Molding and Casting

Import from Eagle

This component is meant to fit over an Eagle component which holds nothing but a phototransistor. It is meant to slot into the shell of my Wisp, where the phototransistor can detect changes in ambient light while still being protected from the rough weather we tend to experience. Therefore, the first step was to import the Eagle drawing, and creating this component around that. I call this PCB the photo-home, since it is where the phototransistor will live.

Small but functional

The requirements for this component is that it must be small, not too cumbersome, yet effective at both letting light through and keeping wind and rain out. I call this component the eye-lid, as it is a lid that slots over the sensor, which is essentially the "eye" of the Wisp. I wanted it to be possible to fasten the eyelid and the photo-home components to the shell with screws, to ensure that everything is sturdy and secure. I therefore drew a circular shape around the electrical board, and extruded it up, to fit around the board. I then created a model that leaves enough space for the phototransistor and any soldering, and leaves space for the screws as well.

Modeling the mold

I modelled the mold around the eyelid shape. The requirements for the mold are that the two halves fit together with minimal risk of misalignment, that there are holes for the air to escape when using the mold for casting, and that the mold is thick enough and big enough to be sturdy, yet also not too big, which would be a waste of material.

Categorizing milling bits

I thought we had 4 types of milling bits for our Modela. a 1/64" end mill for traces on PCB, a 1/32 end mill for outlines on PCB, a big bit for rough wax cuts and a finer bit for detailed wax cuts. I never realized that we have 5 different bits for wax cuts, and that the small bits for PCBs can also be used for wax if you have something with very fine details! I sorted them all, and looked them up on the internet, to get more specific information.

Adding tools to Fusion 360 CAM

I found an excellent guide written by Hashim Nabil, on Fusion 360 CAM from Fab Academy 2017, which helped me go through this process.

After sorting our milling bits, I added them to the Fusion 360 library, to ensure that I had the correct ones. For example, I was unaware that most of our milling bits have 4 flutes, while a few only have 2. This is important information for the CAM process. Like Hashim, I went to Manage-Tool Library-New Mill Tool, where I typed in all the information about the Modela milling bits in our lab.

Creating first Setup

By clicking Setup - New Setup, I opened a new box where I chose the start point for my project to be at the front, left-hand corner. The orientation in CAM is different from in the modelling environment, and I had to return to the modelling environment to rotate my design 90 degrees before I could continue. I entered my stock dimensions to be 178mm, 77mm and 39mm, which are the dimensions of the blue wax brick I have available to me. I then set the model to be offset from x and y by 10mm and from z by 0 mm, as my design already has height added to it in the design process.

Creating roughing toolpath

Selecting 3D-Adaptive Clearing opened a new window, with lots of options. I selected the 1/8 flat end mill at first, since it is the default roughing tool, but it turned out to be too large for my model, so I opted instead for the 3/32 ball end mill (we do not have a 3/32 flat end mill). I selected OK, and the software generated an image of my toolpath, I then checked the simulation to see if there is a risk of collision, and it looks fine.

Post Process Configuration

Now was the time to process the toolpath and export it. I right clicked the roughing toolpath and selected "Post Processing". I then selected Roland Rml, and clicked "post. This lead to half an hour of errors and problem solving.

Error: Spindle Speed value

When I clicked post and saved my path, I repeatedly got the same error. "Error: Spindle speed exceeds minimum value." I tried changing the spindle speed to various values, from 4000-10000, but nothing worked. I either exceeded the minimum or maximum value no matter what I put in. I searched the internet for answers but found nothing. Eventually I checked the post processing code, and discovered that while other Modela machines have different max and min spindle speeds, the MDX-15 and 20 only accept a spindle speed of exactly 6500. I changed my spindle speed to 6500, hit post, and this time, it generated the code with no errors.

Using the .prn file

Now that I had a proper .prn file, I need to see how to use in on our MDX-20. Hashim, in the guide I referenced, had a newer machine, so he used a program called Roland V-Panel, but this program does not seem to be available for MDX-20. Most likely, I will need to use Modela Player, Which is proprietary software which comes with the MDX-20. Unfortunately, the screen card on the computer that has Modela Player on it has crashed, and I am unable to connect to it.

