Flattening a plane sole with aluminium oxide finishing films
This Juuma no. 4 smoothing plane had a slightly hollow sole, and encountered a piece of metal against its front left edge during planing (see image.) The resulting dent on the front led to a small bulge on the bottom edge. This provided an opportunity to use our new Alox finishing film for dressing the sole.
We selected a sheet of Alox finishing film with 100 µm particle size (about P 150) for the initial flattening, and a sheet with 40 µm (about P 400) for removing the grooves left by the coarser sheet. The sheets have a size of 230 x 280 mm and are comparable in size to normal sanding sheets. Since no matching glass plate was at hand, we decided to use a granite measuring plate with dimensions 457 x 152 x 51 mm. This also allowed us to have both sanding sheets affixed simultaneously to the surface. Since the surface of the granite slab is a little less than the area of two abrasive sheets, we trimmed the rough sheet to 150 x 280 mm and the fine sheet to 150 x 170 mm, and affixed both to the slab. The Alox finishing sheets have a self-adhesive backing. Make sure that the surface the sheets are being fixed to is completely flat and free of grease.
Having dribbled a few drops of water on the finishing film we were ready to go. We moved the plane in a circular motion over the magenta grinding surface with 100 µm grit size.
After about 15 minutes this is what the plane sole looked like. The dark stripe along the side of the sole shows where the abrasive has abraded the surface and where not. There was still lots to do, especially since in the vicinity of the plane mouth the entire width of the sole must be abraded. It is worth mentioning that progress tends to slow down. That has two reasons: first, the surface that is contacting the abrasive is increasing; and second, the abrasive grit has worn away. It's time for a fresh sanding sheet.
The spent sheet was easy to remove; smaller remnants of glue were removed with a spatula and alcohol.
After affixing a new sheet, we continued. As you can see, we temporarily removed the frog (the steel screws) to prevent water from getting between the frog and the rest through capillary action. This was to avoid subsequent rusting. Look at the bright spots either side of plane mouth, and the larger light strips in the middle of the back, where the abrasive has not yet abraded the surface. What appears on the front of the sole as bright spots is diffused light, which shows that the abrasive did its job here. The black strip at the edge is swarf (grinding sludge.) It is clear that we are already making really good progress, but we’re not done yet. This was a stage where progress had once again slowed down and things even got a little difficult. Time once more for a fresh sanding sheet.
Once the abrasive action had reached the deepest point near the plane mouth, we switched from a circular grinding motion – even on the coarse magenta sheets – to a back and forth motion to obtain a uniform grinding pattern. Then the plane was placed a little further to the right and with the same movements – back and forth – we continued with the fine sanding on sheets with a grit size of 40 µm (about P 400).
The finished plane: you can still see a dark stripe at the back. That is where the abrasive had not abraded the sole. The inward curve is no longer measurable with the means at our disposal, since it is less than 1/50 mm, and most of the sole has a very good finish. We decided at this point to stop because any additional work would not significantly improve the performance. The plane at this point would not experience any restrictions as far as its functionality was concerned. Of course, you can also smooth out the last area at the rear, the drive for perfection knows no bounds. After you have flattened your plane, dry it well and lubricate it to avoid any rust forming later. The whole process as illustrated took about 60 minutes. It is up to you how much time you want to spend on the job. If you want to speed things up change more frequently to fresh sanding sheets. But the process will then become slightly more expensive.