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.

Nice.... I have a power grinder, but ordered one of these stones from Stu this week, the price was right, and was ordering other stones in any case. Not sure I'll use it to regrind a bevel, but a heavy touch-up or two maybe!

I have also ordered one of these stone just recently with the intention of keeping my primary bevels on things in order. I have a grinder, but I don't have any jigs nor do I enjoy its noise levels and heat. It is on its last legs anyway. I've been following your experience with the stone and will be intrigued to see how it performs for me. Cheers for the summarising review.

As a note I was experimenting flattening a SiC oilstone using a basalt rich paving block and river sand (not beach sand which is mainly seashells and limestone). Considering these little stones are normally quite hard, it chewed through pretty well as long as I kept adding fresh sand. A quick scrub with a stiff bristle brush and it was ready to go without any real signs of clogging from the sand. Might be an even cheaper alternative if someone has an old oilstone and wants to flatten things quickish but doesn't want to lash out the few dollars for the performance stone.

Tom.

Thanks Eddie.

That's a v. informative and useful rundown.

You can get a cambered roller for your jig to help with crowning a scrub blade.

As for the pros and cons re a grinder, for anyone regularly rehabbing or reshaping blades it's a lay down misere. There are wheels that dramatically reduce the risk of bluing and are fast at doing what you need to do. Yes, the cost is a consideration ... when you get sick of hand work.

Flattening the backs is a whole new ball game!
.

Actually, rehabbing chisels hasn't been that bad.

It takes about five-ten minutes to regrind a good bevel, fifteen - twenty for a REALLY buggered one (chips etc). The trick is to make sure you dress the stone frequently - and as I mentioned, it only takes a few moments once you know what you're doing.

Since I've been picking up about 1 chisel a week - all the Titans I can find at markets or eBay - that's not so bad.

But I agree. If I had more than 10 or so to do, I'd just buy a flaming grinder.

I should add another thing though - today I discovered that it's fantastic for flattening chisel backs. Stays nice and flat, and much less stuffing around than sandpaper and glass (as well as cutting more quickly).

I'm gonna see if I can persuade Stu to list, or send me, the slightly larger version for my jointer irons. A slightly larger size would probably speed the grinding up a bit, too.

Yeah, back flattening I start with my #120 Shapton if there's pitting (but I only do 25-30mm).

If I can source coarser 3M abrasive film, what works really well is to whack some of that on Woodpecker honing plates. They're long and v. flat. Have only been able to find 40 micron at the coarsest and one strip of that will last a modest flattening exercise.

How wide are your jointer blades?

I received my stone of the same variety in the mail just recently and have had a bit of a play to get a feel for this seemingly menacing rock.

Out of the pack it does indeed feel like a small block of fine cement. The first task I put to it was to flatten the back of a 5/8" Swedish made HCS vintage chisel. Maker unknown; it has the little crown logo stamped on the front and a name on the back I could not read. I soaked the stone and readied my squirty bottle with water. Initially the stone felt as though I would be lucky to have a chisel left after the first stroke. It certainly chomped into it for those first few swipes but the brittle tips on the SiC grains soon broke off and the cutting settled down. I was careful to direct a forceful squirt of water onto the surface after every dozen strokes or so to avoid glazing as much as possible. It is easy to see the small fragments of steel that have parted company with your blade being flushed away with the water which lets you know it is still cutting readily.

I didn't see the miracle steel removal rates while flattening the back. Sure it was quick, very quick, but I'm not sure the surface can be kept fresh enough all the time to remove metal from such large surface areas at the initial devouring rate. Doing so also requires a lot more pressure on the chisel, which will distort it somewhat and partially undo all the hard work you have done so far. Taking a step back and accepting the moderate but still impressive abrasion rate while applying a bit less pressure and a good flushing every dozen strokes seems to work the best for me. The stone did not dish any appreciable amount however I tried my best to work the stone evenly and settle the whole of the cutting face down to its normal level. I am yet to notice any real glazing.

