1.05.2008

pressing business

I took apart an HO Model Die Casting 4-4-0 that was waddling down the track. Using a collet chuck and a DTI I was able to determine that the factory bored axle hole in one of the metal hubs was ~ .020" off centre.

I turned a cavity into a piece of 1" diameter steel rod stock chucked in the 3-jaw to form a cup chuck that the tyre rim would sit in without play. Loctite was used to hold the wheel firmly in place. I braced the wheel hub with the nose of the tailstock drill chuck overnight so it wouldn't shift while the Loctite cured.


Using a tiny endmill, I bored a new axle hole of larger diameter in the true centre of the wheel.


I then created a brass bushing to fill the gap between the larger hole and the existing shaft diameters from a length of brass rod held in an ER16 collet.


With my miniature arbour press (see previous post) I was easily able to fit the bush to the axle and the bushed axle into the newly bored wheel.

Checking with the DTI on the tyre rim I found the eccentricity to now be ~ .003", which was equal or slightly better than the remaining "stock" wheel measurements.

The locomotive no longer waddles but it does suffer from poor traction due to its rigid, uncompensated chassis design, light weight, and small motor. Unfortunately the driving axle is the forward one, which precludes any easy changes to the suspension which would improve operation.

2 comments:

DEV said...

BobCatt,

I am also doing S scale now, hopefully to Proto:64 standards. What are your thoughts on equalizing a model steam engine? Specifically a 4-4-0 and a 2-8-0?

I like your method of truing this wheel, a simple method with the right tools....

bobcatt said...

A compensated (equalised) chassis is the only way to achieve maximum traction and ensure good electrical pickup, especially in the case of a 4-4-0. It is not necessary to use complicated coil spring solutions; in fact, it would be better to avoid springs altogether. Fix the trailing axle (and gear it to the motor), allow the forward axle to move up-and-down and tilt side-to-side (total play ~.040" in S), and tie the floating pony truck to the lead axle with a lever arm balanced ~1/2 way.