Shopbot it is!

Due to time constraints, I could not fix the computer for the Modela, so I had to improvise. I decided to try using our Shopbot instead, after confirming that we have the appropriate collettes for this sort of milling bits.

Creating sbp files

The sbp files are made in exactly the same way as the .prn file, except this time I picked "shopbot" and "Opensbp" as the post processing function. I added a second and third path to the design, the medium one done with a 1/16" flat head end milling bit, and the finest done with a 1/32" flat head end milling bit. I chose flat mills because I recall Neil mentioning in his lecture that the flat mills tend to give a smoother finish than the ball end mills. I did not want to skip the 1/16" milling pass, as the 1/32" end mill only has two flutes, which means that it has less chip clearance than the 4 fluted mills. These paths are parallel paths, and I configured each of them to mill both parallel to the x and y axes.

Preparing wax

I glued the wax onto our sacrificial layer using a hot glue gun. I made sure it was properly aligned with the y and x axes and that if was flush with the bed, as opposed to tilted in any way. I zeroed the x and y axes carefully, using the fixed steps to get as close as possible to the correct corner of the wax. I then zeroed the Z axis using the Z-axis zeroing plate.

Roughing: 3/32"

I was quite nervous when I started the roughing path, as it was my first time milling wax, and the first time I used a path generated directly from Fusion 360. Everything went very well. I warmed up the spindle, had my hair tied back, and had a second person with me to ensure that all safety protocols were in order. The rough cut took about 15 minutes to mill, with the machine carefully carving out the pattern.

Parallel 1: 1/16"

The second path went just as smoothly as the first. Now all the features were a bit smoother, but still not as detailed as I needed them to be. This only took 3 minutes to mill.

Parallel 2: 1/32"

The final pass went perfectly, leaving a very smooth and detailed wax negative in which I could now cast my mold. This pass took about 7 minutes to mill, as the bit is half the size of the previous parallell pass, and has slightly more area to cover.

Mold Release

I sprayed my wax mould with Mold Release, to make it easier to remove later. If you forget this step, you may damage the rubber mold when you remove it from the Wax mould.

Oomoo

I chose to use Oomoo as my mold material, since it is a fast-curing and relatively environmentally friendly material. I stirred part A and part B separately before combining them and mixing them thoroughly. I wore closed shoes, eye protection, an apron and gloves, as per the safety sheet for the material.

Vacuumbag

I decided to try using the vacuum bag to remove air from the material. I practiced first and prepared it, to prevent the bag from going into the material, but it still didn't go as well as I had hoped. According to the material sheet, this type of Oomoo takes 75 minutes to cure. I decided to give it that time, even though the mold is very small, just to be certain that I wouldn't ruin it upon demolding.

Improper mixing

It appears that my first batch was improperly mixed. The result was a sticky, tacky mess that was much too soft. I looked up instructions on how to do it better, and made a second batch. This second batch came out much better than the first, but that may also be because I was careful to "paint" the mixture into the corners and holes of the mold. I then cured both attempts in my oven at home at 60°C for 3 hours.

(The next day, my co-student Bergþóra was mixing her batch of Oomoo and it was much too thick for her as well. perhaps we need to revise our storage of the materials.

Miscast

Unfortunately my first test of the mold was unsuccessful. I used smoothcast, but the mould parts became misaligned, due to the small size and poor alignment structure. Therefore it was crooked, but gives me hope that a better mold would successfully be able to create this part. I revised my fusion design and prepared the cut paths again, but due to time constraints am unable to re-mill this project.

Hard mould - Soft cast

Since one of my co-students had some leftover Smooth Cast later, I decided to try casting my mold again with hard material, since my cast would only be better if it were flexible and rubbery as opposed to rigid. Once it had cured, I cast some Sorta-Clear in it, and it came out much better than the miscast earlier. It is still a little misaligned, but within the margin of tolerance. It should keep the phototransistor quite safe.