CBN Grinding Wheel

[Nifty Nev]

Have recently purchased a Vicmarc Bench Slow Speed Grinder, for my turning tools. I would like to upgrade to CBN Grinding Wheels. However I see the grit sizes are 80, 180 and 240. Should I use the 80 grit for the sharpening of skew chisels, parting tools and roughing gouges. What should I use to sharpen bowl gouges, spindle gouges? Is the 180 grit sufficient or would the 240 grit be a better set up for a better edge.

Would appreciate if forum members could advise.

[issatree]

Hi Nev,
I'm not sure if CBN Wheels are meant to go on slow grinders.
Can't really see why not, but you will surly get some answers.
If so I wood think 80 & a 240 wood be the go.

[Evanism]

Ive an 80 and 180 on a normal grinder. Id loooooovvveeee the 240 on a slow grinder.

[Nubsnstubs]

You can use CBN on either slow or fast speed. Even though the ads say that heat doesn't build up with CBN, I've found that it does and will also turn your tool end blue using the faster grinder. It's also said they are balanced from the factory, but with the higher speed, it takes more effort to get them to run true. I switched from high speed to low speed as I already had both grinders.

I have 80 for shaping, and 180 for sharpening using a slow speed grinder. Both are 8" being used on a 6" Baldor 1/4 HP grinder. It works for me, and maybe I will get a 220 grit later for grins. .............. Jerry (in Tucson

[Woodturnerjosh]

Absolutely get the CBN! I have them on my Vicmarc slow speed grinder and love them!
They only had the 80 and 180 when I bought mine and I use the 80 for scrapers and rough shaping other gouges and the 180 for sharpening skews, roughing gouge, bowl and spindle gouges, parting tools, everything else. If I was buying again I would get the 80 and 240.

The reason CBN is advertised as having less heat build up is they have a high thermal conductivity compared to aluminium oxide. When you are grinding with Al-Ox the piece you are grinding (tool, chisel, whatever) absorbs about %90 of the generated heat (this is approximate and a little bit more involved but in rough terms a suitable example) whereas a CBN wheel will absorb about %50 of the generated heat. This means the piece you are grinding can still heat but takes significantly longer when compared to Al-Ox

Josh

[BobL]

According to Norton Abrasives the optimum surface grinding speed for CBN is between 15 and 30 m/s

This translates to between 1900 and 3800 RPM for a 6" wheel and 1400 and 2800 rpm for an 8" wheel.
This is not the max speed the wheel can safely be run at but where optimum (coolest/faster) grinding takes place.
Faster grinding means higher temperatures so water cooling can be used where CBN wheels can be run much faster (4800 rpm for the 8" and 6400 rpm for the 6")

I have my 8" 80g CBN wheel on a variable speed 1HP 3P grinder and it will run from near zero to 3600 rpm
The main things I use it for is for touching up MW lathe HSS cutting tips and for that I use ~3000 rpm.

Slower rpm around 1400 are very useful for sharpening small things like small drill bits

[Yanis]

Absolutely get the CBN! I have them on my Vicmarc slow speed grinder and love them!
They only had the 80 and 180 when I bought mine and I use the 80 for scrapers and rough shaping other gouges and the 180 for sharpening skews, roughing gouge, bowl and spindle gouges, parting tools, everything else. If I was buying again I would get the 80 and 240.

The reason CBN is advertised as having less heat build up is they have a high thermal conductivity compared to aluminium oxide. When you are grinding with Al-Ox the piece you are grinding (tool, chisel, whatever) absorbs about %90 of the generated heat (this is approximate and a little bit more involved but in rough terms a suitable example) whereas a CBN wheel will absorb about %50 of the generated heat. This means the piece you are grinding can still heat but takes significantly longer when compared to Al-Ox

Josh

There is also the wear vs cut issue. The crystals of aluminium oxide degrade over time such that the edges round over causing it to rub as opposed to cut which produces more heat. It also depends on the grade of aluminium oxide. White will run cooler since it breaks more easily (it is more pure) and reveals sharp edges. This ability to break is called friability.

On the other hand CBN is much much harder, and does not get blunt like the aluminium oxide crystals so will cut much cooler thus not producing as much heat in the first place.

All very good reasons to go CBN.

John

[derekcohen]

Have recently purchased a Vicmarc Bench Slow Speed Grinder, for my turning tools. I would like to upgrade to CBN Grinding Wheels. However I see the grit sizes are 80, 180 and 240. Should I use the 80 grit for the sharpening of skew chisels, parting tools and roughing gouges. What should I use to sharpen bowl gouges, spindle gouges? Is the 180 grit sufficient or would the 240 grit be a better set up for a better edge.

Would appreciate if forum members could advise.

