Apologies that this is a bit later than planned [see end of previous article for the reason].
In the last episode I detailed how the new prototype castings were coming from two sources. One a complete CAD to brass package via Shapeways and the other printed waxes via Precision Wax and then on to C Harding Castings to be turned into brass bits.
Both have, in my opinion, produced acceptable results with some of the bits coming out better in different methods.
This first image is the Shapeways collection which are a bit harder to photograph as they insist on coating the castings with a gloss coat as they seem to think everyone is doing shiny stuff.
This next group is the wax from one source, cast at another method. Some of these need some work before they can be used as patterns as they need the casting runners cleaning and altering. I have two set of these so I will be using one set to complete a model and finish the instructions.
Here we have the Salter Safety valve castings from the two methods. One of the risks with the wax route is that the waxes this process produces are very brittle (the ones i can print aren’t but we won’t go back to that just now) and both the examples were broken up and cast or broken as they came out of the ceramic mould and whilst they could be assembled to make a model they are pretty useless as a pattern. The complete one piece casting is the Shapeways example that has been lightly abraded in my grit blaster to take the shine off. This is how they should have all looked and how the waxes looked when I mailed them. I think the Shapeways part gets to to be the pattern for this bit! I would make the observation here that the photos are cruel enlargements and that the actual parts are pushing the minimum thickness limits of the process.
Probably by the time you read this the example of each part selected to be a pattern will have been cleaned, prepped and tested on a model to check the sizes and is probably on its way to the casting company. What they do now is: –
- Make silicon moulds around the patterns sent (I think they use a third party for this)
- Cast wax copies of the parts from these moulds making as many as we order
- Add runners to these and assemble them in tree shape
- Pour a fine ceramic moulding liquid around the tree and cure it
- Heat it to remove all the wax
- Pour in brass and wait for it to cool
- Break up the moulds and clean the parts
- Ship the parts
How long this takes I am not sure but I will be having a conversation about that tomorrow. I suspect it will be two to three weeks. Meanwhile my plan is to use the second set of castings I have to complete the instructions and to have the kits packed and ready to go just needing the castings dropping in and the lid putting on.
I never really intended them to be a sales item but having been asked several times it looks like they may become one. It won’t be a cheap set and it will probably be done to order given that the supply of NGG16 kits has now dried up with the demise of Backwoods Miniatures.
However, whilst getting a set for a customer a little upgrade happened. In the original version the expansion link was simplified to provide a pivot hole in the middle but this meant that the valve gear could be set in forwards or backwards but couldn’t move between the two.
Well, with a bit of CAD work the expansion link has been redesigned to feature the prototypical pivots allowing working valve gear.
There is still some fitting work to do of the fiddly and frustrating variety to fit nickel silver wires as the pivots but then if you are fitting working valve gear on an NGG16 you are probably certifiable already.
More progress reports will follow when there is something to report
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