You might recall that the cap iron is the 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.
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:
A précis of the debate is below:
- As the title of the thread indicates, there is a 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. Having said that, the tradition for using the cap-iron to control tear-out – discussed later in this article – does indeed seem to have stalled, at least in the UK, and a number of experienced woodworkers who were trained as apprentices in the 50s and 60s were not taught it at all. While it is a stretch to claim internet forums rediscovered the technique, it is fair to say that they have made ensured its renaissance .
- 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 face upwards instead
- the final group say that tear-out is not a common problem when using a sharp plane and besides is easily rectified with a scraper and therefore does not justify all the energy that gets expended discussing the setting of cap irons.
Obviously this single thread 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?
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 have been 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.
What Bailey describes in his invention4)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:
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’ 5)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 the contributors 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 6)published by C J Hampton of Record tools fame has, on page 20:
And on 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 edge of 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 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 have a similar 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 a commonly selected angle (50°) was given its own name (“york pitch”) compared to the standard pitch (~45°)7)other less common angles are Middle Pitch (55°) and Half Pitch(60°) that are often used on moulding planes..
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.
As I have tried to illustrate in the sketch 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 talented woodworker and a natural explainer of things. In the video below he does a practical demonstration of these ideas:[youtube width=”75%” height=”75%” autoplay=”false”]https://youtu.be/1bhh6kxXZOQ[/youtube]
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 .[youtube width=”75%” height=”75%” autoplay=”false”]https://www.youtube.com/watch?v=pdk4uRYioZg[/youtube]
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.
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
setting up your cap iron
- polish the curved section at the front of the cap iron to reduce the resistance when the shavings are deflected by it:
- 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.
EDIT March 2017
As we have seen from the above, if you are experiencing tear-out you can try setting your cap iron very close (“as close as you can get it”) 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. 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.
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.
And on that final point, it worth noting that many woodworkers regard chatter as a minor problem – uncommon and easily avoided – even with thin irons. However, that did not stop manufacturers from coming up with increasingly ingenious inventions in attempt to prevent it all together. You can read about two of these inventions here: The Record Stay-Set Cap-Iron and the Millers Falls two part lever cap.
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. Having said that, the tradition for using the cap-iron to control tear-out – discussed later in this article – does indeed seem to have stalled, at least in the UK, and a number of experienced woodworkers who were trained as apprentices in the 50s and 60s were not taught it at all. While it is a stretch to claim internet forums rediscovered the technique, it is fair to say that they have made ensured its renaissance|
|3.||↑||in a bailey bench plane the bevel faced down towards the wood surface, in bevel-up planes they face upwards|
|4.||↑||you can see the full patent specification here https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US72443.pdf|
|5.||↑||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|
|6.||↑||published by C J Hampton of Record tools fame|
|7.||↑||other less common angles are Middle Pitch (55°) and Half Pitch(60°) that are often used on moulding planes.|
|8.||↑||Note 4 thousands of an inch is a tiny distance – about 0.1mm|
|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|