bench planes – the cap iron

You might recall that the cap iron is the steel metal part that is held on top of the cutting iron with a shallow bolt.   Thus joined the parts are secured on to the frog by pressure from the the lever cap.1)in old books planes with this arrangement are often referred to as double irons for reasons that are obvious when you discover that here is a long tradition of planes with a single cutting iron, held into place with a wooden wedge.    In modern USA parlance the term used is chip breaker which I believe originated with electric planning machines.

plane drawing

What is it for?  You’d be forgiven for thinking this would be a straightforward question, but as the below discussion illustrates, this is not so:

http://www.ukworkshop.co.uk/forums/where-did-the-knowledge-about-the-capiron-get-lost-t93672.html

The thread is very long,  but in the spirit of thorough research, I read all 300 posts and can  provide you with a handy précis below:

  • As the tile of the thread indicates, there is the view that understanding of the cap-iron function was somehow lost from common knowledge during the early part of the 21st century 2)if you read the thread, the argument is made convincingly that what actually happened was a combination of the amplifying effect that internet forums have on unsubstantiated but oft repeated opinions; and contemporary interest in a design of “bevel up” planes that do not use cap irons is the most likely explanation – in other words the knowledge was not lost at all.
  • The “forgotten” purpose of cap irons involves a phenomenon called ‘tear out’ that occurs when plaining  (this is where small sections  of wood are pulled – rather than cut – from the surface leaving an uneven finish).
  • Some people believe adjusting the cap iron so that it is very close to the blade edge reduces tear-out
  • others think that this makes no difference
  • some people agree the cap iron does reduce tear-out if you adjust the cap iron as above, but that the result is the plane is more likely to choke with shavings, and that this outweighs the benefit
  • others too think that planes with cap irons are an unnecessary complication and you can do everything you need with  a “bevel up” plane 3)in a bailey bench plane the bevel faced down towards the wood surface, in bevel up planes they point upwards instead
  • the final group say that tear-out is not a common problem using a sharp plane and besides is easily rectified with a scraper and therefore not worth all the energy that gets expended discussing the setting of cap irons.

Obviously this single thread – whopper that it is – is not necessarily representative of the gamut of views on cap irons, but it gives you a sense of the air of mystery that surrounds them.

So what is going on?  To get to the bottom of it, we we have little choice but to science the shit out of it 4)thanks to Matt Damon, the Martian for this quote

Historical perspective

Let us begin by looking to Mr Bailey  who – as the inventor of the eponymous design used by Stanley, Record and many other plane makers – must be something of an authority on the matter of plane design.  He did not invent the cap iron and indeed refers to it in a patent from 1867 as follows:

Figure 3 is a longitudinal section of the plane-iron and cap-iron, as ordinarily constructed, that is to say, with one bend, a, only near the bearing-edge of the cap iron. (my emphasis)

US72443 - front page

US patent # 72443

What Mr Bailey describes in his invention5)you can see the full patent specification here https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US72443.pdf is the modification that we are now familiar with in bailey planes, where there is a small hump at the end of the cap iron:

curve

He says:

My object is to use very thin steel plane-irons, and in so doing I find that they are liable to buckle under the pressure of the cap, which causes them to chatter, and makes them otherwise imperfect…

When thick plane-irons are used, their stiffness may resist the pressure of the cap sufficiently to prevent buckling or rising of the plane-iron from its bed; but in thin steel plane-irons which I use, the pressure of the cap upon the projecting portion of the plane-iron causes this portion to yield slightly, and of course produces buckling at some point behind, and generally close to the fulcrum. To prevent this buckling or rising, and still use the thin steel plane-irons, I put an extra bend in the cap, so that it shall have a point of impact with the thin steel at the place where it tends, from the pressure on its projecting edge, and the fulcrum behind that edge, to rise from its bed, and thus I effectually prevent buckling and chattering, whilst I can avail myself of the economy of thin steel for the plane-irons…

The difficulty experienced from the construction of the cap iron with the single bend …, is, that it allows of vibration of the cap-iron and the plane-iron while in use, such vibration being productive of what joiners term chattering, and consequent defective operation of the plane.

