Uneven abrasive on metallic CBN stone possible??

[huntyukon]

Hi again guys,

I just got this metallic CBN stone in 125/100um and it looks and feels to me as if there is more CBN abrasive on the outsides then in the middle of it. Is that even possible with metallic bond stones? I hope it’s somewhat visible in this picture (you might want to zoom in for better view)



I think, even in the picture, it’s visible how the centre of the stone has less abrasive and more copper showing. This is after I dressed the stones in ferric chloride and then SIC powder, btw.

Is that even possible? The stone was a fairly good deal, too. Did I end up with a dud? Or am I imagining things?

Hopefully someone with experience with metallic bonds has an answer or solution.

Thanks a bunch in advance, guys.

 

[Mr.Wizard]

I have zero experience with this type of stone, but it follows the pattern of an incompletely lapped stone, or of a convex lapping job from a lapping base out of flat or letting the stone plow as you slide it back and forth. How flat was the stone when new? How flat is is now? How much material in total did you remove in the dressing process?

 

[huntyukon]

I see. That’s totally possible and makes a bunch of sense. The stone had more “bald” spots when I got it. I only lapped for a minute or two and didn’t check how much material I removed. I just followed instructions I was given.

I guess this means that I have to lap in order to “true” the stone again?

Thanks for the response, btw.

 

[Mr.Wizard]

I see. That’s totally possible and makes a bunch of sense. The stone had more “bald” spots when I got it. I only lapped for a minute or two and didn’t check how much material I removed. I just followed instructions I was given.

On more conventional stones there may be a "crust" that is denser, rougher, or more completely filled in the case of treated stones. Lapping through this crust often results is a noticeably different performance. I don't know if metal-bond abrasives can be affected in this way or not, but as long as you don't eat up a significant portion of the abrasive-filled layer I suggest you lap again and see if it improves. Before or as you do check that you are not preferentially wearing down the ends of the stone and making it convex instead of flat.

 

[Mr.Wizard]

huntyukon One more thing. When lapping on loose grit are you using enough water to keep the grit in suspension, rather than clumping? I recall trying to lap a porous stone that I had not pre-soaked, which resulted in it sucking the water out of the SiC suspension causing the SiC to cake up and fail to dress the middle of the stone, not unlike your photo.

 

[huntyukon]

huntyukon One more thing. When lapping on loose grit are you using enough water to keep the grit in suspension, rather than clumping? I recall trying to lap a porous stone that I had not pre-soaked, which resulted in it sucking the water out of the SiC suspension causing the SiC to cake up and fail dress the middle of the stone, not unlike your photo.

I admit that it was a bit of a learning curve when I lapped my Boride stones for the first time. After a couple stones I think I figured out a good water-SIC ratio. Nevertheless it’s possible that I still had a little bit of clumping happening.

That said I just noticed that the Glas I was using wore slightly unevenly. My brother had been using it a fair bit, so that might be a cause as well.
I will look for a granite tile of some sort now. That should work better, I think.

Mr.Wizard to the rescue, once again! Thank you kindly!

 

[DeadboxHero]

Hi again guys,

I just got this metallic CBN stone in 125/100um and it looks and feels to me as if there is more CBN abrasive on the outsides then in the middle of it. Is that even possible with metallic bond stones? I hope it’s somewhat visible in this picture (you might want to zoom in for better view)

View attachment 2511430

I think, even in the picture, it’s visible how the centre of the stone has less abrasive and more copper showing. This is after I dressed the stones in ferric chloride and then SIC powder, btw.

Is that even possible? The stone was a fairly good deal, too. Did I end up with a dud? Or am I imagining things?

Hopefully someone with experience with metallic bonds has an answer or solution.

Thanks a bunch in advance, guys.

Don't overthink it. Go sharpen something.