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Posted by Dave Baker on May 26, 2008, 5:35 pm
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>
>> Peter Neill wrote:
>
>>> As for wheel grit types, there is as much debate here as the HSS vs
>>> Carbide debate. In general, a harder steel needs a softer wheel, and
>>> vice versa.
>>
>> Why is this? I'm curious. Is it because lost grit gets embedded in a
>> softer workpiece and then wears away the grinding wheel?
>
> It's because ...........
Actually although grinding wheel choice can seem very much like a black art
there are some very similar analogies to other types of machining once you
understand the processes involved. A good example is picking a milling
cutter to suit the material being machined.
1) Material hardness. The harder this is the tougher the cutter needs to be.
In grinding terms the grit hardness of the common abrasives in ascending
order are Aluminium Oxide, Silicon Carbide, CBN (cubic boron nitride),
Diamond.
2) Tool life. Again directly related to material hardness, the tougher this
is the more often you're going to need to sharpen or replace the tooling.
Tough material = frequent sharpening = soft bond for a wheel that breaks
down quickly and exposes new grit edges.
3) Material that clogs the cutter such as aluminium. You want a milling
cutter with widely spaced teeth such as a two flute instead of multi flute
and in grinding you want a wheel with widely spaced grains of abrasive and
more porosity. Coolant/lubricant also make a similar improvement in both
cases to wash away the swarf and keep the tool clean.
4) Desired surface finish. The better this needs to be the finer the feed
and similarly the finer the grit. Again coolant always helps.
5) Stock removal. For high stock removal use deeper cuts and frequent
sharpening. In grinding use coarser grit and softer bonds.
In a way it's really quite straightforward if you think of a grinding wheel
as an abrasive cutting tool that's not so much different to a lathe tool or
a milling cutter.
--
Dave Baker
Puma Race Engines
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