level, you'd see that you're working the edges of the belt more than the middle.

Instead of grinding from the ricasso to the tip and back, try starting in the middle of the blade, moving to the tip, then to the ricasso, then to the middle.
 
Instead of grinding from the ricasso to the tip and back, try starting in the middle of the blade, moving to the tip, then to the ricasso, then to the middle.
Click to expand...
That's tough on a 2" belt with a 2" blade! :D I don't "think" I'm pushing too hard. I don't feel like I use heavy pressure at all, but then again before I got glasses in 4th grade I didn't know I had bad vision, you know? I'll check it out over lunch and report back in. Unfortunately trying to correct it once it's there causes a lot of dips in the blade. :grumpy:

Coote machine does have a detachable platen, so I could resurface it, but I'd just as soon slap a piece of pyroceram or plate glass on it, too.
 
Steve even on a 2" blade I run from the plunge to the tip..
the belts themselves will cause some of a problem..and it's how you get the dips out. now,, if you have a disk, that's the cats a$$ for some of that.. :D

you have to bow and bend down to the whims of the equipment too you know.. :D
 
Chiro75 said:
That's tough on a 2" belt with a 2" blade!
Click to expand...
Like Dan said, you can still work the entire blade. I'm still convinced that the problem is because you are using the edge too much - In other words, you're not perpendicular to the platen.

Here's a great test for this: Get a piece of barstock (high carbon 1095, etc.) and "go through the motions" just like you would to grind a knife. While you are grinding, watch the spark trails below the steel. They're the "evidence" of where your platen is truly contacting the steel. In a perfect world, it would be a solid, 2" wide "beard of sparks". But more likely, you are only contacting the edges of your platen. Try it and see for yourself.


You have to do some pressing against the platen when you grind and because of that (unless you are a machine :rolleyes: ) you are by nature pressing unevenly against it. You just learn to compensate for it.
 
equal pressure is needed for grinding, it doesn't take much uneven pressure toward the center of the blade to cause dips and hollows. Believe I know all about that. :) Also keep the blade as flat as possible, left handed grinding is always my hard side. Good luck and happy grinding.

Bill
 
Ahhhh....vindication. Shined a light on the platen with a metal ruler across it and guess what? My platen is dipped in the middle and higher on both sides. Now, the question is, what would cause this? Obviously it's from the belt, but would heat warp it? I've run my grinder for somewhat extended periods of time and the platen can get pretty warm to the touch, or is it simply from belt wear? Guess I'll be getting some plate glass or pyroceram. My Grind Fu is still strong! Yes! :rolleyes:
 
Chiro75 said:
My platen is dipped in the middle and higher on both sides. Now, the question is, what would cause this? Obviously it's from the belt, but would heat warp it? I've run my grinder for somewhat extended periods of time and the platen can get pretty warm to the touch, or is it simply from belt wear? :
Click to expand...

unless it real thin ... it's wear
place the straight edge up right and do the same with the light
you'll have wear from just below your rest and up..
more of the time working with grinding profiles and such you'll be hitting the center more than the out sides..dipping it.. :)
 
My platen is a big, rough piece of steel about 3.5" accross. Not good for 2" grinding.
I snagged some scrap aluminum from the school machine shop, then used the grinder to size it up, and screwed it in using recessed screws, works great.
now my problem is just rock-n-roll. :(

_z
 
Steve, if you have to replace your platen or surface grind it back to true, I would suggest getting some of the graphite platen cushion that K&G sells. It eliminates bump and gives a nice smooth finish.
Scott
 
If your platen will adjust far enough off the belt. How about attaching a piece of heat treated O1 to use as a platen. I did that with a bader platen using D2. It is drilled and tapped to the old platen material. Heat treated the steel only to around 45rc. Surface ground flat and square. Then squared up plumb on the new platen to the grinder.
 
Chiro,
Save yourself some trouble and order the platen from Darren.

If you don’t want to wait for it, find a glass shop and have them cut a piece of fireplace door glass the size you need and epoxy it to the platen.

Be sure to have them grind the edges smooth.

Joe Foster
 
For a temporary fix I took the platen off and got my morning workout by draw filing it flat again. Took about 20 minutes, but was n experience I'd rather not repeat! :D What a world of difference it made! Actually took me a few minutes to get the hang of grinding again. With the trench in the platen I had before, the blade would just sort of settle in and it was pretty easy to control. So, I wasn't used to a flat platen and it took a bit to get a feel for it again. :rolleyes: MUCH better grinds, though! Whew! I thought I was going back in time with my grinding again! :D I also switched to a new 80 grit belt, so I felt like I was on a brand new machine, again! Thanks for the help, guys. Begs the question: why not use a hardened steel platen instead of Al, to begin with?
 
I usually buy about 4-6 pieces of 2" by 7" pyrex glass from my glass shop. These guys save chunks of broken stuff they repair and cut platten stock for me. I've made knives and tools for these guys and they are happy to recycle material for me. Get the glass guys to radius all outside edges. Long radii top and bottom, and short EQUAL radii on the sides for balanced plunge cuts.

I use an all plastic non fiberous double stick carpet tape to hold the glass on the steel faces. I have a couple of small screws at the bottom of the steel platten that keep the glass from sliding down the steel face. If you don't have some support for the glass on the bottom you will experience an exploding platten and a cut belt, it is very exciting.

Belts stretch from the out side into the middle, which might be part of the problem. I'm finding less of this stretching on the 3M Gator belts, and they seem to be superior for flat grinding.

Centripical force on the high speed belt tries to throw the belt away from the platten.

The next step is to finish flats on a disc grinder. I use a horizonal DC machine for this process.

For Wilton Square Wheel machines: mill away some of the side of your cast platten for relief on both sides of the platten, and you will have clearance to grind with the edge of the belt on both sides of the platten.
 
i think the biggest problem your having is, YOUR NOT GRINDING ON A KMG!!!! :D :p
there i said it ..i feel better now :D
 
chisel said:
i think the biggest problem your having is, YOUR NOT GRINDING ON A KMG!!!! :D :p
there i said it ..i feel better now :D
Click to expand...


Hehehe, along the same lines, just switch over to hollow grinding, then you don't have to worry about a flat platten. ;)

From what the others were saying about it being you, I would still have agree with them. Not that its is all your fault, but I bet you were more careful grinding the bevels after you modified your platten especially if it didn't feel the same.

I was told that one of the first things they look for when judging knives for the ABS tests is a dip in the blade 2 inches from the plunge cut. It is a common thing that just takes time to get right.
 
You must log in or register to reply here.
Back
Top