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Sharpening Impulse Hardened Saw Teeth

Sometimes a willingness to venture “outside the box” yields great rewards.  This is one of those times.

Like probably most of you I have a number of modern saws with impulse hardened tri-faceted teeth.  The upside is that these teeth can be very long lasting.  The downside is that they are brittle and prone to snap off whenever encountering an exceedingly hard material, such as a nail.  I have several saw blades with a gap-toothed grin.  Fortunately, the blades are almost always replaceable.  Unfortunately, until recently, my experience had been that they were impossible to sharpen due to the impulse hardening that made the files skate off of them without affecting any improvement.  I found this frustration to be true for any of the facet-tooth saws I have, whether actual Japanese saws or the Stanley western style saws that employ Japanese-style teeth.

While working at my daughter’s house a while ago with my old-ish Z-brand saw I hit a nail good and hard.  Much to my surprise only one tooth snapped off, but a couple dozen were mushroomed (I’m not good enough with that camera to get a nice pic).  I had never before seen this damage.  Before, the teeth just snapped off.

I certainly had new replacement blades in the drawer, but since the teeth were intact (except for the one) I decided to bring it back to the barn and give it a try to sharpen them.  Using abrasives, first sandpaper then one of my small whetstones, I flattened the back side of every damaged tooth.  Since most of the saw’s use was for rough carpentry and yard work I went ahead and cleaned it up pretty well.

However, when it came to re-shaping the damaged facets even my diamond needle files mostly skated over the hardened tips.  But there in my small container of whetstones for my carving tools was a diamond shaped aluminum oxide “India” stone.  The cross section was exactly like that of the file normally used to sharpen Japanese-stye saw teeth.  I also had a needle-taper stone of the same material.  They both came in handy.

Setting up the sharpening station just like every other saw I’d sharpened in the past umpteen years, “filing” with the “India” whetstones worked like a charm.

In less than a half hour I had the task done.  Prior to the sharpening the saw would still cut after a fashion, 51 strokes to get through a 2×4, but after the sharpening it made it though the same lumber in exactly 1/3 of the strokes, leaving a very nice kerf surface.

It is a good day when you can go to bed after learning something you did not know when you woke up that morning.