Hi All,
Its been a really fine almost summer day here so I have not been in the workshop. I however did manage to look out some of the masters I made for my 4-6-0,in particular the slide bars, crosshead and connecting rods. The effort of having castings made is only justifiable in my mind if one is going to need a number of identical parts. If I was only building one model then the time to make the masters, the waiting time for the castings to come back plus the expense would not be justified. However if the parts have wider use, or a lot are needed on the model then a casting may be a worthwhile investment. The outside connecting rods on the Manson 4-6-0 are a case in point. I wanted to build more than one engine and the time saved by getting the motion parts cast was fully justified.
View attachment 36048
Here is a quick snap of them on the beastie. These are cast in nickel silver by the lost wax process. The masters are brass. See this picture which shows the masters, an un-fettled set of castings and the slidebars assembled.
When making the master you have to remember that for the metal casting to be made a wax copy has to be made first. This wax is made in a rubber or silicone mould from your original. Some casters still use a vulcanised rubber mould for the waxes. If this is the case your masters will need to be soldered together with at least 188 solder. I still use tin/lead and have had no problems with it. If your caster uses an RTV, room temperature vulcanising, rubber or silicone mould you can get away with a lower melting point solder. You may also be able to use plasticard masters but check with your caster first before spending time on the patterns! The finished waxes are then dipped in a ceramic slurry which hardens and when heated the wax burns out leaving a cavity into which the molten metal is poured.
The original must allow for the shrinkage which happens both in the wax moulding and in the metal casting. The allowance is 2-3% For small parts you can ignore the shrinkage but for these rods which are 79 mm long I made the master 81 mm long.
Another consideration is in how the part will be moulded in wax. In all probability the wax will show a part line where the two parts of the mould join. You have to design into the pattern where this line will appear. On the components shown above the part lines are along the edges. I leave a little extra allowance to let me file off the part lines and end up at the correct shape. It is important to tell the caster which way you want the part to be moulded. I always give the caster a sketch of each part showing how I want it placed in the mould. Also beware that the waxes can be distorted when they are removed from the mould. If this is not spotted before being invested then the final casting will also be distorted. Not a big problem on a connecting rod with a slight bend but I have seen the valve spindle guides from a kit where the spindle needed to be bent in an arc to fit!
Also to be designed in is where the metal feeds into the mould. The feed has to be big enough to let the metal flow quickly into the cavity and support any vulnerable part of your pattern. The Slidebars above are extended and joined by a spreader which keeps them parallel and straight. I have spent too long with commercial castings for working valve gear trying to straighten them out to be useable.
Where two parts are to fit together and slide, crosshead and slidebars, you must allow a little extra fitting metal. On the Crosshead I made the slides very slightly wider than required. The slidebars were made to fit. I then soldered 5 thou shims to the inside faces of the bars which meant the crosshead would no longer go in but let me file the castings back to give a nice easy running movement.
The hole for the piston rod in the stuffing box I left at full size. My caster knew about it and made sure that the wax also had a hole. The final castings just need the hole cleared by running a drill through and then easing with a taper reamer just to get a nice running fit on the piston rod.
On this prototype I managed to get away with only making one set of motion as top and bottom slide bars are the same. The crosshead is handed but by not putting the bolts in it I was able to fit the appropriate ones and drill out the upper oiling access hole. The big end on the connecting rod was made with the cotter and oil box on both top and bottom. The redundant parts were easily removed to hand the items.
This has been a wee bit rambling but I hope that this helps Mick. That Y class tank you showed us looks an interesting beastie.
Ian
other than those minor issues I have no problem with white metal and may go that way as well, I wonder if there is a cost implication in casting some in brass and other in WM ? 
