Cap irons or Chip Breakers Do What?

Great Question to Consider

Hello

I have a question about chip breakers. I notice on Krenov style wooden planes and other kits out there employ the use of a chip breaker. Is this necessary? I see most traditional wood body planes do not use chip breakers. And it also seems they are primarily used for depth adjustment on metal planes. Hope you can shed some light on this for me. Also thank you so much for everything you do. You have truly changed my life as a wood worker and as a man. You are an inspiration!

My answer

This is the part of the bench planes where Americans erroneously name the cap iron a chip breaker; a part used in the planing machine known in the US as a planer and in the UK as a thicknesser or a thickness planer, nothing to do with bench plane parts. This started happening in the UK too, unfortunately. Oh well. This ‘chip-breaker’ is the cap made from plate steel locked beneath the wedge or the lever cap that flexes the two-component cutting assembly to reduce vibration by causing tension in the cutting iron and the cap iron and so prevent chatter. It neither causes the wood to chip nor prevents the wood from chipping. Actually, the reference you made about wooden planes having single irons is more incorrect than correct. Almost all bevel down planes developed and used over the past three centuries have been bevel-down, two-part assemblies with the blade and the cap iron locked together with a single setscrew. So, wooden bench planes from 1700’s used cap irons and retained them all the way through to today. Now when you go to wooden bodied planes with thick irons you can use them without cap irons as I do with home made special planes. Remember that wooden bodied planes do not flex and vibrate like metal planes do within the body itself. There is no conflict between the wooden bed and the iron as there is with all metal bodies. In actuality, wooden planes absorb vibration within the body of wood itself and this serves as a dampener if you will.
On then to the term chip breaker. I have thought about this before and put it down. If the cap iron were to be called anything other than a cap iron I would suggest tensioner or better still diverter. Semantics or not, the term chip breaker applies to a planing machine that is a long bar directly adjacent to the cutting blades that stops the wood being planed from lifting and splitting wood away from the cutter before the cutter cuts. Because of the power of a machine and the blade hitting the wood, there would be a tendency for any weaker area of grown or rising grain to surface fracture. The chip breaker applies pressure and holds the wood down as it passes into the cutter. The cap iron on the plane does no such thing.
Now then, one last thing. All bevel up planes are cap iron free for obvious reasons. So, on wooden bodied, usually low angle planes, you need no cap iron.

Thanks for the Question. I like these!

20 thoughts on “Cap irons or Chip Breakers Do What?”

  1. I agree that the cap iron does help stiffen the iron, especially on thinner irons like the original Stanleys. However, that is not its primary purpose. It is actually there to prevent tearout and can do an amazing job at that if set properly. You have to set it ridiculously close to the edge of the iron so you can only see a sliver of reflected light (within about 5 thousandths of an inch or so), otherwise it will have no effect. It allows you to take moderate shavings (2-3 thousandths of an inch) with a smoothing plane against the grain or with reversing grain without tear out. It’s quite amazing when you get the thing set up right. The cap iron’s use for this purpose is well documented in historical woodworking texts, including Nicholson’s Mechanic’s Companion (1812). Nicholson states:

    “To prevent the iron from tearing the wood to crossed grain stuff, a cover is used with a reversed basil, and fastened by means of a screw, the thin part of which slides in a longitudinal slit in the iron, and the head is taken out by a large hole near the upper end of it. The lower edge of the cover is so formed as to be parallel or concentric to the cutting edge of the iron and fixed at a small distance above it.”

    There is some amazing info on the web about the use of cap irons, including a video done by 2 Japanese Professors studying it’s effect. They video the action with a microscope so you can see what the cap iron is actually doing to the shaving as you plane the wood. The link to the video and a much better explanation of setting a cap iron is here:

    http://www.woodcentral.com/articles/test/articles_935.shtml

    1. I think it’s an interesting discussion, and I have seen this go back and forth, but I can rarely find where this applies to life itself. I have said before that it seems the end product in much of this is the length and width of the shaving at xx thousandths of an inch today and not the levelling of two planes to make a joint flush or to simply smooth a surface of wood. That’s the shift in our woodworking culture.

      1. Hello Paul

        I like your work very much. But in this case I think
        your explanation only belongs to planes with UK or US style. In the
        woodworking culture of north western Europe there are planes with and
        without chipbreakers. As I know the first chipbreakers were half circle
        metal bars about 1/2 ” wide welded to the iron slightly above the
        cutting edge on the mirror side. This had no effect on blade tension
        etc. . Because these plane irons were very difficult to sharpen the cap
        iron evolved from this Design. Every teaching book for joiners in
        germany covers the topic of chipbreakers. The classic german bench
        plane set (all wooden planes) consists of several planes with and
        without chipbreaker. Since more than hundred years these are:

        Scrub plane, rounded iron without chipbreaker.