The 80 grit is only for fast re-shaping a profile. You could use a 180 grit for this on HSS. I would get this and a 350 grit for sharpening (or, if you cannot get one this fine - I've seen them on sale in the USA - get the finest available - 240 grit?).

Regards from Perth

Derek

[Woodturnerjosh]

If you're using scrapers you'll find the 180 won't give you much of a burr. The 80 is still the best for this application...

[Pat]

I run 80# & 180# high speed dry wheels and 240# Tormek wet wheel. Normally the 80# is for shaping, leave on the ways of the lathe for 10 minutes and then put the final edge on the torment. I will not be returning to alox wheels.

[Allan at Wallan]

So there you go Nev, look at all that information that you
gained free of charge. This is why being in the Forum is a bonus.

Now I look forward to seeing your chisels at the club when we resume
in January to see if they are "razor" sharp.

Allan

[brendan stemp]

All good advice. I have 2 180 grit CBNs on my Vicmarc 8" slow grinder and like the setup a lot. There are 2 things I would add. 1/ It is very hard to sharpen freehand on CBN wheels so the use of jigs for bowl gouges and platforms for your skews and scrapers is almost essential. 2/ I think the longevity of the CBNs has not quite been proven. If they cost 4 times that of Al Ox. do they last at least 4 times? Is there anyone out there that can answer this question? I have noticed the wear on my widest CBN and wonder how much life it has left.

[Christos]

....... I have noticed the wear on my widest CBN and wonder how much life it has left.

There are a few questions that come to my mind straight away that I believe will give some guild lines. When did you install it? How much sharpening you do daily? I think most of us would be aware that you are not a hobby wood turner but it does give a reference for the rest of us.

The third question and I don't think this can be answered unless actual measurements were taken at the installation. How much wear on the wheels?

[NeilS]

If they cost 4 times that of Al Ox. do they last at least 4 times? Is there anyone out there that can answer this question? I have noticed the wear on my widest CBN and wonder how much life it has left.

As indicated in this post (http://www.woodworkforums.com/f8/cbn-grinding-wheels-159869#post1560180) back in Oct 2012, I wrote the thickness of the grinding matrix on the side of my diamond and CBN wheels when I first installed them in April 2011. At the 18 month mark I couldn't detect any wear, but it is a while since then so must do another reading, although I haven't been in the workshop as consistently over the last three years as I was in that first 18 months of use.

Robo Hippy (aka Reed Gray) has been using these wheels as long as anyone I know of, so it would be good to get a comment from him as he is also a production turner.

[Nifty Nev]

Thanks every one for your input. No just a simple matter. But a 80 and 240 grit CBN wheel have been ordered.

[NeilS]

As indicated in this post (http://www.woodworkforums.com/f8/cbn-grinding-wheels-159869#post1560180) back in Oct 2012, I wrote the thickness of the grinding matrix on the side of my diamond and CBN wheels when I first installed them in April 2011. At the 18 month mark I couldn't detect any wear, but it is a while since then so must do another reading, although I haven't been in the workshop as consistently over the last three years as I was in that first 18 months of use.


I managed to get around to taking another measurement of the thickness of my diamond wheel (WoodRiver) and found that the thickness of the outer 'rim' had hardly changed from the 6.95mm as measured back in April 2011. The latest reading was 6.9mm.


368217

However, I did find when I put a straight edge across the face that it was slightly dished from sharpening just gouges on it (you can see some back-lighting peeping through in the next photo). By my measurement it had dished by 0.06mm. As I'm not sharpening any straight edges on it I won't be trying to correct that, but may try to use more of the shoulders in future.



368222 368223

I hadn't marked up my CBN wheel with a base measurement. As the one I have (from D-way) has a micro-layer of CBN abrasive that has been electroplated to the body of the wheel I will need a micrometer to take any useful measure. I do have an old imperial micrometer somewhere (measures in thousands of an inch), which I will try to find to take a reading, but my intuition is that the CBN layer is so thin that it will be difficult to get a useful set of readings that indicate rate of wear.

Stay sharp!

[Christos]

Thanks for the update. May have to take more measurements in the future so that more can be determined as to how it wears.

[BobL]

but my intuition is that the CBN layer is so thin that it will be difficult to get a useful set of readings that indicate rate of wear.

Yep it will be hard to measure.
For a more representative measurement I'd be looking to measure it multiple locations around the rim rather than in just one location because the one location might be the place that got a small ding in one way or another

80 grit means the particle sizes are on average 200 microns so a loss of 0.06 mm (60 microns) means it has on average not even worn through the first layer of CBN.
Not that that I expect that there will be many layers bonded to the wheel. I expect that there will be only a few layers.

I would expect that like any grinding surface that the CBN wheel would lose its high spots and any less bonded grit quickly and when these are gone that it will take much longer to wear away. What you have probably measured is just the loss of the high spots.

However these things will not last indefinitely especially if that are maltreated.