Thus, in Leonard Bailey’s metal plane design, the cap-iron plays a part in reducing the possibility of the thin irons he was using from flexing and creating ‘chatter’ 6)this is where pressure on the blade causes it to flex back and forward so that it does not maintain even contact with the wood .

However, it is important to note that Bailey is trying to resolve an issue that is created by his attempt to use very thin irons (traditional wooden planes typically had thick tapered irons and did not suffer the same problems).

In fact it seems that the cap-iron was introduced at least 100 years before Bailey’s suggested improvement.   There was a discussion on the old tools list that mentions an advert in the Pennsylvania Chronicle (Philadelphia) placed by S. Caruthers in 1767:

“double iron planes of late construction far exceeding any tooth planes or uprights whatsoever for cross grained or curled stuff”

You can find the original thread archived here, along with many other learned and interesting comments on old tools: http://swingleydev.com/ot/get/71734/thread/

Thanks to bugbear in the above mentioned ukworkshop thread we know that one of the seminal works on how to use bailey planes properly, the 1934 edition of Planecraft 7)published by C J Hampton of Record tools fame has, on page 20:

cap_iron_1934

And AndyT of the same thread:

The cutting iron having been sharpened, the top-iron is screwed fast at the required distance from the edge, say for coarse works one-sixteenth, and for fine work, one fortieth or fiftieth of an inch.

 Holtzapffel, Turning and Mechanical Manipulation Volume 2, 1847 p 497

The position of the break iron is of great importance. The nearer its edge is to that of the cutter, the harder will be the work of planing, and the thinner the shaving, supposing the plane to be set “fine,” ie with its edge projecting but slightly beyond the sole. Hence it is usual to set the the break-iron one-sixteenth from the edge for the first roughing-down process, and then to re-sharpen the blade and set the break iron but very slightly above the other, and thus to finish the work.

 James Lukin, Carpentry and Joinery for Amateurs, 1879 p25

Thus when the jack-plane is required for heavy work, that is to say, for hacking down a rough and uneven surface, the edge of the break-iron should be about 1/8 inch from the edge of the cutter, but for finer work it should not be more than 1/20 inch from the latter; and in the smoothing-plane the distance between the edges of the two irons should be less than this – indeed so slight as to be perceptible but nothing more. The higher the break-iron, the easier the plane will be found to work, and the lower it is the heavier the plane will work, but the cut will be cleaner.

Francis Young, Every Man His Own Mechanic, 1882, p 166

For fine work the cap iron of the jack plane should stand back from the edge of the cutting blade almost 1/16 in.; whereas for rough planing, the distance may be increased to almost 1/8 in. The smoothing plane and the trying plane require the cap iron setting back from the cutting edge about 1/32 in. Steel smoothing planes will require a finer set than 1/32 in. when used on hardwood. No hard and fast rule can be given for setting the back iron; it is one of the points that will come to the worker by experiment and experience, and the above measurements are given as a general guide.

William Fairham, Woodwork tools and How to Use Them, 1922, p97

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.

 Nicholson’s Mechanic’s Companion (1812).

So we know that the cap iron was being advertised as an aid to working with difficult wood over 200 years ago, and that in order to benefit various authorities said you had to set it close to the blade. There is no consistent direction on how close – perhaps the clearest instruction being from Planecraft, namely “as close as you can get it”.

How so, though?

The exciting news is that there is actually some proper scientific research to refer to here, but first the theory.

The theory of how cap irons reduce tear-out

Once the plane blade starts to cut into the surface of the wood, the shaving rides up the blade and acts as a lever on the fibres ahead of it – the fulcrum being the leading edge of the mouth and the blade itself acting like a wedge.   The leverage can cause other fibres that are connected to the shaving to be pulled from the wood before they are cut by the edge of the blade (“tear-out”).