Once I honed the back on my 800 and then 6k King stones, I tried grinding the primary bevel on the other side of the stone. Admittedly, the bevel was a mess, and to avoid excessive work, I used the bench grinder at work to take the majority of the no longer required steel away. Having said that though, once I started using the stone on this bevel I was impressed, on this small area the thing ate the steel away, and just kept eating it until I had a lovely flat primary bevel to work from. I removed about 0.5mm of steel in just a few minutes. The difference in surface area makes a huge difference. This was the main reason I bought the stone; to keep primary bevels in order without going to the grinder. My A2 blade from my LN LA block plane has been time consuming to maintain for this reason and now I have the solution.
After the bevel was formed I checked the flatness and noticed some appreciable dishing in the stone.I suspect that what I have taken off is the initial friable particle layer of the stone and failed to work the stone evenly, which is a lot easier to muck up when shaping a small bevel area with a honing guide to wear away the stone. Once the cutting surface has settled I expect this dishing to subside a bit.

I have not tried to flatten the stone yet. I haven't quite had time as I have been using it at work amongst a wing rebuild. I don't think this is anything to worry about. Stu's enclosed care sheet for the stone is very helpful in this area.

Overall I am very happy with the stone. I will soon learn to recognise it's feel and use it to my advantage. I can imagine this stone best used on a mount in a sink with a gentle stream of water constantly flowing over it. Possibly a circulating water tub system might be the next thing for the waterstone fanatics to try? :D
I understand this might sound a bit jumbled but they are my initial thoughts. It's been a long day and my English dejumblerer may not be functioning correctly.


Cheers, Tom.

Very similar to my experience.

With a small amount of contact with the stone, it cuts "magically" quickly.

When that increases, it slows down significantly.

Logical I suppose.

I imagine it would struggle with the modern, thick cryogenically treated blade. But I might be wrong, please let me know your experience.

Also, if it's got to the point where it's dished, I recommend you deglaze it more often. It slows down in its effectiveness very quickly if not deglazed very regularly (as regularly as you would flatten a traditional waterstone, IMO). I deglaze it every ten minutes or so, with heavy use.

I agree that it's not that great for flattening blade backs. I did that for a while, but I found course grit sandpaper much faster. Still much better than regular waterstones though.

A bit of an update.

Now that I've finally got all of my tools in to shape, the stone is really coming into its own.

While rehabbing a large number of very old, very blunt / chipped / warped blades with the Sigma is a pretty monumental task, it can certainly be done.

But the real advantage becomes apparent once your blade is already in order. As soon as you notice your microbevel is starting to get a bit wide, just stick the blade back onto the Sigma for thirty seconds and it'll recede back to the edge.

Very little fuss. Just involves changing the little quick-microbevel-adjuster-knob thing on the Veritas MK II (did I mention how handy these are?), rubbing the blade on the stone a bit, and you're good to go.

Or you could buy a decent, well-tuned grinder for $300, and a bench to put it on, and jigs and diamond dressers and wheels...

Did I mention this stone costs $30?

Now here's a curious thing.

I just received another of these stones from Stu, in a slightly larger size.

Initially, the surface is noticeably rougher to the touch than my existing two otherwise identical stones. And it cuts FAR faster. I mean, MUCH faster. I wasn't expecting any difference from my existing stones but I immediately noticed that something curious was happening here.

Now if I could get the stone cutting like this all the time, it really would replace a grinder.

However, once this initial "crust" is removed by flattening with 36 mesh SiC grit on glass, with a little water, the surface becomes smoother. The stone still cuts, of course, and faster than any other stone I've used. But it's certainly not as impressive as the stone in its unaltered state.

Has anyone else had this experience? Might there be some other flattening technique or method I could use to maintain this type of surface?

And yes I know this is becoming an unhealthy obsession... :)