Goal
Create hollow carbon fiber tubes to make a bike frame that is lightweight and structurally sound.
Design
Carbon fiber tubes were made by wrapping continuous carbon fiber tow wetted with epoxy resin around a rotating mandrel. The two rotational axes and linear axis were controlled with NEMA stepper motors and a microcontroller running GRBL. A script was used to generate gcode based off of the length of tube, wrapping angle, and cross-sectional area.
To prevent the problem of mandrels being difficult to remove from cured tubes, mandrels were 3D printed with a water-soluble material, PVA, that could simply be washed out. The mandrels were affixed to the rotating axis with a hex shaft to minimize the stress seen by the PVA.
Finished tubes were wrapped with a peel-ply and breather before being vacuum bagged to remove air from the part and compress the carbon fiber to the mandrel.
The CNC tube winder was completed with the Bike Builders of Berkeley and in particular Zachary Liu.
Tube Winder Evolution
In making dozens of tubes for different process improvements and testing, the tube winder took many forms over the two years I worked on it.
- The linear gantry was improved with a second rail to better distribute the weight of the carbon fiber tow, epoxy resin bath, and rotating tow head.
- A resin bath was integrated to automate the application of resin.
- The mandrel grip was finalized at a hex shaft, an improvement from earlier captured-nut and chuck iterations.
- Speeds and motor voltages were improved over time to make a 2ft long tube in approximately 20 minutes.