Setting a Cap Iron
How to Make Your Bench Plane Perform At Its Best

by David Weaver

    Most amateur woodworkers understand that the cap iron (also referred to as a chipbreaker) on their bench plane is supposed to do something close to what its name implies - break shavings before they become tearout - but until recently, few understood how the cap iron actually works with the cutting edge, or how close to the edge of the cutting iron it must be set to have a beneficial effect.
    Thanks to Bill Tindall and Steve Elliott's work to revive a video and related research compiled by Professor Yasunori Kawai and Honorary Professor Chutaro Kato, Faculty of Education, Art and Science, Yamagata University, in 1989, we have microscopic views of the cutting action of the plane iron and the effects of chipbreaker placement on the quality of the finished surface, a phenomenon that the authors call the "cap iron effect." Thanks also to Mia Iwasaki for her dedicated work to translate the original Japanese research paper and video captions into English, and to Wilbur Pan for enhancing the video preserving the enhanced version on Vimeo.
CLICK HERE to view the Kato and Kawai video.
Effect of closely-set cap iron on plane shaving. Breaking the chip. This highly magnified view of a plane iron (1) and cap iron assembly, excerpted from the video by Professors Kato and Kawai, shows the chip-breaking effect of a closely-set cap-iron (2) with a 50° bevel on the leading edge. Note the lack of tearout behind the cutting edge despite the downward sloping wood grain.

    This article is a brief discussion of setting up a cap iron on a common (Stanley Bailey or similar type) metal bench plane, but there will be some references to the older style cap irons that are flatter, without the "hump" that the Stanley-style stamped bench plane cap iron has. The objective of using the cap iron is to get a good surface for any practically planed wood, and to do it with a common plane, for several reasons:
  • The satisfaction of using the cap iron mechanism properly to get a good surface, using only through shavings-those made with straight, end-to-end passes of the plane, making it easier to produce or keep a uniform flat surface and do it quickly.
  • To save money on planes that you may not need.
  • To avoid scraping, especially on medium hardwoods where scraping degrades the surface and higher angle specialty planes yield a surface with less polish
  •     Most of the discussion surrounding the use of the cap iron is related to smoothing, and most of the following discussion is in regard to smoothing. However, if you dimension wood by hand, the cap iron is at least as useful in preventing tearout or at least mitigating it in steps prior to smoothing.
    Tearout, chipbreaker set back.
    Now you see it, now you don't. The interlocked grain of this mahogany sample board (above) shows evidence of heavy tearout after planing by a Stanley No.5 jack plane with the cap iron (a.k.a. chipbreaker) set back approximately 1/16" from the cutting edge of the plane iron. Tearout is drastically reduced or eliminated (below) when the same board is planed with the cap iron set back just .1 mm (~.004") from the cutting edge.

    photos by ellis walentine
    Tearout with chipbreaker set close.

    Start with a Functional Plane
        There are a lot of articles on the Internet and in books, guides, etc., describing how to set up a plane. Everything following this point assumes you already have a plane that is set up to take a continuous shaving with the grain without relying on the cap iron to do anything. Starting with a plane that is not capably taking shavings in this "easy" scenario will make it difficult to figure out whether you're making steps forward or otherwise when you do start to use the cap iron to control the surface quality in a more difficult scenario.

    Preparing the Cap Iron - Undercut the Front Edge and Polish the Front
        Thanks to Bill and Steve and everyone who helped with the translations of Kato and Kawai's work, we know that the cap iron will work with good effect when the front edge of the cap iron is beveled at a 50 to 80 angle to the plane iron with which it is matched. Good effect means that instead of allowing the iron to lever a shaving such that it breaks below the level of the cut, the cap iron exerts forward pressure on the shaving, preventing it from levering wood before the cutting edge is able to reach it. This action helps reduce or eliminate tearout.
        If you have a common bench plane with the standard Stanley-style cap iron, the stock profile already works well in my experience. You can decide later if you want to adjust it, but you don't need to do any reshaping or grinding initially. Less is more to start.


    illustration by
    ellis walentine

        The first step in preparing a cap iron is to accurately undercut the leading edge. (See illustration above). This has always been part of setting up a cap iron, even if it's not going to be that close to the cut. You want a thin knife-edged line of contact between the cap iron and the back of the iron. Setup articles might have more information on this, but I go with "less is more" in this case (i.e., there's no reason for the undercut to be drastic), and prepare this undercutting on a medium (1000-grit) stone, side sharpening with the cap iron edge about " onto the stone. The undercut is created by allowing the remainder of the cap iron to drop below the surface of the stone.
        If you're going to stick with the stock profile on the front of the cap iron, all you need to do is polish the profile that's already there The shaving will contact it very close to where it meets the iron. 1/16" is plenty. A polish here may help feeding of the chip later, so it's nice to work to and through the polish stone or high grit sandpaper (e.g., 2000 grit) you have on hand. You don't want a burr to be left on the cap iron, and since it's soft steel, the burr can be persistent. Polish it off while retaining the undercut knife edge.
        If you're working with the older, flatter style of cap iron, you want to preserve as much of the bow as possible while making a clean bottom, and then apply the angle of your choice to the face and polish all the way to the edge. If absolutely necessary, you can bend most western cap irons, but if you don't need to, there is no reason to do it.

