A casting that looks fine on the drawing can still come back with shrinkage holes, warped faces or no material left to clean up at machining. Most of these problems are designed in, and many can be caught at the RFQ stage if you know what to look for. This guide covers the main casting DFM checks for draft, wall thickness, fillets and machining allowance, with the reasons each one matters.

What a casting drawing should tell the foundry

A casting teaching lecture from the University of Technology, Iraq lists what the foundry engineer must decide for a part: the parting line, gate and riser location, draft, machining allowance, casting tolerance, and cores and locating points.

If the customer drawing shows only the finished machined part, several of these are left to you. That is not wrong, but each one is a decision that affects cost and rejection risk, so it should be written down and agreed before you quote.

Draft: give the pattern a way out

Faces that run in the direction the pattern or die is pulled need a taper. The same lecture gives draft of about 1 degree for sand casting and 2 to 3 degrees for permanent-mould processes. A permanent-mould design guide from Batesville Products recommends 3 degrees in general, with the minimum depending on depth, and notes that cored holes need their own draft based on depth and diameter.

Watch for drawings where a face that must be cast is shown dead straight with a tight tolerance. Either the drawing allows draft, or that face must be machined. Ask which.

Wall thickness and section changes

Uneven walls are the biggest source of shrinkage and distortion. The Aluminium Guide's casting design notes say the best way to avoid hot tears and distortion is to avoid lopsided distribution of metal and keep a uniform wall thickness. Where heavy sections cannot be avoided, risers or other costly foundry remedies are often needed.

Thin sections bring the opposite problem. The same source notes that misruns and cold shuts happen when metal freezes early in sections that are too long and narrow or too thin. Batesville recommends a wall of at least 0.180 inch for aluminium permanent-mould castings and advises avoiding isolated thin or thick sections.

Large flat areas are another risk. The lecture notes they can warp during cooling and suggests breaking them up with staggered ribs.

Fillets and corners

Sharp internal corners act as stress raisers and can lead to cracking and tearing. The lecture gives typical fillet radii of 3 to 25 mm, smaller for small castings. Batesville recommends at least 0.030 inch on slightly rounded corners for its process.

There is a limit, though. The Aluminium Guide warns that if a fillet radius is made too large, the extra metal at the junction can create an isolated hot spot, which brings back the shrinkage risk you were trying to avoid.

Uniform walls, sensible fillets and enough draft prevent more casting rejections than any amount of inspection after the fact.

Machining allowance and casting tolerances

Every machined face needs extra stock on the casting. The lecture states that allowances depend on casting type and grow with size, ranging from about 2 to 5 mm on small castings to more than 25 mm on large ones. Batesville notes its machine stock allowance is specified per surface, not per dimension.

For as-cast dimensions, ISO 8062-3:2023 is the reference. According to the ISO page, it specifies general dimensional and geometrical tolerances and machining allowance grades for castings, for all cast metals and processes, and does not apply to 3D CAD models used without indicated dimensions. If a drawing says only "casting tolerances as per standard", ask which grade.

Also check where the machining datums sit. If the first machining set-up locates on a face that crosses the parting line, the shift between mould halves can eat your allowance. Batesville gives wider tolerances for features across the parting line than for those on one side.

A quick casting DFM checklist before quoting

  • Is the parting line marked or agreed?
  • Does every pulled face have draft, or is it to be machined?
  • Are walls reasonably uniform, with no isolated heavy bosses?
  • Are internal corners filleted, and not so large that they create hot spots?
  • Is machining allowance shown per surface, and is the tolerance grade named?
  • Are casting datums and locating points defined for the first machining operation?

For example (illustrative), a pump housing drawing with no draft note, no tolerance grade and no casting datums gives you three clarification points before you even look at the bore sizes.

How SpecReview helps

Upload the drawing PDF, STEP model or both, and SpecReview lists DFM and rejection risks with a short reason why each matters, along with spec gaps such as a missing tolerance grade or draft note. Each point is tagged as coming from the spec, the geometry or an assumption, so you know what to confirm with the customer.