Prior to the 17th century, English woodworking tools looked pretty much the same as the the tools in the rest of Europe. Often the tools were actually imported from Europe. But two major events occurred that pushed the design of English woodworking tools forward to make the country a tool powerhouse. The first began on September 2, 1666. The Great Fire of London destroyed huge swathes of the city. The skilled labor needed for the reconstruction taxed the existing system of guilds and equipment supply past the breaking point. Apparently pretty much anybody who could swing a hammer could find employment in one phase or another in the woodworking trades. The second event was the Industrial Revolution in England. By the middle of the 18th century, English woodworking tools reached most of the maturity and function that they have today.
Why is this relevant to tear out? Because roughly around the middle of the 18th century, rolled iron and steel were available, affordable, and people were figuring out how to make things like nuts and bolts. The invention requires making a slot in your iron (also giving rise to the name for these irons as "cut irons") and screwing on a secondary or double iron also known as a "cap iron" or "chipbreaker." The cap iron still needs to be of steel with some spring to it so that it can stay clamped tight at the very edge of the back of the iron. Whereas previously a shaving could ride up the back of a single iron, now the shaving would ride up onto the cap iron and get turned at a much higher angle and thereby broken - losing the strength to cause tear-out. This is a monumental, revolutionary concept. For the first time, a given plane would not need to possess a superb, tight mouth in order to prevent tear-out. You also didn't need a very high angle. More importantly, as your wooden bodies wore and the mouth of the plane opened, you still had a way of controlling tear-out without fussing with plugs and other things. You could now mass-produce high quality planes, ones that didn't require the handwork and skill needed to produce a whisper fine mouth. The planes also lasted longer. The purpose of a cap iron is to turn and break up a shaving so the shaving doesn't have the strength to cause tear-out
I have in my collection several of what I would consider a few fairly early cap irons. None are 18th century. Even if the plane itself dates from the 18th century, determining if the cap iron is original is nearly impossible. Let's just say they're early.
Probably the oldest cap iron in the group. This American cap iron C. 1840 by W. Butcher comes from a Knowles patent plane and predates Stanley by a generation. The thread for the cap iron screw was formed by punching a rough hole in the material and threading the inside.
This cap iron probably mid-19th century comes out of a much earlier jointer plane by Christopher Gabriel. Tapping steel without a good machine shop is tricky so it was pretty common to instead solder in a brass nut to the cap iron, as was done here.
And the 21st century newcomer - Ron Hock cap irons. A modern Hock iron is considerably thicker than a stock Stanley iron, and lends to the overall rigidity of blade.
According to Toshio Odate hand planes in Japan were single iron up until about 1900, when a secondary iron was introduced. As with a single iron plane, in a double iron Japanese plane the iron is held by the wedge sides of the plane. The cap iron is tapped into place and is held in position by a rod across the body of the plane. The cap iron itself fits closely at the blade end of the cap iron, but is bowed at the back end so you have to springiness to have it lock in place under the rod. It's a very clever solution that allows you to retrofit a cap iron to a single iron plane. Modern Japanese planes use cap irons which are made and ground like a dull plane iron.
There are two real problems with cap irons. The first is if the back of the plane iron or the cap iron aren't flat. If there isn't some spring to the cap iron when you put it all together, you can end up with a gap between iron and cap iron. Shavings can get caught underneath the cap iron and the plane clogs up. With old, corroded, Stanley planes, this sometimes happens but you can usually tap the cap iron so the curve in the cap iron is a little more pronounced and you are back to working. The heavy cap irons of Norris and Hock don't really have this issue.
The second issue, which is frankly the main reason why cap irons sometimes get a bad rap, is that unless the cap iron is positioned literally a fraction of an inch behind the iron, it'll be too far away to do much good on a thin shaving. We're talking about a sixty-fourth of an inch or less. If the iron is slightly cambered or curved, then it's even harder to get the cap iron in the right place to be effective. In general, for heavier work with a thicker shaving, the cap iron is not that critical and works great. Being able to take a thicker shaving without tear-out is huge. But if you're trying to improve performance on an incredibly fine shaving to begin with, a whisper setting is kind of critical.
