I've always had a fascination with gears. I think most machinists feel the same way. The engines that I have built over the years whether hit and miss single cylinder or V-8's generally only require a several gears, two for the crank to cam drive and maybe a couple for a distributor drive. These are generally of the spur gear type which can be made quite easily in the home shop. If miters are required they can be made in the shop or purchase through many outlets. Generally if someone designs an engine that requires helical gears they will specify what is needed and where they can be purchased.
Several years back I built a couple of functional 1/3 scale transmissions, the first being a Borg-Warner T-5 and after that an early 50's Ford 3 speed that waa used in the passenger cars. Although the full sized BW has helical gears I hadn't got to that point in my machining career.
When I got into 3D printing I printed objects that could be found on one of the many sites that offer free .stl files or at a reasonble price considering the amount of work it takes to make a comples model.
As with my metal models I was soon bored just printing someone else's handiwork so I started printing models from the 3D models that I had made from my own metal designs.
The first transmission that I printed was a ¼ scale version the BW transmission that I had built. As you know you can only print things so small and have them operate effectively. The BW has all the gears and on the cut-away version you can slide the gears into their operating positions by hand. There's no way to make the required links and levers in ¼ scale and have them workable.
The next printed transmission project was a 1/3 functional replica of the Ford model T transmission although in all honesty it does have some metal parts. Even with the larger scale extruded plastic just isn't strong enough to take any abuse.
At his point I was kind of hooked on transmissions, why, because they have gear in them so the next one on the agenda was the Ford 3 speed, circa 1939. This is what they refer to as a top-loader, meaning the gears are assembled in the case through the top of the transmission. I gathered pictures from the internet an searched for specific dimensions to start making my drawings. These transmissions were the precursor to the transmissions that were used all the way up into the 50's so with that in mind I used the internal gears and shaft dimensions from the drawings I had made for the metal model of the early 50's transmission.
Creating the gears was quite an easy task. I wish it was that easy to make the metal ones. In my searchings when learning about helical gears I discovered a website called 'The Engineers Edge'. They have information and spread sheets for numerous engineering parts, from screw threads to helical gears. All you have to know to develop a helical gear set is the shaft center distance and the diametral or module pitch. You enter a set of numbers for the DP/Module and something close to the ratio/tooth count you require and it gives you the calculated shaft center distance. If it's greater than what you need you drop a tooth on one of the gears and see how that changes the center distance. Once you're close to the center distance and ratio you need you start tweaking the helical angle and you can come up with almost a perfect center distance. So now you have DP/Module, tooth count and helix angle you go, in my case, to Soliworks and open up the gear toolbox. You create a gear with your numbers and it produces a model for you. You can then alter that model by adding splines or whatever you need. I can't attest that the gears models are 100% accurate but all of the gears I have printed mesh almost perfectly without any massaging. Even at this reduced scale.
I printed up 2 versions. One with the fully enclosed case and one cut-away version. Following are the Youtube links to the videos I made of making, assembly and operating the transmission.
