gernoff
Just to gloat, I have been thinking of doing the same thing myself....... I live in Montana and just to acquire a decent sized surface plate implies criminal shipping charges. I just got my Davidson D600 comparison autocollimator. Measures to 1/10 of a second of arc. Just as you described using 10 second levels, this autocollimator compares two mirrors, on a stationary reference and the other the moving measurement.
So, since I love to keep getting more and more projects started at once (they are all times to be finished before I die), I was pondering buying a bigass chunk of granite locally and lapping it in.
Flame away people.........
Thats the spirit!
. Just be aware that it took me a fair effort to remove a guesstimated 0.0003" over one 6" x 20" area! I dont know what you mean by 'bigass', but if the chunk o granite wasnt formerly a surface plate, you might have to bring out the big guns. Like an angle grinder maybe!
Pete F
My first question, having seen the first pictures, is how is your plate being supported?
Its being supported on a stand that came with the plate. It provision for both four and three point mounting. Im using four points on the floor (which is a concrete slab) and three points to support the plate.
bebop
As you've noted the level is a bit longer then convenient for checking the plate, you can use this to your advantage. The linear sensitivity of a level is inversely proportional to the length of the measurement base, which means that at the 10" ref length of the level one dev = .0005 if you were to put some "feet" on the base of the level the same level will move 2 dev for .0005 rise if the distance between the "feet" is 5.0" if the "feet are 2.5" apart .0004 rise will move the bubble 3 dev. by reducing the apparent length of the level base in your level will allow you to map the plate with finer discrimination, but not necessarily greater precision. the proper lapping of a plate would require a cast iron plate about 12x18 for the bigger areas, a 6 x 6 cast iron block for local high spots and Silium (sp) abrasive and lots n lots of elbow grease. as Gernoff noted an autocollimator provides a high order of precision but its still alot of work and calculations. I use electronic differential levels and a mapping program on my laptop. The "British flag " pattern Forrest described is referred to as a Moddy flatness check or an 8 line calibration pattern in Mill spec GGG-p-463c
Cheers Don
Re the level. If ive got you correctly about the feet, then its along the same lines as the 'table' i mentioned in my reply to Forest. I understand what you mean about the change in sensitivity if you were to move the feet closer together. It would effectively change the angle that the vial sees for a set change in height. Unfortunately my level already has feet per say which makes things a little more awkward, hence the table (with feet) idea.
Re the lapping. It sounds like youve done this before
. Id be interested to read more on the abrasives used, do you have any links? I tried to search Silium but didnt have much luck.
Can i ask. When youre lapping. How do you protect the lap from the heat from your hands whilst rubbing? It was an eye opener to see just how much my camel back bent when using that! Silly when i think of it now, when scrapping it, id use rigger gloves to transfer it from bench to plate and back again, yet i didnt think that using it as a rubbing block (with bare hands!) might be a problem!
Will have another go (using the union jack style method) at mapping the plate today, staying in from edges as Forest suggested (along with oven gloves or some other such measure to try and keep heat out of the level). Ill aim to keep the level as point to point as possible and try and put some numbers to the readings.
Thanks all for the input.