        A shorter Jack plane type plane, curved iron no chipbreaker.
        So called double plane, with double iron (iron + chipbreaker)

        Smoothing plane with double iron

        Try plane with double iron

        Toothing plane, single iron very high pitch

        Kind of shoulder plane one with single and one with double iron.

        In general one can say that for coarser work we use single iron planes, for fine work we use double irons.In
        these days at least german joiners work hardly at the bench anymore.
        The german hobby woodworkers are often oriented to the UK and US style
        of woodworking. They normally want metal planes from the US and Canada
        and so the traditional knowledge about our wonderful wooden planes and
        how to use them is lost. I hope you appreciate a “continental” view on
        this topic and will excuse errors in writing and style, because english is not my native language.

        1. Thank you so much for this response. i want to hear more about this and would like images, drawings references too. Very helpful for me and for others.

    1. Between 1/8′ and 3/16″. The lower the pitch the thinner the iron. in general. The fact os that thick irons are in no way a new thing. All the old planes from the early 1700’s were thick ironed planes where the iron tapered from thick at the cutting edge to thin at the top. When rolled plate we call flat stock came into being this changed the industry and most irons changed from being hammer forged with drop hammers to rolled steel stock. As far as I know only one of the thick ironed modern day plane makers still uses hammer forged irons forged using a hydraulic drop hammer and that is Clico in Sheffield who make the much revered Clifton Planes.

  2. Peter GeePeter Gee

    You might find the “Complete Book of Woodworking Tools” from the well-know German maker, E. C. Emmerich interesting to look through – the link is under the main details headed as a “bonus download”.

    http://www.ecemmerich.com/index.html

    It is in English so should give you some interesting details.

    As a side issue, I see very little written these days in the UK regarding wooden planes, which is interesting considering I, and probably you too, were introduced to the craft at school using wooden plane pretty much exclusively.

    Anyway, I appreciate your blog/website and highly commend your video tutorials to everyone. Well done! Keep up the good work for a long time to come – that is an instruction! 🙂

  3. Hi, my name is Wojtek, I’m from Poland. In polish we have a term “łamacz wiórów”, which is translated straight from US “chip breaker” and literaly means “the thing, that breaks chips”. This is only way to name this part of a plane. I have just realised that we use this fairly descriptive term to describe the activity, that “this thing” doesn’t do… Silly, but true : )

    Thank you very much Mr Sellers for all you do. It is so helpful and inspiring.

    1. Strange, maybe it’s a regional or jargon thing? The only Polish term I have ever used to refer to the piece in question is “odchylak”, which would roughly translate to something that tilts (inclines?) another thing; “the tilter” or “the incliner”. Which seem pretty spot-on.

  4. Someone linked my article above, I realize I’m only about 3 years late to this blog post. The cap iron, I’m sure, was added to reduce tearout as it cost a great deal more than just thickening the iron.

    In terms of significance, some of the understanding of why it’s so useful is lost when we go to power planing and jointing, there is no need to hit a thickness mark without taking tiny shavings, for example. So, I can’t say much about the usefulness of the cap iron if someone never does dimensioning by hand. In wood that is less than favorable, it cuts dimensioning time in half at least, and the worse the wood, the larger the gain. It also takes the risk out of dimensioning to a mark in highly figured wood.

    The last thing that it does handily is make a common bench plane a platform to plane anything, including what is probably the worst – ribboned/quartered wood where the early wood is very fragile and seemingly ready to break out easily.

    Another example might be in using a badger plane to raise a panel. You can do it very quickly regardless of the orientation of the grain in an old double iron badger plane if the cap iron is set. It becomes a bit more tricky to do with no cap iron, and slowness in a lot of finish planing or scraping will be the result (speaking of working something someone might make furniture of, like cherry or oak, and not so much pine or poplar that’s always used for demonstrations). The modern solution is a router bit set, though, and there’s very little left to understand the context of the double iron.

  5. James A. Clarke

    Hello Mr. Sellers
    Your explanation regarding Cap Irons, Chip Breakers, etc. ……”(they) DO WHAT?”…..couldn’t be more wrong! They do what their name implies! They BREAK the shaving as it passes up thru the Throat in order to prevent Tear Out ahead of the Cutter! Please review the many Technical (University) Studies, and the plethora of info from several experts – Leonard Lee, Ron Hock, Christopher Schwartz, Thomas Lie-Nielsen, to mention but a few.
    Please do your homework before you try to answer a technical question from one of your Cult Followers, about the esoteric design/function of the tools you use, and stick to doing the woodwork itself!
    I was amazed at the cult-like exchange from your follower, and your answer..”…thank you so much for all that you do. You have changed my life…” This is woodworking for Gosh Sakes! Not a religion! But the original sin is you are failing, miserably, your Flock with erroneous edicts and proclamations!

