Back irons, pt 1

Back iron. Chip breaker. Call it what you will. It's part of the hand plane puzzle (unless you use only single iron planes).

The back iron from my old #3 smooth plane.

The back iron has at least two purposes. First, it helps to stiffen the cutting iron. If you use a modern Bailey style plane you likely have a flimsy .080" thick cutting iron. Compare that to a .187" or even .250" iron of days gone by.

The iron assembly...a flimsy .080" Stanley iron.

However, even many old thick cutting irons have a back iron. The second purpose of the back iron is where it gets its second name: chip breaker.
To put it simply, the chip breaker disturbs the shaving as it comes off the bevel of the cutting iron. It forces it to turn, and reduces the chance that it will pull on the fibers in front of the cutting edge. That's a complicated way of saying it helps reduce tearout.
(by the way, I think one of the best explanations of how a chip breaker works is Jim Kingshott's Handplane Basics video)
Probably the trickiest part of setting up a plane is getting the back iron properly set. Let's get going on that with a basic explanation of how the iron and back iron interact with each other. I think it's time for some incredible high-definition graphics, don't you?

1) Back iron before being tightened. 2) After tightening. 3) Proper relationship. 4) Bad fit. 5) Acceptable fit.

That's a pretty awesome picture, eh? 1 & 2 represent what happens as you install the back iron and tighten the screw. As the screw is cinched down, pressure is exerted on the leading edge of the back iron which holds it tightly against the cutting iron. As that pressure is exerted, one of conditions in 3,4, or 5 will occur. 3 is the ideal fit, notice the angle at which the leading edge of the back iron meets the cutting iron. 4 shows what happens with a poorly prepped back iron...this WILL cause clogging and a sudden halt to your hand plane activities. 5 is acceptable, you are not likely to clog up the plane with that fit.

That does it for part 1, just a basic view of what the chip breaker is all about. In part 2, I think we'll go through the prep process of the back iron for my #5. I never set it up properly, so it's time to rectify that situation!