How-to guide

From part to finished product: the machining process of a double head lathe

A double head lathe machines both ends of a shaft, fitting or housing in the same cycle. This guide walks through seven steps, from stock preparation to final inspection, and gives the parameter ranges we actually run in the shop.

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Double head lathe machining process from raw bar to finished part

On this page

1
What matters before you cut metal
2
How a double head lathe differs from a single-spindle lathe
3
Stock preparation and material selection
4
Workholding choices that hold concentricity
5
Turning parameters and tool selection on both spindles
6
Seven steps from raw bar to inspected part
7
When a double head lathe is the right call

Key takeaways

What matters before you cut metal

Opposed spindles change the setup, not the physicsTwo spindles mean the second end is machined without re-chucking. Concentricity depends on spindle alignment, not on operator feel.

Bar stock suits most jobs under Ø65 mmAbove that, the bar whip and the chuck jaw swing force you into castings or forgings with a pre-turned stub.

Thermal drift shows up in the second hourA 2 °C spindle rise moves a 300 mm part by roughly 4 μm. Let the machine idle through one warm-up cycle first.

In-process gauging beats end-of-line sortingMeasure at the transfer point. Scrap caught there costs one part, not a full batch.

Machine setup

How a double head lathe differs from a single-spindle lathe

A double head lathe carries two opposed spindles on one bed. The main spindle turns the first end while the subspindle waits on the other side of the part. When the first operation finishes, the subspindle picks up the workpiece, and both ends are cut without the part ever leaving the machine envelope.

That single transfer is the whole point. On a conventional lathe, flipping a shaft means re-chucking, re-zeroing and accepting whatever runout the jaws introduce. On a double head machine, the second end inherits its position from the spindle axis. Concentricity between the two ends typically lands inside 0.01 mm when the spindles are aligned.

The trade-off is setup time. Aligning two spindles costs more than dialing in one chuck. Once the alignment holds, cycle time drops because the part no longer waits in a queue between operations.

We run opposed-spindle work on our mill-turn centers, which share the same logic: one fixturing, two cutting directions. For parts under Ø65 mm and 400 mm long, a double head lathe usually beats two separate lathe operations on both cost and concentricity.

1
Best fitShafts, bushings, fittings and housings with tight end-to-end concentricity.
2
Poor fitParts needing access from a third direction, or features on the part's mid-section.

Material

Stock preparation and material selection

Stock form decides how much of the cycle is cutting and how much is waiting. Bar stock feeds through the spindle bore and needs no pre-operation. Castings and forgings need a pre-turned stub so the subspindle has something concentric to grab.

For turned parts, we work mostly in 6061-T6, 303 and 316L stainless, 1045 and 4140 steel, and C36000 brass. Aluminium 6061-T6 cuts at 300–600 m/min with carbide, which keeps cycle times short. Stainless 316L runs at 120–200 m/min and work-hardens if the feed is too light. Keep the feed above 0.08 mm/rev on 316L or the next pass will rub instead of cut.

Bar diameter should sit within 0.05 mm of nominal for collet work. Oversize bar springs the collet, undersize bar slips. Both show up as taper on the finished diameter.

Leave 0.3–0.5 mm of radial stock on diameters that will be finish-turned after the transfer. Any tighter and the second operation has nothing to clean up.

1
Aluminium 6061-T6300–600 m/min, 0.15–0.30 mm/rev, flood coolant.
2
Stainless 316L120–200 m/min, 0.08–0.15 mm/rev, never let the tool dwell.
3
Steel 4140150–250 m/min, 0.10–0.25 mm/rev, check for scale on hot-rolled bar.

Fixturing

Workholding choices that hold concentricity

Collets suit bar work up to Ø65 mm. A 5C or 16C collet closes evenly around the bar and repeats within 0.01 mm. Jaw chucks handle larger or irregular stock, but each set of soft jaws has to be bored in place at the clamping pressure you will actually run.

For castings, a three-jaw chuck with bored soft jaws is the usual answer. Bore the jaws to the casting's locating diameter plus 0.05 mm, then clamp at the same pressure used in production. Clamping harder than the bore pressure pulls the part off center.

When the part has a finished bore, an expanding mandrel locates on that bore and gives the best concentricity of any workholding method. It costs more to build, so it earns its place on runs above a few hundred pieces.

Keep the subspindle clamping pressure below the main spindle pressure on thin-wall parts. A bushing with a 2 mm wall will ovalize at 25 bar but hold round at 15 bar.

1
ColletBar stock to Ø65 mm, repeatability about 0.01 mm.
2
Bored soft jawsCastings and forgings, clamp at production pressure.
3
Expanding mandrelParts with a finished bore, best concentricity.

Cutting data

Turning parameters and tool selection on both spindles

The first operation does the bulk of the metal removal. Rough at 2–3 mm depth of cut per side on aluminium, 1.5–2.5 mm on steel, then leave 0.3 mm for the finish pass. A 0.4 mm nose radius insert handles both passes on most jobs; drop to 0.2 mm only when a small internal radius forces it.

