The technique we were taught to get sides parallel was to measure the thickness of each corner with a micrometer. Mark whichever one is the thinnest, then scrape that entire face except that corner. Repeat many times, and the new face will converge on parallel with the old one. Since you don't actually care how thick it is, you don't even need to use something as advanced as a micrometer. Something like an uncalibrated height gage would work just as well.
In the class, we kind of cheated to get the right angles, by using a right angle block on the surface plates. We did that last, on the thin face, so we could easily hold the wide sections flat against it.
If you were going to do it completely from scratch, you'd need a surface plate plus 3 blocks (A, B, C). Using the normal techniques, flatten one side of each of the blocks (sides A1, B1, B2). Next, place A on the surface plate, A1 down. Ink the unfinished face, A2. Put B1 down on the surface plate as well. Keeping A1 and B1 flush with the surface plate, rub A2 against B2. Scrape the highpoint, then repeat with B2 and C2, etc. It would be fun to do, but I haven't even found time to start with the little surface plates I got. =\
no subject
In the class, we kind of cheated to get the right angles, by using a right angle block on the surface plates. We did that last, on the thin face, so we could easily hold the wide sections flat against it.
If you were going to do it completely from scratch, you'd need a surface plate plus 3 blocks (A, B, C). Using the normal techniques, flatten one side of each of the blocks (sides A1, B1, B2). Next, place A on the surface plate, A1 down. Ink the unfinished face, A2. Put B1 down on the surface plate as well. Keeping A1 and B1 flush with the surface plate, rub A2 against B2. Scrape the highpoint, then repeat with B2 and C2, etc. It would be fun to do, but I haven't even found time to start with the little surface plates I got. =\