Thread: post ht grits and speeds

Been off the scene for a while, but finally got belt grinder on order and will be about 2 months to get built.
In the mean time, can anyone recommend running speeds (rpm) and belt grit sizes as blade gets closer to finished thickness.
Sorry if this has been answered before elsewhere here. I'm a grinder newbie and wouldn't even know where to get belts from and what types. The grinder I'm getting has a vfd so quite good control of speed.

Thanks
Joe

Go to USA Knifemakers and find the Tracy's editorial on belt recommendations. This should have most of your answers.

Not my area of expertise but I think you may find a lot of info in the sticky thread in the workshop :

http://straightrazorpalace.com/works...wers-here.html

The short and sweet answer is go slow and cool the blade often. Do not wear gloves that you can feel the heat build up and even then cool the blade before you think you should.

Mechanically a larger grit rides across the steel with less surface area than the finer grits. The more surface area contact on the blade the more heat generated. As you step down through the grits, the finer grits also break down easier leaving more surface area contact hence more heat, and the more important to slow down and cool often, especially after heat treatment. The VFD is a handy device. Rather than get into RPMs and belt speed, you will likely find yourself running in the 10-30% range of motor output.

Again, slow and careful. Another example is that if you finished your blade at 60 grit and then heat treated. You now have those scratches to remove and they are harder to remove than when the steel was annealed. It will take more pressure at 100-120 grit to remove them and that risks additional heat. Once you are past about 400 grit equivalent or 60 microns, things will go faster but you risk the blade being hot before you know it and can get to the water bucket.

There are water cooled wheel arrangements. They are messy but they work. The wheels turn slower than you might think would get any work done.

It's a combination of the operator's pressure on the material, grit size and surface area and sensitivity to heat build up that will help the most.

Originally Posted by Mike Blue

The short and sweet answer is go slow and cool the blade often. Do not wear gloves that you can feel the heat build up and even then cool the blade before you think you should.

Mechanically a larger grit rides across the steel with less surface area than the finer grits. The more surface area contact on the blade the more heat generated. As you step down through the grits, the finer grits also break down easier leaving more surface area contact hence more heat, and the more important to slow down and cool often, especially after heat treatment. The VFD is a handy device. Rather than get into RPMs and belt speed, you will likely find yourself running in the 10-30% range of motor output.

Again, slow and careful. Another example is that if you finished your blade at 60 grit and then heat treated. You now have those scratches to remove and they are harder to remove than when the steel was annealed. It will take more pressure at 100-120 grit to remove them and that risks additional heat. Once you are past about 400 grit equivalent or 60 microns, things will go faster but you risk the blade being hot before you know it and can get to the water bucket.

There are water cooled wheel arrangements. They are messy but they work. The wheels turn slower than you might think would get any work done.

It's a combination of the operator's pressure on the material, grit size and surface area and sensitivity to heat build up that will help the most.

Thanks, sound advice.
What grit progression do you recomend as the grind progresses before hand finishing becomes necessary.

JG

Originally Posted by Deckard

Thanks, sound advice.
What grit progression do you recomend as the grind progresses before hand finishing becomes necessary.

JG

I will stop machine grinding at 5-9 microns and that is on a hard felt wheel. I have a largish buffer on a VFD so I can go very slow with full torque and use hard felt wheels and buffing compound to finish surfaces. I might burnish (a hardened steel rod) some exposed surfaces for different effects.

Sometimes visible scratches, ala satin finish, can look good. Sometimes you want a high polish and sometimes the steel does not go along with your wishes. Stay flexible

Or you could do a lot of pattern welded materials and etch them dark a gray. That can be used to hide a lot of scratches, but once you see them they look like crap so you still have to do the finish work.

As soon as you are done, Mr. Oxygen begins his work patinating everything. They are only "perfect" once. Besides, with enough magnification there are still going to be scratches. LOL.

Originally Posted by Mike Blue

Do not wear gloves that you can feel the heat build up and even then cool the blade before you think you should.

At the same time, be careful. This weekend I was so focused on grinding the face of a hammerhead that I accidentally touched a knuckle to the 40 grit belt running at 22 m/s.

You do not want to do that.

That does get your attention, doesn't it Bruno. Been there, done that, got the tee shirt.

Originally Posted by Bruno

...accidentally touched a knuckle to the 40 grit belt running at 22 m/s. You do not want to do that.

+1. I wonder Bruno, if you saw the touch coming for about two full seconds before you stuck your knuckle into the belt? It seems common that tool operators had some knowledge of the accident before it happened. At those speeds, even a glove wouldn't make much difference. It will still take weeks to heal.

Although, in my capacity as a Member of the Order of Grinder Injuries, you are now promoted to play with 24 grit belts. The next award comes with a ribbon indicating the degree of ripped flesh. The higher grit belts are more like paper cuts or cauterized while abraded so they aren't nearly as dramatic and prize-worthy. LOL