Castings

[neathabbeyironworks]

Just a couple of quick questions regarding castings. Allowance for shrinkage when casting cast iron is 1%. Is this still important when the components are small? Also how much extra should I allow for machining? For a driving wheel, on a loco some faces will need to be machined smooth, how many mm extra should be added to the casting dimensions to allow for this? I am also casting horn blocks. Would it be a advisable to cast a bar across the ends of the horn blocks to minimise distortion during casting and during machining?

[Jo]

While the allowance for shrinkage is small for CI you need to leave "extra" above that to machine off to leave a smooth finish on the finished item. If you are having the items sand cast then you will want to leave a few mm as not only is there the roughness of the sand but also the first mm can harden.

Most important is to leave a draught angle on the pattern so that the pattern can be withdrawn from the sand. This means the thin end of the draught angle needs to have that extra bit on it so you don't get marks from the sand and the wider end will be even wider still. The pattern should have not sharp corners and should be painted with a suitable non stick paint (I use blackboard paint) Also think about how you are going to machine the casting: An additional spigot can sometimes make things a lot easier to hold.

The hardest bit on a horn block are the webs at the back they are very thin and therefore likely to be extremely difficult to cast. I've had foundries refuse to cast items with thin webs because they are very difficult to cast intact.

I wouldn't bother with a bar across the end of a set of 7 1/4" hornblocks. I would look to screw all of my patterns onto a single sheet of wood bigger than the box they are going to use to help the foundry not loose any of the patterns and it makes it easier for them to place the runners/sprues and feeders.

Jo

[ennech]

I Go along with Jo's comments. As regard pattern painting don't use gloss paint as the sand sticks to it. For machining I leave 2 to 3 mm. I would take the advice of the foundry before you fasten small patterns to a board. I did this early on and had to remove them as the foundry preferred them loose! Neathabbeyironworks? are you really in South Wales? if so you have an excellent foundry handy on the Gower.

[abby]

Some confusion here as this is the gauge1 board yet jo mentions 7 1/4" gauge horn blocks , if you are "Neathabbeyironworks" does this mean that you are a foundry ?
I have found most gauge 1 iron castings to be of poor quality requiring a great deal of work to get anything useable . Most G1 builders prefer castings in gun metal or brass.
Some good quality investment cast iron wheels are available but at a very expensive cost. personally I use my own investment cast brass wheels with steel tyres and I use a 2.5% shrinkage allowance plus an extra mm on any face requiring machining.
Dan.