Week 8: Molding and Casting

The task for this week is do design a 3D mold, mill it and cast it. I viewed it a great opportunity to create a part for my final project. For my tube amplifier, I decided to make a volume control knob out of Hydrostone. I have already purchased the Alps potentiometer, so I briefly sketched all the key dimensions.

Initial draft of the potentiometer knob

Design a mold

This week's assignment has clearly gotten to the limits of my spatial imagination, so I needed a great 3D tool, in which I would also need to be experienced in. I chose a completely new to me Fusion 360 and decided to take a systematic approach for learning it. After spending 2 days on Paul McWhorter's tutorials "Learn Fusion 360 or Die Trying" (which I highly recommend for newbies in Fusion), I felt like I was ready to go.

After making a 2D radial profile of the knob, I revolved it around the shaft axis. I also added a hole for the side screw that would tighten the knob to the potentiometer's shaft.

Initial draft for the potentiometer knob

Fusion 360 Sketch of the knob

For this design, I have used as much of Fusion's 360 power of constraints and parametric design as I could. This was a great help, since later in my design, I needed to change some of my dimensions. Once the knob design was finished, I have measured the dimensions of my brick of wax to mill (38.7x77.1x177.8mm) and used it to invert parts for my future silicon mold (OOMOO in our case). Now, the task could have been made from a single part mold, however I needed to have a sidescrew, so I decided to design a mold out of three parts. By the way, I would like to thank my instructor, Zach, for his tips and for his previous project, which was a great starting point ( Zach's mold project).

I then proceeded with the second inversion in order to design the milling plan for my wax brick.

Invertion: modelling OOMOO mold

2nd invertion: modelling wax mold

Mill the mold

Once the file was ready, I have approached the Shark milling tool and loaded a 1/8 ball tip end mill. However, before the milling, I need to create toolpaths. The parameters that I used in Vectric Cut3D tool were as follows:

Roughing toolpath

Finishing toolpath

I then proceeded with milling, which took around an hour for the roughing toolpath and around 2 hours for the finishing. Finally, the milled wax piece was ready.

Milling the wax

Milled piece of wax

One may find a few tracks on it, which was a consequence of a too large fine stepover (30%).

Casting the OOMOO mold

After a bit of a postprocesing work, I quickly prepared a 1:1.3 (A:B) mass ratio of OOMOO material and poured it in. In 2 hours, OOMOO became solid, so I carefully extracted it from the wax.

OOMOO is filled

2 hours laters: OOMOO is cured

One can find that the side edge screw didn't come out nicely, so I used Zach's suggestion and sticked an old broken mill into one of the OOMOO pieces instead.

Casting the knob

I then connected all the pieces together with a rubber band and prepared a 22:100 (water:plaster) of a Hydrostone material. I carefully mixed it together and poured it in:

Getting the form for hydrostone

Hydrostone is curing

After around 2 hours, I came back and my knob was ready. It took me quite a bit of time to sandpaper the piece. I then prepared a hexagonal tube piece and glued it to the knob. After adding a potentiometer, the end result looked as follows:

Unfinished knob

Backside of the knob with the installed shaft holder

Final knob piece

Issues that I faced

My first attempt of mixing OOMOO was unsuccessful. I have poured a heavier component into a lighter one and did not mix it well, as I was worrying too much about the bubble creation. As a result, even curing it overnight didn't help much, and I had to remove the partially solidified material and pour it again.
Always measure your endmill dimensions and take them into account in your design. My deepest parts were 33mm which made me super nervous a few times during a milling. At first, I even had to pause the milling and change the end mill to a more approapriate one (which has no changes in the diameter)
If you use the Shark tool, make quick dry runs in the air above your piece. For some reason, my first passes were somewhat random and away in XY from my actual sample, while second ones were as requested.
One can find mill traces in my wax design. This was a consequence of a too large of a stepover of 30%, which I could have easily reduced to 15% without doubling the milling time, since my feedrate was only 25%.
When you have deep holes in your design, make them sufficiently wide. For the side screw, I have designed a 4mm hole, which was not enough for OOMOO to get in, so I had to substitute it with the broken piece of mill.
Make sure you shake and wiggle your OOMOO and plaster (hydrostone) solutions once you poured them into your molds. You don't want those bubbles, which I, luckily, had only a few of.

Used/learned software tools

Fusion 360. Finally.