Metal Thickness

1. Whenever possible, use a wall of reasonably constant thickness throughout.
   Satisfactory wall thickness range from 1/4 inch to 3/4 inch for most sand cast irons. All common metals expand when heated as their molecular activity increases. After pouring, the liquid loses heat through the mold by conduction and immediately starts to build the metallic solid shell. If the casting's wall is both reasonably thin and uniform, sound castings can be expected. But if the casting has heavy walls, thick local section, abrupt transitions, or massive junctions, the center metal may become porous unless additionally fed. This is called center-line shrinkage or internal porosity. If design parameters require thicker castings, consider austempered Ductile Iron to reduce section thickness.
   
   
2. Make all unavoidable transitions gradual between sections of different thickness.
   Avoid sharp corners or abrupt steps that can cause feeding or structural problems. Sharp internal corners (hot corners) can lead to internal porosity defects.
   
   
3. Round all corners generously.
   Rounded corners (external) and fillets (internal) are trade marks of a casting. Not only do they enhance appearance but they distribute strains. Either carry the common thickness around a liberal radius or thin it slightly. Use generous fillets and radii with impunity.
   
   
4. Use metal sparingly.
   Pressure tight castings must be completely sound and free of porosity -- they must not leak! Examples are hydraulic cylinders, valve bodies, and manifolds. Design the desired system (for example, a hydraulic manifold) as a series of passages in free air and then clothe them with metal as thin and uniform as will do the job. Drilling passages and foundry porosity problems can often be eliminated, thus providing a sound casting.
   
   
5. Consult us and our (or your) machine shop on recommended machining allowances.
   We typically recommend 1/16 inch to 1/8 inch depending on casting size. Excess machining can influence casting soundness. Inadequate machining allowance can lead to machining rejects and excessive machining tool wear.

Flanges, Ribs, and Bosses

1. Use external in preference to internal flanges -- the latter may require a core at additional expense.
   External flanges on the part-line, or at right angles to the parting line, are the most practical, and do not typically create extra expense.
   
   
2. Avoid diagonal flanges unless they justify extra expense of coring.
   
   
3. Some castings can utilize internal and/or external bosses.
   This supplemental local metal will allow the body of the casting to be desirably thin and uniform.
   
   
4. Place bosses or flanges where they will draw -- either vertical or horizontal with the parting line.
   All other angles will probably require cores.
   
   
5. Carry minor horizontal bosses to the parting line.
   This will eliminate the expense and bother of a core. Holes can be drilled and spotfaced to eliminate draft and to provide a seat.
   
   
6. Use pads to limit the area to be machined.
   Filleted pads look good, define the surface to be finished, offer a major economy in machining and prevent marring the surrounding cast surface's patina (texture). These are often raised 1/8 inch to 1/4 inch plus finish.
   
   
7. Gussets, ribs and fins carry forward the general principles of flanges and bosses.
   The principle difference is that these are used in multiple. They are also most efficiently handled at right angles to, or following along, the parting line. Allow extra draft and generous fillets when designing multiple ribs or flanges, particularly when they are close together.
   
   
8. Consider locating the parting line at right angles with the axis or center line.
   This will allow any number of fins, ribs, or gussets parallel with the center line to be molded without cores. Draft encroachment distortion, shared by the cope and drag, will be reduced.
   
   
9. Emphasize the parting line for appearance.
   Grinding away any flash or mismatch will develop -- rather than hide! -- the parting line. A slightly protruding fin looks okay when only its edge is dressed.

Closures

1. When precision is modest, cast-in dimples will locate the holes to be drilled.
   By using the specified diameter and drilling into the pattern to its full conical point depth, dimples can be cast in any surface that will draw (approximately parallel with the parting line). Pre-drill when these are on surfaces to be machined. If used intelligently, dimples can often save the expense of a drill jig, the time wasted and inconvenience of using it.
   
   
2. Evaluate the degree of precision and finish that is really required.
   Tight tolerance control can lead to additional casting and machining cost and may also increase tooling costs.

Screw Threads

1. Specify that threads, both male and female, be machined.
   The exception is when threads are both coarse and sloppy. Threads are difficult to cast successfully, even with special processes and handling. Small imperfections -- bumps, out-of-roundness, warp or pitch -- will require corrective machining. Surface metal is hard on tools! The male may have parting line flash or suffer slight part-line shift, and the female, even when accurately cored, tends to have minor but intolerable variations that must be corrected.

Spokes

1. Specify the number of spokes as a multiple of three.
   The most common lathe chuck self-centers its 3-jaws. Inevitably, the casting needs to be chucked internally by its rim for external clean-up, grooving or machining, a system that will also ensure a uniform rim and approximate dynamic balance.

Hub-type Castings

1. When a core is required, get your money's worth!
   Relieve the centre area of the wheel hub internal cored diameter. Since the wheel's hub is stabilized only by its ends, the fat part of the core will relieve the unnecessary center, reducing boring by half, and saving casting weight and machining cost.
   
   
2. Consult the machinist and foundry as to how much detail can be cast in.
   There is no question about any shape that need not be machined (oil collector grooves, for example), but some machinists prefer to turn or bore to diameter and then mill the fussy detail. On the other hand, any detail that can be cast is executed only once by the patternmaker -- NOT on every piece until the death of the part. Coring will normally reduce costs vs. accomplishing the same with machining.
   
   
3. Locate the parting line where it will do the most good.
   Unless thoroughly familiar with the art of casting, discuss this with the foundry and pattern shop. Any surface perpendicular to the part line requires draft (typically 2° minimum).
   
   
4. Use green sand "core" whenever possible.
   Homogeneous appendages are automatically molded in drafted cavities in the pattern, thus preventing coring and of course saving $'s! In general, a green sand "core" should not be deeper than its width / diameter. Deeper green sand cores (pockets) can cause excessive casting roughness and lead to internal porosity problems.

Nomenclature

1. Always specify identification and/or part numbers.
   Supplying spare parts must be anticipated. Part numbers assist in ordering accurately and in identifying your part through our many processes in the foundry. Pattern impressions (cavities) should also be identified. Manufacturers I.D. and date cast might be other lettering you should consider on the castings. Locate lettering (recessed or raised) on a surface parallel with the part-line.

Don'ts

1. Never thicken a casting with unnecessary finish (extra metal to be machined away) in hope of getting below the bad into the good.
   This wishful thinking will intensify the problem of center-line or internal shrinkage! The opposite approach -- minimizing the wall thickness to near finished dimension -- is generally the right answer.
   
   
2. Don't insist on doing it all yourself.
   This leads to static thinking and often limits the product. Good ideas abound - the cost is only listening. It is all too easy to let one's thinking get trapped up a dead end street! Talk to the Tooling Development division at our foundry -- they have a wealth of knowledge and ideas to share with you!
   
   
3. Don't discourage the machine shop and pattern shop from participating.
   Their involvement will inspire better work and cooperation all the way down the line. Playing coordinator provides satisfaction, is good management and will improve the product. Best of all, it will make everyone from salesman and engineer to the foundryman and machinist come up smelling like a rose!