CNC Spring Machine Innovation: How Coil Parts Are Actually Made
This page explains what changed inside CNC spring machines, which parts still belong on them, and when a lathe or mill does the job better. Written for design engineers and buyers who need a decision, not a brochure.
What a CNC Spring Machine Actually Controls
A CNC spring machine takes wire from a spool, straightens it through rollers, then bends, coils, cuts and sometimes grinds it in one continuous cycle. The wire never gets clamped in a vise the way a milled part does. Instead, tooling axes move into the wire path at programmed moments, and the wire itself is pushed forward by feed rollers.
Two numbers describe most of the machine: wire feed length accuracy and angular repeatability of the coiling axes. Feed accuracy decides free length and pitch. Angular repeatability decides leg angle, hook orientation and the position of any bent feature relative to the coil body. If either drifts, the spring still looks right and fails inspection.
Modern camless machines replaced mechanical cam plates with individually driven servo axes. That change is the core of CNC spring machine innovation. A cam machine locks one motion ratio into hardened steel; a servo machine lets you rewrite that ratio in the program. Setup for a new part drops from hours of cam swapping to minutes of parameter entry.
The trade-off is real. Servo axes need tuning, and a badly tuned machine produces a spring that varies from the first piece to the thousandth. Mechanical cams were slow but stubbornly repeatable. On long runs of a simple compression spring, an old cam machine can still hold its own.
- 1Feed rollersSet wire length and pitch; wear shows up as drifting free length
- 2Coiling axesPush tooling into the wire path; control diameter and leg angle
- 3Cut and grindSeparate the part and square the ends when the drawing calls for it
How Multi-Axis Motion Expands the Geometry You Can Specify
A basic two-axis spring machine moves tooling in and out along one plane. Add a rotating quill or a servo-controlled wire guide and the wire can be twisted as it feeds. That is what makes a torsion spring with a bent leg, a double torsion spring, or a spring with an off-axis hook possible in one cycle instead of three operations.
The practical effect for a designer is fewer assembly steps. If a spring needs a hook at 90° to the coil axis, a two-axis machine may need a secondary bend station or a manual touch-up. A machine with a rotating wire guide can form it in the same pass, which removes a handling step and a chance for the angle to drift.
This is where the comparison to milling gets useful. A machined coil spring is cut from bar or tube on a lathe, so the coil is a thread, not a wound wire. It holds far tighter tolerances on diameter and pitch, and it can carry features a wound spring cannot: a hex on one end, a cross-hole, an internal bore.
Wound springs win on cost per part and on fatigue life, because drawing wire through a die leaves beneficial residual stress. Machined coils win on geometry freedom and on small quantities. Neither replaces the other; the drawing decides.
- 1Wound wireCheap at volume, good fatigue behavior, limited end features
- 2Machined coilTight diameter and pitch control, free end geometry, higher unit cost
- 3HybridWound body plus a machined end fitting pressed or welded on
What Wire Materials Do to the Process Window
Music wire and oil-tempered chrome-silicon wire coil predictably. They are stiff enough to hold a bend, so the tooling pushes and the wire stays where it was put. Spring back is known and can be compensated in the program. These are the materials a spring machine was designed around.
Stainless grades behave differently. 302 and 304 work harden as they bend, so the second coil of a tight spring is harder to form than the first. 17-4PH in the solution-treated condition coils well and then ages to high strength, which is why it shows up in medical and aerospace springs. 316L is softer and galls against tooling, so feed rollers need to be kept clean.
Beryllium copper and phosphor bronze coil easily and conduct well, which suits contact springs and current-carrying parts. Titanium wire is the difficult one. It has a narrow window between forming cleanly and cracking at the bend, and it springs back hard, so tooling radii usually need to open up compared with steel.
Inconel and other nickel alloys sit at the far end. They need slower feed rates, more tooling clearance and often a heated forming station. If your part is Inconel wire under Ø1 mm, expect to talk through the geometry before anyone quotes it.
- 1Carbon and chrome-siliconPredictable spring back; standard tooling and feed rates
- 2Stainless 302 / 304 / 17-4PHWork hardening changes the window from the first coil to the last
- 3Titanium and InconelWider bend radii, slower feed, sometimes hot forming
Where CNC Spring Machine Innovation Meets Tolerance and Inspection
A wound spring is not a precision-machined part in the usual sense. Free length on a compression spring might be held to ±0.25 mm on a good day; wire diameter is whatever the mill shipped. Where CNC machines have improved is in the features around the coil: leg angles, hook positions, ground ends and the consistency from piece to piece.
