One of the crappy things about using old planes is that a tremendous percentage are worn out. A steel mitre plane (or "infill" to use the modern phrase) unless made by a modern maker will probably be at least 150 years old. Norris, Spiers, and a few other makers continued making mitre planes up until the mid twentieth century but those are rare beasts. The average mitre plane you come across will be pre 1850.
Rotten wood can be replaced, but the most important feature of a mitre or a shoulder plane is a fine mouth. And not just a kind of fine mouth, the finest of mouths, especially if you are using the plane on end grain. The planes in the picture have mouths (with the irons withdrawn) ranging from a fat 1/64" to a fat 1/32". That's very fine.
Let's talk about fine mouths for a second. First of all it is pretty well understood that a super fine mouth on a smoothing plane breaks the shaving and reduces tearout. All well and good. But what about mitre, shoulder, and block planes? All of them are bevel up and used primarily for planing endgrain. Certainly there is no need for a fine mouth if the shaving is endgrain and will disintegrate on its own.
So why do unaltered historical examples of mitre and should planes have such extremely fine mouths?
There are two dimensions that concern us: the open space from the front of the blade to the lip of the throat - the effective mouth. And the absolute mouth opening when the blade is removed. As you can see from the photographs the mouths of these planes - a late 18th century mitre plane and a C. 1920's Norris shoulder plane are ridiculously fine and I would say this is typical of any infill in good condition that I know of. You do get planes that are worn out, planes where someone has widened the mouth, but for any infill plane in basic decent condition a very fine mouth is to be expected.
As we've stated before, planing endgrain doesn't require a plane with a fine mouth, but there are two very important reasons for having a fine mouth, especially when planing with a bevel-up plane.
Extending the iron sole of a plane as far as possible under a bevel up blade gives the blade more support and makes it less likely to chatter. Steel-soled planes can do this easily, but cast planes can't - unless they have a steel sole (like the shoulder plane in the picture). With a bevel down iron, there is a lot of support in the blade to prevent the very tip from bending and chattering. On a bevel up plane, on the other hand, the iron wants to bend and chatter around the edge of the sole. The more support the sole gives the iron, the more strength the iron has at the cutting edge -- and the better the plane will work. Cast mitre planes, by the very nature of a casting, cannot get as close to total support as a steel-soled dovetailed plane, where the steel sole can taper to a knife edge.
Controlling the cut:
If you are planing endgrain, especially if you are holding the plane in one hand and wood in the other, and you hold the plane perfectly against the wood when you start your stroke, you can determine the exact thickness of your cut by setting your plane iron. But if you are even slightly off and the plane is tilted on the wood, your shaving thickness will increase depending on the size of the plane mouth. The second drawing shows an exaggerated example of this. The practical effect of this is that you try to take a fine shaving and your plane jams, skids off the end, and takes an uneven chunk off the edge. Worse, you can damage that nice low angle cutting edge on your iron. A very fine mouth mitigates this and makes the plane easier to use, even if you aren't perfectly sitting on the wood. There is simply less space for the wood to jam into.
These points are small and minor. I understand that. But I get frustrated when someone compares the performance of a worn out 200 year old plane to an new modern plane, possibly of a lesser design. If you are in the market for a mitre plane, or a shoulder plane, make sure the overall mouth is minuscule. Also make sure that the iron and wedge match the plane. It's not at all uncommon for an old infill to have a replaced blade and/or wedge. Just normal use can cause this. Mitre planes had tapered irons and the original iron and wedge would have been fitted together so that you get continuous contact on the bridge. When properly fitted, the iron will set properly, hold its setting, and be easily adjusted. An ill fitting wedge just won't work right. If a parallel iron had been used to replace what was supposed to be a tapered iron, you will never get proper action without adjusting the wedge. Depending on circumstances, you will probably have a replacement iron with the original wedge. If you do and they don't fit, just put the original wedge in a safe place for when you resell the plane, and make a new wedge. Most shoulder planes used parallel irons so any replacement should fit it properly. Check before buying.
Many bevel up planes have cosmetic issues that don't matter, including damaged wooden parts (easily replaced) and misaligned wedges (easily adjusted). But - unlike a bevel down plane - bevel up planes with wide mouths can't be fixed with a thicker iron. You might like the feel of the plane but it won't get the action you would have gotten two centuries ago. Flattening a sole of a bevel up plane can easily, accidentally, widen the mouth. The steel sole behind the blade forms a knife edge and can be damaged. Unlike cast planes which can easily warp over time, steel planes stay pretty flat. A few pits and dings aren't worth worrying about. I would stone down any raised dings, but otherwise leave the sole alone. Before you try to flatten anything see how the plane works.
I was curious about why you are referring to them as mitre planes? I was under the impression they were always called infill planes and mitre plane was a task specific plane. To me a mitre plane is a rather large plane with a skewed blade made specifically for shaving or tuning up a mitre joint with the aid of a shooting board. In my searches for hand planes as a collector(Norris spears is on the very top of my want list) I have never seen the infill plane referred to as a mitre plane.
Let me clarify.
Mitre planes, various styles shoulder planes and rebate planes, chariot planes, and thumb planes are all metal bodies planes with bevel up irons. Steel bench planes are another form of metal bodies plane.
"Infill planes" is the modern term used to describe almost all metal planes of a non-stanley type design. most of which are characterized by wooden infill in a steel or bronze body.
Historically however I can't find any pre- WW2 references to "infill planes" and the term I see was "Steel planes" or "metaled bodies planes"
Mitre planes are earliest form of infill plane. The feature of the extremely fine mouth is found on all infill or steel bevel up planes.
Norris BTW made all these various styles and types of steel plane, their shoulder planes and smoother being the most common. Very few Norris Mitre planes have turned up, but they do exist, the reason for the rarity is that after 1850 or so, long before Norris began making planes, the English mitre was superseded by the introduction of steel/infill bench planes and demand for mitre planes shrank to mostly keyboard instrument makers.
To get back to your comment, skew mitre planes are pretty rare, most are not skew. Early mitre planes were the first form of metal bodies planes and in general were used for general smoothing, not for shooting (although that was a use too). and yes. Mitre planes are one style of infill plane although not all mitre planes are infill.
If you can have just two infill planes my choice would be a smoother and a wide shoulder plane.
Thank you for the clarification and perspective on the subject.
I have yet to find an infill that price and condition are right. Have contemplated attempting to make my own. As a professional woodworker I always have the early edition Stanley No.140 that I restored within reach at the shop. A wider infill shoulder plan would certainly be a welcomed shop partner to my 140.
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