One way to reduce the leverage exerted by the shaving is to adjust the frog so that there is a very small opening between the front of the mouth and the blade, thus effectively reducing the length of the unsupported shaving.

You can also see how taking thin shavings would will have a similiar effect, since the shavings are weaker and break before they can lever out any other fibres in the wood.

The other techniques involve causing the shaving to break early, for instance by raising the ‘angle of attack’ of the blade.  That is to say, if you  increase the pitch of the blade it causes the shaving to bend more sharply than would normally be the case, and this weakens the fibres immediately in front of the blade and, thus weakened, they are less likely to lever out fibres they are attached too.

One way to accomplish this is to use a modified frog that seats the blade at a higher angle than normal, and indeed so common was this technique in days gone by that the preferred angle (50°) was given its own name (“york pitch”) compared to the standard pitch (~45°).

The argument follows that the cap iron has the same effect, presenting a steep surface to the shaving and thus creating an effective higher angle of attack.

shavings

As I have tried to illustrate in the hyper-realistic artist’s impression above, this effect is more pronounced the closer the cap iron is to the edge of the blade.

If you imagine the grain in the drawing above running in the other direction you can see how tear-out gets worse when you encounter grain that runs in the opposite direction to the plane’s forward motion.   This is because the tears will follow the grain direction removing material from behind the point that the plane blade has already passed and leaving an uneven surface.

In case my draftsmanship has left you none the wiser then I commend to you the excellent Richard Maguire, who is a very talented woodworker and a natural explainer of things.  In the video below he does a practical demonstration of these ideas:

http://www.theenglishwoodworker.com/cap-irons-tear-out-video/

So that is the theory, and here is some research that shows it actually happening at a microscopic level.  In Professor Kato’s study shavings of 0.004”  reliably caused tear-out in his experiment, and this was prevented by adjusting the machine’s chip breaker to be 0.004” from the blade edge 8)Note 4 thousands of an inch is a tiny distance – about 0.1mm – a gap that might be tricky to set in practice..

There is a translation on Steve Eliot’s web site 9)also contained is a lot of Mr Eliot’s interesting independent research into the effect of wear on plane blades.

http://planetuning.infillplane.com/html/chipbreaker_study.html

So according to this evidence and the experts quoted above, adjusting the cap-iron to be very close to the cutting edge can help reduce tear-out.

Avoiding tear-out in practice

It seems clear that one important factor in avoiding tear-out is skill in picking timber that is not likely to be a problem in the first place.
I suppose that if at some point you will encounter woods that are hard to plane without tear-out then perhaps it is good to know a bit of the theory, but it does not take long watching and listening to real woodworkers to see that what really counts is the skill and experience they have acquired over years of practice.
In this light, one thing that intrigued me is that skewing the plane while in use is sometimes recommended to avoid tear-out in some circumstances.  This is counter-intuitive, since skewing the plane reduces the angle of attach 10)think of walking up a steep hill by climbing directly up it, rather than walking along a winding path that snakes up the hill: the indirect route is easier because it is at a lower pitch.     My explanation for this – again based on observation rather than woodworking knowhow – is that skewing the plane increases the shearing effect (imagine cutting into a tomato with a slightly blunt knife, you would naturally use a slicing action to start the cut rather than simply pushing down).  It seems skilled woodworkers learn to feel/see/hear where there is a change to the grain and adjust their approach accordingly, sometimes pushing the plane head-on (e.g where the grain changes to run towards them) and skewing it where the grain is in their favour.   It is no doubt one of those areas where practice beats theory.

setting up your cap iron

Even if you are not worried about tear-out there are a couple of things you can do to your cap iron that might make it work more effectively:
  1. polish the curved section at the front of the cap iron to reduce the resistance when the shavings are deflected by it:
    polish

    polished version on the right – fine sandpaper  followed by a little autosol does the job

  2. slightly undercut and flatten the front edge – this will ensure a close fit to the blade and prevents bits of shavings jamming in the gap between the two and clogging the plane.  It is easily done by holding the cap iron as in the picture below and rubbing left and right on some sandpaper or another flat abrasive surface.
flatten

Raise the abrasive the abrasive a little so the underside of the cap iron is undercut slightly, making for a tight join when cap iron is pressed to the blade.