If the cap iron is set really close to the cutting edge like you're supposed to do, but the plane doesn't have the right geometry, the plane itself can clog because the shavings have no place to go. This is really less of an issue on a well-performing plane. I've experienced this problem occasionally, but the problem occurred because I was just not setting things up correctly in the first place.
I should mention that there's another benefit to a cap iron on Stanley planes. The Stanley stock irons and cap irons are quite thin - but bolted together under tension like they're supposed to be, they offer a better performing, effectively thicker, iron.
The W. Butcher cap iron, which came with my Knowles patent plane, is part of one of the earliest American metal planes, predating Stanley by at least a couple of decades. In the American drive for mass production punching a hole and threading it must have been seen as an awesome cost saving, even it it is pretty ugly. Detail of the punched in hole for the cap iron screw on the Butcher cap iron
Some of the cap iron in this blog entry pre-date the ready availability of milling machines. And even after these machines were available, smaller makers, or the smaller sub-contractors that actually made the cap-irons, didn't have the latest and greatest equipment. Stanley manufacturing was always based on precise machine tools. As for the other makers - not so much. Here are a few examples: The cap iron screw below was probably made by hand on a small lathe. The screwdriver slot was made by hand with a saw and followed with a tapered file. Hand filed cap iron screw Two 19th century cap irons with soldered on brass nuts
Alex Mathieson was a giant tool maker of the 19th century. While I have seen forged cap irons before, this one takes the cake. From the forging marks we can see that it was made in stages, not just stamped out in one piece. And the entire cap iron is tapered like a plane iron. This Mathieson cap iron was forged from solid. The cap iron is tapered and the boss for the screw is integral with the rest of the body.
Record introduced a two-part cap iron so that you could lift off the bottom section for sharpening and not lose the precise cap iron setting. They are a bit fussy, I never cottoned to them, and I don't have any original ones in my collection. Record Stay-Set Cap iron from the book "Planecraft"
This closes out the series about the engineering of thin shavings and how to prevent tear out. Basically the first line of offense is a very sharp blade that's capable of taking a thin shaving. Then a plane body that's contacting the wood right in front of the iron. You also want a plane with a bed angle that's appropriate for the material. And finally, especially for general work, a cap iron. Certain situations, like for highly figured, or stringy wood benefit from scraper and toothing planes.
I think this series has been a longest topic I've ever written on in ages. My series on Grinding might have been up there also. I think I need to write about some lighter topics for a little while.
L-R - Panel Plane by Robert Towell c. 1830's;American Knowles patent plane C. 1840;Mathieson smooth plane C. 1860;Hock Cap Iron;W. Butcher cap iron from Knowles plane;Stanley 605 1/2 c. 1900; Norris A51 C.1914; Tsunesaburo 2005, in the back: Gabriel jointer C. 1800 - Howarth cap iron
Group photo of most of the cap irons mentioned in this blog; L-R W. Butcher;Stanley,Norris;Hock; Tsunesaburo ; top - Howarth
Join the conversation
05/31/2023 Gerry Cox
I've enjoyed your series, Joel. Thanks!
A question: W. Butcher worked in Sheffield, right? So his cap irons wouldn't be American but imported to America.
And a comment: one of the several advantages about low-angle planes is that there's no need to fuss with a cap iron! Couple that with an adjustable mouth and a heavy plane iron and you're ready to take really fine shavings!
Best,
Gerry
06/02/2023 Thomas Walter
Question: If the cap iron (aka - chip breaker) is essential on a bench lane, how can a bevel-up plane work well without one?
Walter - On bevel up planes the bed is typically around 20 degrees + a bevel angle on the blade of 25 degrees - giving you similar performance of a bench plane - BUT many people routinely sharpen at a higher bevel angle giving one the benefit of a high angle plane. For basic smoothing overall bevel down planes with cap irons are far more popular than larger bevel up planes.
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A question: W. Butcher worked in Sheffield, right? So his cap irons wouldn't be American but imported to America.
And a comment: one of the several advantages about low-angle planes is that there's no need to fuss with a cap iron! Couple that with an adjustable mouth and a heavy plane iron and you're ready to take really fine shavings!
Best,
Gerry