How much space do you need to leave between parts in your design for them to fit together? Print this set of blocks and pegs to help get an answer.
Start with the
Block_x_2.stl unless your process is super-accurate. Then print the pegs. By testing the which hole the peg fits in, you'll get a sense of how much space you need to leave in your design for parts to fit together.
BlockPlus.stl is useful for more accurate printing processes, like SLA or SLS. Only rarely have I needed the negative sized holes in
More details on my blog post, The Fit Testing Block. There you can read my experiences trying this with nine different printing services and materials.