Two Numbers That Decide Machining Cost
Every dimension on a casting drawing carries two decisions that are easy to overlook and expensive to get wrong. The first is how much metal is deliberately left on surfaces that will be machined. The second is how much variation the foundry is allowed before the part is rejected. Too little allowance and the tool never cleans up, leaving skin and subsurface defects in a bearing face. Too much and you are paying for iron by the kilo only to turn it into swarf, adding cutting time and tool wear. Getting both right is one of the clearest signs that a designer and a manufacturer are actually talking to each other.
What Machining Allowance Has to Cover
Allowance is not a single number applied everywhere. It has to absorb the casting's own dimensional variation, the surface roughness and skin left by the mould, and the distortion that occurs during cooling and heat treatment. A large bed casting moving through a stress relief cycle will shift differently from a small bracket that goes straight to the machine. Section size matters: bigger castings vary more, so they need more metal left on. The surface also matters — a face that will be hand scraped needs less than one that must be milled through a hard skin. Experienced manufacturers hold tables of working values by casting size and process, and those tables are worth asking for.
Dimensional Tolerance Classes for Castings
Casting tolerances are coarser than machining tolerances and always will be. International standards define tolerance grades for moulded parts, graded by nominal size and by the capability of the process, so a resin sand moulded part can be held tighter than a large green sand one. The practical mistake buyers make is copying machined-part tolerances onto a casting drawing, which either forces the foundry to quote a premium process or produces a drawing nobody can meet. Specify casting tolerances where the as-cast surface is final, and specify machining tolerances only where metal will actually be removed. Keeping those two separate clarifies half the arguments at goods-in.
Choosing Datums Before Anyone Cuts Metal
A datum is the reference from which everything else is measured, and on a casting it must be a surface that exists reliably on every part and can be located in a fixture immediately. Good datum design uses surfaces produced in the same mould half, avoids parting lines and gate stubs, and gives the machinist three points that locate the part without arguing. Bad datum design sends machinists chasing one feature to another, compounding error with each setup. If the drawing and the machining process agree on the datum, inspection becomes straightforward, and the tolerance stack that worried the designer mostly disappears.
Where Designers Create Their Own Problems
Some geometry is simply hostile to casting. Abrupt changes in wall thickness create hot spots and shrinkage; a long thin rib beside a heavy boss will crack or distort; deep pockets with no draft make moulding miserable; sharp internal corners concentrate stress and crack in service. Generous fillets, tapered transitions and ribs sized to match the walls they meet cost nothing and prevent a great deal. So does leaving coring out where a pocket can be machined instead, if the volume justifies it. A half hour with the manufacturer at drawing stage regularly saves weeks of arguing about scrap rates later.
Specifying for Manufacture, Not for Argument
The drawings that produce good castings tend to share habits: material grade cited to a standard, hardness range stated where it matters, machining allowance given per surface or by a stated general rule, tolerance class referenced rather than invented, and inspection requirements named up front. Equally important is stating what does not matter. A cosmetic standard applied to a hidden internal surface adds cost with no benefit. Clarity about which faces are functional allows the foundry to concentrate its control where the part actually works.
The Payoff in the Machine Shop
Well-specified castings arrive at the machine with an even amount of metal to remove, sit on their datum without shimming, cut consistently because the hardness is controlled, and finish inside tolerance on the first setup. Poorly specified ones need packing, extra roughing passes and judgement calls by the operator. None of that shows up in the purchase price comparison, which is why the cheapest quotation is not always the cheapest part. Working with a manufacturer that reviews drawings for castability before quoting is the least expensive quality measure available, and it is usually free.
Weight, Cost and the Commercial Side of Allowance
Allowance has a price tag that is easy to miss. Foundries quote by weight, and machinists bill by the hour, so every extra millimetre left on a large bed casting is paid for twice: once as iron and once as cutting time. On a multi-tonne part, trimming allowance by a couple of millimetres per face can meaningfully change both the casting price and the machining cycle. The counterweight is risk: cut allowance too fine and a slight shift during heat treatment leaves a face that will not clean up. The sensible answer is to set allowance from the manufacturer's own capability data rather than from a generic table, and to review it after the first article proves how the casting actually behaves.
References
ISO 8062-1, Geometrical product specifications (GPS) — Dimensional and geometrical tolerances for moulded parts, International Organization for Standardization.
ISO 8062-3, Geometrical product specifications (GPS) — Dimensional and geometrical tolerances for moulded parts, International Organization for Standardization.
ASME Y14.5-2018, Dimensioning and Tolerancing, American Society of Mechanical Engineers.
ASM Handbook, Volume 15: Casting, ASM International.
ISO 2768-1, General tolerances — Tolerances for linear and angular dimensions, International Organization for Standardization.
About the Author
Rowan Pike writes about workshop tooling, fixturing and fabrication practice. He spent years building jigs for agricultural equipment before turning to technical writing and still prefers a bolt-together fixture to anything that needs a welder.
