Multi spindle CNC screw processing explained

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  Key takeaways
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  How multi spindle CNC screw processing removes material
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  Cycle time math you can do before you ask for a quote
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  Part features that suit multi spindle screw processing
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  Where multi spindle CNC screw processing stops being the right answer
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  Qualifying the process: what to send, what to ask
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  Multi spindle screw vs cam screw vs Swiss-type vs single-spindle lathe

A multi-spindle screw machine holds several spindles in a rotating drum or turret. Each spindle carries one bar of stock. When the drum indexes, every bar moves to the next station and a different tool engages it. On a six-spindle machine there are six stations, so six tools cut six parts at the same moment.

That is the whole trick. On a conventional lathe the part stays put and tools come to it one after another, so cycle time is the sum of every operation. On a multi-spindle machine cycle time is set by the longest single station, plus the index time between stations. If turning takes 8 seconds and drilling takes 5, the cycle is roughly 8 seconds plus index, not 13.

Modern machines are CNC, not cam-driven. Each station has its own slides and servo axes, so you can program independent feeds, spindle speeds and even synchronized cuts between two tools on the same bar. Older cam machines still run in some shops because they are fast and cheap to run once the cams are cut. CNC versions win when the part family changes often.

The engineering consequence is simple: this process pays you back in time, not in accuracy. It does not make a part more precise than a good single-spindle lathe. It makes the same part faster, and the faster you run, the more the fixed setup cost is spread across the order.

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  Drum indexingThe drum rotates 360° divided by the number of spindles. Index time is typically 0.3–1.0 s depending on machine size and load.
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  Simultaneous stationsEach station has independent slides, so one bar can be turned while the next is drilled and a third is threaded.
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  Bar feedStock is fed as bar, usually 3–12 m lengths, which removes the per-part loading step that a chucking lathe needs.

Start by listing every operation on the part and the time each one takes on a single-spindle machine. Then group them into stations. The cycle time of the multi-spindle machine is the time of the heaviest station plus the index, assuming the drum has at least as many stations as you have groups.

A worked example. A stainless 303 fitting needs facing (3 s), OD turning (7 s), drilling Ø6 mm (5 s), tapping M6 (4 s) and parting off (2 s). Total on one spindle is 21 s. Spread across five stations, the longest is turning at 7 s. Add 0.5 s index and the cycle is about 7.5 s. That is roughly 2.8 times the output from the same floor space.

The load balance matters more than the total. If one station needs 12 s and the rest need 4 s, you have wasted four stations and the machine runs at the speed of the slow one. Rebalancing usually means moving a chamfer or a groove to a different station, or splitting a long cut across two tools.

Parting off is often the hidden constraint. It is short, but it happens on every part at the same station, so any dwell there multiplies across the whole order. Keep the parting tool sharp and the feed consistent, or the cycle you calculated on paper will not hold after two hours of running.

The process likes parts that can be made from bar and that have most of their features on the axis of rotation. Think connectors, fittings, bushings, valve bodies, sensor housings, standoffs, inserts and small shafts. Diameters below 3 in (76 mm) are the normal window; above that, bar weight and spindle load start to work against you.

Threads, grooves, cross-holes, hex flats and knurls are all standard. Cross-drilling and slotting need a station with a cross slide, so tell the shop early if your part has them. A single cross-hole that arrives late in the quote can force a second operation on a mill, which erases the cycle-time advantage.

What does not suit the process: deep 3D contours, features at odd compound angles, large flat faces, and anything that needs a lot of material removed from a solid block. Those parts want a 5-axis mill or a mill-turn center. A multi-spindle machine can do a light milling cut on a cross slide, but it is not a milling machine.

Tolerances are honest, not magic. On a well-maintained machine with stable stock, diameters and lengths hold around ±0.005 mm (0.0002 in) on critical features, and surface finish lands in the Ra 0.8–1.6 μm band for normal turning. If a callout is tighter than that, the shop has to add an in-process check, and you should expect the cycle to slow down.

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  Good fitRotational parts from bar, Ø under 76 mm, cross-holes and threads, volumes from a few thousand up.
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  Marginal fitParts with one or two light milling features that can be grouped on a cross-slide station.
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  Wrong fitDeep cavities, long unsupported bores, large flat surfaces, or features needing 5-axis simultaneous motion.

Volume is the first boundary. Below roughly 2,000–3,000 pieces, the setup cost of cams, tooling and first-article proving can outweigh the cycle-time saving. At that point a single-spindle CNC lathe, or a Swiss-type lathe for small parts, is often cheaper per part because it needs less dedicated tooling.

Length-to-diameter ratio is the second. A part that is long and thin will deflect no matter how many spindles are cutting it. Swiss-type lathes solve that with a guide bushing right behind the tool, so slender medical and electronic pins usually go there instead. Multi-spindle machines handle shorter, stouter parts better.

Material matters too. Free-machining grades like 303 stainless, 12L14, brass C36000 and 6061 aluminium run cleanly and hold tolerance at high feed. Sticky or work-hardening grades like 316L, 17-4PH and titanium TC4 (Ti-6Al-4V) will run, but feeds drop and tool wear climbs. Budget for more inserts and more frequent tool changes on those jobs.

Finally, geometry that changes between parts in the same family is a cost. Every new variant needs its own setup and its own first-piece approval. If your annual demand is split across six near-identical variants, ask whether one common blank plus a second op would be cheaper than six dedicated setups.

Send the 3D model, the 2D drawing with GD&T, the material grade, the annual volume and the surface finish callout. If you can say which dimensions are functional and which are reference, the shop can group operations better and may cut a station. Mark critical-to-function features clearly rather than tolerancing everything at the same level.

Ask three questions of any supplier. How many stations will the part run on, and which station is the constraint? What is the first-article plan, and what do you measure on every piece after that? And what happens to the tooling if we move the part or end the program? The answers tell you whether the shop understands the load balance or is just quoting a cycle time.

At GreatLight we run 127 high-precision CNC machines, including 16 simultaneous 5-axis machining centers and 16 mill-turn centers, so a part that does not fit the screw process can still be quoted on a machine that suits it. That matters when one drawing has both a turned body and an off-axis feature. You get a quotation and a free DFM analysis within 12 hours, and production can start within 24 hours.

Inspection is the same standard across processes: raw material check, in-process monitoring, and 100% inspection before shipment, with reports on request. For high-volume screw work the in-process check is where the money is saved, because a drifting tool is caught before it becomes a bin of scrap.