My home workshop's been a surface plate short, as well as 300 quid for acquiring one. A well used but decent 2nd hand polished granite tombstone would set me back £ 35. Could Messrs Whitworth & Maudsley grind their own (literally), so could I whilst stumbling upon three free pieces of 8 x 16" polished white marble. The 190 year old method still works! Pumice, carborundum, silicon carbide or even sand paper will get you going. Having my DIYed "FastWarp" indicator* provides both aural and (with a dial test indicator, 1/1000 of a mm) visual indication of twist. Other measuring stuff (top - down): a Homebrewed 430 mm two-footed twist gage, a 500 mm knife-edge and a 300 mm SS straightedge. NOTE: These are necessary as this method only guarantees flatness for SQUARE plates - mine are rectangular, and cannot be ground at 90 degrees to each other without the overhang creating even greater problems. Meccano "Squeegee" dye roller winged from a butchered laser printer axle. Measures so far: within 10 microns across the surface, but this´ll be improved further. I tried and measured polished granite floor tiles, 10 mm plate & mirror glass with even bigger deviations than my semi-finished ones. Best thing is - when finished I´ll have THREE flat plates, two of whom could be donated to fellow machinists. Work time from roughing 'til finished app 3 h/ plate. * Started its life as an IKEA cheapo Teflon skillet, bottom sawed out and faced, drilled and tapped for 5 M4 bolts as adjustable "legs". My 15 seconds "FastWarp test": Set & fix outer 4 screws for flatness on surface, turn the gadget 90 deg and check for instability- any rocking (> 4 microns) will be heard when lightly poking the outer bolts/ <4 mu indicated for play. -All you really need is a pile of junk and some imagination! H & S caution: -Be certain to bring any & ALL birds out of your workshop before turning/ heating teflon - they´ll croak from the horrible-smelling fumes. Any questions?