Accurate CNC machining for investment castings

Investment casting, also called lost-wax casting, starts with a wax pattern that is coated, de-waxed and filled with molten metal. What comes out is close to the final shape, but not close enough for most mating surfaces. As-cast surfaces typically sit in the Ra 6.3–12.5 μm range, and the process carries its own tolerance band on top of that.

The casting supplier controls wall thickness and general geometry. The machined features control what the part actually does: bearing bores, seal faces, bolt patterns, dowel holes, spigots. Those are the features your assembly feels. A few thousandths in the wrong place and the bracket will not seat, or the shaft will not spin freely.

We machine castings across aluminium, stainless steel, 4140, 17-4PH, titanium and copper alloys. The setup changes with the alloy, but the planning logic does not: understand the as-cast condition, pick datums that survive, then cut.

Stock allowance is the first number to settle. Too little and you cut into the casting skin, where sand, oxide and chill zones live. Too much and you pay for cycle time and risk distortion in a thin wall.

For most investment castings we plan 0.5–1.5 mm per machined face on small parts, and 2–3 mm on larger ones where casting warpage is harder to hold. Castings that will be heat treated need more, because quenching moves the geometry.

Here is a practical starting point. Every job gets checked on the first article before we commit to a full run, because no two foundries pour the same way.

One more thing. If a face only needs to be flat and clean, a light skim is enough. If it is a locating surface, plan the allowance so the finished face lands in the middle of the tolerance band, not at the edge.

• 1
  Cast surface finishRa 6.3–12.5 μm as-cast, so any sealing face needs a cut
• 2
  Heat-treated castingsAdd allowance; distortion after quench is normal
• 3
  Weld-repaired areasLocal hardness changes; expect tool wear

You cannot locate a casting on an as-cast surface and expect repeatability. The first operation has to create the datums the rest of the part will use. Usually that means a soft-jaw or fixture setup that clamps on a stable, non-critical area and machines a reference face and two reference edges.

Once those datums exist, every later operation is a known quantity. On a 5-axis center we can often reach five faces in two setups instead of four, which matters when the part is awkward to hold.

Clamping force is the quiet failure mode. Investment castings are often thin and ribbed. A vise tightened like it is holding bar stock will push the walls in, and the part springs back after unclamping. Use light pressure, support under the ribs, and if the wall is under 3 mm, consider a fitted nest rather than jaws.

For high-volume runs, a dedicated fixture pays for itself in the first few hundred parts. For one-offs, soft jaws and a probe touch-off are usually enough.

Castings are not homogeneous. Gas porosity can sit just under the skin, and the cutter will find it. You will hear it before you see it: a change in pitch as the insert passes through a void, then a rougher finish on the next pass.

Small, scattered porosity in a non-critical area is usually acceptable if the drawing does not call out a pressure or sealing requirement. A void in a bore that holds a seal is a different conversation. That part gets scrapped or reworked, and the foundry should hear about it.

Hard spots are the other common surprise. Chill zones, weld repairs and unmelted inclusions all cut differently. Carbide grades that work on 6061 will not survive the same feeds in a repaired zone. On 17-4PH and tool steel castings we plan a more conservative stepover and watch insert wear on the first few parts.

Interrupted cuts are normal on castings because you are crossing from skin into core, often over a cored hole. Keep the tool engaged, avoid dwell marks, and do not climb into a thin web at full width.

A tight tolerance on a casting is not the same problem as a tight tolerance on a billet. The material is the same, the geometry is not. Residual stress in a casting releases as you remove stock, and the part moves.

We plan around that. Rough, then let the part sit or run a stress-relief pass, then finish. On bores that must hold ±0.005 mm, we leave 0.2–0.3 mm for the finish pass and take it in one continuous cut rather than two.

Temperature matters more than people expect. A casting pulled from a cold truck and measured immediately will not match the number you get after it stabilizes. For tight work, let the part reach room temperature before the final inspection.

If the drawing demands a bore tolerance tighter than the process can hold repeatably, say so early. Sometimes the answer is a different fit, a bushing, or a design change. We would rather raise it at quoting than at final inspection.

Every casting job we run gets 100% inspection before shipment. That means raw material check on arrival, in-process monitoring during the cut, and final inspection against the drawing. Reports are available on request.

What belongs on a casting inspection report depends on the feature. Bores get a diameter and roundness check. Mating faces get flatness. Bolt patterns get position. If a feature is not called out on the drawing, we still check it if it affects assembly.

For first articles we recommend a full dimensional layout, then a shorter check on production parts. It keeps cost realistic without hiding drift.

One practical note: castings vary batch to batch even from the same foundry. If you send us a new lot, expect us to treat the first piece as a first article again. It takes less time than reworking a full run.