Tool Grinding and CBN wheels? OK or Not?

[BFHammer]

For my wood lathe HSS tools I use 8" CBN (Cubic Boron Nitride) wheels mounted on a slow speed bench grinder at 1750 RPM. These work great for my wood lathe tools, 80 grit for roughing in and 120 grit for maintaining a great edge.

Being new to metal lathe tools - is there a reason or reasons that this would be a bad idea for grinding HSS lathe bits? It makes sense to me on the surface but I'm wondering if speed is an issue or are there other problems. In the metal lathe world of the inter-webs I never see this method mentioned.

Thanks,
Mark

 

[4GSR]

They are used all the time in the production environment for sharpening HSS endmills, drill bits, etc. Just not worth a flip for grinding carbide. In fact, they are not made for grinding carbide, just HSS.

 

[Bob Korves]

For a sharpening touch up of a tool bit, the slow speed grinder will work well. For roughing out a bit from a blank, it will be S-L-O-W... HSS is not particularly prone to losing its hardness by grinding to higher temperatures, even heating it to dull red only softens it slightly. Do not worry at all about blue color developing on the bit when roughing it out.

 

[4ssss]

I've used them in OD/ID grinders to grind hardened steel. They work very well.

 

[mikey]

I sent a query to Ken Rizza of Woodturners Wonders about this:

Me:
I am a hobby machinist who prefers using HSS tool bits on my lathe. Are your CBN wheels an appropriate tool for shaping metal lathe tool bits from square HSS tool blanks or will it be too a slow process? If you think it is a good choice, which grits would be appropriate for rough shaping and finishing?

Ken:
CBN is fine for that- 60 or 80 grit for rough shaping and 350 for finish work.

Given that a CBN wheel is intended for use with HSS, I would give it a go and then report back on your experience if you would. (That means show us the tool, hint, hint!) There are some distinct advantages to using a wheel to grind lathe tools; a non-explosive CBN wheel that needs no dressing is very attractive.

 

[BFHammer]

Thanks for the replies guys and Mikey thanks for reaching out on that.

I do like the CBN wheels for the reasons you described, solid (no exploding wheels) and they stay pretty clean. Hopefully I will get time this weekend to give it a try. I will report back - you guys can critique what will be my very first attempt at grinding a HSS bit. I have read and read and watched and watched - nothing left now but to just do it!

 

[mikey]

Hope your grinder has a wrap around rest to provide support on the right side of the bit. This helps to grind the all-important rake angles. If you have another grinder with aluminum oxide wheels, practice with mild steel keystock before trying HSS - it helps.

Good luck - we'll be waiting to see what you come up with. No pressure, of course!

 

[benmychree]

For a sharpening touch up of a tool bit, the slow speed grinder will work well. For roughing out a bit from a blank, it will be S-L-O-W... HSS is not particularly prone to losing its hardness by grinding to higher temperatures, even heating it to dull red only softens it slightly. Do not worry at all about blue color developing on the bit when roughing it out.

I have always heard that in grinding HSS tools, that repeated heating and quenching is not good practice, as it tends to cause fine cracking at the cutting edge and subsequent breakdown of the cutting edge, although I think this much more important in grinding mill cutters than lathe tools, where I have never seen any evidence of it causing a problem; I agree that the blue color is not a problem, but holding onto the tool bit gets to be a problem as heat builds up and travels back to where the fingers are, although grinding the lathe tools in their holders does make it a non problem.

 

[Cavediver]

I
Given that a CBN wheel is intended for use with HSS, I would give it a go and then report back on your experience if you would. (That means show us the tool, hint, hint!) There are some distinct advantages to using a wheel to grind lathe tools; a non-explosive CBN wheel that needs no dressing is very attractive.

Care to elaborate on that statement? If that's better left for a different thread I'd be happy to start one.
(Written by a guy who grinds with wheels and is looking to switch to a belt due to YOUR previous posts )

 

[mikey]

Care to elaborate on that statement? If that's better left for a different thread I'd be happy to start one.
(Written by a guy who grinds with wheels and is looking to switch to a belt due to YOUR previous posts )

Heh, heh, not enough advantages to make me even think about going back but the key ones to me are that 1) the wheel doesn't snap like an old belt will, leaving you stranded without a back up and 2) the hollow grind can make honing easier and more accurate because you have two edges orienting the tool on the stone. It is very quick to touch up a hollow ground tool between uses because you're only hitting the edges.

On the other hand, a belt sander is far faster when shaping a tool, is easier to avoid facets with due to the flat platen and grinds very cool compared to a solid wheel. I won't go back to a wheel myself but I want to encourage others to grind tools, regardless of the machine they use.