// Slots from a little narrower to a little wider than the material.
// Push a scrap of the same sheet into each slot: if the one marked 0 is
// snug, $kerf and $thickness are right. If a + slot fits best, your
// material is thicker than you said; if a - slot does, it is thinner.

/* [Material] */
$thickness = 3; // [1:0.5:10]
$kerf = 0.15; // [0:0.01:0.5]

/* [Slots] */
// How many slots
steps = 7; // [3:2:11]
// Change in width from one slot to the next
step = 0.1; // [0.05:0.05:0.3]
// How deep the slots go
depth = 15; // [8:1:30]

pitch = $thickness + 8;
width = steps * pitch + 4;
deltas = [for (i = [0:steps - 1]) round((i - (steps - 1) / 2) * step * 100) / 100];

difference() {
  rect([width, depth + 12], r = 2);
  for (i = [0:steps - 1])
    translate([pitch / 2 + 2 + i * pitch - ($thickness + deltas[i]) / 2, 12])
      rect([$thickness + deltas[i], depth + 1]);
}
engrave() for (i = [0:steps - 1])
  translate([pitch / 2 + 2 + i * pitch, 4])
    text(str(deltas[i] > 0 ? "+" : "", deltas[i]), size = 3.5, halign = "center");