Allowances for castings

[neathabbeyironworks]

I am in process of obtaining castings for loco wheels and cylinders, all in cast iron. I realise I have to make allowance fir shrinkage during cooling of the fresh casting. Is 1% sufficient for castings for a 7.25 inch gauge loco? Also these castings will need to be machined, so how much should I allow for machii ing down to a dimensionally accurate smooth machined surface?  I know my questions on these boards appear a bit random, but I am trying to plan ahead so I minimise bad language and metal ending up in the recycling skip.

[stevep]

It doesn't matter what gauge you are working in, the shrinkage allowance should be based on the material used for the casting.

[mattb]

Shrinkage is only one part of the story there will be tolerances that need to be accounted for from the pattern making process and on the assembly of the molds any cores. The casting industry talk about this as CT tolerances and there are pdf copy's online of ISO8062 if you do a google. For reference a large UK casting supplier states CT12 for Green sand and CT10 for Airset molding processes. CT12 would be +/- 3mm (6mm total)on a 100mm feature so pretty significant. Obviously these are standard numbers for production runs and take a lot of things into account. Probably worth having a chat with your foundry to get the guidance and understand their process but ultimately you need to make the call and for me I would always play it safe as the cost of a few mm of extra iron and the time to machine it its preferable to scrapped castings.

[jordanleeds]

Whilst not scientific I always add 1/8" on top of the prescribed allowance to give more a more generous shrinkage and machining allowance

[ettingtonliam]

Shrinkage allowance for cast iron (IFIRC) is 1/8" per foot, so yes, 1% for shrinkage is pretty well spot on. There will be 'draw' on the pattern which I used to make about 1/16" per inch, and a machining allowance which I used to make 1/8" on all dimensions. This allowed for any errors I might have made in the pattern, and for taking off any 'skin' on the casting, and small surface defects.

For a typical 7 1/4" wheel, I'd make the pattern about 1/4" large on diameter, and for the cylinders about 1/4" long on length and 1/4" small on the bore.

[jasonb]

When I make patterns intended for iron I add 1%, well actually I draw out actual size in CAD and then scale the pattern by a factor of 1.01. This applies to all sizes from a small bearing cap not more than 25mm long to large 430mm dia flywheels weighing over 20kg.

As for machining allowances if the surface will pull out of the sand without the need for draft then I usually go with 2.5mm unless the part is quite thin in which case I add a bit more to reduce the risk of chill. If the surface is on the side of a pattern and needs some draft angle to be able to remove it then I go with 2.5mm at the narrow end and whatever the draft angle makes it at the split line.

If the part is a cylinder and needs a core then I'd go with at least 3mm on the radius as with the best will in the world a core can shift and leave you with less metal on one side than the other

Depending on the part 2 to 5 degrees draft. For example the outside rim of a flywheel may get 2degrees draft but a self coding crankcase or box bed may have 2deg external but 5deg internal

Some people allow a bit less for machining but I'd rather pay a little more for the extra weight of iron and know I will be able to machine the part I want from the casting. More so as yours will be a one off whereas a commerical casting could be modified if it was found to need more material before being put into production.