All to often we see video’s online showing how to lap planes flat and one of the main points that always stand out is their reluctance to show how to tackle the larger planes. The obvious answer to this is simply that this is no easy task and requires far more effort and work to undertake correctly.
Secondly there is usually no engineers surface plate that is used to reference the accuracy and many dubious ways this process is undertaken. Generally though most workshops would struggle to warrant the costs of a lab grade granite bed to test the results of lapping their plane on something less accurate. That’s the conundrum many people have yet to solve!
As a little experiment I took a used No: 7 plane to a couple of local engineering companies to see whether they could flatten the plane on a surface grinder. Firstly there was a matter of holding the plane and supporting the entire length which generally requires making something for this purpose, secondly the sole would be placed upside down and removing the frog etc which apply pressure to the sole. This method has to be questioned for accuracy and probably why manufacturers of planes rarely make perfectly flat planes. Also there is the cost aspect, not a cheap job that’s for sure and usually far more than the plane is worth.
The simple fact is lapping a plane correctly requires a perfectly flat surface and the plane has to be assembled ready to use, working the plane by hand with lots of effort. The flatter it is the finer it will work, providing your have tuned the rest of the plane correctly.
The next time you see an old hand plane know this, they are never flat unless they are refurbished correctly. You can compare prices but never the flatness or the back breaking work it’s going to take, even if you are fully equipped for this particular job.
This is only one extra aspect of refurbishment of old planes we undertake so that you don’t have to.