    Setting the Cap Iron
        Setting the cap iron to the iron is a crucial step that demands good lighting and precision. Assuming you're working with the Stanley-style iron and bench plane, you should have a mirror polish on the back of the iron, so that it will reflect light strongly. If putting the polished edge of the cap iron against the polished back of the iron makes it difficult to differentiate between the two, you can color the edge of the cap iron with a marker or marking fluid. I haven't found this necessary, though.
    Setting the cap iron by eye. Setting the cap iron by eye. With practice, it is quick and easy to set the cap iron close to the cutting edge. First, tighten the screw by hand and then advance the cap iron until just a tiny sliver of reflected light is visible on the back of the polished plane iron.

    photo by steve elliott

        You can set the cap iron any way that is easy for you, but here's the method that works for me:
  • Put the cap iron and iron together, with the cap iron set back 1/16" or a little bit less than that from the cutting edge.
  • Tighten the cap iron screw partially with your fingers until there is some tension on it. There shouldn't be much of a gap between the cap iron and the iron at this point (any gap will affect the projection of the edge when it's finally tightened out).
  • At this point, with the cap iron on top, and with your light source reflecting off the back of the iron, start to push the cap iron slowly toward the edge of the iron.
  • If you overshoot, check the cutting edge of your iron. You might need to fix anything you've damaged. Within a few iterations, overrunning the edge should be something that you do rarely or never again.
  • If you're not comfortable pushing the cap iron into position with hand pressure, you can get it close and then tap the opposite end of the cap iron with a small hammer (e.g., in the iron slot).
  • Your "closest" setting will be the setting where you push the cap iron almost all the way to the edge, but can still just see some light reflecting off of the back of the iron. (See photo above.) If you push it any closer, you'll overshoot.
  • Finish tightening the cap iron.
  •     This seemed fidgety to me at first, but within less than a week it was quick. Shortly after that, it became a 10-15 second process. This super-tight setting is probably not one you'll need that often, but it is a very capable setting in eliminating tearout, and it is a good starting point from which you can ease the cap iron back when a close set is not needed.
        From this point, knowing the level of closeness in the cap iron setup that you need to have is better learned through experience than reliance on something strictly methodical or formulaic.

    Putting the Plane to Work
        If you aren't having any trouble planing the wood at hand, then experimenting with a super-tight cap-iron setting isn't necessary. If you have wood that isn't that agreeable and that is causing tearout trouble, the tight setting is a good starting point. The "super tight setting" is described above as the first setup target. It is something that will help you deal with the worst wood without doing anything other than making straight through shavings, where you work the plane straight through the length of the board without skewing or changing direction. The plus is that your through shavings don't have to be unreasonably thin and you don't have to stop every few swipes to sharpen to avoid tearout. This setting corresponds to the .004" settings that Kato and Kawai published. It is my opinion that it's pointless to measure something like this, but knowing the measurement is worthwhile. Knowing that the measurement specification exists does not mean you have to confirm that you're doing the same thing.
        When you plane with this close cap iron setting, the shaving will do one of three things:
  • It will look the same as it does without the iron set close. In this case, the cap iron may not be doing that much, and the set might not be as close as you expect. Or, it might be close enough, but the shaving might be too thin to get worked by the cap iron. If you have no tearout, though, you can't argue with success. If you never smooth with a coarse shaving (in the .003"-.004" range), this may be all you experience unless you've set the cap iron so close as to create a jam.
  • It will straighten out, if the shaving is thick and hard enough to have structure from the point of the cut to where it's worked by the cap iron. If you're taking a heavy shaving in something that you otherwise have tearout with, this is a really nice setting to have. The shaving will come out straight or straighter. This doesn't mean that the shavings will come out looking like pointers, in some cases it will just be that some parts of them are straightened or wavy instead of curled neatly.
  • It will bunch up and fold over. So far, I haven't seen any situation where this is better than a shaving that straightens, and, even if you can continue, the surface might be degraded. For an example of what it looks like, plane the face of a quartered white pine board and examine the surface of the board. You can clearly see that the surface is being crushed. Either the cap iron isn't set properly, or the edge isn't polished or there may be other feeding problems.
  •     If you don't need the cap iron, the first type of shavings is preferable. If you're having issues with tearout, the second straightened shavings are preferable. I can't think of a case where you'd want the third type. Either tune the cap iron or back it off from the edge just a little.
        When you plane a fairly thick shaving (for example a .003"-.004"-thick smoother shaving), if the cap iron is flexing, breaking or altering shavings in difficult wood, then you will notice that there is more resistance. The plane shouldn't bull you around (i.e., be so hard to push that you cannot work with it), but you will have to push somewhat harder; after all, you are literally forcing the shaving back into the surface of the board to prevent it from lifting. The first time I set a cap iron properly several years ago, I gave up figuring that the additional resistance meant that I was doing something wrong.
        Trust the surface and the chip, and revise as necessary if the plane is bulling you. If the surface is good and there is no bunching, then you don't need to worry about the setting. If the surface is not good, and the chip shows no evidence of being straightened, then a closer setting will likely help.
        There are many ways to mitigate tearout. First and foremost, the advice is often to sharpen the iron and reduce the shaving thickness. These often help, but if you are working wood that has been sitting through a temperature or humidity change, or if you are smoothing wood that has been hand dimensioned up to that point, taking a thicker smoother shaving with through shavings is a nice luxury to have. You will not have to plane many panels that are not perfectly flat with .001" shavings to realize that it might be nice to work a little bit faster. And, with through shavings, you don't threaten dips and swales all over your board.
        Once you've set yourself up to be able to take thicker shavings, you may have a tendency to bear down on the plane with more weight. All this will do is make it seem several times harder to plane the wood, and you'll errantly equate it to a dull plane or obnoxious wood.
        Make sure that when you set the cap iron close, that any additional pressure that you apply is forward, in the direction of the cut, and not downward such that you're creating extra friction on the sole of the plane. You will be surprised how many times you may think a plane is dulling, when the real issue is that you are working against your own weight.
        One other thing that I think you'll find is that a stock cap iron and a stock iron on a good plane will not chatter; as in, will not at all. I get no chatter with cocobolo with knots in it. I have seen others mention that they're satisfied with their setup, but they get chatter. There should be none. The force exerted by the lever cap on the cap iron and supported on the frog ,or more accurately returned in kind by the frog to bedded side of the iron, should eliminate any issues with a chattering iron.

