Week 2 · computer-controlled cutting

A press-fit kit that builds robots

This week was about making a press-fit construction kit with the laser cutter. Since I work on robots that can take different shapes, I thought it would be fun to make a kit that could become several little robot bodies. I designed bars, notched disks, joint disks with angle markings and a clevis joint held together by pins. The parts all come from one Python-generated cut sheet. The important dimension is the slot width, which I calibrated with a test cut rather than assuming it would fit.

Finished brown laser-cut cardboard quadruped, photographed beside a laptop — The completed cardboard quadruped, built from the press-fit kit.

Step 1: characterize the kerf

Laser-cut press-fit test comb and several cardboard kit pieces on the shop table — The tolerance comb and the first batch of parts. I used test slots to work out the press fit before cutting the full kit.

I cut the test comb first and tried fitting a scrap of the same MDF into each slot. The best fit was the 3.4 mm slot for material that measured 3.7 mm with calipers. That suggests about 0.2 mm of kerf plus 0.1 mm of interference. I used those values for the rest of the kit.

# top of kit_v3_1.py
t, k, FIT_PRESS = 3.7, 0.20, -0.10     # measured thickness, kerf, interference
SW = round(t - k + FIT_PRESS, 2)       # = 3.40, the drawn width of every press-fit slot
FIT_RUN = 0.05                         # extra clearance for the parts that must spin

For a different material, measure the thickness, update t, cut a new comb and choose the best-fitting slot. Then adjust FIT_PRESS so SW matches it and regenerate the sheet. There’s no need to edit the SVG manually.

Step 2: the sheet

The full kit v3.1 cut sheet with bars, cookies, clevis plates, cheeks, pins, lollipops, heads and feet laid out — The whole kit on one 458×403 mm sheet. Black is cut, blue is engrave.

Part	Qty	What it does
Bars S50 / S75 / S110	4 / 4 / 3	16 mm wide struts, 9 mm end slots, mid-bar mortises every 25 mm
Cookies H3 / H4 / H5	1 each	44 mm disks with 3, 4 or 5 radial slots; hubs and hip plates
Clevis joint	4	1 fork plate + 2 cheeks + pins. The disk is captured between the cheeks; pins clamp it
Lollipops	4	24 mm disk on a stem, engraved every 15°. The thing that rotates inside the clevis
Pins	14	8×12 mm, press through the cheeks. Push in to lock a pose, back off to spin
Heads, feet	2, 4	feet are a sole plus a fin tab that mortises into the bar

Most of the parts connect with perpendicular 9 mm slots. The clevis took more work: its pocket has a radius of 12.05 mm (12 mm for the disk plus FIT_RUN), and the opening is a 120° wedge so the joint has room to swing. I initially thought the smaller opening I'd drawn would allow about 35° of movement. Once I checked the cut geometry, it turned out to be closer to 12°. I widened it before cutting the revised version.

Cut settings

Machine	xTool laser in the CBA shop
Material	3.7 mm MDF (prototype: single-wall cardboard)
File	`kit_v3_1.svg`, 1:1, mm units. Black 0.2 mm strokes = cut. Blue = engrave (lines and filled labels)
Cut layer	power / speed / passes as used on the shop xTool, fill in
Engrave layer	power / speed, fill in. All text is outlines, not fonts, because the driver dropped fonts
Order	engrave first, then inner cuts, then outer cuts, so parts do not shift before their ticks are done

Laser cutter working on the cardboard construction-kit sheet — Cutting the kit on the xTool. The engraving and outlines are visible on the cardboard.

Step 3: where it broke, and the fix

My first joints used a snap fit: push the disk through a narrow opening and it clicks into a round pocket. That was fine in cardboard, but MDF doesn't flex in the same way. The disk didn't stay put. I changed it to a three-layer clevis, with the disk held between two cheek plates and press-fit pins keeping everything together. That's the v3.1 design in the files.

Early circular snap-fit joint beside a slot-tolerance test comb — An early joint test. The disk sat in a circular opening, but this was not the final clevis design.

Hand holding a cardboard joint prototype with a slotted bar — Trying the first joint geometry by hand before revising the connection.

Step 4: build the quadruped

Partially assembled cardboard robot on a workbench beside the laser-cut parts — An earlier assembly on the shop table, while I was testing how the leg joints would fit together.

Step	Do this
1	Build 4 clevises: cheek, fork plate, lollipop in the pocket, second cheek, then 3 pins through the stack. Leave pins loose for now.
2	S50 bar end slot onto each lollipop stem, ×4. These are the legs.
3	Foot on each leg: fin tab through the sole, leg bar into the fin slit.
4	Chassis: S110 on edge, H4 cookie on the front end slot, H5 on the rear, cookies flat.
5	Hips: each clevis tail slot up into a cookie side notch, mouth facing down. Two per cookie, one each side.
6	Set all four lollipops to the same tick, stand it up, then splay to pose. Push the pins home to lock.
7	Neck: spare S50 upright in the front notch; head slit onto its top.

The first quadruped had all four hips attached to one central bar, which left the legs almost in a single plane. Its stance was only about 1 cm wide. Using the round disks ('cookies') as hip plates spread the legs to about 13 cm, and the robot could stand much more reliably.

Completed cardboard quadruped seen head-on, standing on a desk — The finished quadruped, now standing with its legs spread farther apart.

Same parts, other animals

Laser-cut cardboard snake made from repeated linked disk joints — The same kit assembled as a snake, with the joints alternating along the body. The CAD overview shows some other configurations.

Vinyl: a mission patch

I also used the vinyl cutter for a sticker based on a drawing I already had: two Mews curled around the moon, combining my favourite Pokémon with To the Moon, one of my favourite games. I cut it in two layers and put it with my NASA and ICRA stickers.

Step	Do this
1	Photograph the drawing flat, trace the outline and the inner shapes as two vector layers (light blue, dark blue).
2	Cricut, standard vinyl setting, one sheet per color, roughly 60 mm wide.
3	Weed each layer with tweezers. The faces are the fiddly part; go slow.
4	Transfer tape on the dark layer, align over the light layer using the moon as the register mark, burnish, peel.

Blue vinyl sheet on a Cricut cutting mat with the design outline faintly visible — The blue vinyl on the Cricut mat after cutting.

Completed two-tone blue vinyl sticker beside NASA and robotics stickers on a laptop — The finished sticker on my laptop.

Reproduce it

pip install shapely
python3 kit_v3_1.py        # writes kit_v3_1.svg and prints the part inventory
python3 quad_steps.py      # regenerates the 7-step, 4-view build figure (needs trimesh, pyrender)

# If you ran out of pins (I did), this cuts 18 spares on an 84×57 mm offcut:
#   pins_only.svg

To remake the kit, measure the sheet, cut the comb, choose the slot and set t and FIT_PRESS before regenerating the SVG. If the joints don’t rotate, try increasing FIT_RUN to 0.10 and recut the fork plates. If the pins are loose, try making FIT_PRESS 0.05 more negative.

Files

kit_v3_1.py	the generator, every dimension a variable
kit_v3_1.svg	the cut sheet as cut, 458×403 mm, for 3.7 mm MDF
pins_only.svg	18 spare pins
quad_steps.py	renders the build steps from the kit geometry

The kit can be assembled in several ways, has engraved angle ticks and part labels, and is generated parametrically. I used Python and Shapely for the cut sheet, an xTool laser cutter and a Cricut for the sticker. I used AI assistance while developing the generator and CAD assembly diagrams.