Designer's Notebook: Make Your Own Hardware

Although pattern making for complex parts is still best left to the pros, making a wooden pattern to then have a foundry make a metal sand casting is well within the abilities of the typical DIY individual. Making a water-cooled exhaust manifold is usually well beyond the DIY'er, but simple cleats or one of a kind fittings are mostly within the realm of the handy person.

A basic understanding of how a foundry makes a sand casting should be understood. The "sand" used to make the casting is a special product and not the typical beach type; clays and other additives make it stick together.

For example, let's take a simple flat plate pattern as shown in the sketch. Note that the sides are tapered (this is called "draft"); the part must be able to be withdrawn from the sand mold. The person who makes sand castings is called a "molder". He uses a two part box called a "flask" to hold the sand mold. This flask is divided into two halves, aligned by guides; the flask does not have a fixed top or bottom. The lower part of the flask is called a "drag" and the upper section "cope".

Using the simple flat plate pattern, the molder lays it on a flat board. The drag portion of the flask is inverted over the pattern lying on the flat board and the molding sand packed firmly over the pattern and into the flask drag. Another flat board is put over the exposed open end of the drag and the entire affair rolled over and the original flat board removed to expose the top of the pattern.

The cope (upper half of the flask) is slipped over upright locating pins and the cope firmly packed (rammed) with sand. A flat board is put over the top open end of the cope. The cope is then slid upward by the guide pins and off of the drag. The cope and board are then flipped over so the part with the pattern impression is exposed.

The wooden pattern is then carefully withdrawn from the sand mold. A good molder will draw the pattern from the mold with almost no sand being disturbed. The molder cuts "gates"' (paths in the sand) for the molten metal to enter the pattern cavity. A "sprue" or hole for entrance of the molten metal is cut through the cope sand directly over the gate in the drag. The mold cavity and gates are cleared of any loose grains of sand. The cope is then inverted and placed over the drag with the guide pins aligning the two.

The sand mold is then ready to be poured with molten metal. After cooling, the casting is removed from the sand, the gate cut off, and you have a metal duplicate of the wooden pattern.

The resultant casting will be smaller than its pattern. In manganese bronze the difference will be about 3/16" to the foot, but this will be unimportant for the typical cleat or similar fitting. If, however, the part being made is to fit to another part, shrinkage should be allowed for. Special "shrink rules" are available but calculating the amount of shrinkage is not that difficult.

Wooden patterns may be made from almost any stable wood; soft white pine is easy to carve and for short runs is commonly used. The important thing to remember about a pattern is that it must be pulled from the sand mold smoothly; it must be tapered or have draft, no back hollows. A pattern may be split in two halves with the "parting" line on the same plane as that of the cope and drag junction. In truth, the parting line does not have to be in this plane but to keep things simple, we're describing a pattern that has the above requisite or is in the drag only as our flat plate example.

Examples of split patterns.

On the left is a split pattern that was made to slip over a boom tube of a very small dinghy. The split pattern halves are aligned by small dowel pins tapered to slip out easily but align the two halves accurately. On the right is a fitting made for the bow of a sailboat. The forward portion is a cleat while the aft vertical portion can be drilled for the forestay pin and other fittings required.

Sharp corners cannot be tolerated on sand cast patterns. Interior junctions should be filleted, made from any hard setting compound that can be easily applied and formed. Fillet shape wax is frequently used by pattern makers but since small quantities may be difficult to obtain, for small patterns paraffin wax will suffice. A heated fillet tool (simply a steel ball on a handle) is used to apply fillet wax, melting it and forming it into a junction. Just be certain all edges are well rounded and all junctions flow smoothly together with draft or taper.

The finished pattern must be sealed. For years orange shellac made from scratch was used. Sand mixes became more complex and special coatings were developed but are expensive and difficult to obtain for most. Lacquer-based finishes also work well. Usually two or three coats of paste wax, well rubbed over the pattern, will suffice for short runs. If in doubt, check with the foundry that will make the part.

Sand castings for marine use are generally made from a brass compound, red brass, manganese bronze or related alloys. Aluminum is easy to cast but does tend to deteriorate, especially in a salt-water environment.

The patterns shown are but a couple of examples of what can be made from sand casting patterns. We've made patterns and sand castings for most anything that can be imagined; cleats, lifting eyes, bow handles, vents, hawse pipes, exhaust port flanges, water pickups, steering wheels and columns, throttle fittings, windshield brackets, struts, fins, rudders, etc. Some parts will have back-draft, holes, or other conditions that may need to be free of molten metal. An insert in the mold cavity must be used called a "core" that is made from special sand packed into a wooden core box, removed and baked in an oven. The resultant core is inserted into the mold cavity.

This short discourse is not intended to cover the pattern nuances required for patterns with cores; it can be a complex subject. The would-be pattern maker contemplating making patterns that require cores should obtain a good book on pattern making and foundry practice.

The handy DIY person can make simple parts, however. It's fun and castings can be made for parts that can't be bought but do fill a particular need.

Try it; you'll like the challenge and end up with a one-of-a-kind part or fitting.

Anchors

-ArtDeco

Photos sent in since the last WebLetter...

eMail of the Month

Subject: Thank You
Date: 27 January 2009

Dear all at Glen-L, thanks for all the information that you sent. My project is going slow, but it is going. Stem is connected to the breasthook, side braces are done, transom is half way and hopefully will be ready for mid-February when I plan to build the platform.

My big problems are second floor flat, no garden, no driveway, and two girls that are in education (the girls are not the problem but paying for education is). But hey, life is too short for negative attitude, so we will do it.

Oh, sorry, my name is Alex, I live in London, and I was born by the river Danube in Serbia. In the eighties my brother and I built a 13-footer in plywood and fiberglassed it with polyester, but I always wanted to build with epoxy, and this Power Drifter is just an educational exercise. Plan is to build a classic sailing yacht in cold molding, and Mediterranean retirement.

Dreams. Yes dreams. Smell of wood when cut or sanded, dreams become reality. Freedom at last. Thank you.

Wishing you well.