    A Good Compromise
        When I first found I couldn't induce tearout with the super-close setting, I wanted to use it a lot. Since then, I've backed off some; there's no reason to threaten the surface quality on softer woods by smashing the chip back into the surface of the wood with the cap iron if there's no issue to begin with.
        I can only offer what I see as a general setting that usually works when I don't know whether or not the wood will cause a problem, or if I'm coming from hand jointing or thicknessing a board and want to take thick shavings with my smoothing plane to expedite things.
        The setting I'm describing is one where the cap iron is set just a bit further from the edge than the close setting discussed above. If the shaving is thin (.002", etc, which is thin for practical purposes despite the fact that you can work much thinner than that if you'd like), then this setting doesn't do anything and probably doesn't need to. If you want plane a thick shaving to speed up your work, then the cap iron still prevents tearout generally, and when examining the chip, you will still see some evidence that it's influencing the shaving. It's sort of a backstop setting: It mitigates tearout when the shaving thickness increases, but it doesn't require you to adjust the set of the cap iron for every thickness of shaving, as you might need to do if you've set your cap iron as close as you possibly can.
        In a short while, you'll figure out what you want to do. You might want to not set the cap iron close at all and wait for tearout. You might want to start with the close set and then back off from there, or maybe you'll favor the settings I do where you can just set the plane once and not have to adjust the cap iron until you have to sharpen again.

    Troubleshooting Jammed Shavings
        The most common complaint from users the first time they set the cap iron close to the edge of their iron is that shavings are jamming in the mouth of the plane. This is most likely for one of the following reasons:
  • Shavings are getting between the iron and the cap iron. This indicates that the cap iron is not seating perfectly on the back of the iron. Hone the mating surfaces until they fit tightly when assembled and clamped in place with the lever cap.
  • Shavings are jamming between the cap iron and the front of the mouth opening. The fix for this is either to move the frog back or open the mouth with a file if the frog can't be moved back more. You won't need the mouth to be set tight if the cap iron is working properly. It could also mean that the cap iron is not polished enough. If you have an unseen burr or something of that sort on the leading edge of the cap iron, it can cause chips to hang up and jam. I've had only one issue with a burr early on, but I have heard several other people mention it.
  • The shavings are being bulldozed into an accordion pattern, instead of bent and allowed to escape. This means the cap iron is set too close. Move it back from the edge some and try again.

    Thanks to Bill Tindall and Steve Elliott for acquiring and bringing this important research to light.

    This article is based on the research paper,Wear of Knife Used for Hand Plane III: The influence of the cap iron, by Professor Yasunori Kawai and Honorary Professor Chutaro Kato, Faculty of Education, Art and Science, Yamagata University, Yamagata, Japan, published in the journal of The Japan Wood Research Society, Vol. 35, No. 10, p.886-895 (1989). A slightly edited English translation of this paper by Mia Iwasaki appears on Steve Elliott's web site along with several other pages of plane and chipbreaker studies that Steve has compiled.

    Creative Commons License The video is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

    Thanks to Wilbur Pan for enhancing and captioning the original video based on a translation of the original Japanese captions by Mia Iwasaki, and for uploading it to Vimeo for posterity. The video may be accessed HERE.

    Thanks to all the members and visitors on the WoodCentral Hand Tools forum who contributed to the many productive discussions of the implications of this research for hand plane users. To review some of those discussions, please visit the following links:
  • April 26, 2012
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  • May 26, 2012

    . . . David Weaver

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    © 2012 by David Weaver All rights reserved.
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