HG20 variant, front view with AIO cover and OSSM branding
HG20 variant, side view showing capstan rope routing and motor cover
The AIO version shown in these pictures mounts the control board in the front with cables routing to the motor in the back, based on a concept by @thearmpit. The AIO Cover part used here is a custom design not covered under CC BY-NC-SA 4.0 license.
What is a capstan drive?
A capstan drive uses rope and a pulley to transmit motion. For an excellent explanation of the mechanism, watch this video by Aaed Musa:Capstan Drive Explained
Learn how capstan drives work and why they’re effective for high-torque applications.
Why choose capstan over belt?
The standard belt-driven OSSM can experience slipping at higher torques. While this may result from build errors, the capstan drive eliminates this issue entirely.Comparison to standard OSSM
- Advantages
- Disadvantages
- Same form factor as standard OSSM
- Equal or quieter operation
- Only 13cm effective rail length loss (450mm rail = 32cm stroke)
- 2.7x stronger (32kg vs 12kg capacity)
- Same maximum speed
- No belt slipping
- No custom PCBs required
- PitClamp compatible
- Compatible with all custom end effectors
MGN12H vs HG20
There are two rail variants for this build. The HG20 version is recommended if your budget allows.- HG20 (Recommended)
- MGN12H
MGN12H rails and carriages reach their limits with 30cm+ cantilevers combined with heavy toys (~1.5kg and up). This results in louder operation, bearing wear over time, and deflection issues.The HG20 version addresses all of these and includes additional improvements such as a bigger drum and better rope guiding for smoother operation.Additional cost: ~40€ more than the MGN12H version.
Configuration options
AIO vs normal version
- AIO (Recommended)
- Normal
The AIO mod version mounts the control board in the front of the motor head with integrated cable routing. This is the recommended configuration.
Pulley versions
- Bushing Pulley (Recommended)
- 3D Printed Pulley
The bushing pulley is superior in every way:
- Better durability
- No deformation over time
- Supports rail lengths up to 450mm (500mm likely possible)
Supported rail lengths
Rope selection
Required rope: Liros DC pro 161 (or equivalent Dyneema DM20, SK99, SK75 with max 1mm diameter)Why rope selection matters
Why rope selection matters
The rope must have minimal stretch under load. Testing by community member @neos demonstrated that generic UHMWPE rope from AliExpress stretches excessively and breaks under load. The specified Dyneema rope maintains consistent tension for reliable operation.
Alternative rope specifications
Alternative rope specifications
If you cannot source Liros DC pro 161, look for:
- Material: Dyneema DM20, SK99, or SK75
- Maximum diameter: 1mm
- Verified low-stretch construction
Print settings
Pre-configured .3mf files are included for most parts with optimal settings.
Bill of materials
- HG20 (~380€)
- MGN12H (~345€)
HG20 fastener list
HG20 fastener list
The M5x110 fully threaded hex cap can be sourced from AliExpress.
Assembly guides
Detailed step-by-step assembly guides are available as PDFs:HG20 Assembly Guide
PDF assembly instructions for the HG20 variant.
MGN12H Assembly Guide
PDF assembly instructions for the MGN12H variant.
Printed parts and CAD files
All printable files and STEP sources are hosted on GitHub. Pre-configured .3mf files include recommended slicer settings.- HG20
- MGN12H
Printable parts (STL/3MF):
STEP source files:
Community support
- MGN12H
- HG20
MGN12H Discord Thread
Join the discussion, share feedback, and get help with your MGN12H Capstan build.
Special thanks to everyone who provided suggestions and helped overcome challenges during development!