I am contemplating building the 3.5 inch Pennsylvania A3 Switcher and ordered the excellent book by Kozo Hiraoka.
Before diving into this long term project, I decided to first see if driving wheels could be casted locally (Cape Town, South Africa). Only one of the three local casting companies showed any interest, and this compary only do high end investment casting. To save some money, it was suggested that I provide the wax patterns. A minimum of 10 patterns were required for 5 wheels, to allow for any casting failures.
Even though a bit pricy, I decided that learning a new skill and going through the whole process, rather than importing wheel castings, would be worth it.
Recycled wax was obtained from a local business specialising in lost wax casting of bronze art statues.
Making wax patterns opened up a whole new world to me. Wax casting using a silicon mould had to be mastered...
First a 3D model of a casting pattern was made in Fusion 360.
The 3D model was then used to cut a pattern out of MDF on my 3 axes milling machine.
The MDF pattern was first painted and sanded. A mould was then prepared for the bottom half of a two part silicon block mould. Nuts located in moulding clay were used as locating features for the top half of the mould.
Silicon was then mixed and poured to cover the pattern by about 10mm. This forms the bottom half of the mould. The silicon has low enough viscosity to allow any air bubbles to escape naturally before it cures.
A mould was then made to cast the the top half of the silicon mould. The bottom half of the mould was mounted at an angle to prevent any air from being trapped in the mould when the wax is being poured. A sprue for pouring the wax was added so that poured wax will rise from the bottom of the pattern. An air vent was also added to the top of the pattern.
The top half was partly cut to allow easy demoulding with the sprue and air vent intact.
Wax melts at around 55 deg C, so it was easily melted on the kitchen stove in a pot suspended in boiling water. The mould was preheated in the stove oven set at 70 deg C. After the wax was poured, the mould was returned to the (now switched off) oven to cool slowly for at least four hours to minimise the effects of shrinkage.
Six wax patterns done, four to go. Wax shrinkage is mostly taken up by the wax in the sprue and vent.
Sprues and vents were removed from the ten wheels with a knife heated in boiling water. Any remaining imperfections were also filled with wax using a heated screw driver.
The ten wax wheel patterns were then delivered to the casting company for investment casting using mild steel. Two weeks later the castings were ready for collection.
Nine of the mild steel wheels came out perfectly. One wheel has a partially filled spoke. This wheel will be used to practise all the machining operations.
Now I have enough wheel castings for two locomotives! So if anyone is interested to purchase four or five mild steel castings, let me know...
I found the whole process most satisfying and rewarding!
