Scratch building in HO scale railroad hobby is generally limited to buildings, structures and rolling stock (passenger cars, freight cars etc.).
Scratch building in terms of locomotives is generally limited to making existing store bought loco more authentic looking, or improving performance, or weathering.
There are tutorials for G scale but I could not find anything for HO scale.
I was put off by high price of even the entry level HO scale locos. Ditto for motor bogies (power blocks).
Here I have attempted to construct a HO scale locomotive grounds up.
The only things that are store bought are the wheels and tracks.
Step 1: Preparation and Research
I started with reading articles on re-motoring and model loco repair to know how these commercial locos work. I also saw scanned copies of Athearn model loco exploded views on company website. These videos, articles and pictures gave me insight on how commercial HO scale locos work.
When constructing a model loco from scratch, major frustration comes when all wheels do not touch the rails all the time. There is a very good article on 3 point stabilization here. I read this article and found it very useful.
Step 2: Initial Ideas
I started with Balsa model first because its easy to cut, shape and glue. There are 2 major ways of transferring power from motor to wheels - gear drive and rubber band drive.
I thought of starting with band drive since it is simple to achieve. There is no need to aligning gears and its hard to find a gear that is small enough to be slid on wheel and yet be big enough to be used as drive gear.
For power, I used DVD tray motor.
For pulley on wheel set, I used rubber grommet from DVD player. I cut out extra rubber from this grommet and used some glue to affix it to wheel set axle.
I used simple balsa wood tabs to construct makeshift wheel set holder.
Then I drilled a slit for passing band from below to motor above the wooden platform (chassis). I found position for motor on the chassis and hot glued it in place.
I added a thin paper tube to one wheel set and used it at other end of the chassis to act as second set of wheels for the loco.
Step 3: Initial Ideas...
Balsa model was not stable on the tracks and all 4 wheels never touched the rails.
My next idea was geared model.
I made a styrene chassis and filled it with BBs. I poured two part epoxy resin in it and let it set. This added needed weight and added rigidity to my model.
Then I used cog and worm gear assembly to transfer motion from motor to wheels. I had an old CD player motor lying around that was perfectly shaped for this.
I mounted wheels on Dremel and pushed an abrasive disc against the wheels to create a slight groove and increase roughness.
This helped to solve slipping issue a bit.
But this Locomotive as well was prone to derailment and slipped a lot (still). :-(
Step 4: Final Idea
My final idea is a hybrid of rubber band drive and gears. This is partly an idea that was used in old Athearn locomotives (check here).
I rolled a strip of paper around shaft of a wheelset till its diameter was just smaller than wheel diameter. (Sorry no pictures of the process). I used rubber mold making material to make a mold of it. I removed paper strip rolled on the wheelset and replaced the wheelset in the mold. I poured two part acrylic resin and I got a resin cylinder around wheelset shaft.
Then I created a styrene chassis , filled with BBs to add weight and added tabs with holes to hold wheels.
I used a plastic tube (ball point pen refill tube) for main drive shaft. I inserted a straightened paper clip for rigidity.
I added gear on main drive shaft to transmit motion from motor (DVD tray motor again) to drive shaft.
I created a simple rubber band drive for wheels (see picture for reference).
Step 5: Loco...
Here is the final loco. I created a styrene body for loco and painted it.
Added a few LEDs for light and made a styrene carriage to carry batteries (in battery case). I connected battery carriage to loco using a thin twisted pair of wires. These wires connect loco and battery carriage and connect battery to loco motor.
This loco runs on 4 rechargeable batteries. I have not added any switch or remote control yet. But this is a possibility in future :)