Wooden Planes – Tapered Irons (and how a back bevel can save the day)

I use old wooden bench planes daily.  I’ve got a full set of iron bench planes as well, but find that the wooden bench planes are much more pleasurable to work with.  They’re lighter, which is a plus when doing a lot of planing and wooden jointers are, usually, significantly longer than their iron counterparts.  And, the soles of wooden planes tend to burnish the surface being planed.

Recently I was asked if I found that antique American bench planes were designed only to be used in softwood.  “Well, of course not!” I responded.  I mean, look around you at the myriad of things built from hardwoods, from building frames to washbowl stands, during the period in which only wooden planes were being used.  But then I started thinking about it a little more.  Some of my wooden planes do seem to work better while planing softwood.  So, I started looking at their geometries.  I found that both iron and wooden American bench planes are, more often than not, bedded at 45 degrees.  (Most wooden molding and specialty planes are bedded with 50 or 55 degrees.  Wooden bench planes that were designed exclusively for use in hardwoods are generally bedded at 50 degrees, but angles as high as 60 degrees are not unheard of.)   

After finding that the bedding angles were the same, I began looking at the irons.  I have several wooden bench planes that use irons that are the same thickness throughout their entire length.  But most of my wooden bench planes have tapered irons.  That is to say that the iron is much thicker at the cutting edge than at the top.  The reason is that the portion of the iron below the chipbreaker screw slot has been forge welded into the upper portion of the iron, which is generally very soft steel.  It is not uncommon for the cutting edge to be two to three times as thick as the upper part of the iron.  But how does this affect performance, assuming that all of the irons are sharpened to their optimum?

A moderately tapered iron – about 2 degrees of taper

Well, the answer in reality is pretty simple.  It’s well known that a lower cutting angle produces better results (with less effort) in softwoods.  When the degree of taper is subtracted from the bedding angle, the total included angle or effective cutting angle is lowered.  In the case of the Japanese plane which uses an iron that tapers in the opposite direction (because the iron itself is wedged into the plane body), the taper angle would be added to the bedding angle, thereby raising the effective cutting angle.  So, once again, geometry provides the answer.

The actual cutting angle is 43 degrees, due to the 2 degree taper of the iron

Then it dawned on me.  Over the years, I had back put a back bevel on some of the planes that I used mostly in hardwoods.  Most folks who do a lot of hand planing, hone irons instinctively, depending more on feel than the protractor.  As most of my readers know, I’m very cautious about back bevelling irons as it increases the amount of energy required to push the plane, exponentially.  As I get older, I want to work less, not more!

The long and short of it this:  If you want to improve the performance of your old wooden bench plane with the tapered iron in hardwood, put a back bevel on it.  There, I’ve said it…back bevelBejasus, bite my tongue!

And just one short note of caution:  bevel length looks suspiciously long on tapered irons.  Don’t allow yourself to be deceived.  Make sure that your primary and secondary bevel angles are correct and that you’ve got adequate clearance.

Go find yourself a nice old wooden bench plane.  Tune it up and have a ball.

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