Review: The 120 Grit Sigma Power Stone - The Black Beast
You might have been following my trials and tribulations with this particular stone, largely broadcast on this forum. In particular, I decided that I didn't want to buy a bench grinder to sharpen woodworking tools. I was advised that there's only one real alternative - the Sigma Power Stone. What follows is, more or less, my review of said stone, after about a month of solid use.
I suppose the question I should set out to answer is: can this stone really be used as an alternative to a bench grinder?
The answer is... sort of.
It doesn't cut as quickly as a bench grinder. That's probably not surprising. It takes, by my estimation, about three times as long to regrind a bevel (assuming you've got a secondary bevel of about 1-2mm you need to remove) than a high-speed grinder does.
The stone that's currently available on toolsfromjapan isn't wide enough for use on jointer irons (though I know that a slightly wider version will be made available soon).
Plus, I still haven't figured out how to use it to grind a scrub radius.
In addition to that, there's a fair bit of fussing around to get it to work as it should. Because it's so hard - it's made out of silicon carbide fused together in a porous block, without the binder traditional waterstones employ - it stays flat for a very, very long time. However, while the pores on a traditional clay-based stone actually disintegrate as the stone wears, these stay resolutely present. As a result, they quickly clog up with metal and abrasive, resulting in a very flat burnished surface that doesn't cut very well.
The solution to this problem is to sprinkle some #36 silicon carbide grit on the surface of the stone, with a little water, then grind it against a sheet of glass. This declogs the pores and flattens the stone somewhat (not that it really needs it). It's just like flattening a regular waterstone, but more awful (you'll see what I mean). The grit comes with the stone, or can be found cheaply from an online lapidiary supplier.
After a little bit of practice, the deglazing procedure only takes a few moments. And while it does need to be done almost as often as the flattening of a course waterstone, the payoff is that it won't bugger your blades by going out of flat. It also cuts much, much faster. Finally, the hardness allows you to apply a lot of pressure, and sprinkle #120 mesh silicon carbide to speed things up even further. While this is inadvisable if, like me, you're using a Mk II honing guide with a relatively soft brass roller, it works a treat if you're using an older Eclipse style guide or are brave enough to go freehand.
So, in short, it's a vast improvement over flat grinding with a traditional waterstone (and I've spent plenty of time doing both).
But is it really an improvement over a grinder?
In a lot of ways, yes. First, this stone costs about $30, including postage, from toolsfromjapan.com (a website which, incidentally, I wholeheartedly recommend). A bench grinder costs, at the absolute bottom end of the range, $100 for a six inch with an alox white wheel appropriate for tool steel. If you want to buy a decent, well balanced, half decent model, you can easily spend $300 or more. You also need to buy tools to dress the wheels, and need to make or buy your own jigs and tool rests. A bench grinder also needs a bench of its own, to stop it rattling about - so you need quite a bit of extra space for one. And, if you're a newcomer like me, you can easily blue a blade (not fun) and ruin its temper. Finally, grinders make a terrible amount of noise, smell and mess.
A $30, 8" rock that makes no noise or mess other than the occasional splash doesn't seem so bad.
It's worth noting, though, that you'll really need a honing guide of some sort. I don't think that even the most experienced woodworkers would feel comfortable applying this sort of pressure freehand. I purchased the Mk II for $90 for this reason, and can't recommend it highly enough (though it, too, has its quirks). It's such a useful product that I don't really consider it part of the "stone versus grinder" calculation - you probably want one anyway!
So in summary...
There's no denying that it's not efficient enough for use in a commercial setting. And if you really like power tools, I can't imagine you'll want anything to do with it.
You've really got to be a bit of a masochist. But if you're a darksider, then chances are you're that anyway. So if you're like me - a beginner to woodworking and sharpening, with more patience than money and not much space - then I can't recommend it enough.