Trilobular Screws for Printed parts

I was tired of taping holes in the prints that I was making so I decided to go out and get some self taping screws. I did some research and came across these screws.

Trilobular screw

I have never heard of Trilobular screws before and they are claimed to be specific for soft material such as plastics. I rolled the dice and picked some up.

Drum roll please …….

They work great. They have great holding strength (not imperially tested). They screw in fine and do not seem to cause any issue with repeated screwing and unscrewing. I did need to beef up my screw holes though. I previously had a 6mm cylinder with an 2.26mm hole (I am using a #4 screw) in the middle as a standoff. I found that this seem to have some deformities when the screw was inserted. I increased the cylinder to 8mm and it was much better. I would guess I can do some tests to compare the difference between tapped/machine screws vs. these Trilobular screws but I can tell you that the time saving eliminating the need for tapping is great.

Desk fan – Prototype 1

I have always wanted to make an electric motor from scratch and I finally got started. Here is the first prototype that I came up with.

20140322_181132 20140322_181206 20140322_181152

(Sorry the video is rotated apparently wordpress cant rotate a video)

I did not want to mess with brushes so this is a single coil brushless motor with a neodymium magnet in the rotor/fan blade.

I am using a Hall Effect sensor attached to an Arduino to sense the position of the rotor and time the switching of the coil connected to an H-bridge.  I also have a pot read as analog input on the Arduino to adjust the timing from when the Hall senses the magnet to when it pulses the coil.

As you can see it works. Albeit it’s a bit slower than I would like. In the next prototype I will make the rotor propeller shaped so it can move are in a useful manner and I will make the motor more powerful (somehow).

Evolution of a design

How do you do design when you are unsure of what the final product looks like?  In a word (or two or an acronym and a word or whatever) “3D printer”

The problem:

Child safety locks for drawers are quite limited when on drawer/cabinet design. Some cabinets do not have a cross member above the drawer to attack the latching part of the lock. So how do you lock these drawers you improvise with you 3D printer.

Getting to the solution:

I had some ideas on what I wanted the design to look like. There is room on the side of the drawer in this case to allow for a lock. In this case instead of thoroughly thinking through all the potential pitfalls in the design I just started testing ideas. Here is my iterative process of design.


Step1: Rough out the idea on what I want it to be. This puts something physically in hand to see if the idea has any hope of working. This one seems like it could work and it has the flexibility that could work without breaking.

Step 2: The first prototype was way too short. It was not able to be depressed by hand when the drawer was closed.

Step 3: Wow I nearly tripled the length from step 1 to 2 but it’s still way to short. Also I question why it was so wide when clearly other locks are much skinnier.

Step 4: This is getting closer; I added the hook at the end. This is actually where something interesting and unexpected happened. The lock was behaving with an extra benefit. When locked (drawer all the way in) and the drawer was pulled on without releasing the lock by depressing it, the lock hook end actually deformed a bit to form a wedge that secured the drawer in place even more than just the hooking action I expected. This one was also a bit too tall.

interesting behavior

Step 5: Shortened the height to give a bit of wiggle room for the drawer. I also extended the shape on the wedge on the end to take advantage of the unexpected behavior.

Step 6: Final design. I made the long extension a bit thicker to increase rigidity and prevent de-lamination of the top and bottom.

Here is the final design installed in the cabinet.


Well there you have it. The process of design with a 3D printer can be iterative because why not. To print successive designs takes only time and a few pennies of material.