The importance of having a flat sole on a woodworking plane is necessary if you want to minimise tearout and is one important factor in making the plane perform to a high standard, this is not the only factor but I’m addressing this specifically in this article.
The plane’s sole flatness is important if a woodworker intends to use hardwoods with any grain character that might travel in various directions and to keep tearout to a minimum limiting the use of scrapers, filling, and sanding. This requires the user to be able to take the finest of shavings resulting in a seriously smooth refined finish.
In truth, the testing for true flatness can only be done when using a high-quality engineer’s 5″ thick lab-grade granite surface plate which we use here in our workshop to test the planes for flatness in conjunction with a feeler gauge, this is by far the best method when the planer is fully assembled and in an upright position.
Most manufacturers of woodworking planes will use a surface grinder to flatten a sole and the sole’s accuracy will generally depend on the quality of equipment used and how well it is set up. Note that the sole will not have any components fitted to the sole and it will be upside down during this process.
This is why when using surface grinders on an inverted sole with its opposing forces a sole’s casting needs to be of sufficient thickness to help counter these forces and why those premium planes are usually heavier than those old Stanley or Record planes which were never truly flat after they were first manufactured.
This is why hand-lapping a plane was always undertaken by Cabinet makers and often missed by woodworkers who didn’t need to worry so much about the final finish, for example in construction where the wood was painted or not seen.
When a plane is rotated with the sole facing upwards the forces within it change and the longer the plane sole the harder it is to test the sole straightness because it is upside down, hence methods such as using rulers or straight edges are not accurate, especially when it comes to identifying any form of twist. To illustrate this get two rulers, sit them on a flat surface about two inches apart, and lift one end of one of those rulers at one end, they are both straight but not along the same plain. Think about flattening a board, you would use winding sticks to identify high and low spots and any twisting. When testing in fractions of a millimeter it’s difficult, if not impossible to do this by eye.
Using a straight edge does help with one important aspect, to identify any hollowing of the throat that comes about from using rotary wheels that may have been used on the sole. The leading edge for the throat is supposed to hold the wood fibers down firmly in place as the iron cuts and lifts the shaving. I guess over the years people have seen others using straight edges thinking they are testing for sole flatness when in fact they were looking at the throat.
Having flat soles on a surface is the equivalent of working with plumb levels, working with squares, and precise angles whereby the more accurately you work the better the end product.
Of course, planes with uneven soles do play a role where less importance of finish is concerned on rough work or to save your best tools from any abuse.
If you ever wondered why antique dovetailed planes were so highly prized it was because of the construction using two sides that were like straight edges connected to a precision ground flat plate steel sole. These antique planes over many years can wear with use and often need correction today but when found with little use are usually very accurate.
Lapping planes:
I’ve seen many videos online that seem to choose the smallest of planes to demonstrate this process but ultimately the accuracy always depends on the equipment used, methods, and experience when larger planes are concerned and the time/effort one is prepared to put into it.
A standard vintage bench plane’s accuracy can vary from 0.10mm to 1mm before lapping flat whilst twist is common, I have seen worse which end up being split up for parts needed here. Many will know we work to a standard of 0.01mm for small planes to 0.05mm for larger jointer planes with the emphasis being the three points of the toe, front of the throat, and the heel being in plain. It’s important to recognise the larger the plane the more it is likely to minutely flex according to the pressure applied.
The obsessional quest for a perfect 100% zero flatness on vintage jointer planes therefore would take many more hours because it would require zero pressure and lots of very fine sanding materials. Commercially this would be unviable and the benefits beyond what we are doing here would be indistinguishable.
Rust & Pitting.
An important aspect beyond getting a sole flat is the removal of any deeper rust and pitting areas that can affect the surface of the wood, we also try to remove this beyond the flat stage if it isn’t too deep. Light pitting can range between 0.2mm – and 0.5mm deep which we will undertake, heavier pitting beyond this is usually unviable, and once recognised we will usually dismantle these planes for parts. Ultimately where the pitting is and the sole flatness can also come into effect because this can correspond with what material is being removed first.
So as you can see raw unrestored planes have hidden pitfalls and of course the hidden costs of materials and the time it’s going to take to bring them back up to our standards.
And why comparing our vintage planes with unrestored examples is like comparing wood with a fine piece of furniture.