EDIT March 2017

It seems some people struggle to get a good fit by slightly undercutting the front part of the lever cap (see comments below).  Here is another method to try:
“Having sharpened the back-iron to a fine edge on the oilstone, screw it fairly tight to the cutter, leaving about 1/16″ [1.5mm] of the latter exposed. Next holding the pair very firmly on the bench, draw a hard bradawl, or the broken off end of a sawfile, along the edge of the back-iron; this will strip off a fine shaving of metal, and the operation should be repeated until it is impossible to see the slightest trace of light between the irons when the edge is held in a strong light, and the joint at the side, which is looked through, is shaded by the hand.”George Ellis - Modern Practical Joinery (1902)

Conclusion

As we have seen from the above, if you are experiencing tear-out you can try setting your cap iron very close to the tip of the blade to see if it helps.  There is a knack to it, but it is not difficult – you can set the cap-iron pretty close before tightening the screw and then gently tap the screw to make final adjustments 11)Incidentally, one of the things you may notice when you experiment is the way the shavings escape from the plane changes according to how fine the cap-iron is set:  Too close and the shavings crinkle up like an accordion, very close and the shavings straighten up, slightly less close creates a sort of wavy ribbon effect.   Apparently similar effects are seen with high-angle planes, too, but I haven’t got one so can’t try it out myself!
The disadvantage of a very close setting is that – like high angled planes – it is harder to push the plane when set up this way.   No doubt experience will tell you when it is needed.
We will never know for sure why the cap-iron was invented, but it seems plausible that it was initially introduced to emulate the effect of a high angle plane but with the added convenience of being able to revert back to a normal setting when tear-out is not a problem (thus creating a much more flexible tool).
Subsequent alterations to the design were made to resolve issues with thin irons, namely their increased propensity to ‘chatter’ compared to thicker irons.
Finally, It should be noted that many woodworkers regard chatter as a very minor problem – uncommon and easily avoided, even with thin irons –  but that did not stop manufacturers from coming up with increasingly ingenious inventions in attempt to prevent it all together.  Look out for a future post on 2 part cap-irons and 2 part lever caps!

References   [ + ]

1. in old books planes with this arrangement are often referred to as double irons for reasons that are obvious when you discover that here is a long tradition of planes with a single cutting iron, held into place with a wooden wedge.    In modern USA parlance the term used is chip breaker which I believe originated with electric planning machines.
2. if you read the thread, the argument is made convincingly that what actually happened was a combination of the amplifying effect that internet forums have on unsubstantiated but oft repeated opinions; and contemporary interest in a design of “bevel up” planes that do not use cap irons is the most likely explanation – in other words the knowledge was not lost at all
3. in a bailey bench plane the bevel faced down towards the wood surface, in bevel up planes they point upwards
4. thanks to Matt Damon, the Martian for this quote
5. you can see the full patent specification here https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US72443.pdf
6. this is where pressure on the blade causes it to flex back and forward so that it does not maintain even contact with the wood
7. published by C J Hampton of Record tools fame
8. Note 4 thousands of an inch is a tiny distance – about 0.1mm – a gap that might be tricky to set in practice.
9. also contained is a lot of Mr Eliot’s interesting independent research into the effect of wear on plane blades
10. think of walking up a steep hill by climbing directly up it, rather than walking along a winding path that snakes up the hill: the indirect route is easier because it is at a lower pitch
11. Incidentally, one of the things you may notice when you experiment is the way the shavings escape from the plane changes according to how fine the cap-iron is set:  Too close and the shavings crinkle up like an accordion, very close and the shavings straighten up, slightly less close creates a sort of wavy ribbon effect.   Apparently similar effects are seen with high-angle planes, too, but I haven’t got one so can’t try it out myself!