    Have a Nice Day!
    James A. Clarke – Hilton, NY

    1. Paul Sellers

      Oh, right! Still, I see from your tone that you disagree slightly. ‘fraid you’re wrong too though. It’s not the cap iron that breaks off the shaving it’s the fore part of the sole of the plane that prevents the shaving from lifting too far ahead and tearing off ahead of the cutting edge. Nothing ‘breaks‘ per se, when set up properly, the wood is parted from the main body of wood by shaving with the cutter and the shaving is simply diverted upwards into and through the throat as a continuous shaving for as long as it stays in one piece; the cap iron (not chip breaker) diverts the shaving upwards. Nothing really being ‘broken‘ at all. Also, many planes were made with no secondary iron to”break” the shaving, so how is it that they worked for so long, centuries, without a “chip breaker”?
      So, with no chips and nothing chipping, and there are no chips ever, and nothing breaking off or away, it’s still in my view and the views of hundreds of thousands before me, a cap iron and not a chip breaker. You cite manufacturers and journalists as experts, not at all always the best source for independent reviews or information in my view.

  6. James A. Clarke

    Mr. Sellers
    Again, you have a tendency to give short “Flip” incorrect answers to matters that you are obviously deficient on!
    Case in point – Please review Wood Central/David Weaver article regarding research by Professor Yasunori Kawai, and Hon. Prof. Chutaro, Yamagata University Japan, 1989 –
    “Breaking The Chip” – Video showing plane and cap iron assembly forming and resultant visual “chip-breaking” effects, controlling/minimizing/eliminating tear-out! If that doesn’t convince you, also see Kees van der Heiden, The Netherlands, 2014. Also see Leonard Lee (Sharpening) book covering the same issues from a scholarly, controlled, microscopically observed standpoint. Also see Scott Wynn’s “Woodworker’s Guide To Handplanes” ..stating that “the chip is broken backward before it has a chance to lift and split ahead of the cut..reducing tearout…” I could go on since there are numerous authoritative researched sources on this subject!

    Also – Why do you think that Stanley, Sargent, Millers-Falls, Gage, Siegely, and several others, as well as Clifton, Lie-Nielsen, Veritas spent so much effort on this aspect of their respective designs. To control chatter on thin cutter blades? I don’t think so! You, yourself, have already answered that part of the equation – To merely utilize a thicker Cutter! No, this is more esoteric than “chatter”.

    P.S. Much literature also states that the names – “Top Iron”, “Cap Iron”, “Chip-Breaker”, and some others are all the same thing!

    Best Regards – James A. Clarke – Hilton, NY

    1. Paul Sellers

      I have read and seen many if not all the arguments I know of over the years. My reasoning stands in the face of them with regards to the nomenclature. I was in no way “flip” in answering you, I simply countered the aggressive way you approached the matter with a short, concise and clear answer according to my experience and my understanding. I don’t agree with many things said by those presenting what are mostly their opinions. Time to park this now.

    2. James A. Clarke

      Mr. Sellars
      I will agree to disagree with you!
      The matter is now “parked”!
      Best Regards – James A. Clarke

  7. I thought the term ‘Chip’ here meant ‘Shaving’ (whether long, short, narrow, wide, thick, or thin), and ‘Breaking’ in this context meant ‘Curling’ (as in waves breaking), not ‘Breaking off’.

  8. I was taught woodworking in a London school in the 1980s. I can tell you that we called that thing a “cap iron”. In the 1938 Marpels catalogue https://www.toolemera.com/bkpdf/MarplesCat1938.pdf it’s part of the “double iron” assembly and called a “top or back iron”. If course, we say rebate and in American it’s rabbit.

    The japanese research is interesting but to prevent tear out you need to set the cap iron extremely close to the edge and take an incredibly fine shaving. That strikes me as the exceptional case and not the normal one. Presumably the irons would need to be almost dead straight with sharp corners, would would leave tracks in the wood under normal conditions. So whatever the primary purpose is of the cap iron, it isn’t there to prevent tear out.

    I imagine that almost all manual working knowledge was passed down orally. My grandmother, born in 1912 in Croatia, and a peasant, farm worker and farm cook for her whole life. She was almost illiterate but boy, was she great with her hands and knew lots of practical things.

    Thankyou Paul for showing, telling and writing down your knowledge and experiences. Otherwise it would be lost.

    Jack

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