Thursday, March 26, 2015

Chassis MkIII

So after a little bit of teething problems I've managed to assemble the third version of the 3D printed chassis for the 24HP Hudson-Hunslet diesel locomotive I've been building. The first stage was to assemble the mechanical (rather than electrical parts of the chassis) to see if the design changes had made it easier to build. You may remember that the main problem with the second prototype (the first to be printed in brass) was that fixing the worms and gears in place was a bit of a lottery and the more likely outcome was that you would end up gluing the whole thing solid.

Well I can report that the changes to the chassis design have made assembly a lot easier. You can now fit the layshaft and worms without any risk of locking anything solid, and the etched spacers mean that getting clearances right it also nice and easy. The axles are still a little awkward to fit and it is possible to lock everything solid, but as long as you are careful the chassis now goes together fairly easily. Adding the electrical components is still quite tricky though.

The main problem is that because of the lack of space I'm forced into using a surface mount resistor to protect the motor and these are tiny. They need soldering to a piece of copper clad board (with a gap cut in it to separate the two ends) and then pickups soldering to one end and the motor wire to the other. The piece of board that came with the resistor is essentially the same size as the resistor and is very fiddly to use. I did try though, but at some point I managed to form a solder bridge across the resistor, which meant the motor got the full 12v and was damaged. The extra power caused the motor to get so hot I burnt myself and had to wait a few minutes before I could safely handle it. The heat must have melted something internally as the metal case was now live to the positive terminal, and even a small amount of power caused it to get dangerously hot.

Fortunately I had a replacement motor and resistor so could replace the electrical side of things. While doing so I changed how the resistor is mounted and used a 9mm offcut of the copper clad sleepers I used to hand build track recently. This gives me a lot more room to play with, was much easier to fit, and is still hidden once the body is put in place. The final step was to add the etched brake shoes which fold and then glue to the support arms included in the chassis. Anyway the end result of all that is this...

I'm hoping that with a little more running in and maybe a slight tweak to the pressure of the pickups and it will run nice and slowly, but I'm really happy with how this chassis built up (my ham fisted electronics aside). I've updated the artwork for the etched components so hopefully it won't be long until I've completed the design of this locomotive and can think about making it available as a kit.


  1. This is looking good. Do you have to thin the paint down to stop it obliterating detail?