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Odd thought just went by:

There are sandpaper-based sharpening systems ("scary sharp" and it's cousins), generally using fine wet/dry sandpaper on a flat surface like plate glass.

There are motorized versions of this, typically spinning a disk of sandpaper mounted on a similar flat plate. And I know narrow-belt sanders are sometimes used by knife makers.

It just occurred to me to wonder whether an appropriately mounted/jigged sander I already have on hand -- belt or disk -- could be (ab)used for this purpose.

So: Outside of the usual problems of holding the blade reliably at appropriate angle and not overheating and the like, can anyone suggest reasons this wouldn't work? I suspect buying or building a proper slow-speed grindstone would be sufficiently better to justify its cost, but I could see this possibly being useful on a worksite....

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    There are belt-sander sharpening systems specifically marketed to woodworkers in case you haven't seen any, so for sure you can sharpen on a belt sander. For things other than turning tools though I think 'sharpen' might be overstating the effectiveness in some cases, when it comes to the quality of edges we often need. So they'd be more about bevel shaping/edge formation, not getting tools fully ready to use — more a substitute for grinding than a standalone sharpening system. But in both cases you can fit something that can strop an edge to razor sharpness (e.g. paper wheel or leather belt).
    – Graphus
    Commented Dec 4, 2016 at 23:33
  • @Graphus The only caveat would be solutions that don't use typical abrasives on paper with adhesive. Trizact by 3M comes to mind. However, concerns about flatness are still valid. Great for knives, curved blades and the like.
    – BrownRedHawk
    Commented Dec 6, 2016 at 12:44
  • @BrownRedHawk You can make or fit fabric or leather belts that you can load with compound and strop to get a superb edge, don't think it's feasible on a chisel though. A typical plane iron just about since you can stroke along the edge as you would with a knife and the corners aren't critical like on a chisel. A very tiny back bevel that'll accidentally be formed doing on that side isn't a killer on a bevel-down plane but could prove an issue on bevel-up planes. Obvious solution then is to finish the back by hand... but then why not do the bevel side by hand while you're at it? :-)
    – Graphus
    Commented Dec 6, 2016 at 14:03

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For flat blades (chisel, plane) you're going to struggle getting to a fine finish. Most sand-paper sharpening systems have you placing the paper on glass or granite, so as to have a dead-flat reference plate. The metal plate inside a belt-sander is not likely to be flat enough for sharpening. The flexible pad in a random-orbit sander will certainly not be flat or rigid enough to give you a flat edge.

That said, a belt sander is great for rough-shaping or re-shaping a blade, say if you need to get past a big nick in the edge. It is only effective for the roughest work though, after which you'll need another more precise sharpening method.

Turning tools and knife blades have their own set of rules and requirements, which I am not qualified to answer.

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  • Good point. For what it's worth, granite "surface plates" flattened to within a thousandth of an inch are surprisingly affordable... if you can avoid paying shipping fees, anyway.
    – keshlam
    Commented Dec 6, 2016 at 5:36
  • @keshlam That kind of flatness is actually surprisingly un-needed for a sharpening/honing surface. You can plane a piece of wood with a no. 4 and just check it for flat with the edge of the plane and that's flat enough for 99% of sharpening or woodworking tools.
    – Graphus
    Commented Dec 6, 2016 at 14:08
  • Also true. My point was just that if anyone does want a rigid, machinist-flat reference surface, it isn't expensive. For most things the top of the tablesaw or a sheet of float glass are more than flat enough. As the old joke goes, "Measure with a micrometer, mark with a crayon, cut with an axe..."
    – keshlam
    Commented Dec 6, 2016 at 20:26
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    Given that the same size Starret granite block with .0001” tolerance is near $200, I would seriously doubt the $35 plate is that accurate. Now, that said, woodworking doesn't need anywhere near that type of accuracy, especially if you're using sandpaper, which has far more variance to its own thickness than even a cheap slab of granite. A cutoff from the local cabinet shop will be more than flat enough for any sharpening, and I'm sure you'll be happy with your purchase.
    – ench
    Commented Dec 16, 2016 at 4:54
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    @keshlam - "Measure with a micrometer, mark with a crayon, cut with an axe..." you've met my father-in-law??
    – FreeMan
    Commented Dec 16, 2016 at 15:04
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This is a bad answer by Stack Exchange standards, but since I just discovered someone had tried it...

At http://www.finehomebuilding.com/2007/07/16/5-job-site-tips-and-time-savers-bench-vise-sharpening-and-power-tool-supports-2 there is indeed a demo of a carpenter doing the rough grinding on a damaged chisel using a belt sander. Note that is only the rough shaping, taking out notches made by running the chisel into a nail; he moves to a 400/1200 diamond stone for actual sharpening and puts the final polish on the microbevel by stropping.

One interesting trick there, which might not be a bad idea with other powered grinding: He actually keeps his fingers on the blade, an inch or so back, and quenches it in water whenever it starts getting too hot to hold. That guards against letting it get hot enough to lose its temper.

He also points out that eye protection is essential, hearing protection is recommended, and you should make sure that if the blade catches it's thrown away from you rather than toward you.

So, yeah, apparently it can indeed be made to work as a grindstone if you're trying to recover a seriously abused blade and don't have a better alternative available. But it really is only suitable for grinding, not sharpening.

Good to know, for emergencies and "jobsite repairs", but not recommended as a normal technique for fine woodworking.

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