GreatLight machines coil and wire parts alongside milled and turned work, so wire-formed features often get finished on a CNC. A spring that needs a flat ground end, a machined seat, or a threaded stud on one end can be wound first and then held in a collet for the secondary operation. That keeps the datum consistent.
Tolerance on the machined features follows the shop standard, ±0.005 mm on critical dimensions with 100% inspection before shipment. The wound portion cannot reach that, and it is worth saying so early. Mixing the two tolerance classes on one drawing is the most common source of argument between a designer and a spring shop.
For small wire, the finished surface on a machined seat typically lands at Ra 0.8–1.6 μm. A ground spring end is coarser. If a mating bore needs a specific fit, machine that bore and let the spring end stay as-wound.
- 1Wound featuresFree length, pitch, leg angle; consistency matters more than absolute tolerance
- 2Machined featuresSeats, flats, threads, cross-holes; ±0.005 mm on critical dimensions
- 3GrindingSquares the end for seating; does not make the coil body more accurate
Wound Spring or Machined Coil: Which Fits Your Part
Read the row that matches your drawing, then check the note column.
| Part condition | Better process | Why |
|---|---|---|
| Simple compression spring, 1,000+ pieces | CNC spring machine | Low unit cost, fast cycle, wire stays uniform |
| Torsion spring with bent legs | CNC spring machine | Rotating wire guide forms legs in one cycle |
| Coil with a hex end or cross-hole | CNC lathe | Wound wire cannot carry that end geometry |
| Wire under Ø0.3 mm | CNC spring machine | Machining wire that thin distorts it |
| Prototype, 1 to 5 pieces | CNC lathe or mill | No tooling setup; geometry changes stay cheap |
| Diameter tolerance under ±0.02 mm | CNC lathe | Machined coil holds diameter; wound wire springs back |
| Inconel or titanium wire | CNC spring machine | Machining wastes expensive bar stock |
| Spring plus machined housing in one part | CNC mill-turn | One setup keeps the coil axis and bore concentric |
The Rule We Use on the Floor
If the part is wound from wire and the ends are simple, run it on a CNC spring machine. If the drawing needs a machined end feature, a threaded stud, or a diameter tolerance tighter than ±0.02 mm, machine the coil on a lathe instead and accept the higher unit cost.
Questions Engineers Ask Before Quoting
Can a CNC spring machine hold ±0.005 mm on coil diameter?
No. Wound wire springs back after the tooling releases it, so coil diameter typically varies by a few hundredths of a millimeter. The ±0.005 mm figure applies to machined features on the same part, such as a seat, a flat or a bore.
If the drawing puts a tight tolerance on the coil diameter itself, that is a signal the part should be machined from bar rather than wound.
What wire range can be formed?
Spring machines cover roughly Ø0.1 mm up to Ø16 mm depending on the model and the material. The upper end drops for stiff alloys. Inconel and titanium wire are usually limited to smaller diameters unless the forming station is heated.
Below Ø0.3 mm, handling becomes the hard part. The wire tangles, the feed rollers mark it, and inspection needs optical measurement rather than calipers.
Does the coil need a secondary machining operation?
Only if the drawing calls for a feature the winding process cannot produce. Ground ends, machined seats, flats, threads and cross-holes all fall into that group.
We wind the part first, then hold it in a collet on a CNC for the secondary cut. That keeps the coil axis and the machined datum aligned instead of stacking two setups.
How do you inspect a wound spring?
Free length, coil diameter, pitch, leg angle and wire diameter are checked against the drawing, with optical comparison for small wire. Machined features are measured with the same equipment used on any other turned part.
Inspection is 100% before shipment. Reports are available on request, and raw material certificates come from the wire mill.
Can you quote from a drawing with no spring experience behind it?
Yes. Send the drawing and note which dimensions are functional and which are reference. That single distinction changes the process choice more than anything else on the sheet.
Quotation and a DFM analysis come back within 12 hours, and there is no minimum order quantity, so a single prototype is fine.
What happens if the wire material is not in the standard list?
We check the mill certificate for tensile strength and ductility before quoting. Some grades coil well but need a stress-relief bake afterward, which adds a step.
If a grade is genuinely unsuitable for winding, we will say so and suggest either a machined coil or a different alloy rather than run a process that will not hold.
Send the Drawing, Get a Process Recommendation
Upload your spring or coil drawing and we will tell you whether it should be wound, machined, or both, with a quote and DFM notes inside 12 hours.
12-hour quote100% inspectionNo minimum order quantity
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