CBN Maximum RPM?

[Tom Hamilton]

I saw the other CBN thread, but it's all about QC on products now.

I’m thinking of getting a cbn grinding wheel and I have an 8” Delta single speed grinder that runs at 3600rpm.

I see the ones over at CSA are limited to 2200RPM.

Are CBN wheels only meant for slow speed grinders?

Thanks

 

[Bill Boehme]

I saw the other CBN thread, but it's all about QC on products now.

I’m thinking of getting a cbn grinding wheel and I have an 8†Delta single speed grinder that runs at 3450rpm.

I see the ones over at CSA are limited to 2200RPM.

Are CBN wheels only meant for slow speed grinders?

Thanks

There are some disadvantages to a grinder running faster than 1800 RPM -- here's a few reasons why:

1. Using a high speed grinder motor (a two pole motor that operates at a bit less than 3600 RPM) means more heat and less control unless you are able to refine your touch sufficiently to overcome the faster removal of metal.
2. A slow speed grinder motor (a four pole motor that operates at a bit less than 1800 RPM) runs much smoother than a two pole motor for two reasons -- lower torque ripple and less intrinsic vibration from mass imbalance.
3. A slow speed grinder is noticeably quieter. This is just my personal preference since I care about preserving the hearing in my one good ear.
4. SAFETY: The centrifugal force that is trying to pull the spinning mass (grinding wheel, pulley, chunk of wood) apart is about four times greater on a high speed grinder compared to a slow speed grinder. Any speed restrictions must not be ignored.

 

[robo hippy]

They are probably safer on high speed grinders than standard matrix wheels. They are spin and bubble balanced before they go out the door. They will work fine at high speeds. I think the main reason people use the slow speed grinders is because there is less intimidation. Maybe also because at high speeds, more so with the standard wheels than the CBN, there is more of a tendency to burn the steel. Not a real problem with HSS. You do need a lighter touch at higher speeds so you don't 'grind' instead of sharpen.

robo hippy

 

[Bill Boehme]

They are probably safer on high speed grinders than standard matrix wheels. They are spin and bubble balanced before they go out the door. They will work fine at high speeds. I think the main reason people use the slow speed grinders is because there is less intimidation. Maybe also because at high speeds, more so with the standard wheels than the CBN, there is more of a tendency to burn the steel. Not a real problem with HSS. You do need a lighter touch at higher speeds so you don't 'grind' instead of sharpen.

robo hippy

Things like this fail from metal fatigue. Operating over the stated RPM normally accelerates fatigue. It may work OK on day ONE, but beyond that, there is one less day until ...

Safety factors such as a limitation on maximum operating speed have some built-in fat to take care of the unknown. Trouble is, we don't know what the unknown is so we know enough not to knowingly mess with the unknown. Know what I mean?

 

[n7bsn]

D-Way gives no max RPM on their web-page, and from talking with Dave 3450 is fine (slower is too).

Personally D-ways are cheaper, have more surface area, and Dave is a friend

 

[MichaelMouse]

More energy means more fracture, more release from matrix, and other things. Doubt that it make a difference to a turner, but in a machine shop, perhaps. http://www.industrialdiamondcbn.com/mp-cbn.htm

 

[Tom Hamilton]

Hi All -

I asked the same question of Dave Schweitzer by email, and as noted by n7bsn Dave says 3450 is fine.

He said that's what he uses his at, but notes that they are more aggressive at the higher speeds, like Bill said.

I got concerned about the max RPM after seeing the 2200RPM max from CSA.

There you are, another sale for Dave.

Tom

 

[Matt Lewis]

Read the ABCs link at the bottom of the entry at this link - http://tri-colorturners.com/wordpress/?p=334.

 

[Hu Lowery]

interesting read

Read the ABCs link at the bottom of the entry at this link - http://tri-colorturners.com/wordpress/?p=334.

Not my most pressing need but I am very interested in CBN for several reasons, primarily grit reduction to be honest. After reading the article that I got to following the nested link I have to suspect that since they still use resin or electroplating that there might be multiple electroplating techniques in use too.

I don't understand the 2200 RPM limitation from one manufacturer. Not understanding it strikes me as a very good reason to pay attention to what they say. They know what they built a lot better than I do. If choosing that brand I will stay with that limitation. If another brand doesn't give a limitation then I would assume there was a difference in design or construction even if I couldn't see it. When I first bought a 24,000 RPM die grinder many years ago I had some of the bonded stones around that claimed they were limited to 3500 RPM. They indeed disappeared in a puff of dust when I spun them at 24,000 at arm's length and right angles to my body. While I don't see a CBN wheel doing the same, I have to wonder about surface bond and the wheel remaining true under the far greater stresses than it is rated for. I don't have the math but overdriving the wheel 65% will more than double the load on it I believe.

Before the age of computer modeling and design and when I was a bit younger and less cautious I might have trusted the "fudge factor" built into designs. I no longer trust engineer's caution to protect me from my lack of it. Production cost considerations have often slashed the fudge factor razor thin, even to zero in some cases with a certain designed in failure percentage deemed acceptable. As manufacturers have grown more reckless I have grown more cautious.

Hu

 

[Bill Boehme]

Sometimes we think of metal as being indestructible if the loads on it are not obvious, but when a large metal wheel is spinning at very high speed, the centrifugal forces at some point will be sufficient to cause the piece to disintegrate.

Machined cast iron pulleys are very common and are much stronger than die cast zinc or machined aluminum of the same size, but not necessarily up to the same RPM. The mass of cast iron is a significant limiting factor in maximum speed and for all pulleys, the maximum speed is affected by its diameter. The same limiting factors are valid when it comes to to maximum speed of metal body abrasive wheels. If you see a limiting speed then heed it. If you don't see that information, then find out and don't go on some uninformed WAG from someone speculating. Just because somebody sells something and gives an opinion -- that is all that it is -- an opinion. If they state manufacturer's rated performance data, then you have trustworthy information.

 

[DOCworks]

Our Wednesday night group had a D-Way CBN on a 3450 Ryobi grinder and it worked fine, except it took a lot of metal off very fast even with very little pressure. I found a "used" 1800 rpm grinder on craigslist, one from woodcraft, for $40 and donated it. Much easier to use and control "sharpening" instead of grinding metal. The CBN wheels do not heat the metal as much as a fragile type wheel and there are less sparks, but you still leaving metal on the grinder, what you don't see or get is the wheel dust. But except for cleaning or leveling the surface of the wheel (not CBN) I don't see that much difference in clean up. It's the fact that the wheel diameter stays the same and lack of vibration that sold me. I can reset my bowl gouge setting in 10 sec. and it's always the same even after I've moved it all around to do some of my other gouges. That is worth the price to me. I like most of the CBN's on the market, but I get mine and the clubs from D-Way for the same reason n7bsn does, I like Dave. He was extremely helpful and patient when I decided to move to CBN. He keeps trying to get me to try his gouges too, but, well...we'll see...

 

[robo hippy]

After having 1 inch wide wheels for years, I found the 1 1/2 inch wide wheels as convenient as variable speed on my lathe. Dave's tools are as good as any on the market.

robo hippy

 

[Patrick Miller]

d-way tools

I have a 180 grit D-way sold CBN wheel and a number of Dave's gouges, a skew and a scraper- all are the highest quality and worth to me every cent I spent on them.