Truly Flat Blades
Posted 28 March 2012 - 10:28 PM
Posted 29 March 2012 - 06:10 AM
It seems to me you should be able to sort this out on the grinder. I don't use a disc sander--strictly a 2x72 belt grinder for me. One thing I wonder is whether you've got any kind of backing on your platen. Some makers use a graphite, ceramic, or even a canvas backing on the platen. It reduces friction, which has a whole lot of benefits, not the least of which is wear and tear on your platen. I don't use anything like that--I just run my belts directly against the platen.
At any rate, it seems to me that one of three things is causing you to finish up at the grinder with a crowned blade:
1) If you're using a soft backing like canvas, the mechanics of the grinding are going to crown your blade for you--naturally, as you press into the canvas, the edges will sort of roll out top and bottom. I'm assuming that's not what's going on.
2) If you're using a hard backing or the platen itself, it is very likely that you've worn a bit of a "dish" into the grinding surface. Chances are, you stand in the same position and hold the knife at the same place just about every time you grind. The friction from the back of the belt being pressed against a hard surface (platen or backing) will eventually wear away a shallow dish, which will then crown your blade. I'm guessing this is your culprit.
3) Lastly, you can't ignore technique. I know you know how to grind, and I know your blades don't have all kinds of weird bevels and facets on them when you come off the grinder. If your grinding passes are all at the same angle and you have a flat grinding surface, your blade should be flat. If you've got a flat grinding surface and you're coming off the grinder a blade that looks "true," and it looks as though the blade should be flat, then it's got to be #2 above.
As I think about it, one other thing to think about is the mechanics of your platen and belt. Is your platen slightly proud over the idle wheels? If not, your belt itself might be "dishing" ever so slightly between them. Just an afterthought.
Hope that's useful.
Posted 29 March 2012 - 07:41 AM
Posted 29 March 2012 - 11:22 AM
Posted 30 March 2012 - 05:39 AM
Posted 30 March 2012 - 07:22 AM
The hot rolled platen that comes with the machine is too rough to grind well and the edges are not uniform, so do not do a good job on the plunge lines. I use the original hot rolled platen for miscellaneous grinding.
I made a second platen from a piece of surface ground O-1 steel. The O-1 platen is used ONLY for knife blades. About every six months, I will surface grind both platens to re-flatten them.
Another method of hand sanding a blade flat, is to glue a piece of sandpaper to a flat surface and then rub the blade on the sandpaper.