CAD

Guide

• Design
• 3D print / Laser cut

What questions need to be answered?

• What are the dimensions are needed for the silicone bracelet?
• What are the dimensions are needed to create the housing for the casting process?
• What are the dimensions are needed to create different parts of the mould?

Dependencies from Electronics:

*To make calculations easier and add some clearance to the PCB i’ve rounded up the measurements.

Other measurements:

Software/Hardware used

• Fusion360
• Laser cutter
• Ender 3 - 3D printer

What I did

• A backing for the PCB

There’s two issues that i needed to solve here. The PCB board is very flat, and the human wrist isn’t. And the other problem is having a battery contacting the person’s skin (with skin humidity etc).

To save space the battery and it’s holder were placed on the underside of the circuit board, thus potentially bringing it into contact with the wearer’s skin (probably not a good thing on many levels).

So I had to design a backing to the PCB board that would transition the bracelet from a flat profile to a curvature that was similar to a wrist. And have a cavity to contain the battery and holder safely away from the wrist.

To hold the PCB and backing to the silicone bracelet, I designed a channel around the backing piece, where a silicone lip on the bracelet will fit inside and hold them together. This will also facilitate the easy replacement of batteries in the future.

• A inner boundary wall to the casting housing as registration key

I will be using the same casting container to cast the mould for both parts of the final mould. For positioning and registration of the second part of the mould, i decided to add a temporary inner wall to the first mould casting. for the second half I would take it out and the void left would give a step in which it could use as registration points.

Modeling the simple wall was no problem, only when the complete 3D model was placed into the slicer software did I realise it didn’t fit onto the bed fo the 3D printer so wouldn’t be able to print. So I had to cut it into 2 parts and add ‘puzzle’ tabs on one side and cutouts on the opposite side for them to fit together after printing.

• Casting housing

This 2D designed ‘press-fit’ box, will provide support for the casting of all parts individually and together at the final casting. The first half of the mould will include a 10mm 3D printed ‘boundary wall’. This will provide a registration key in the second mould half once taken out of the housing for the second cast. Tape would temporarily hold the box together and prevent any leaking of the mould casing material to come out.

• Bottom half of the final model + core

The design of the bottom half of the final model (in positive). This will define the seam line, and include what will be the breather holes. At the very bottom, the core’s foot will be sticking out, giving space in the completed mould to place the core.

• Complete model + core

For the second half of the mould, (but will be the bottom half of the completed mould), a complete model with the core’s foot sticking out will be placed into the first half of the mould.

• Core

This small piece will ‘create’ the cavity in the final casting. It will (hopefully), mean that there is an equal thickness of material throughout the bracelet. The channel around this core needed to be double the height of the core’s ‘feet’ that had been combined with the other models. The first half of the measurement was to allow the core to be fixed mechanically (as opposed to chemically with glue) without any double sided tape (this wold add height) in the mould by using the ‘lip’ that was created in the bottom by the other models. The second half of the measurement was for the silicone lip to be created and, in the same way, hold the circuit board and backing in place.

BOM

Tests

• Small silicone test

The purpose of this test would be to get a feel for the dimensions and thickness of the silicone product. As well as test if the proposed fixing method would work. To do this i will have to design and 3D print the positive of the test piece, make a small mould to cast it. Not forgetting to design and laser cut a container for the moulding process.

Questions this test would help to answer:

• Will the thickness of the silicone strap be robust enough/ feel good.

• Which of the different diameter ‘nipple’ fixings will fit and perform best at holding the strap ends together.

• Are the proportions of the strap (width, thickness etc) aesthetically pleasing and non-intrusive.

  • The test piece

  

  This consisted of a section of proposed strap, 3 mm thick, with three 3mm holes on one side, and a selection of different diameter ‘nipples’ to pass through those holes and hold the two ends together.

  • The housing for the mould

  

  Creating a simple box (without a lid) cut out of 5mm acrylic. This was my first time cutting acrylic on the laser cutter, and it was a bit nerve racking as I could see all the flashes of the laser through the acrylic, when it cut through to the bed. Setting fire to the Fablab would not be a good idea!

See CASTING for more details on casting the parts.

Mistakes & Issues

Focusing Issues with the laser cutter

Trying to cut a couple of small test squares of the acrylic i had some difficulty. Settings on the labs guides for this thickness just wouldn’t cut all the way through. I re-calibrated the Z-axis from the top of the material, and then from the bed (not a good decision!) when the results still had problems. A check from Henk, and we managed to cut through the material using some default settings (10mm/s, 80% max power, 20% min power).

In the end, these settings weren’t quite enough to cut all the way through, so I raised the power to 90% to be sure it cut all the way through (i probably could’ve got away with 85%). I believe the original problem was that i wasn’t as accurate as i needed to be when zeroing the z-axis of the laser. It’s amazing that such a small discrepancy in the focusing of the laser can make a big difference.

Too big for the 3D printer to print

See above. Designing the 3D models of the parts i need, only to find that they are too big to be printed on the 3D printer bed. So i had to divide the model into 2 and add some ‘puzzle’ piece type connections to the ends so they could be assembled into 1 after printing.

Some people (including the nice lady at the Special FX materials store) suggested printing the mould instead of casting it. This would obviously take a long time to print, but it would also not fit inside the 3D printer’s work area (I tested just to make sure). Although this approach might work in the future on projects with smaller dimensions.

EVALUATION

My starting point for the 3D Computer Added Design was minimal at the start of the course, as well as non-existent experience with Computer Added Manufacturing. Its been a very enjoyable experience learning this and my final project, with so many parts and processes needed show how far i’ve come in the time. Especially since everything worked out without too many problems and fits together nicely :).

FURTHER DEVELOPMENT

• The design for the backing plate for the battery would need to be altered depending on what sensor (e.g skin humidity) was used as it would need to have direct contact with the user’s skin.
• More holes in the strap - For better fitting and smaller wrists.
• Proportions of the bracelet and its fixing - The proportions and thickness of the bracelet currently give it a ‘robust’ / ‘chunky’ look. Tests with similar casting materials (and shrinking the circuit board with smaller components) would allow for thinner material thickness and width dimensions.
• More diffusion of light without losing brightness - In a combination developments in material choice, circuit design and CAD there are possible ways to give the device more of a ‘bright glow’ rather than a point of light source.
• Research the use of 3D printed mould pieces - A comment mentioned by several people was ‘are you printing the mould’. So it sounds to me that with the rise of 3D printing, a lot of projects print their moulds now instead of casting it like i did. Possible drawbacks of using this would be the time it takes to print, and it if a hard/ inflexible mould would suit the final casting form I was trying to get.

FILES