; Print the kit flat, then fit wheels to the axles and push in split rivets.
(define-component tractor-chassis ((number axle-radius 3.2mm) (number body-clearance 0.3mm))
  (let* ((axle-height 5mm)
         (body (box 30mm 50mm 10mm))
         (engine (translate 0 -11mm 10mm (box 22mm 23mm 10mm)))
         (chimney (translate 0 -15mm 20mm (cylinder 2.5mm 12mm)))
         (cabin (translate 0 14mm 10mm
           (difference (box 24mm 18mm 23mm)
             (translate 0 0 3mm (box 26mm 12mm 16mm)))))
         (axle-bores (repeat-compound i 2
           (translate 0 (+ -15mm (* i 30mm)) axle-height
             (rotate 0 90deg 0
               (cylinder (+ axle-radius body-clearance) 34mm :align '(center center center)))))))
    (difference (union body engine chimney cabin) axle-bores)))

(define-component tractor-axle
  ((number axle-radius 3.2mm) (number axle-length 47.2mm)
   (number mouth-radius 1.8mm) (number cavity-radius 2.35mm)
   (number lip-depth 2.2mm) (number socket-depth 5.5mm))
  (difference
    (cylinder axle-radius axle-length)
    (translate 0 0 -1mm (cylinder mouth-radius (+ axle-length 2mm)))
    (translate 0 0 lip-depth (cylinder cavity-radius (- socket-depth lip-depth)))
    (translate 0 0 (- axle-length socket-depth)
      (cylinder cavity-radius (- socket-depth lip-depth)))))

(define-component tractor-wheel
  ((number radius 10mm) (number width 6mm)
   (number axle-radius 3.2mm) (number running-clearance 0.4mm))
  (difference (cylinder radius width)
    (translate 0 0 -1mm
      (cylinder (+ axle-radius running-clearance) (+ width 2mm)))))

(define-component tractor-rivet
  ((number mouth-radius 1.8mm) (number snap-interference 0.25mm)
   (number head-radius 4.8mm) (number head-thickness 1.6mm))
  (let* ((stem-radius (- mouth-radius 0.2mm))
         (barb-radius (+ mouth-radius snap-interference))
         (barb-base 4.3mm)
         (slot-bottom (+ head-thickness 0.8mm)))
    (difference
      (union
        (cylinder head-radius head-thickness)
        (translate 0 0 (- head-thickness 0.2mm) (cylinder stem-radius 3.1mm))
        (translate 0 0 barb-base (cone barb-radius 1mm 1.9mm)))
      (translate 0 0 slot-bottom (box 1.1mm 8mm 5mm)))))

(model
  (meta :title "Snap-together tractor")
  (params
    (number wheel_radius 10mm :label "Wheel radius" :min 8 :max 12 :step 1)
    (number wheel_clearance 0.4mm :label "Wheel running clearance" :min 0.2 :max 0.6 :step 0.05)
    (number snap_interference 0.25mm :label "Rivet snap interference" :min 0.1 :max 0.35 :step 0.05)
    (toggle print_layout true :label "Print layout (changes export)"))
  (let* ((axle-radius 3.2mm)
         (body-half-width 15mm)
         (wheel-width 6mm)
         (wheel-side-gap 2mm)
         (wheel-end-play 0.6mm)
         (wheel-center (+ body-half-width wheel-side-gap (/ wheel-width 2)))
         (axle-half-length (+ body-half-width wheel-side-gap wheel-width wheel-end-play))
         (axle-length (* 2 axle-half-length))
         (rivet-head-thickness 1.6mm)
         (body-lift (- wheel_radius 5mm)))
    (part chassis
      (translate 0 0 (if print_layout 0mm body-lift)
        (tractor-chassis :axle-radius axle-radius)))
    (part axles
      (repeat-compound i 2
        (let* ((axle (tractor-axle :axle-radius axle-radius :axle-length axle-length)))
          (if print_layout
            (translate (+ -28mm (* i 56mm)) 46mm 0 axle)
            (translate (- axle-half-length) (+ -15mm (* i 30mm)) wheel_radius
              (rotate 0 90deg 0 axle))))))
    (part wheels
      (repeat-compound side 2
        (repeat-compound i 2
          (let* ((sign (- (* side 2) 1))
                 (wheel (tractor-wheel :radius wheel_radius :width wheel-width
                   :axle-radius axle-radius :running-clearance wheel_clearance)))
            (if print_layout
              (translate (* sign 32mm) (+ -15mm (* i 30mm)) 0 wheel)
              (translate (* sign (- wheel-center (/ wheel-width 2)))
                (+ -15mm (* i 30mm)) wheel_radius
                (rotate 0 (* sign 90deg) 0 wheel)))))))
    (part rivets
      (repeat-compound side 2
        (repeat-compound i 2
          (let* ((sign (- (* side 2) 1))
                 (rivet (tractor-rivet :snap-interference snap_interference
                   :head-thickness rivet-head-thickness)))
            (if print_layout
              (translate (+ -18mm (* side 24mm) (* i 12mm)) 46mm 0 rivet)
              (translate (* sign (+ axle-half-length rivet-head-thickness))
                (+ -15mm (* i 30mm)) wheel_radius
                (rotate 0 (* sign -90deg) 0 rivet))))))))
  (verify
    (tag eleven-printable-pieces)
    (metric pieces (stl connected-component-count))
    (expect pieces (= 11)))
  (verify
    (tag closed-surfaces)
    (metric bad-edges (stl non-manifold-edge-count))
    (expect bad-edges (= 0))))