Keep a film scanner adjustable

The scanner is an assembly of printable parts, not a single hero mesh. Rail, channel, film insert, tunnel, cover, and moving lens carrier meet at named interfaces. Read each source as a contract between parts before treating it as a finished object.

The interfaces are the model {#interfaces}

Three relations govern this mission. fit_clearance expands a female rail channel around its male rail. The film-format branch derives aperture dimensions from one selected value. thread_clearance separates the helicoid ridge from its matching socket. Keep those names at the point where the two features are derived; copying a number into each part makes later adjustment unsafe.

Make the rail and channel agree {#rail-fit}

The first source is a small two-part coupon. A triangular rail profile is extruded into the base part. The tunnel part derives channel_h and channel_w by adding twice the shared clearance before extruding the matching cutter. This is deliberately smaller than the final scanner: it lets you inspect one male/female relation without confusing it with film format or focus motion.

Change rail tip width, rail height, or clearance only through its named parameter. A changed rail with an unchanged channel is a broken interface even if each part still renders.

Branch by film format {#format}

film_format is one select control. Nested if expressions derive a frame width and height for 135, 120 6×4.5, and 120 6×9; the insert blank and aperture cut then consume those derived values. There is no copied 135 insert or copied 120 insert. This keeps the aperture centered on the same rail and tunnel datum.

The worked scanner subassembly expands that pattern with insert clearance and a base plate. Read the branch first, then follow frame_w and frame_h into the two cutter dimensions. Do not change a downstream box dimension to make a format fit: correct the branch relation instead.

Inspect the final scanner {#scanner-final}

The final scanner retains independently printable base rails, lower guides, upper clamp, tunnel, top cover, and moving lens carrier. The film insert is a two-piece stack: lower guides carry the supportless male rails; the upper clamp derives the matching female channels with named join clearance. The base/tunnel and top-cover rails use the same clearance relation introduced in the coupon.

The helicoid is also a pair. The top cover cuts its socket with a female helical-ridge; the carrier fuses matching male ridges, both using pitch, depth, and thread clearance derived from the same controls. The preview proves part placement and branch structure only. It cannot prove a smooth sliding rail or a turning thread.

Calibrate the moving interface {#coupon}

The calibration source is a bounded mechanism study. It exposes one female helicoid ridge, a central bore, repeated radial stops, and a sampled focus knob. repeat-union makes stop geometry; repeat-compound keeps witness ticks grouped without fusing them. The common, intersection, and xor shapes are comparison witnesses, not extra scanner parts.

Start with pitch and thread clearance. Print a short threaded sample and turn it through a few stops before changing the full carrier. Do not infer a production fit from a preview or from a Boolean that happens to compile.

Print fit before finish {#print}

Print two small coupons before the complete scanner: a rail/channel section and a short helicoid/socket section. Use the intended material, nozzle, layer height, and orientation. Record the clearance that slides and the thread clearance that turns, then set those named controls in the full assembly. This is the handoff from geometric intent to a machine-specific fit.