"Cookie cutter walls, magnet pockets and keyholes: the rules a printable part checks while you type"
30 September 2026
Until this week, most generators on alpaco.it checked your numbers in one place: the server. You typed, the page sent the settings, and if the geometry could not work, the server answered with a 400 and a sentence. The sentence was fine. What happened around it was not. The preview stopped updating, and if you typed 120 into a field with a minimum of 20, the server saw 1 first, then 12, and refused both before you had finished the number.
Now every generator page checks the value in the page, as you type, and puts the reason right under the field. The server still checks everything, because it is the part that knows the geometry. The page just knows enough to stop asking it silly questions.
Cookie cutters: walls have to step outward
A cutter printed upright is three walls stacked on each other: a thin cutting edge at the top, a mid step, and a thicker base wall at the bottom that you press on. You pick each one, 0.4 to 1.6 mm for the edge, 0.4 to 2.4 mm for the mid wall and 0.8 to 3 mm for the base.
The rule between them is the one people break. Each wall must be at least as thick as the one above it. A mid step thinner than the cutting edge leaves the edge hanging out over air, and FDM does not print over air. Same for the base under the mid step. The base also has to stay lower than the whole blade, which runs from 8 to 40 mm, or there is nothing left to cut with. And a flange under 1.2 mm snaps when you press it into the dough.
Miniature bases: the magnet sets the height
Round bases are easy. The rules sit on the ovals and on the magnet:
- An oval can't be wider than it is long. At that point it is the same oval turned 90 degrees.
- If you leave the oval width at 0 and let the generator pick it, the length has to be at least 25 mm.
- The bevel stays at least 1 mm under the total height.
- A magnet pocket needs a base 3.2 mm tall: 2 mm of magnet and 1.2 mm of plastic.
Wall hooks: the head is wider than the shank
A keyhole or a screw hole is a head and a shank. The page now refuses a screw head narrower than its shank. Nobody types that on purpose. But both fields are editable, people fill them in whatever order they like, and for a few keystrokes the head is 3 mm and the shank is 4.
The hook is also where testing embarrassed me. I ran every preset on the site, a bit over 190 of them, through the page and the server. One of them failed, "coat hook, keyhole-mounted". I had put its top keyhole at 62 mm on a 78 mm plate. With the default slot and head, the keyhole ended at 79 mm, one millimetre past the top edge, and the server refused a preset I had shipped myself. Nobody reported it, probably because anyone who clicked it saw an error and picked a different hook. The keyhole sits at 60 mm now.
Spacers, and the 0 that means auto
Some fields use 0 as a switch. 0 means auto or off, anything else is a size, and between 0 and the minimum there is a gap where no value makes sense. Those fields now say "0 (auto) or at least N", and a value in the gap snaps to 0 or to the minimum when you press Enter or leave the field.
Spacers got a height range per style. A washer is 1 to 6 mm tall. A round or hex spacer is 2 to 60 mm, and it can't be taller than six times its outer diameter, because a 5 mm spacer 40 mm tall breaks in your fingers.
How I checked it
For every page I loaded the defaults, typed into the fields, then tried 25 random combinations and compared what the page allowed with what the server built. No page blocked a value the server accepts. The other direction is where the work was. Before this round the box page let through 20 combinations that the server then refused, the hook page 14, cookie cutters and gears 10 each. Cookie cutters and miniature bases are at zero now.
The box, gear and funnel rules depend on geometry the page does not compute, like whether a hasp fits between the latch slots. For those, the server's reason appears under the field you changed last, and the preview picks up again as soon as you change something. The box is next, since it is the most used generator and has the most rules the page cannot see yet.