NeilS
8th October 2019, 01:17 PM
Higher grit CBN grinding wheels, like #360 and #600, are becoming more available and the price on them has come down on them so that they are now comparable with the lower grit wheels like the #180.
So, if the price is not a factor now between the higher and lower grit wheels, what (if any) is the benefit of going one way or the other?
I have had a #180 CBN grinding wheel for a while now and recently purchased a #360 8" CBN wheel. I decided I needed an extra wheel so that I could have dedicated settings for my various grinds and thought I would give the #360 a try to see if it provided any additional benefits.
My #180 has performed well and has settled down to a less aggressive grind after some use. A #600 wheel seemed to be too much of a step up from that, so I settled on the #360.
My #120 diamond wheel continues to give me good service. It has a resin bonded matrix and despite the coarser grit rating it gives what seems to be a finer grit finish on the tool than the #180 CBN. I think the diamond wheels are underrated and too much credence has been given to the myths, or at least exaggerations, about the iron in the tools eating up the diamond. But, I decided to go with another CBN wheel so that I could compare apples with apples.
Straight off the #180 and #360 wheels, the bevel grinds on the tools are indistinguishable. The following are two Ellsworth gouges (same steel, etc) with the bottom off the #180 and the top off the #360 wheel.
462670
But, what about the finish off these two grinds. The piece was some soft pine with an end grain knot.
462671
As you an see, the finish off these two gouges after being freshly ground was also indistinguishable.
So, no advantage there.
Next was an endurance test. How long would each grind hold its edge before the curly shavings stopped coming off the edge and only produce dust particles. This is far beyond where I would go with normal turning, but a measurable point of some sort.
The average result was 7mins 15secs off the #180 wheel and 8mins 30secs off #360 wheel on very old and hard red gum. Not an earth shattering difference and in the hands of someone else the result might be different.
My impression was that the gouge off the #180 wheel cut more aggressively to begin with then dropped away sooner. The gouge off the #360 wheel was more even from beginning to end, but did a bit more work (removed more wood, but I didn't measure/weigh that) overall by the time it stopped cutting. Going back to the grinder immediately the edge performance dropped away might change the analysis, but would be more subjective.
On the above results, I didn't find a significant benefit from the #360 over the #180 wheel.
Other factors, like how quickly a #180 would sharpen a tool over a #360, might become obvious once both wheels are fully broken in. The #360 is still grinding quite aggressively, as these new CBN wheels do, but it is too soon for me to know if that will become a factor.
Out of interest I took a microscope image of the fresh edges off the two wheels. As you can see, their burrs are quite different and partly explains why I found the #180 cut more aggressively to begin with but then dropped away in performance. The edge off the #360 is quite refined. If turners are refining their edges with a hone on tools like skews, a #360 or #600 wheel may obviate the need for that. The last image shows the resolution of the images measured in 0.5mm increments.
462673
#180
462674
#360
462675
Res 0.5mm increments
So, if the price is not a factor now between the higher and lower grit wheels, what (if any) is the benefit of going one way or the other?
I have had a #180 CBN grinding wheel for a while now and recently purchased a #360 8" CBN wheel. I decided I needed an extra wheel so that I could have dedicated settings for my various grinds and thought I would give the #360 a try to see if it provided any additional benefits.
My #180 has performed well and has settled down to a less aggressive grind after some use. A #600 wheel seemed to be too much of a step up from that, so I settled on the #360.
My #120 diamond wheel continues to give me good service. It has a resin bonded matrix and despite the coarser grit rating it gives what seems to be a finer grit finish on the tool than the #180 CBN. I think the diamond wheels are underrated and too much credence has been given to the myths, or at least exaggerations, about the iron in the tools eating up the diamond. But, I decided to go with another CBN wheel so that I could compare apples with apples.
Straight off the #180 and #360 wheels, the bevel grinds on the tools are indistinguishable. The following are two Ellsworth gouges (same steel, etc) with the bottom off the #180 and the top off the #360 wheel.
462670
But, what about the finish off these two grinds. The piece was some soft pine with an end grain knot.
462671
As you an see, the finish off these two gouges after being freshly ground was also indistinguishable.
So, no advantage there.
Next was an endurance test. How long would each grind hold its edge before the curly shavings stopped coming off the edge and only produce dust particles. This is far beyond where I would go with normal turning, but a measurable point of some sort.
The average result was 7mins 15secs off the #180 wheel and 8mins 30secs off #360 wheel on very old and hard red gum. Not an earth shattering difference and in the hands of someone else the result might be different.
My impression was that the gouge off the #180 wheel cut more aggressively to begin with then dropped away sooner. The gouge off the #360 wheel was more even from beginning to end, but did a bit more work (removed more wood, but I didn't measure/weigh that) overall by the time it stopped cutting. Going back to the grinder immediately the edge performance dropped away might change the analysis, but would be more subjective.
On the above results, I didn't find a significant benefit from the #360 over the #180 wheel.
Other factors, like how quickly a #180 would sharpen a tool over a #360, might become obvious once both wheels are fully broken in. The #360 is still grinding quite aggressively, as these new CBN wheels do, but it is too soon for me to know if that will become a factor.
Out of interest I took a microscope image of the fresh edges off the two wheels. As you can see, their burrs are quite different and partly explains why I found the #180 cut more aggressively to begin with but then dropped away in performance. The edge off the #360 is quite refined. If turners are refining their edges with a hone on tools like skews, a #360 or #600 wheel may obviate the need for that. The last image shows the resolution of the images measured in 0.5mm increments.
462673
#180
462674
#360
462675
Res 0.5mm increments