Drive units - chassis exploration

First step towards a fully functional chassis including motor and gearbox is making the spacers. I turned them from 6 mm brass rod, to a length of 9 mm. Their length was mainly based on what I found in other kits. It needs to be proved correct.

I drilled them all the way through and tapped them M2.0. To prevent them from sticking out of the frame ends I milled 1.0 mm off one side.

My cheapy single fluted Chinese spiral mill delivered unexpected good work. The milling marks were so minute that I could not feel them with a nail test and altogether vanished after a few strokes on 1200 grit emery paper.

Then I added all three axles before installing the gearbox. I immediately ran into the next problem. Can you spot it?

The two rear wheels overlap.

So I set to work. I had basically two options: turn the wheel to a smaller size or putting the two rear axles further apart. The first option had my preference for three reasons.

• The Alan Gibson wheels were a tad oversize so turning them down was an attractive option as it would not only solve the problem but I would end up with wheels closer to scale
• The two rear axles were already a bit too far apart as provided per the etch, so taking them further apart was not to my liking.
• Filing the holes of the axles eccentric is not a very controllable process whereas turning wheels is very repeatable and predictable.

To turn the wheel I needed a mandrel to clamp the wheels tightly down. As turning is not the subject of this page I will not go into detail how I made it.

Some details on the features of the mandrel:

• I provided a central spigot to mount the wheel. It is 3.20 mm wide, just a bit more than the standard 1/8" axle (3.18 mm) so the wheel would sit very tight
• The backplate is about the size of the plastic centre of the wheels, smaller than the metal tyre to allow the RP25 tool to get behind the flange.
• An M1.2 hole was tapped in the front of the mandrel. The bolt serves to prevent the wheel from slipping. In practise one bolt did not suffice. It broke the adjacent spoke of the wheel centre. So I added a second one to hold the wheel.

Turning was done with a Fohrmann RP25 tool, here matched to the wheel after completing the wheel for the sake of the photo.

The black line on the tool indicates the front face of the wheel.

One complication while turning was that doing so the tyre heated up considerably. Chances were that the tyre would come off the plastic centre. So after I found that out the hard way I first gave them a drop of thin cyanoacrylate glue at the back of the wheels which by capillary action drew into the separation between plastic and the metal and permanently fixed them together. Turning was done ever so carefully. Only the lightest of cuts and slowest of speeds and ample time to cool down, all served to keep the stress and temperature in the wheel to the lowest possible level.

And with about 0.25 mm to spare they now go together well. Tight but clearance is clearance. The right wheel broke a spoke and will be replaced.

At long last I could finally trail fit the gearbox/motor assembly.

And yes it fits well.

An attempt at a sprung chassis

I always wanted to build a sprung chassis. Springing allows the wheels to maintain rail contact when the track is not entirely flat. This reduces contact problems and the chance of derailments.

So I made two frame plates from scratch and actually built a fully functional sprung chassis for the LTM 51. I described that build on a separate page.

A sprung chassis

So far I was very doubtful about a sprung chassis but now I have built one I am pretty happy with the results. Yet giving it careful consideration I found that the supposed advantages of the springing did not outweigh the added complexity of the build. I kind of chickened out. So in the end I decided to build two conventional rigid frames.

Gearbox and motor

It is not easy to fit a suitable combination of gearbox and motor in the confined space of this model. With an etch-only like this there is nothing in the way of instruction of some sort to help you out. I chose a LoLoader of High Level Kits with a motor of Tramfabriek. I described this in detail on an a separate page.

Coupling rods

The coupling rods provided with the kit were simply horrible. So I had to find an alternative. Read the detailed story on a separate page. In the end I had this improvement

Bottom: the original coupling rod. Top: 3D printed in aluminium and cleaned up

Conclusion

Having explored the variants of the frame and having decided on motor and gearbox I could now commence the final build of the (rigid) chassis.