Table of Contents
CAD
First, i need to create a digital design of the dial wheel with the following specs:
- 50mm outer diameter
- 8mm thickness
- knurled rim
- center hole to mount it on the shaft of the rotary encoder
Steps
- Design “positive” model of the dial wheel
- Add cast-on sections
- Subtract from a solid to get the negative model
- Split result into two halves
- Add registration marks for proper alignment of the two halves
This process is actually cheating – typically you mill a positive from which you can cast a “negative” mold. That mold in turn is used to cast the actual positive. The mold is typically made from a flexible material, like silicone. In my case, i hope i can directly cast my final part in the milled wax mold – let’s see if it works…
Execution
I open the model in FreeCAD, so i can export it as a STEP file from there. OpenSCAD can only export to mesh formats, which, when imported to Fusion 360 will be more difficult to handle. FreeCAD can parse scad models into its internal geometry model and export step files, which are more easily handled by Fusion than meshes.
CAM
Model Import
The STEP model can be easily imported into Fusion 360
Tools Setup
I will use 2mm and .6mm flat end mills. The parameters for 1/8” and 1/32” tools from Bantam Tools’ site on machinable wax roughly apply:
1/8” flat end mill:
| Parameter | Value |
|---|---|
| Feed rate | 1500 mm/min |
| Plunge rate | 500 mm/min |
| Spindle speed | 16,400 RPM |
| Max pass depth | 3 mm |
1/32” flat end mill:
| Parameter | Value |
|---|---|
| Feed rate | 1500 mm/min |
| Plunge rate | 500 mm/min |
| Spindle speed | 16,400 RPM |
| Max pass depth | 1.5 mm |
Fusion 360 Tool Library
Milling passes
- Roughing pass (2mm end mill)
- Facing pass for surfaces in Z-plane (2mm end mill)
- Finishing pass for “low detail” vertical faces (2mm end mill)
- Finishing pass for the tiny vertical surfaces of the teeth (.6mm end mill)
1. Roughing pass
First, we use a adaptive clearing strategy to efficiently clear most of the material, but leave some stock for the detail passes.
2. Z-Facing pass
Using a facing strategy, we finish all surfaces in the XY-plane.
3. Vertical finishing
Still using the 2mm tool, we finish the “low detail” vertical surfaces, namely the registration marks. I choose a contour strategy for that job.
4. Teeth Finishing pass
In the final pass, we use a .6mm mill to finish the fine details of the knurled perimeter, also with a contour strategy.
Milling the Mold
Setup
Using the “measure” tool in Fusion, i determine the total dimensions of the model (58x58mm). I then cut some milling wax to that size, roughly 60x60mm. Finally, i adjust the stock in Fusion to be slighly larger than that (62x62mm) in order to compensate for any inaccuracies.
So we’re ready to regenerate all toolpaths and run the post-processor to generate g-code for the Othermill. Before i send the code to the machine, it is very important to run a simulation in order to detect any possible collisions upfront.
Milling
Having fixed the wax block onto the sacrificial plate (using double-sided tape 🤭), we can start milling:
Casting
The resin we have fortunately declares itself as weakly exothermic – good for my wax mold. It already has lots of chunks in it, but it had that already when i used it last time and it worked when carefully pouring only the liquid parts.
Unmolding
Two weeks later:
(one or two days would have been enough)
Questions & Notes
- Would the machining marks not have impressed onto the final piece, if i had used the typical “two-step” molding process?
- I apparently left too little extra resin in the funnel, so it sucked in air and created that dent. Might have been neccessary to attach another funnel to the “outlet hole” to establish equal pressure at both ends. I’m also wondering why it didn’t suck in air from the “outlet”.