My first custom design in OpenSCAD. The final part is bigger than my dimensions, though. Not quite sure why.
OpenSCAD uses mm for its dimensions. A common mistake is also using diameters in a circles radius field.
I don’t think it’s OpenSCAD itself but rather the slicing process. When I load it into Slic3r it says it will be 53mm, but the printed part is more like 60mm.
Then your printer is likely not calibrated properly. E-steps, for all axis are likely incorrect but tuned for those incorrect steps. (I’ve seen it happen)
print at 20mm x 20mm cube, make sure that it is 20x20
Well, I would agree with you about calibration except I’ve done this 25mm pyramid and the size is correct. http://www.thingiverse.com/thing:24238
I have found that if your outside dimensions are ok but your hole dimensions are not that’s a clue – small holes means too much extrusion large or small perimeters means wrong step value calibration – at least often; other things can cause both, of course. Try printing a cal piece with holes or at least an open area and make sure the inner and outer dimensions are right.
Pla shrinks about 2% so if you print a calibrationcube 2020mm and it turns out perfect it does not mean a cube 100100mm will be correct. In your case it is calibration of e steps that is wrong so your item comes out larger than expected.
0 all axes, mark with a pen, enter 100mm travel on X. Mark again and measure the distance actual travel.
Adjust e steps and when satisfied repeat on Y axis.
By doing this I managed to end up within 0.02mm on 100mm travel.
7mm off is likely not an issue with E steps/mm, but with X and Y steps/mm. Use the values http://calculator.josefprusa.cz gives you. Scratch that. Since the simple does not use toothed belts, you’ll need to calibrate steps/mm, for example by jogging one axis 100mm and comparing that against how far it actually moved.
Also, did you check (e.g. in Netfabb) that your stl actually has the dimensions you’re expecting?
The formula is simple
(old steps value * software move) / hardware move = new steps value