Sunday, 26 May 2019

A quick make, 3d printing threads.

I've had this piece of plastic tube for ages, I use it to protect my small model rocket launch rail rods that need to be kept straight and protected from bends. It has had numerous bits of gaffa tape over each end which had slowly turned it into a sticky mess. I've been trying to get to grips with 3d printing threaded items, it's been a challenge until I realised I was expecting to much by applying machining tolerances to the 3d prints. Once I started under sizing the internal threads by around 0.25mm on the radius I started having some success. So now my rod protecting pipe has a nice end-cap one end and a screw lid on the other and I've learnt some new stuff on the journey!

Friday, 24 May 2019

Hackspace Magazine! Keeping track of my articles.

It's been a privilege to write for the Raspberry Pi foundations Hackspace Magazine since last October and I have been meaning to create a post about various articles I have written but never got around to it. So I thought I might create this post with links to all the issues I have article in and a little description of the articles and keep adding to it as (hopefully) the articles keep getting commissioned and published!

Issue 12, Where it all began! I had a "design your own rocket" tutorial in here using the amazing opensource Openrocket design and simulation package.

Issue 14,  An article on Manual Milling machines and beginning machining.

Issue 15, An article on Lathes and their use and a review of a Neke Laser Engraver.

Issue 16, An article on Making a multi headed screwdriver using the lathe and mill and a review of the Eleksmaker Eleksdraw pen plotter.

Issue 17,  Part one of a two part introduction to laying out PCB's for professional fabrication with Kicad.

Issue 18, Big issue for me this one! I had the feature article on Space, featuring lots of maker/amateur satellites, a focus on Libre Space Foundation and a tutorial on setting up a SatNOGS ground station. Also had a tutorial on making a "Slim Jim" antenna and also the second part of the Kicad two part tutorial.

Issue 19, Tutorial on Marking out and Centre punching accurately for metal work and I also wrote the space of the month article on the fabulous Prishtina Hackspace.

Issue 20, Tutorial on Drilling and Tapping threads.

Wednesday, 8 May 2019

DIY Screw Switches for Rocketry

In high power rocketry its good practice to have some kind of keyed switch or remove before flight system that isolates the avionics from power. Important as often these systems are connected to small wells filled with explosives that are fired at points in the flight to split airframe sections and deploy recovery parachutes etc. Isolating the system (killing the power) enables the flyer to handle, carry and prepare other aspects of the rocket and only arm the system when necessary.

I've used small 3.5mm stereo jacks and sockets and when the stereo jack is removed 2 pins are connected under a spring steel load to which I have soldered a power cable. I have also experimented with rotary key switches. I have never flown key switches and recently read about some failures using rotary key switches and I've always wondered at what force/speed would it be possible that my clamping stereo jack socket might separate in flight and and kill power?

I then came across the idea of screw switches. A screw switch essentially is a system where a circuit only becomes connected when a screw is tightened. They are often made by hand by epoxying a nut onto a metal bar which is insulated and another metal piece added on top which will be connected when the screw/bolt is tightened. I wanted to try and make one that was accurately repeatable so I could make a multiples but also smaller, tougher and neater.

So I quickly decided on using double sided copper clad fr4 and a PEM nut press fitted to one side. I CNC routed the boards with a hole to receive the PEM nut and 2 holes for M2 mounting bolts. As the M2 mounting bolts would connect the 2 sides of the FR4 I routed a clearance area which generously clears the head of the M2. I fitted a PEM nut (press fit using a 1.5 ton arbour press) to the first prototype and continuity checked that the two sides of the board where still electrically isolated. To make the switching screw I have epoxied another PEM nut onto an M2 bolt and when fully tightened it connects the 2 sides of the board. On the other end of the M2 bolt I made a small punch mark to disrupt the last couple of threads on the bolt which means it cannot be fully unscrewed.

As the device is copper clad I routed the board to have a tab on one end  (at the top of the picture below) onto which power wires can be soldered either side. Therefore when the M2 bolt is fully tightened it connects the wires and allows current to flow.

It then just needs to be mounted in a position inside the rockets electronics bay where its possible to have a small hole in the airframe allowing a small screwdriver to be inserted to open and close the switch. As a mockup I made a small 29mm bulkhead and added a standoff to mount the switch too.
Finally one of the great benefits of a CNC is repeatability, I have now knocked up a few of these and they sit awaiting deployment in upcoming rocket builds.

EDIT!! So some people where asking questions and struggling to see how this worked so I've done a little video explanation and demo! 

DIY Rocketry Screw Switch from concretedog on Vimeo.