Surface speeds for dry grinders?

[woden]

I was just pondering the different surface speeds you can get with grinders depending on the rpm and wheel diameter. What sparked this off was a comment by Joel Moskowitz in his article on grinding in this month's Fine Woodworking that an 8" high speed grinder has a surface speed that will burn edges for sure.

Now of course that's in America where high speed grinders seem to run at 3500 rpm or thereabouts. If my dodgy calculations are correct, an 8" wheel should have a circumference of 0.64m which would give you a surface speed at 3500 rpm of ~ 37 m/s. In Europe, with lower rpms, the surface speed would never be this high.

And Moskowitz uses a new fangled 60-grit Norton 3X wheel. So in America 8" top speed grinders seem to be out even with one of the coolest running wheels going.

In the UK/Europe a high speed grinder seems to run at ~ 2800 rpm – a good bit less than the American equivalent – and thus half-speed versions on the market run at about 1400 rpm. Take Creusen, they do grinders at 2850 or 1425 rpm.

Given that Creusen's rpms are probably par for the course when it comes to European grinders, the different possible surface speeds I've worked out are:

30 m/s for an 8" wheel at 2850 rpm

23 m/s for a 6" wheel at 2850 rpm

15 m/s for an 8" wheel at 1425 rpm

11 m/s for a 6" wheel at 1425 rpm

Now elsewhere I've read that 11 m/s is just too slow for substantial stock removal when removing major nicks or reshaping bevels. So if that surface speed and the 37 m/s are out which of the options left do you think are preferable in the heat/speed trade-off? And to make all else equal lets say you would always use the coolest/coarsest stone available – like the 46-grit Norton 3X that Philly did a thread on.

 

[woden]

I wonder if somewhere in the mid to high teens is what's best in terms of surface speed in m/s?

If a lot of people find a 6" dry grinder too fast, and those spin at 23 m/s at the stone surface, and whereas some think a 6" on a slow grinder is too slow at 11 m/s then maybe something in between is the best option. Over at the American Association of Woodturner's forums I've noticed lots of posters with slow 8" grinders. With the American half-speed motor doing 1750 rpm then they'd be using a spinning surface running at 19 m/s. Even then some felt they could do with slower.

The option that would get you into the mid teens in the UK would be a 1425 rpm grinder with an 8" wheel. With this you'd be looking at a surface speed of 15 m/s - maybe the best compromise amongst all the possibilities out there?! :?

 

[Joel Moskowitz]

Turners in the US and elsewhere mostly use HSS (high speed steel) which can be ground at almost any speed without fear of burning. So a high speed 8" grinder is perfect for turners.

 

[woden]

But, Joel, isn't there some talk amongst turners now that if you keep heating HSS excessively that micro-fractures in the steel can develop. This, it's claimed, can lead to the edges of turning tools cracking.

If you're FWW's Joel Moskowitz, then nice article on grinding. Have you ever tried Norton's 46-grit 3X wheels? I ask because I've only been able to source either those or the 100-grit versions in the UK and not the 60-grit.

 

[Joel Moskowitz]

But, Joel, isn't there some talk amongst turners now that if you keep heating HSS excessively that micro-fractures in the steel can develop. This, it's claimed, can lead to the edges of turning tools cracking.

If you're FWW's Joel Moskowitz, then nice article on grinding. Have you ever tried Norton's 46-grit 3X wheels? I ask because I've only been able to source either those or the 100-grit versions in the UK and not the 60-grit.

I've never hear the micro-fracture thing specifically on HSS but I have hear that bandied about on grinding in general. I don't know about turning tools but I have seen no evidence of it on regular tools of any steel alloy.

Thanks - I'm that guy. 46 3X wheels run a little cooler than the 60 grit. (BTW I stock and sell all the grits and we ship to the UK all the time) I happen to like the 60 grit wheel - a good compromise between running cool and getting a decent general texture when you are doing general grinding. but the 46 wheel does run cooler and many prefer it.

 

[woden]

I suppose you might have to adjust to using a somewhat coarser bench stone when beginning to hone after coming off the 46-grit wheel. Or is the surface left by the 46-grit really all that rough? Maybe just a bit extra work on your first honing grit is all that's needed to make up for this.

I usually go straight to a 600-grit diamond stone when coming from the grinding wheel. I'd had doubts that this was like attempting to run before you could walk but I've since read that some people start on a 1200-grit diamond stone after the grind! I wonder could you get away with this when using a wheel as coarse as the 46-grit 3X?

I'm sending you a pm about the Norton 3X wheels you sell.