CBN wheel sharpening recommendations

[Christian DiPaola]

Hi,
I am in the process of setting up a grinder and jigs for sharpening lathe tools. I'd like to get a set of CBN wheels (which are expensive). But I figure the cost is worth it if I don't need to buy a new grinder. The trouble is I don't have an 8" grinder. I have a 6" 3200 rpm grinder. I also have an old walker turner double arbor belt driven set-up where I could attach a 1700 rpm motor.

2 main questions:

1. Is an 8" wheel "better" than a 6" if you're using CBN (which does not wear like a standard abrasive wheel)?
2. Is 3200 rpm OK or am I still better to have the 1700 rpm variety.

Thanks,
Christian

[Prashun Patel]

Personally, I think 3200 is too fast. My CBN - which is 180 grit is very aggressive even at 1750 RPM.

I do think an 8" is preferable to a 6" because the hollow will be slightly less. FWIW, I use an 8" wheel on my 6" grinder. With the right bushing it spins fine on the smaller arbor. I run a 6" blue AO wheel on the other side of my CBN, and I do not notice any imbalance. I planned to do this 'temporarily' 3 years ago, and haven't needed to alter the arrangement.

[Thom Sturgill]

D-Way sells both 8" and 6" wheels and Dave has stated that 3600 RPM motors are OK. HSS is not subject to losing temper by bluing as it requires a much higher temperature and CBN tends to run cooler anyway.

The biggest difference between 6" and 8" is the depth of the hollow in the hollow grind achieved with a wheel. Many of us grind off the lower portion to create a rounded heel on the gouge. I would think that a 6" wheel would require more metal be ground off. But also that that only effects the first time, after that you are simply moving the same profile back a hair.

For items that might still get honed (a skew perhaps?) the deeper hollow just means more honings can be done before regrinding. Depending on the included angle of the edge, some claim it creates a weakened area behind the edge though I 'm not sure that that is really an issue.

[Reed Gray]

Well, I don't remember how to do the math, but a 6 inch wheel at 3200 is pretty close in feet per minute to an 8 inch grinder at 1750. The nice thing about the CBN wheels is that they are spin and bubble balanced so they will run more true than standard wheels. Many turners use 6 inch grinders because they already have one, and don't want to buy another grinder.

robo hippy

[allen thunem]

what part of the world do you hale from??

[Christian DiPaola]

what part of the world do you hale from??

Rochester, NY originally. Currently live in central MA.

Thanks to everyone for the advice.

C

[Thom Sturgill]

Well, I don't remember how to do the math, but a 6 inch wheel at 3200 is pretty close in feet per minute to an 8 inch grinder at 1750. The nice thing about the CBN wheels is that they are spin and bubble balanced so they will run more true than standard wheels. Many turners use 6 inch grinders because they already have one, and don't want to buy another grinder.

robo hippy

Material past the a point (per minute) is circumference x rpm. So Pi x Diameter x rpm . SO:

6" x 3200 x 3.14 = 60288 in/min
8" x 1720 x 3.14 = 43206 in/min

The 6" grinder is moving half again as fast as the 8" slow grinder at the edge.

[Leo Van Der Loo]

Not quite Robo, or better not nearly so, a full 6 inch wheel at 3450 rpm spins 65032.5 inches past a given point, and a 8 inch wheel at 1725 rpm 43383 inches, or about 5420 feet for the 6” against 3615 feet for the 8 inch wheel, that is a big difference.

The other big difference, is that any small imbalance in the grinder gets compounded exponential by the power of, as in doubling the speed of the wheel here, something alike to a car doing 50 miles an hour and then do that at 100 miles and hour, and you will notice the difference all else being equal.

Of course IMO if you need to grind just regular HSS there is no need for anything but a regular grinding wheel, now if you want to spend your $$ on a better grinding setup, I would start with a better slow speed grinder, and after that maybe some better wheels, but then it isn’t my money they want to spend (waste IMO )

OK Thom you type much faster than I, besides I had thee in between also, but we are pretty close in our figures, I using 3.14 times the Dia. of the wheel, times 1725 rpm and the same with the larger wheel and 3450 rpm

[Thom Sturgill]

OK Thom you type much faster than I, besides I had thee in between also, but we are pretty close in our figures, I using 3.14 times the Dia. of the wheel, times 1725 rpm and the same with the larger wheel and 3450 rpm

I used the speed quotes above, rather than the normal 1725 vs 3450.

[Reed Gray]

Well, thanks for the math, or is it geometry or as one teacher I had said 'gee-I'm-a-tree'. Leo, I figure CBN wheels are like true variable speed. Once you try them, you will never go back. Yes, they really are that much better in every way I can think of. Even at the extra cost, you get a far better value for the $ amount you spend. No need to clean or true them up. No need to balance. No risk of them ever blowing up. I expect mine to out live me. They cut with less heat.

robo hippy

[Leo Van Der Loo]

Well, thanks for the math, or is it geometry or as one teacher I had said 'gee-I'm-a-tree'. Leo, I figure CBN wheels are like true variable speed. Once you try them, you will never go back. Yes, they really are that much better in every way I can think of. Even at the extra cost, you get a far better value for the $ amount you spend. No need to clean or true them up. No need to balance. No risk of them ever blowing up. I expect mine to out live me. They cut with less heat.

robo hippy

Apples and oranges Robo with the variable speed comparison, that is to adjust your cutting speed, that needs to change, and we used V belts for that and still do somewhat, the variable speed makes it quicker and without the large steps in speed.

CBN and other type of grinding wheels just put an edge on steel, and the end is a sharp edge with any of them, unless you start using steel that a normal wheel can not cut, like carbide, the second or two that you might save using one wheel over another might add up for a professional turner, but is hardly of any consequence for a hobby turner.

Regular grinding wheels will cut regular HSS just fine, my tools never get a yellow or blue edge on them from grinding on everyday carborundum wheels, and even if it did HSS was developed to withstand high edge temperatures when cutting, like drill bits and metal turning bits and there were no CBN wheels needed, it was only much later when carbides were used and even later with powder metal use, that de need for CBN wheels came to be.

[Bill Blasic]

JoHannes Michelsen sells 6" & 8" CBN wheels up there in your part of the country.

[Reed Gray]

Leo, it really isn't apples to oranges. Maybe it is rotary phone to key pad phone. Really much better. Like I said, if you try one, you will see why.

robo hippy

[Scott Hackler]

I bought my 8" 180 grit wheel from D-Way for the following reasons:

1) never ever ever ever will reduce in size. I have my Wolverine jig, slide permanently marked for the exact spot I need to do my bowl gouges. That distance will never change, unless I change my grind.
2) the sharpening surface is much wider than the standard stone wheels
3) NO MORE DRESSING the stone. I hate having to dress the wheels because of the dust and the fact that the wheels wear unevenly and have to be dressed
4) the finish off the CBN wheel is akin to a honed surface. I want a razor sharp tool and had been honing. I feel like I don't need to...now. My 180 CNB puts an edge that makes the 180 AO wheel look like it is a 80 grit.
5) Dave and D-Way is a good guy and answered all my questions. His prices were comparable and I believe with the bushing and shipping I spent $215.

Worth every penny to me.

[Jon Nuckles]

I spent money on Norton blue wheels and a Oneway balancing kit and still wasn't happy. That was money wasted. The CBN wheel I bought from D-Way is some of the best money I ever spent on a turning tool. It really is a game-changer.