Casting Iron - The Background

I ran into a not so unique situation when I researched casting iron parts for my car. All the "experts" I contacted who run foundries said that it would cost around $4500 to make a template and mold and another $500 to pour the iron for the three exhaust manifold to block spacers that I need.

They made it sound so damn complicated and difficult that it was quite discouraging. Something sounded wrong to me. There has been sand casting going on for over 2000 years so it can't be that complicated and difficult. Everything that I read about it indicated it was a pretty simple process. Make a wooden template, pack it in a split sand box, add some pour holes and vent holes, pour in melted iron, cool, machine the unwanted parts off.

I researched sand casting on the Internet and came up with some great links from hobbyists who had there own mini foundries. They designed and built their own furnances for typically under $200. All I had to do is find someone who has done that in my area. I figured out that model steam engine builders usually cast a lot of their own parts so they must have a foundry or access to one.

I eventually contacted Rupert Wenig who was gracious enough to offer his help even though he lives 180 miles from my house. He is a member of the Alberta Metal Enthuasists Network which is a great bunch of guys who go out of their way to help you.

I was given very good advice on how to make the wooden template and invited to a "pour" of aluminum and zinc-aluminum about a week later. I figured that I would try to quickly whip up a template out of a 2x6 and see what type of problems I would encounter.

Here is a pdf file of a side view of what the exhaust spacers will look like. They are basically the shape of the exhaust outlet on the heads and 1 7/8" thick. The pdf file shows the parting line for the casting running through the mounting bolt holes.

Some quick rules:

  • cast iron will shrink 1/16" per foot from the template size. This means that the template has to be 5% larger on exterior dimensions and 5% smaller on interior dimensions versus the final product. The term for this is shrinkage.

  • Any area that will be machined after casting should have an extra 0.125" added because the iron will react with the silica to form a very hard layer and you'll want to machine past it.

  • In order to remove the template from the sand without breaking the sand mold, vertical lines are not allowed. A small angle anywhere from 1-3 degree has to be made into the template.

  • No sharp corners are allowed on the template.

The first thing I discovered when making a template was that table saws are very inaccurate instruments for cutting when dealing with hundreds of an inch. I was attempting accuracy of 1.125". I asked the local carpenters at work to thickness plane my 2x6 to 1.125". They said no way, the closest they could get it would be 1 1/8". The result was 1.100" which is too small for my purposes.

I got out my Dremel tool and went to work on one template. I tried steel routing style bits but found that the small drum sanders worked the best. Too good, it was very easy to take too much material off. I figured that I would have more control if I put one of the small drum sanders in my drill press. This worked out very well but definitely indicated the need for better control of movement like found on a mill.

I had a split pattern (2 piece) that was raw wood (should be painted and a layer of hard wax) that I brought with me to Rupert's farm to test.

template

2 piece wooden template

The templates have small rectangular extensions that run out both sides for holding the sand core plug. The sand core plug is used to make the exhaust runner opening.

Just a note on the template's warpage: right after the 2x6 was planed, it was perfectly flat. Within 3 hours, it had warped slightly concave. Two days, after carving the template, the wood warped the opposite direction and now was slightly convex. A drier piece of wood would of helped.

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Copyright July, 2011 Eugene Blanchard