Multi Process Five Axis Machining for Gas Turbine Hardware

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  Key takeaways
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  What multi process five axis machining means for turbine hardware
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  Reading the drawing before the first cut
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  Building a datum chain that survives three setups
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  Setting feeds and finishes for casing features
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  Checking the part against the datum, not the machine
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  Six steps from drawing to shipped casing
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  When to use multi process five axis versus 3-axis setups

A gas turbine casing is a ring of features that must line up: split-line flanges, bore diameters, dowel holes, mounting pads at compound angles, seal grooves. No single setup reaches all of them. Multi process five axis machining means we plan the order of operations so each feature is cut from a datum that still exists, and so the part is not moved until the tolerances that depend on that setup are finished.

The alternative is a stack of 3-axis setups with re-fixturing between each one. Every re-clamp adds position error. On a 600 mm casing with a ±0.005 mm bore-to-bore callout, three re-clamps can burn the whole tolerance band before the cutter touches metal. Five-axis work cuts the count of setups, and that is where the accuracy comes from.

We run 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix matters for turbine work because a casing may need a 4,000 mm envelope for the roughing operation and a Ø400 mm rotary table for the angled pad finishing. The route picks the machine per operation, not per part.

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  Rough on the largest machineUp to 4,000 × 400 × 150 mm travel for oversized casing sections.
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  Finish on simultaneous 5-axisRotary table carries the part; the tool stays normal to the surface.
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  Turn round features on mill-turnBores and seal diameters held in one chuck instead of two setups.

Turbine hardware in our shop usually lands in one of four material groups. Stainless 304, 316 and 17-4PH for casings and brackets that see heat and mild corrosion. Inconel and titanium TC4 (Ti-6Al-4V) for hot-section components. 4130, 4140 and 4340 for structural mounts. Aluminium 6061-T6 and 7075 for test hardware and tooling plates that never see the gas path.

Material choice changes the cutting data, not the route logic. Inconel work-hardens fast, so we keep radial engagement low and never let the tool rub. Titanium moves heat into the cutter, so coolant delivery and feed per tooth get checked before the first pass. On 17-4PH we plan for the heat-treat state: machining in condition H1025 behaves differently from annealing stock.

One thing to settle before quoting: is the blank a casting, a forging or plate? A casting arrives with 3–6 mm of stock variation across the surface, so the first operation has to establish a datum from the casting itself, not from an assumed corner. Forgings are closer but still move after stress relief. Plate is the most predictable and the slowest to buy in thick sections.

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  Ask for the blank drawingStock condition decides how much the first setup can remove.
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  Confirm heat-treat statePre-hardened stock cuts differently and may need a stress-relief step.

The fixture plan is the real deliverable on a turbine part. We pick one primary datum, usually the largest machined face or the split line, and machine it first so every later setup can locate on it. Secondary datums come from dowel or bolt holes that are drilled in the same operation, not from edges that the cutter might have left with a burr.

For a casing with a Ø400 mm rotary table available, the common pattern is: setup one establishes the split face and two dowel holes; setup two holds the part on that face and finishes the inner bore and flange faces; setup three indexes the rotary table to the compound-angle pads. Three setups, one datum chain.

Soft jaws and dedicated fixtures both work. Soft jaws are faster to make for one or two pieces. Above about ten pieces, a dedicated plate with hardened locating pins pays back because the load/unload time drops and repeatability improves. We keep the fixture drawing with the process sheet so a re-run two years later starts from the same numbers.

Do not let the fixture clamp over a surface that will be finished later. It is a small mistake that shows up as a clamp mark on a sealing face, and it usually means a rework or a scrap decision at final inspection.

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  One datum, three setupsEvery later setup locates on the first machined face.
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  Clamp on rough surfaces onlyKeep jaws away from sealing faces and bore walls.

Roughing removes the bulk. On stainless we run carbide end mills with a 0.3–0.5 mm finishing allowance left on every tolerance surface. Depth of cut can be aggressive when the setup is rigid; the limiting factor is usually chatter at the top of a thin casing wall, not spindle power. When a wall rings, we reduce radial engagement before touching the feed.

Semi-finishing brings the part to 0.1–0.15 mm of nominal and corrects any distortion the roughing released. This is where a casting that moved during the first cut gets pulled back into shape. Skip it and the finishing pass has to remove uneven stock, which pushes tool pressure around and moves the bore.

Finishing controls the callouts. Bores and seal diameters typically land at Ra 0.8–1.6 μm on our machines; sealing faces that need better go to Ra 0.2–0.8 μm with a dedicated finishing pass. Take light depths, 0.05–0.1 mm, and keep the tool path continuous around the bore. A dwell mark in a seal groove is a leak path later.

Angled pads on the rotary table are cut with the tool normal to the surface. Ball-nose cutters leave scallops that depend on stepover: 0.05 mm stepover on a 6 mm ball gives a finish that usually needs no hand work, 0.2 mm stepover leaves visible ridges. Match stepover to the print finish callout rather than to habit.

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  Rough: leave 0.3–0.5 mmEnough stock for the semi-finish to correct distortion.
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  Finish: 0.05–0.1 mm depthLight passes hold bore roundness and finish.

Every part is inspected before shipment, and the inspection report follows the same datum chain used in machining. On a turbine casing that means the CMM program is written from the primary face and dowel holes, not from a convenient edge. If the print calls out bore-to-bore position, the report shows that distance, not just the individual bore diameters.

In-process checks catch drift before the finishing pass. We measure the semi-finished bore and the two key pads, compare against nominal, and adjust the wear offset for the finishing operation. On a small batch this takes minutes. On a scrapped casing it costs weeks.

Final inspection covers raw material certification, dimensional report, and surface finish where the print specifies it. Reports go out on request. For turbine work we also record the as-machined condition of any sealing surface so the customer can decide whether a coating step is needed.

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  CMM from the process datumReports match the setup the machinist actually used.
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  Offset adjustment before finishingCorrect drift while there is still stock to remove.