Archive for the 'Sharpening' Category

Reader Andrew Reynolds posed a question which has probably occurred to many of us, about whether fancy tools are worth it. He asks: “I’ve got a really good set of bench chisels and I am thinking of adding some mortising chisels to the collection. There is a very wide range in price that you can pay – the Lie-Nielsen chisels look great but the price is up there compared to some others I’ve seen. The question is, why the big difference? Are the more expensive chisels really worth the price?”

You pay for two things in fine chisels: steel quality and the heat treating process. Here’s the executive summary.

Quality of steel

Traditionally, chisels have been made from simple tool steels such as O1, which is iron, carbon, and a touch of manganese (to allow the steel to quench adequately in oil). Ron Hock‘s excellent book, The Perfect Edge, has a great explanation of the subject for those who want more depth. These steels have good hardness and abrasion resistance, but yield readily to sharpening stones. Their fine grain structure lets them take a very keen edge.

Steel design is a trade-off. Metallurgists can make alloy steels such as A2, which is harder and more wear resistant than O1, by adding chromium, nickel, manganese, and other elements. The chromium is the key here, as it combines with carbon during heat treating to form very hard granules of chromium carbide. These, suspended in a matrix of softer tool steel, give A2 tools their long-lasting edges.

But there’s a price for everything. Those carbides are large and HARD. They don’t grind readily, making sharpening a chore. What takes a minute with O1 tools takes lots of time with A2. If you stop too soon, you leave an edge with a radius approximating the radius of a carbide crystal. It’s not sharp. Also, those tiny carbides can become dislodged from the softer steel at the cutting edge, like stones from concrete, leaving little gaps. So we compensate by leaving more steel at the edge for them to hang on to, by grinding at a larger angle. That makes the edge last longer (it would with O1 also), but also makes the tool harder to push through the wood.

Heat treating

You harden steel by first heating it above the critical temperature (around 1400ºF), so the carbon and other elements go into solution in the still solid steel. Then you cool it quickly (“quenching”), freezing the molecules into a crystal structure called Martensite. The faster you cool the steel, the higher the percentage of Martensite versus other, softer crystal structures. But if you quench too fast, you crack the steel. If you quench, then cryogenically cool the steel with liquid nitrogen, you can convert more of the original structure to Martensite, getting a more durable edge. Next, you reheat the steel to 350–400ºF and cool it slowly, “tempering” it and converting some of the very brittle steel structure to a softer, tougher matrix that won’t shatter when it encounters a knot or gets dropped on the bench.

That heat treating takes time, fancy ovens with inert atmospheres, and cryogenic freezers; all of which cost money. And it pays off best with fancier steels such as A2. So a 1/2″ rod of hardened A2 is $6.50/ft. versus $2.50/ft. for O1.

The bottom line

Which is better? I enjoy sharpening, and like an edge I can shave with. Most of my hand chisel work is paring. So I like O1 tools that are easy to hone and sharper than a witch’s curse. If I’m buying woodturning tools, which don’t have to be so sharp but do have to withstand abrasion and heat, I’ll go for alloy steels such as A2 or M2. But if you’re someone who puts off sharpening as long as possible and then doesn’t mind devoting a lot of time to it, A2 is a fine choice. However, you’ll pay more and may wind up with a less sharp (but longer lasting) edge.