Often an old plane may need to have some work to be done on it before it can be used and having a flat sole and frog is essential. 

We test our planes for flatness with a feeler gauge on a dead flat granite inspection block to the tolerance of 0.038mm or 0.0015” (1.5 thousandths of an inch) – if the plane is very flat you will feel it suck on to the flat surface and 10mm float glass is a good alternative, if an inspection block is not available.  If the sole is damaged or not flat we re-flatten it as follows.

Take some self-adhesive 80 grit Aluminum Oxide paper and stick it to 10 mm float glass, or tape/clamp it to a flat surface such as a Surface Planer bed (do not let any grit get into your machine bearings).  Wind the plane blade into the body of the plane so that you do not damage the blade edge although this probably needs re-honing any way.

After a few strokes on the abrasive, you can see how flat your sole is and therefore how much more work you need to do.  Keep flattening the sole on the 80 grit paper until those hollows or pits have disappeared, testing for flatness all the time.

You will be left with some fairly coarse scratch lines, so now use 120 grit Aluminum Oxide abrasive to remove the scratch marks and keep flattering the sole, moving up through the grits until you have a very clean and flat sole.  Once you are happy with the sole of your plane you need to work on the frog seating. Remove the lever cap and blade; unscrew the two machine screws under the blade to release the frog from the plane body.  The underside of the frog and plane body will have machined faces that should be a good flat surface for the two to make an intimate fit, but sadly they are often not very well machined.

I cover how to match these surfaces and re set-up the plane to achieve the best results on my beginner’s furniture making course - have a look through my website to see some of the other tricks we teach.