Screw machining or CNC turning: which process for your part?

Turning and screw machining make the same kind of parts: shafts, bushes, screws, fittings, sleeves, contacts. The part rotates, the tool removes the material. The two trades overlap, and a workshop may master both, but the machines and the logic of production runs differ.

• CNC turning (fixed-headstock lathe): the part is held in a chuck, sometimes supported by a tailstock. It handles bars as well as blanks (forgings, castings, saw-cut sections), one-offs as well as small runs, from a few millimetres to several hundred millimetres in diameter.
• Screw machining: machining from bar stock on automatic lathes, sliding-head (Swiss-type) or multi-spindle, designed to run high-rate production. In France, the Arve valley in Haute-Savoie is a historic hub: see the Auvergne-Rhône-Alpes page.

These parts are found in connectors, watchmaking, medical (screws, instruments), hydraulics, automotive and technical fasteners: wherever small turned parts are produced in quantity, with demands for precision and consistency.

On a conventional lathe, the part overhangs the chuck and bends under the force of the tool, the more so the longer and thinner it is: beyond a length of three to four times the diameter, support (tailstock, steady rest) is generally needed. On a sliding-head lathe, the bar is held by the collet of the headstock, which slides in a guide bush: the tool always works right next to this support, whatever the length being machined. Deflection stays low, even on very slender parts. As several tools work at the same time, on the main spindle and on the sub-spindle, cycles are short, from a few seconds to a few minutes depending on the part.

These machines are most often fitted with a sub-spindle, which picks up the part to machine its second end, and with live tools for milling, cross-drilling or machining flats, which suits parts with several milled features, such as a connector or a screw with a special head. The part comes out finished, without manual rework, after parting off. Common capacities range from a few millimetres up to 32 mm bar diameter, with some machines going further. Beyond that, you turn to a fixed-headstock lathe with a bar feeder, or to a multi-spindle for very large runs.

The table compares the two most common families; multi-spindle machines are covered in the next section.

A multi-spindle automatic lathe carries several bars, four, six or eight depending on the machine, on a drum. At each indexing, the drum advances one station and a part is completed: each station performs one operation with its own tools, at the same time as the others. Production rates are very high, which suits runs of several tens of thousands of parts, even several million: screws, fittings, sleeves, connector or automotive parts.

The trade-off is a long and costly set-up, and a part suited to the sequence of stations. A multi-spindle is justified for relatively simple, high-volume parts with a steady flow. For smaller runs or more complex parts, a sliding-head or CNC turning remains more economical.

Screw machining costs more to start and less to produce. Setting up a sliding-head lathe takes time (tools, guide bush, bar feeder, multi-channel synchronised programme), but the cycle is short because several tools work at the same time. This is the usual logic: fixed costs ÷ quantity + variable cost per part (see the price of CNC machining).

• One to a few parts: a conventional CNC lathe, simpler to set up, or a multitasking lathe.
• A few dozen to a few hundred: CNC turning is still in the running; a sliding head becomes attractive if the part is thin, long or complex.
• Several hundred to several thousand, and beyond: sliding-head screw machining is generally the most competitive, then multi-spindle for very large volumes.

These limits are orders of magnitude, with no universal value: they depend on diameter, material, complexity and the workshop’s workload. Ask for prices for several quantities and, if in doubt, for both families of machines. At very high volumes, cold heading, a form of forging, can take over for certain geometries.

Bar supply also weighs on small batches: common lengths are around three metres, and some grades can only be bought in minimum quantities. For a recurring need, a framework order with staggered deliveries lets the workshop buy the material in one go and smooth its schedule.

Both processes routinely hold tolerances in the order of a few hundredths of a millimetre on diameters. The sliding head stands out on long, thin parts and on repeatability over a run, because the cut is made close to the support. Tighter tolerances call for a controlled process, or even grinding: to be validated with the workshop, dimension by dimension. For general tolerances, the ISO 2768 tool gives the values from the standard.

An Ra of around 0.8 to 1.6 µm (grades N6 and N7) is common in turning, finer with finishing tools or a secondary operation. The roughness converter lets you switch from one notation to another. Only specify a fine roughness on functional surfaces: it lengthens the cycle.

Screw machining starts from calibrated bars, already toleranced, which limits operations. Free-machining grades (free-cutting steels, brass, 2011 aluminium, 303 stainless) break the chip and machine faster. Common stainless steels, titanium or engineering plastics can also be machined, at lower rates: see the materials for machined parts guide.

In series production, inspection is carried out during production: the operator measures parts at regular intervals with gauges or suitable instruments, which makes it possible to track drift in critical dimensions. If you require traceability of material batches, an inspection report or a first article, state it in the request: these documents have a cost.

1. Start from a standard bar diameter: a maximum diameter close to an available bar size avoids removing material for nothing and wasting cutting time.
2. Allow for the parting-off mark: the cut face keeps a small pip or striations; say whether it is functional and, if so, what finish you expect.
3. Prefer chamfers to sharp edges and radii to internal corners, which spare the tools and make deburring easier.
4. Group milled features (cross holes, flats) in as few positions as possible, to reduce the number of live tools.
5. Tolerance only what is needed: each tight dimension lengthens set-up and inspection, especially when the general tolerances of ISO 2768 are enough.

For a turned part with a lot of milling, a multitasking lathe or a multi-axis machine may be preferable. Watchmaking, in particular, makes heavy use of small-diameter turned parts.

• The dimensioned drawing with tolerances, and the 3D file: length, maximum diameter, length-to-diameter ratio, tight-tolerance areas.
• The material: exact grade, bar condition, any material certificate.
• The quantities: batch, annual requirement, delivery rhythm and price breaks to be quoted.
• The operations: threads, cross holes, flats, knurling, marking, deburring.
• The treatments after machining (passivation, zinc plating, anodising) and whether the dimension applies before or after treatment.
• The inspection: dimensions to check, gauges, frequency, documents expected.
• The packaging: fragile parts, cleanliness, loose or in trays.
• The lead time and expected delivery rhythm, indicating what remains negotiable.

On Meaflow, posting a request is free and commission-free: turning and CNC machining workshops can send you their quote, which you compare before discussing through the messaging system.