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====== What I want to accomplish ======
  * Design the 3d printed parts,
  * Print them,
  * Assemble everything.

====== What I've done ======

I broke out my favourite CAD tool: [[https://www.openscad.org|OpenSCAD]] and wrote a bunch of code. I found an openscad library on [[https://www.thingiverse.com/thing:16627|Thingiverse]] that can generate GT2 pulleys. My belts aren't quite GT2, but they're close enough.

I decided on 8mm shafts because I have a large number of 608 bearings, and they have an 8mm ID. I sized the pulleys to get the required spacing between the stepper shafts and the first axis shaft, around 27mm OD, which works out to a tooth count of 44. The stepper pulleys have 20 teeth. I made the holes for the stepper motor mounts slotted to allow for tensioning the belts. I did the same for arm 1.

It was a bit of a challenge to design the bottom bearing holder. I can't use the bolts holding the stepper together to mount the bottom plate, since removing bolts from a stepper is generally a bad idea. It's important that the bearing be in the correct place as that determines the belt tension. I ended up making a piece that fits between my steppers with my belt tension. Not ideal, but perfect is the enemy of done.

{{:projects:sand_drawing:work_logs:pasted:20200415-051841.png}}
bottom\_bearing\_holder

This fits between the steppers to hold the bottom bearing in place. The steppers are clamped together by a cable tie.

{{:projects:sand_drawing:work_logs:pasted:20200415-052007.png}}
top\_holder

The steppers mount to this, retaining a bearing in the centre.

{{:projects:sand_drawing:work_logs:pasted:20200415-052107.png}}
shaft\_1\_drive\_pulley

This is pinned to shaft 1 and driven by a stepper via a belt.

{{:projects:sand_drawing:work_logs:pasted:20200415-052152.png}}
shaft\_2\_drive\_pulley

This holds two bearings and sits next on the shaft above the shaft 1 drive pulley. It is driven by the other stepper and drives the shaft 2 driven pulley.

{{:projects:sand_drawing:work_logs:pasted:20200415-052532.png}}
arm1\_split\_base and arm1\_split\_end

The smaller hole is pinned to shaft 1. Two bearings are retained in the big end to hold shaft 2.

{{:projects:sand_drawing:work_logs:pasted:20200415-052624.png}}
shaft\_2\_driven\_pulley

Pinned to shaft 2 underneath arm 1. Slightly taller than the shaft 1 drive pulley to allow for a wider belt.

{{:projects:sand_drawing:work_logs:pasted:20200415-052950.png}}
arm2

Pinned to shaft 2 above arm 1.

====== Assembled ======

{{:projects:sand_drawing:work_logs:pasted:20200415-055858.png}}
{{:projects:sand_drawing:work_logs:pasted:20200415-060042.png}}
{{:projects:sand_drawing:work_logs:pasted:20200415-060051.png}}
{{:projects:sand_drawing:work_logs:pasted:20200415-060114.png}}
{{:projects:sand_drawing:work_logs:pasted:20200415-060134.png}}

====== Bill of materials ======

If I were ordering these parts for a build I would order at least 20% extra, especially the washers. They're cheap

^ Item ^ Type ^ Count ^ Note ^
| Printed parts | Various | 8 | |
| Stepper motor | NEMA-17 | 2 | |
| Stepper pulley | GT2-20, 12.35mm high | 2 | My steppers had pulleys from the factory. |
| Belt | S2M belt 120mm | 2 | S2M ≈ GT2.  |
| Belt | S2M belt 278mm | 1 | |
| Shaft 1 | 8mm x 90mm | 1 | Mild steel is easier to cross-drill than stainless. |
| Shaft 2 | 8mm x 33mm | 1 | |
| Bearing | 608 | 6 | |
| Washers | 8mm ID, 15.7mm OD | 5 | OD not critical, but it must not rub on outer race of bearings. |
| Washers | 3mm ID, 6.8mm OD | 12 | |
| Bolts | M3x12mm | 8 | I used all 20mm and cut them to length. |
| Bolts | M3x20mm | 2 | |
| Nylock nuts | M3 | 2 | Nylock not critical. |
| Pins | 2mm diameter | 5 | Roll pins would be best, I used nails. +1 extra for pinning shaft 1 into the bearings. |
| Magnets | 10mm diameter, 5mm high neodymium | 2 | Dimensions not critical. |
| Cable ties | 200mm x 4.6mm Nylon | 2 | Dimensions not critical. |


====== What I want to accomplish next time ======

  * Write the software