Thread: This was a good weekend to test a theory of mine

I've been a proponent of slow is the way to go for a long time. Come to think of it I think I'm pretty well the only one who thinks that. I believe that if your grinder is spinning at over 500rpm it's going too fast. I've thought that for about 25 years.

Way back when my dad bought me my first lathe. We bought a mandrel for the outboard side that held the grinding wheel. It was very soon after that that I came to realize that it was a much superior way to grind tools. Back then lathe tools were made from high carbon steel and there were no grinding jigs to help you along. So burning the tips of the gouges was an easy thing to do. But I found that I could hold the tool for a much longer time on the wheel without any fear of burning the tip and get a much superior and consistent bevel. Since then I always felt that I could grind a tool faster on the lathe than on a regular grinder. Up until this weekend I had never really put it to the test.

This weekend I made a set of chisels and so when it came time to grind the bevels I thought I'd test the theory. I have a variable frequency drive that powers my grinder (from 0 to 120hz or 0 to 6000rpm) as can be seen in the attached pics. On one side is a cheap grey wheel and the other holds the MDF buffing wheel mentioned a few months ago by Lignum. I also have a Makita grinder with the same grey wheel and a cloth buffing wheel on the other side that spins at about 3000rpms.

I found that I could grind the chisel much quicker on the grinder that was spinning at about 10hz or 600rpm. I was able to put a great deal more pressure on the tool and remove a surprising amount of steel without the tool reaching a temp that would boil a drop of water at the tip. Thats about 100 degrees C. At about 160 and you start to burn the tool. Because the temp increase is very gradual (compared to the makita) I could have held the tool on the grinder longer but I didn't want to push it. As opposed to the Makita where it didn't take long at all to boil the water off. I didn't time it but I'd say I was about 20% faster at around 600rpm than with the Makita at 3000.


What I suspect is happening: Since the heat energy generated at 600rpm is probably about 5 times lower than at 3000rpm the steel has a far better ability to absorb the energy and "pull" it away from the tip than at 3000rpm. This allows the user to put more pressure on the tool for a longer period time before it over heats. And because the increase in temperature is more gradual it's easier to tell when it's time to back off and cool the tool down.

Once you have a bevel established there is no reason really to grind at anything above 200rpm.

Also at 500 or lower rpms (unless you've tried grinding at low speed you won't know what I'm talking about so you'll have to trust me), once you get a bit of experience with it you'll actually find it's easier to grind by touch and hearing than by sight. This is something you can't do at 3000rpm. I'm a bit out of practice but I tried to convey to the folks in Toowoomba that you can actually grind a good bevel on a chisel with your eyes closed. It had been about 2 years since I'd had any quality shed time so the attempted wasn't all that successful for them but it is possible.