The RC Massey 8680 is based on a static SIKU model (No. 3278) which is a relatively cheap model with quite good build quality. The construction of the model suits RC conversion well as it is screwed together and can easily be disassembled. However the model, while a relatively large tractor, doesn’t have a huge amount of space for wiring and electronics. In addition the bonnet does not close tightly to the body of the tractor leaving the plastic engine visible between the bonnet and the body. This means that you can’t simple remove the engine and replace is with electronics, you need to modify the engine to hide the electronics.
The Massey 8680 was the first static model I attempted to convert to radio control and it was a reasonably successful first attempt. It was a test bed for many drive train attempts using motors I had collected over the years and different gear and belt drive arrangements. As a result the model was cut and modified quite a few times before I settled on my final design.
Final Design Costs
These are the costs of the components used in the final design.
|Siku Massey 8680 Model||1||€20.00||Siku No. 3278|
|5V Arduino Pro Mini||1||€2.35|
|3.3V Arduino Pro Mini||1||€1.81|
|NRF24L01 Radio Module||1||€0.91|
|TB6612FNG Motor Driver||1||€2.21||Used in the steering|
|Hobby King HK-15178 Servo||1||€2.00|
|Siku Drive Motor||1||€33.00||From Siku Control 32 Fendt 930, hard to find|
|LEDs, Transistors and Resistors||€1.00||Bought in large quantities|
|TowerPro Micro Servo||1||€2.40||Used for lifting arms|
These are additional parts I bought during the design and testing process before I finally got the model perfected.
|XBee Radio Transceiver||1||€20.00||Very good radio but expensive|
|Hobby King HK-15178 Servo||2||€2.00||Used as drive motors and damaged|
This tractor uses a Hobby King HK-15178 servo for the steering. The servo has adequate power for the set up easily pushing the steering to full lock with a full charge however as the battery voltage fails the steering becomes less precise.
To mount the servo I first cut away the lower half of the steering shaft but leaving enough to support the steering wheel. Initially I milled out part of the plastic engine to allow the servo to be mounted inside the engine so that it wasn’t visible with the bonnet down. However I had trouble running wire in my final design so I removed the engine completely. This allowed me to mount the servo under the bonnet of the tractor but the servo and wires are visible through the side of the bonnet.
I found that a lego gear suited the steering rack perfectly so I cut the arm off a servo arm leaving just the toothed socket. I then glued this to the lego gear and screwed it onto the servo. This was a relatively complicated steering set up and took quite a bit of time to perfect. In hindsight a simple bar from the servo arm to the steering rack would have been less elegant but also much less of a headache.
Motors, Gearboxes and Pulleys
Like I said at the start this was my first attempt at converting a die cast model into a radio controlled model and there was a huge amount of trial and error when it came to the drive train.
My first attempt was to glue one wheel to the axle so that it was fixed while the other free wheeled, this gives a kind of differential effect allowing the tractor to corner better. I then roughed up the shaft a bit and glued a lego gear to it. Next I cut a servo arm so that the splined end would fit into a lego worm gear and glued it into the gear. I chose a HK-15178 servo modified for continuous rotation to drive the worm gear as it was cheap and I had a few of them lying around. This set up worked at first but not for too long.
As soon as the tractor bumped into something solid the drive system failed, if I tried to pull something heavy the drive system failed. At first it was just the glue on the wheel or the glue holding the gear on the shaft that failed leaving one of them spinning but after a few tweaks these were quite good connections which caused even more heart ache.
The next thing to start failing was the gearbox inside the servo. The improved glued connections were holding up so well that the weak plastic gears inside the servo started to shred when the tractor hit something solid. I also ended up burning out the control board in one servo as it drew too much current trying to move the tractor. Smoke billowing from the engine bay of a toy tractor is a very impressive sight but didn’t help my build too much.
I should probably point out that unlike a toy RC car, these tractors are heavy and have good rubber tires with lots of grip so it is highly unlikely you are going to spin the wheels when you hit something solid or try to pull an excessively heavy load. So if your drive train is too weak you are going to damage it.
My solution to this problem was to redesign the entire drive train, instead of a solid connection to the axle I decided to use a belt drive. I used another HK-15178 servo modified for continuous rotation except this time it was driving a Meccano pulley. I added a second pulley to the axle just behind the wheel and connected a rubber o-ring between the two pulleys. After rebuilding the tractor this design worked well, the pulley spun when the tractor hit something hard rather than breaking the servo gears but the tractor couldn’t really pull any sizeable loads.
My final solution is to remove all the previous attempts and use a Siku Control 32 gearbox like I used in the Fendt 936 which had been the test bed for that idea. This solution worked well as the gear train in the Siku gearbox is strong enough to withstand the forces of a motor stall. The problem with this design is that the Siku motors are hard to get and are pretty expensive. Also there are no Siku Control 32 Massey tractors so the wheels which come with the gearboxes are coloured to suit John Deeres or Fendts. I got around that problem by gluing part of the Fendt wheel rim into the Massey wheel so that it would mate to the gearbox.
Choice of Radio
When I first started building the RC farm and construction vehicles I used XBee radio modules because they are very good radios and easy to set up. The XBee modules are great but I was beginning to collect so many models that purchasing an XBee for each was simply too expensive. So I decided to switch to using the cheaper NRF24L01 radio module in all my models and so far that has worked out perfectly.
The RC Massey 8680 has a large number of LEDs arranged around the model. It has red LEDs for brake lights, orange LEDs for indicators and orange for the two beacons on the cab. At the rear of the cab roof are four super bright blue LEDs (all I had when I first started this conversion) and on the front of the cab roof are four super bright white LEDs as work lights. Also on the cab but down beside the indicators are another pair of super bright white LEDs used as work lights. Finally at the front of the bonnet there are eight super bright white LEDs for head lights.
RC Massey 8680 Circuit Diagram
Here is the schematic for my Massey 8680 in pdf form. This model uses a 3.3V Arduino Pro Mini in the cab and another in the body of the model. An NRF24L01 radio module in the cab provides the wireless link and a TB6612FNG motor driver in the body drives the Siku control 32 motor.
Massey 8680 YouTube Playlist
There are many videos following the building of the Massey 8680 on the RC Tractor Guy channel and a few videos of the Massey in action. If you want to view the videos related to the RC Massey 8680, follow the link below to visit the playlist on the RC Tractor Guy YouTube channel.
RC Massey 8680 Code
The RC Massey 8680 code can be found in the RC Tractor Guy Arduino library.