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I'm learning as I'm go. If you have some advice please send me an email: tjhowse at gmail.com

projects:sand_drawing:work_logs:cad

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: OpenSCAD and wrote a bunch of code. I found an openscad library on 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.

bottom_bearing_holder

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

top_holder

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

shaft_1_drive_pulley

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

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.

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.

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.

arm2

Pinned to shaft 2 above arm 1.

Assembled

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
projects/sand_drawing/work_logs/cad.txt · Last modified: 2020/04/17 05:11 by tjhowse