Chipbreaker: Harden or no

The first Krenov plane I made I didn't incorporate a chipbreaker. That project was more about learning. It turned out to be a good little block plane. It was initially bevel up 25* iron on a 40* bed which I was able to take shavings on red oak of .003 thickness. To further experiment, I opened the mouth and flipped the iron over bevel down. With a mouth clearance of about 1/32 and no chipbreaker I was able to take wisps of wood from alder and QSWO that didn't register on my Starrett calipers.

I intend to make a whole set of planes in Krenovian style with my remaining bois d'arc. I get my bar stock for the irons/chipbreakers from MSCdirect.com. David Finck doesn't mention anything about hardening a chipbreaker. The color of the chipbreaker in his book looks like it has been oil hardened. The O1 stock comes soft, about 5 RC. I will definitely grind, shape, drill and tap before hardening. So the question: Is hardening the chipbreaker necessary? The only vintage iron I own is a #6 Union. I suppose I could take its iron and chipbreaker and have their hardness tested.

All my chip breakers seem to be malleable steel. In other words, the chip breaker doesn't face the kind of wear received by the edge of a plane iron. In time it may be necessary to reshape the chip breaker. Being hardened would make that difficult.

jtk

Makers seem to be all over the map on this. The traditional bent-sheet-metal Stanley iron and its clones are unhardened AFAIK. Some of the newer "premium" cap irons seem to be hardened to Rc50 or so (guessing from how much of a pain they are to reprofile). I don't think it matters very much to be honest. Mild O1 should be OK.

I think Jim has it right. I'll add a small bit of experience I've had. I got one of the LV chip breakers to accompany a replacement blade. After using it for a while and reading about chip breakers and how the work, I decided to make its angle steeper. I was able to do it readily with a common file. That tells me they aren't hardened. I've also smoothed out the edge and nose of Stanley chip breakers quickly with my diamond stones, again not hardened.

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Originally Posted by Richard Line

I think Jim has it right. I'll add a small bit of experience I've had. I got one of the LV chip breakers to accompany a replacement blade. After using it for a while and reading about chip breakers and how the work, I decided to make its angle steeper. I was able to do it readily with a common file. That tells me they aren't hardened. I've also smoothed out the edge and nose of Stanley chip breakers quickly with my diamond stones, again not hardened.

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Nit picky, but: Saws and scrapers are hardened (both to around Rc50) and can both be filed. From my own experience I think that the older LV breakers (on their "classic" BD planes) are unhardened, but that the newer ones on the custom planes and the replacement blade sets are hardened and tempered to Rc50 or so. They certainly don't grind as easily as the sheet-metal ones in my experience.

Also I have files (Valtitan, Corinox) that can handle Rc60+ steels, but those are pretty uncommon :-).

In my opinion cold rolled is about the right hardness for a chipbreaker.

Chipbreakers are really subjected to some high abrasive forces. Takes some passes with and then without the chipbreaker, all other settings the same, and compare the force necessary to propel the plane. It is noticeable, and adds up to a lot of work over time.

An easy way to confirm this is to notice that the chipbreaker heats up when in use. This is more apparent with wooden-bodied planes that steel planes. This heat is generated when the shavings impact on the chipbreaker. Kinetic energy converted to heat energy. It erodes the chipbreaker as surely as air bubbles on a ship propeller.

If you pay attention to a chipbreaker that is positioned properly and working, over time you will notice pitting has developed. Once again, the result of friction. This is especially quick and noticeable when you plane tropical hardwoods that contain silica particles.

Notice also that the area on the blade's flat directly in front of where it touches the extreme forward edge of the chipbreaker tends to get pitted due to shaving impact, despite the blade's hardness. This tendency is, again, most apparent when planing tropical hardwoods, but occurs to one degree or another with any wood.

A hardened chipbreaker will resist wear longer. Perhaps the friction is lower? I don't know. But clearly a hardened chipbreaker will resist wear longer than a soft one.

A soft chipbreaker will work fine of course, if you polish it occasionally on your stones and oil it in use. Neglect it and friction and tearout will increase.

I suppose it makes sense to err on the safe side and just harden them; though, I don't foresee subjecting my planes to the rigorous heat-generating-use mentioned by Stanley. I also don't intend on working with exotics unless some "falls in my lap."

I'll take pictures of my next builds for not only the plane, but the metal processing. I hope you all find it somewhat entertaining or at least a bit enjoyable to follow.

Doesn't the Chipbreaker need a bit of "spring" to them? To enable them to snuggle down tight to the back of the iron? To better avoid gaps that can trap shavings?

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Originally Posted by steven c newman

Doesn't the Chipbreaker need a bit of "spring" to them? To enable them to snuggle down tight to the back of the iron? To better avoid gaps that can trap shavings?

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It needs to withstand a very small amount of bending without deforming, which mild/unhardened steel can handle just fine. Ironically the old, unhardened Stanley breakers deflect more than the new "solid"/hardened ones do.

What Steven alludes to here is that hardening increases the yield strength of the steel but not its stiffness, which means that hardened steel can be deflected further than unhardened before it permanently deforms. That's why "spring steel" is hardened.