Model loco design

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All contents (including photos and drawings) - ©  Derek Mackenzie

Model loco design

      I started out seriously designing G1 locos back in 2000 (after I retired from Software development) with a 1/32 scale 100% (!) accurate GWR City class loco. I produced the etch but not the castings. By this time I had acquired a CNC mill and modified an old lathe for CNC. With suitable software I was designing 3D parts and starting to cut them on the mill and lathe. It was rapidly becoming apparent that the design was too complex and expensive to produce, and the parts were taking too long to machine.
     Then along came the DEE design and I started to make kits and RtR versions of that along with locos (derived from those parts) that I called DEErivatives. Still everything was taking too long. I regarded it as a hobby (my wife thought it was a very expensive hobby) – it kept me amused! Then along came the ARM1G design and I decided I needed to at least cover my costs (not quite got there yet, but I live in hope) so I did a kit for the SECR H class etches plus castings to match. Since then I have developed a range of locos using the ARM1G cylinder set (slightly modified and beefed up) and developing various parts and techniques influenced partly by the parts used in DEE & ARM1G. By now you might have difficulty tracing the ancestry as the parts develop. There are also a lot of parts that are unique to my models.
     The masters for the castings are designed in 3D (SolidWorks these days) these are then sent off to a 3D printing firm used by my excellent caster. This saves a lot of my time and enables me to produce complex and finely detailed casting masters that were impossible on my CNC machines. The CNC machines are still in use for turned production parts and milled blocks for pumps etc.
      Combining the above with improved etch designs (I now try to get all parts to fit together with slots and tabs, or nuts and bolts, or pins – not always possible unfortunately) and improved techniques as I learn better ways of etching  things,  means that I am now making kits that are far superior to my attempts of just 3 years ago. The designs are validated in SolidWorks which enables me to make sure that all parts fit, work out how to design the pipes and control wires in 3D and provides exploded drawings for the instructions.

       The latest design development is my computerised radio control system – all locos (kit or RtR) now come with radio control. The increase in cost is low as it enables me to eliminate the boiler pressure gauge and the water level electronic unit - both are still available as an extra cost option, but are unnecessary as these functions are reported back (and controllable) on the handset touch control pad. One handset can control many locos simultaneously, but the user might struggle! This system is still under development but I hope it will be available soon. Watch this space – the system will only be available with a loco order.

      Note – where the boiler backhead is plainly visible on tender locos the operating controls are hidden behind a dummy fully detailed backhead. There is a minor loss of boiler capacity, but this compensated by the onboard pump and computer controlled bypass.

      Tank engines are usually fitted with a single flue poker burner boiler for simplicity and ease of operation. This also releases space below the boiler for servos. Servo controlled coupling loops are provided that can be raised or lowered to uncouple from various types of auto-couplers. Servo control of gas flow is provided to enable the loco to sit idling e.g. at stations or signals where appropriate. Tank engines are gas fired only.

       Tender engines are fitted with a JVR C-type boiler – usually with 2 flues and a gas-fired ceramic burner in the firebox (again controllable for idling). Alternatively a vapourising spirit burner can be fitted (no idling control). Servo controlled loops for uncoupling are not normally fitted to tender locos as I suspect these will rarely be used for shunting (there may be exceptions). An axle driven pump is fitted with a computerised servo driven bypass valve.


    There is more information on the ARM1G variant specification page - see here.