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Thread: Grinding stones for air grinder - do the different colours mean anything?

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    Question Grinding stones for air grinder - do the different colours mean anything?

    In regards to grinding stones for an air grinder, do the different colours mean anything e.g. more course or fine etc? Or is it just a case of pick the shape that suits the job at hand and the colour is the colour?


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    Quote Originally Posted by twr7cx View Post
    In regards to grinding stones for an air grinder, do the different colours mean anything e.g. more course or fine etc? Or is it just a case of pick the shape that suits the job at hand and the colour is the colour?
    On bench grinders dark grey is often silicon carbide. Pure white an aluminium oxide. Grey with white flex a mix. Different colours for different metals, alloys, or carbides.

    Not sure what the colours represent in air grinders - all I can say is I rarely use the airstones, preferring the tungsten carbide bur die grinders for steels and aluminium - fast material removal, long lasting - maybe stones for more sensitive rates of removal?

    Die Grinder Bits | eBay

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    For grinding or cutting wheels, there is a spec printed on each disc that will give info on what makes up the disc, and where its strengths may lie (ie large agressive grit, and bond strength holding it all together)



    For vitrified mounted points like you've pictured, there is a standard, but for the sort of stones you get with a dremel or other tool, it's probably pot luck.
    Color may signify various grits for some manufacturers, but your mileage may vary. Is there any spec written on it? To be blunt, if it was for a job that mattered,you wouldn't be using the cheap ones that came in the box in any case.

    If you were to buy some of those 'vitrified mounted points', there are a few options available from St Gobain (Flexovit), and they have a classification system to make a bit more sense of it all.


    Using the convention in the top pic, their spec shows AlOx grit, various grits (36, 46, 60) and PV signifying Pointed vitrified.

    If you have the money and the inclination, get some carbide bits and watch the metal chips fly.
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    I've just got a bit of aluminium that i want to clean up (it's the replacement for the EGR piece prior to the inlet manifold on my TD5 - it's a 60mm pipe but the opening at the mounting flange was only 56mm so I wanted to open it up by removing the lip). I used a metal lathe to take out the bulk of the material but it wouldn't mount dead straight so I stopped short to finish it by hand rather and thought the air grinder might be the easiest for the job. It just needs less than a mm removed from 1/2 of it now. The flange is about 15mm thick.

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    I have no idea about the stone colours But it looks like a very handy bit of kit, Nice
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    Quote Originally Posted by twr7cx View Post
    I've just got a bit of aluminium that i want to clean up (it's the replacement for the EGR piece prior to the inlet manifold on my TD5 - it's a 60mm pipe but the opening at the mounting flange was only 56mm so I wanted to open it up by removing the lip). I used a metal lathe to take out the bulk of the material but it wouldn't mount dead straight so I stopped short to finish it by hand rather and thought the air grinder might be the easiest for the job. It just needs less than a mm removed from 1/2 of it now. The flange is about 15mm thick.
    Mounting in a four jaw or face plate on the lathe might give you the centering needed. or onto a mill with an indexable cutter.

    I find pure aluminium tends to clog the grindstone face, in the worst case running the grindstone on some brick material to freshen the grindstone. Zinc alloy grinds more like steel. One of your coarse stones should do for a small one off job. Have a look at the die grinders though - a good set in the tool shops are around $100 for five shapes - a well known hardware chain sells similar sets for twice the price - even cheaper on the net. I think I would do bulk removal with the die grinder and then refine the surface with one of your stones.

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    Quote Originally Posted by workingonit View Post
    On bench grinders dark grey is often silicon carbide. Pure white an aluminium oxide. Grey with white flex a mix. Different colours for different metals, alloys, or carbides.

    Not sure what the colours represent in air grinders - all I can say is I rarely use the airstones, preferring the tungsten carbide bur die grinders for steels and aluminium - fast material removal, long lasting - maybe stones for more sensitive rates of removal?

    Die Grinder Bits | eBay
    Silicon carbide wheels are always green in my experience. They are used for grinding tungsten carbide cutting tools such as lathe tools. Use tungsten carbide rotary burrs for metal removal and those stones for final finishing.

    If buying rotary burrs make sure you get the solid carbide ones. Cheapies are commonly a TC cutter silver soldered or brazed to a steel shank. If you use them hard then the solder can soften and the cutter head falls off.
    URSUSMAJOR

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    Quote Originally Posted by Bigbjorn View Post
    Silicon carbide wheels are always green in my experience. They are used for grinding tungsten carbide cutting tools such as lathe tools. Use tungsten carbide rotary burrs for metal removal and those stones for final finishing.

    If buying rotary burrs make sure you get the solid carbide ones. Cheapies are commonly a TC cutter silver soldered or brazed to a steel shank. If you use them hard then the solder can soften and the cutter head falls off.
    Agree with the comment about getting solids. And I was surprised at how long they last. I expected all the teeth to be ripped off after an hour of work, but after about 15 hours accumulated work still good as new. In fact they're outlasting the cheap air tools!

    Wiki has carbide as yellow to green to bluish black. One of my fine carbide wheels has a light green tint. The coarse ones are more black than anything. My loose lapidary grit is grey black. So neither of us has seen yellow?

    I've one bench grinder reserved for tool sharpening, writing on the machine to remind me - silicon side for tungsten carbide, and the aluminium side for HSS and Crobalt.

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    Quote Originally Posted by workingonit View Post
    Agree with the comment about getting solids. And I was surprised at how long they last. I expected all the teeth to be ripped off after an hour of work, but after about 15 hours accumulated work still good as new. In fact they're outlasting the cheap air tools!

    Wiki has carbide as yellow to green to bluish black. One of my fine carbide wheels has a light green tint. The coarse ones are more black than anything. My loose lapidary grit is grey black. So neither of us has seen yellow?

    I've one bench grinder reserved for tool sharpening, writing on the machine to remind me - silicon side for tungsten carbide, and the aluminium side for HSS and Crobalt.
    I once upon a time was a fitter and turner-machinist first class. I have trade papers and advanced trade certificate saying so. In my machine shop days I never saw a silicon carbide wheel in anything but green. In fact, they were always called "green wheels", "use the green wheel for that". We had, and I still do, grinding wheels in grey, white and green.
    URSUSMAJOR

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    Quote Originally Posted by Bigbjorn View Post
    I once upon a time was a fitter and turner-machinist first class. I have trade papers and advanced trade certificate saying so. In my machine shop days I never saw a silicon carbide wheel in anything but green. In fact, they were always called "green wheels", "use the green wheel for that". We had, and I still do, grinding wheels in grey, white and green.
    Afraid I'm only Wiki qualified. Green and yellow are the purist, being formed closest to the heat source. I can only guess that would mean sharper grains and better grinding characteristics for tougher tooling.

    I'm guessing you're retired - do you keep up your skills in any hobby way? When I was 50'ish I asked the TAFE if I could do a machining trade course to better understand my machines and methods and was told only if I had an apprenticeship...

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