Model loco design

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All contents (including photos and drawings) - ©  2019 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.

      All my tank locos use a single flue boiler and poker burner similar to the ARM1G design which leaves a lot of space below for the servos and R/C, but limits the power source to gas.

      All my tender locos use a twin flue JVR C type boiler with a ceramic gas burner or vaporising spirit burner. From the T9 onwards all tender locos have a highly detailed dummy backhead with the real controls hidden behind. The backhead is not really visible in a Q1. All the R/C gear goes in the tender operating the loco via rods and Bowden cables where suitable.

      The aim for all locos is to get similar duration to the DEE and ARM1G locos.


      All silver soldering is already done - so it is just a simple screw/glue/solder assembly job, followed by spraying spray acid etch primer followed by mostly black gloss, red for the buffer beams and inside the frames. Apply the transfers then spray varnish to required finish. Transfers (laser printed to eliminate fading) come as large panels including the lining.  The transfers are customised for each loco showing the users preferred company and number.


      Disregarding the City class loco (I learned a lot from that) I regard the DEE (SECR D class) and the GWR 56XX body kit (done for JTT when Dicky Boast was running it) as my first generation locos. They lack the castings and the slots/tabs, holes/bolts assembly features of later locos, and are more of a challenge to put together. The second generation were developed to fit  the ARM1G parts. They are the SECR H class, the SR Q1 and the BR class 08 shunter (my first and probably last attempt at a diesel – there is just too much surface detail). They have more tabs and slots (increasing in order of H class, Q1, 08) than the first generation so gradually become easier to assemble. All of the first and second generation come with photo illustrated assembly instructions. The third generation (LSWR T9, LMS 3F, GWR 14XX, LSWR Adams Radial Tank and all later locos) take the designs to a new level with far more tabs/slots, holes/bolts to simplify assembly, designed for solder OR adhesive assembly, a fully integrated computerised radio control system and assembly instructions with 3D exploded drawings. See here for a sample page. The instructions for ALL locos can be downloaded from a link on the relevant model page, so you can see what you are getting before deciding to purchase. This is also worth doing if you only considering buying a ready to run model.


     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 5 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 as standard. 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 is compensated by the onboard pump and computer controlled bypass.

      All locos are fitted with servo operated coupling loops that can be raised or lowered to uncouple from various types of auto-couplers or scale hooks. Servo control of gas flow (or spirit burner) is provided to enable the loco to sit idling e.g. at stations or signals where appropriate. Tank engines are gas fired only.


       Servo controlled loops for uncoupling are not normally fitted (but are designed in) 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.


       Apart from the 3F, the 14XX and previous loco designs (which were all 10mm scale) all locos will be to 1:32 scale. There may be occasional exceptions for extremely small tank locos. The smaller boiler size required for 1:32 scale should not be a problem for tender locos as the computerised control of water level will maintain the safe level so the limitation on run time is really the tender size. As a bonus the smaller boiler will reach running pressure quicker.

Tank locos are a little more problematic, but where there is a dome, that is made functional to raise the boiler capacity. Occasionally low mounted safety valves will add the complication of needing a feed from the dome for safety reasons - you do not want the SV spraying superheated boiling water in your face!


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