6 Responses to “bench planes – the cap iron

  • David Weaver
    1 year ago

    I’d caution two things in use of the cap iron:

    1) The idea of shooting for .004″ gives an arbitrary and unnecessary target. You use a cap iron for two weeks (and no scrapers or high angle planes) and you’ll learn pretty quickly to set the cap iron distance by eye and sense. It is set different distances from the edge on different planes (on a try plane, for example, maybe about twice the distance of the thickest shaving, but who knows…there’s never a reason to measure. The feedback that you get from the planed surface and the continuous (a tearout free shaving should be continuous once a board is flat) is all you need.

    2) The kato and kawai work was done for the purpose of the super surfacer. We don’t work the same way as individuals, and we will not be doing something in general like setting the cap iron distance equal to a shaving thickness. It presents unnecessary resistance and makes the surface quality lower. We want the cap iron to hold the shaving in place until it is cut, and that is about it. We’re aiming to get a surface that was cut by a 45 degree plane, and one that will look like it on soft or hard woods, not to get a surface that looks like it was scraped or planed by a very high angle plane.

    It’s not the same as a high angle cut or a scraper plane cut. We not trying to break the chip, we’re just trying to hold it in place until the iron can sever the wood.

    In practice, the try plane and smoother should be set with a cap iron (if someone is doing heavy work with planes) that eliminates tearout or keeps it minor enough that is is removed with no additional work in the next step. For example, if someone is measuring shavings, you might want to take a 7 or 8 thousandth shaving out of cherry with a try plane, and you can tolerate very minor tearout if it is something that will be removed in the process of smoothing, but without taking more passes than you otherwise wood smoothing.

    You always want to plane with the grain in general, but sometimes it’s not possible to go end to end and side to side on a board, thus the cap iron is helpful. For jack plane type work, you want to rely more on working with the grain – setting the cap iron too close limits your ability to adjust the shaving depth when you have two more steps to remove tearout. In the worst wood, you might want to set the cap iron just close enough so that it prevents absolute disaster.

    • thanks David – you are quite right about the exact measurements being unimportant compared with the end users experience in setting the cap iron to task, and everything I have read and seen from other experienced woodworkers confirms the same.

  • It’s not necessary to use the Wayback Machine to reach my website. It’s still there at http://planetuning.infillplane.com/html/chipbreaker_study.html and there have been some revisions since 2006.

    Glad to know you find the material there of interest.

  • >It is easily done by holding the cap iron as in the picture below and rubbing left and right on some sandpaper or another flat abrasive surface.

    This is not easy, I have ruined two cap irons using this advice. It will cause a convex edge which will render the cap iron a clogged up mess.

    You have to be extremely careful doing this.

    • I am sorry to hear you had trouble Tom – I have read that other people have had difficulty setting up their cap irons, but I can only speak from my experience.

      A couple of remarks about my set-up might be relevant – I am using a DMT diamond stone, which I know is flat. All of the used oil stones I’ve bought have been worn so they are ‘dished’ along the length to one degree or another – if I had used those then the cap iron would have taken on the same profile and no doubt I would have had issues.

      Second, I have only done this with old Record cap-irons, which like Stanley are made from a bent piece of soft steel and are very thin. This means very little pressure is needed to remove material from the leading edge , which I found made it easy to keep the cap iron flat to the stone. None of my old cap irons had a lot of damage either – just the odd nick and dent – and that may explain my success. I suspect it would be a more difficult job on modern high-end cap irons, which are generally much thicker (although one would hope they did not need adjusting in the first place!)

      Finally, as you can read elsewhere, I flatten the face of my irons – if they were not flat at the point the cap iron beds down then, no matter how much work I did on the cap iron, it would never fit correctly to the blade.

      I hope that extra information helps someone!

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