The second operation runs lighter. Because the part is now held by the subspindle, cutting forces push against a shorter, stiffer grip. Take 1–2 mm depth of cut and keep the feed at or above the first operation's feed so the surface finish stays consistent end to end.

Turning grades matter more than brand. A PVD-coated carbide insert for steel, an uncoated or DLC-coated insert for aluminium, and a tough grade for stainless. Switching grades mid-run without re-checking the first part is a common mistake.

Coolant should be high-pressure through-tool on deep bores. Below 20 bar, chips pack the hole and the boring bar chatters. Above 50 bar, chip evacuation is reliable on bores up to 8× diameter.

1
Roughing2–3 mm DOC aluminium, 1.5–2.5 mm steel, 0.15–0.30 mm/rev.
2
Finishing0.3 mm DOC, 0.05–0.12 mm/rev, target Ra 0.8–1.6 μm.
3
Deep boresThrough-tool coolant at 20–50 bar to clear chips.

Step by step

Seven steps from raw bar to inspected part

Follow these in order. Skipping step 3 is the most common cause of a concentricity failure.

1
1. Verify stock and drawingCheck bar diameter against the drawing, confirm material grade with a certificate, and mark the datum face on the print. If the drawing calls for 316L and the bar reads 304, stop. Material mix-ups are cheap to catch now and expensive after heat treatment.
2
2. Pre-machine the locating stubOn castings and forgings, turn a stub 15–20 mm long to the subspindle collet size, within 0.05 mm. This is the surface the second spindle will grip, so its roundness sets the concentricity of the whole part.
3
3. Align both spindlesIndicate the subspindle against a test bar held in the main spindle. Aim for total indicated runout under 0.005 mm before cutting anything. Re-check after the first warm-up cycle, because a cold machine drifts.
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4. Warm up the machineRun a 20–30 minute warm-up cycle at the production spindle speed. A 2 °C rise in spindle temperature moves a 300 mm part by roughly 4 μm, which is most of your tolerance budget on a ±0.005 mm job.
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5. Cut the first end, then transferRough and finish the first end, then let the subspindle pick up the part. Confirm the grip pressure is set for the wall thickness. Thin walls ovalize above 15–20 bar clamping pressure.
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6. Cut the second endFace, turn and bore the second end to the drawing. Take 1–2 mm depth of cut. Measure the first part immediately, before running the second one. If the offset is under 0.02 mm, adjust the tool offset at the control rather than the work offset.
7
7. Inspect and releaseCheck diameters, concentricity and surface finish on the first part, then at intervals through the run. Record results. We inspect 100% of parts before shipment and issue inspection reports on request.

Decision table

When a double head lathe is the right call

Match the part to the process before you commit to a fixture.

Part condition	Double head lathe	Two separate lathes	Why
Bar to Ø65 mm, one cycle	Good fit	Extra handling	Subspindle transfer keeps concentricity
Shafts under 400 mm long	Good fit	Workable	Both ends cut without re-chucking
Features on the mid-section	Not suitable	Better fit	Neither spindle reaches the middle
Thin-wall bushings	Good fit at low clamp	Risk of ovalizing	Subspindle pressure must stay low
Parts over Ø65 mm	Limited	Better fit	Bar whip and jaw swing limit size
One-off prototype	Setup-heavy	Faster to start	Spindle alignment takes time
Run of 500+ parts	Strong fit	Slower cycle	Transfer time drops per piece

Pick the process before you pick the price

A double head lathe pays off on shafts and fittings where end-to-end concentricity matters and the run is long enough to absorb setup. For mid-section features or one-off parts, a single-spindle lathe or a 5-axis mill is the better call.

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FAQs

Questions engineers ask before quoting

What tolerance can a double head lathe actually hold?

On diameters, ±0.005 mm is achievable with a warm machine and a rigid setup. End-to-end concentricity is looser because it depends on spindle alignment, typically 0.01 mm or better.

If your print calls for 0.005 mm concentricity across a 400 mm part, plan on a grinding operation after turning rather than asking the lathe to do it.

Does the subspindle mark the finished surface?

It can, if the subspindle grips a finished diameter. We either grip an unfinished section or use soft jaws bored to the finished diameter so the contact is even.

On polished or anodized surfaces, the grip area is usually left oversized and finished after the second operation.

How long does setup take compared to a single lathe?

Expect more setup time on the first article because both spindles need alignment and both tool sets need offsets. Once running, cycle time is shorter because there is no queue between operations.

The crossover point is usually a few hundred parts. Below that, two lathes with a buffer between them can be faster to first part.

Can a double head lathe cut internal features on both ends?

Yes, as long as the bore is reachable from each end and the tool shank fits. Deep bores need through-tool coolant at 20–50 bar to clear chips.

A bore that runs the full length of the part from one side only is a better job for a lathe with a long boring bar, or for a mill-turn center.

Which materials machine well on this setup?

Aluminium 6061-T6 and 2024, stainless 303 and 316L, 1045 and 4140 steel, and C36000 brass all run well. Titanium TC4 and Inconel are possible but slow, and tool wear drives the cost.

Plastics like POM and PEEK need sharp tools and light clamping pressure, or the part distorts in the subspindle.

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