11. Molding and Casting¶
group assignment:
review the safety data sheets for each of your molding and casting materials,
then make and compare test casts with each of them
individual assignment:
design a mold around the stock and tooling that you'll be using,
mill it (rough cut + (at least) three-axis finish cut),
and use it to cast parts
Here is the 3D file of the knob:
Individual assignment¶
I am making a custom potentiometer knob mould for this week’s assignment. It will be made from Delrin wax on the Shopbot CNC in a single part, and then I will use MoldStar Silicone as the material to constitute the knob itself.
Rhino 3D design¶
I made several tests for what the knob could look like:
I eventually settled on this design which gives the impression of a cresting wave (the name of the video synth is the oceanique). Here I show the generation of the form but first making a fluted cylinder and then subtracting it with a deformed and tilted torus. I had to truncate the stem because it made a form that was impossible to mill.
Now I subtract a 6mm shaft of the potentiometer from the bottom of the form:
Because I wasn’t exactly sure what to use to generate the mill paths in V-Carve I generated a block the same size as the material I had and subtracted the form I wanted from it in addition to making a positive of the form to test with also:
CNC Milling Prep in V-Carve¶
I started by created the stock in V-Carve.
I first tested with the positive but it wasn’t giving me what I wanted:
I couldn’t figure out how to ask V-Carve to make the inverse of this form…
Here I am working with a model of what I want cut exported from Rhino 3D and this is working for me. I set the home to be the top surface of the material and unchecked discard all data beneath Z=0.
I also tried working with important a block the same size as the stock rather than just a surface but they both equated to the same thing in V-Carve essentially.
Here are the two end mills (132097 and 130007) I’m using from Carbi Universal:
On the left is a flat nose for roughing and on the right is a ball nose for finishing.
I looked up the references to create the tools in V-Carve:
Here I am adding a new tool bit to V-Carve:
I already had the flat nose but didn’t have the ball nose already in virtual form. I took the dimensions from the table and used the speeds and feeds calculator from MIT machine shop (https://pub.pages.cba.mit.edu/feed_speeds/)
Here is the machinable wax I’m using:
Here is the roughing tool pass I generated in V-Carve. I tried different options with regards to generating the path (raster, making the profile with the Y or X, but ultimately didn’t see a great deal of difference between them):
And here is the finishing pass:
A closer look:
I exported the G-code in two files and then loaded one at a time using the Shopbot controller software.
CNC Milling¶
Here I’ve glued the wax to the milling sacrificial surface with hot glue:
I then ran the C5 command to warm up the spindle and did an air pass of the roughing and finishing passes to make sure there weren’t any surprises.
Setting the home using Shopbot control software by moving around with the arrows and then clicking “zero all axes”:
Here is the first roughing pass in action:
Roughing pass all done, the stem in the middle is still intact:
After changing for the ball nose end mill:
I should have not been so ambitious with using the entire length of the endmill as this caused the collet to rub slightly the top surface of the wax - thankfully it was off by less than a millimeter and I was ready to stop it if there was any sign of trouble.
After the finishing pass:
FYI: during clenup I didn’t understand how to dismantle the vacuum and it exploded…
Molding¶
I first calculated the volume of the model to help with the mixture calculating:
The total is 30890mm cubed which is 31cm cubed.
I am using Mold Max 29NV which cures in 6 hrs, and requires a 10:1 mixing ratio.
Because the density of Mold Max 29NV is 23.7in cubed/lb, this is 0.856 cm cubed / gram. This means I will need 36 grams of the silicone to fill the mold. 90% of that is 32.6 grams and 10% is 3.6 grams.
Here is my mold hot off the CNC mill and the two parts ready to mix!
Here is an example that the other students made using this same silicone. It is firm but flexible and has a nice creamy texture:
Here is the safety sheet for mixing. I put on goggles, non-latex gloves and worked under the fume hood being extra careful to wash my hands thouroughly afterwards.
It’s hard to take photos with sticky liquids everywhere - here are parts A and B together in a cup and with a gram scale underneath:
And here is the silicone after 3 minutes of mixing. There are air bubbles but they also appear to sort themselves out after a few minutes of sitting:
Here is the silicone in the mold. Now we wait!
Our lab is closed on Wednesday due to an electricity outtage so I’ll have to wait until Thursday to see how this turned out but I can’t wait!
EDIT Here is the final result ! :
