CBN sharpening wheels

Hello all.
My first post.
As well as a woodworker, I also like to do wood turning.
I have recently bought a slow (1440 RPM) speed 8" grinder for sharpening my HSS tools with two white wheels; the finest being 120 grit.
I am disappointed at the finish this puts on my gouges, chisels etc, and have started looking at CBN wheels.
The choices appear to be very poor in Aust, unless I am not looking in the right place!
Ideally, I would like to buy a combination wheel, with the grits 180/600 on the same wheel
These are available in the USA for USD$200, (with aluminium wheels) but postage is a killer! Can anyone please suggest where I might find this
type of wheel here in Australia, or at least one that has 600 - 1000 grit?

Firstly welcome.

Then a couple of questions.
Why are you sharpening HSS with white wheels at such a low speed?
Why do you want to use such a fine grit CBN ?

Grinding at slower speed reduces the heat generated, thus also reducing the chance of overheating the tool being ground.
From my research, 180 grit CBN is a good compromise between time taken to grind, and the amount of material removed during the grinding process; finishing with around 600 - 1000 grit
puts a mirror - like finish on the ground edge, which translated to a very smooth cut.

Quote:

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Originally Posted by BobL

Firstly welcome.

Then a couple of questions.
Why are you sharpening HSS with white wheels at such a low speed?
Why do you want to use such a fine grit CBN ?

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:whs:

If you bump up the speed of the white wheel to that of a normal bench grinder, you'll get a much finer finish that is usable "as is" or just needs a light hone on a stone if you really want to.

There is practically zero risk of overheating HSS, it was invented specifically to retain its properties even when red hot while cutting steel (it'll hold 60 Rockwell up to 600 degrees C with no issues). Careful grinding will not blue the edge, but, even if it does, the damage is cosmetic only and will not affect its performance.

What they said. The temper problem is for other tool steels, like hardened carbon steel. There is a lot of misinformation on the web. Remember anyone can put up a youtube video or a web page and claim to be an ex-spirt....

As an aside I saw a thingy the other day where he made discs of mdf mounted them on a bench grinder and used polishing soap on the side to do the later grinding, from 850 up. I don't know if it works but I'm thinking of trying it and seeing...

Thanks for the responses by EJ and Damian - they are exactly what I was driving at

HSS takes many hours to soften at high temperature - a typical annealing profile is
Heat to 920ºC, then cool in a furnace at 20° C per hour to 330°C. Air cool.

It's temper [profile is unusual because temps between 500 and 600ºC will not change its hardness, between 450 and 350 ºC it will soften by a whopping 5% and then temps below 300 will reharden it back to original hardness again. But the temps have to be applied for many many minutes to affect these hardness changes. There is no way this can happen when resharpening or even reshaping on a grinding wheel.

A 240 grit CBN wheel running at full speed (3000RPM) should be sufficient to provide a finish suitable for turning. If you want something smooth than this I suggest a belt sander with a fine grit belt rather than a wheel.

Bob, we all know you already know the answer when you ask a technical question, you're just checking to see if the person you're asking also knows the answer :p:2tsup:

I don't have CBN , Wouldn't mind one or two . Ive been looking up some info on them lately.
What I would like is a cheap way of getting slower speeds for my grinders so I could run white wheels slow for the great control of slow grinding . The 2850 speed going like mad take to much off to quick . Keeping straight wings on the 40/40 grind is tricky at high speed
Ive been going from White wheels to linisher with fine worn paper for my turning chisels. Slower linisher would help as well .

CBN sounds good because of it takes so little off . Maybe that's equal to the slow speeds I like ?

I do slow speed grind HSS turning tools with a hand grinder. The old crank things. Its good, works well .
But two hands on the tool would be nice, rather than one on the tool and the other cranking .

bout 5 years ago I gave away/sold 4 of my 5 single phase (SP) grinders and all associated wheels. Son, friend, Menshed and Turning club. The one SP grinder I kept has a Scotchbrite wheel on one spindle and a wire wheel on the other.

I replaced the 4 SP grinders with three, 3 phase (3P) grinders. A HD 10" cast iron pedestal behemoth running on its own VFD while the other two 8" grinders run (alternately) from a single VFD.
I gave the large 10" grinder to my motorcycle restored bro last weekend as I wasn't using it much and it took up a lot of room. The remaining 2 x 3P and the SP are on a turntable.
Shown in the photo are the two remaining 3P grinders - they can be rotated into position via the turntable and on their own axis for ease of access.

C is a home made think kerf cutting wheel saw
D is a green wheel that alternates between it and a white wheel but will soon be reached by an other CBN.
E is an 8" CBN and a 6" diamond wheel (on the side)
F is a Multitool Linisher.
B and A are the lock out electronics that enables the VFD to operate either the gold or green grinder.
Attachment 457993

The VFD works brilliantly on these - I run the linisher and CBN between 2400 and 3200 RPM, The thin kerf wheel is on a step up pulley and runs at up to 11,000 rpm - the green or white wheel doesn't go above about 3200 rpm. My next step when I get some funds is to replace the green/white wheels with another CBN.

The grinders are smooth but if there is a hint of vibe I can tweak the VFD to a slightly different speed and reduce the vibe significantly.
I consider this setup as one of the best mods in my shed

looks nice Bob .
What do you cut with the thin blade ?
If you run a grinder on a VFD does it loose power if you run it slow like 1/2 or 1/4 normal speed ?

I bought one of those old ozito slow waterwheels the other day for $20 on gumtree not because I'm concerned about heat but because I want to use the side of the 40 grit 8"x40mm wheel. At 120 rpm if it does come apart it isn't going to kill anyone...

Note I typed 8"x40mm deliberately because I know it will make some heads explode .. :D

Quote:

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Originally Posted by damian

I bought one of those old ozito slow waterwheels the other day for $20 on gumtree not because I'm concerned about heat but because I want to use the side of the 40 grit 8"x40mm wheel. At 120 rpm if it does come apart it isn't going to kill anyone...

Note I typed 8"x40mm deliberately because I know it will make some heads explode .. :D

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Many white wheels on tool and cutter grinder setups have tools applied on the sides of the wheel so you should be able to run a 40 mm wide wheel at 2800 rpm and easily safely grind lightly on the side. Even a light grind @2800 rpm will easily out grind a heavy grind @120 rpm, wear away less of the wheel, and give a much better finish.

I'm surprised to see you type that given your previous stance on workshop safety.

Yep I've been using the sides of wheels for decades, but it's always warned against because if they do come apart it's messy.

Mind I'm also the person who has never fitted the blade guard to his table saw and almost never uses a riving knife or splitter... :D

For $20 the ozito will be an amusing experiment..

Has anyone tried the soap on mdf thing ?

Quote:

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Originally Posted by damian

I'm surprised to see you type that given your previous stance on workshop safety.

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Yep but note I did write, "and easily safely grind lightly on the side".

Like I said, tool and cutter white wheels have been ground on "lightly" on their sides for decades.
Tool and cutter grinder users are not heaving great lumps of metals onto the sides of these wheels but lightly applying the bits being ground.

Because of the behemoth OH guard on my TS I also don't have a riving knife or splitter on my TS although I do advocate other people use them :D