Screw Machining
Also: automatic screw machine, multi-spindle screw machine, Davenport, Acme-Gridley, Brown & Sharpe, cam screw machine, screw machine products, high-volume turning
Screw machining turns fasteners, fittings and small round parts from bar on single- and multi-spindle automatics: seconds per part, tens of thousands per order.
166 shops tagged Screw machine products among 5,104 CNC machining companies in the Noramark directory
Automatic screw machines are bar-fed lathes built for volume. A single-spindle automatic runs one part at a time like a fast, simple lathe; a multi-spindle machine (five, six or eight spindles) carries several bars through a ring of tooling stations at once, so a finished part drops off at every index, every few seconds. Older machines are cam-driven (Brown & Sharpe, Davenport, Acme-Gridley) and take a skilled setup person most of a day to change over; modern CNC multi-spindles set up in hours and hold tighter.
The process makes brass and hydraulic fittings, fasteners, bushings, spacers, pins, electrical contacts and valve parts by the hundred thousand at unit prices of cents to a few dollars. Tolerances of ±0.05 mm (±0.002") on diameters are routine and ±0.013 mm (±0.0005") is achievable with shave tools and gauging. Free-machining materials are the point: 12L14, C360 brass, 303 stainless and 6061 aluminum run at the speeds that make the economics work.
Quantity is everything. Below a few thousand pieces the setup swamps the run and a CNC lathe or Swiss machine is cheaper. Part diameter tops out around 50-65 mm (2-2-1/2"), geometry is limited to what a handful of stations can cut, and cross holes, flats and milled features are secondary operations unless the machine has attachments. Cam machines hold looser tolerances and looser concentricity between spindle positions than a single-spindle CNC lathe.
At a glance
| Typical tolerances | Standard ±0.05 mm (±0.002") on diameters and ±0.13 mm (±0.005") on lengths from cam and CNC multi-spindles; ±0.013 mm (±0.0005") on diameters with care on CNC machines or with a shave or roller-box tool. Threads to class 2A/2B routinely and 3A with rolled threads. Concentricity between spindle positions is 0.025-0.05 mm (0.001-0.002"), looser than a single-spindle or Swiss lathe. |
|---|---|
| Size limits | Multi-spindle bar capacity is commonly 16-42 mm (5/8-1-5/8"), with some machines to 65-80 mm (2-1/2-3"); Davenport single-spindles take up to 19 mm (3/4") bar. Part length is limited to about 100-150 mm (4-6") by the machine stroke. Minimum diameter is around 1.5 mm (1/16"). |
| Surface finish | As-turned 0.8-1.6 µm Ra (32-63 µin) on brass and free-machining steel; 0.4 µm Ra (16 µin) with a shave tool or roller box on diameters. Parts are normally tumbled or vibratory-finished to deburr and brighten, then plated or passivated. |
| Lead time | Setup-heavy: first shipment in 3-6 weeks, then releases every one to two weeks against a blanket order. Prototypes are usually made on a CNC lathe first, because a screw-machine setup for ten pieces makes no sense. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Free-machining grades first: 12L14 and 1215 steel, C360 brass, 303 and 416 stainless, and 6061 or 2011 aluminum. 1018 and 1045 run with longer cycles and stringier chips; 4140 and 304 are possible when the volume justifies slower feeds and more tool changes. C260 brass and C110 copper run for electrical parts, copper with care for its gummy chip. Delrin and nylon run on the same machines for insulators and spacers. 316, titanium and Inconel are poor fits and usually go to Swiss or CNC turning.
What drives the cost
- Quantity: a cam multi-spindle setup takes a day, so the run has to be thousands to amortize it
- Cycle time: the slowest station sets the pace; one deep drill or long thread can double the part cost
- Material: brass and 12L14 are the natural fit; stainless and 4140 cut slower and wear tools
- Tolerances below ±0.025 mm (±0.001") that need shave tools, gauging and slower feeds
- Secondary operations: cross-drilling, slotting and milling off the machine, deburring, plating
- Bar cost and scrap: the remnant and the parting-off loss are a larger share on small parts
- Sorting and inspection: 100% gauging or vision sorting on critical parts adds cost per piece
When to use it
- Fittings, fasteners, bushings, spacers, contacts, valve bodies and pins in quantities of 5,000 and up
- Brass, free-machining steel and aluminum parts where price per piece decides
- Simple turned geometry: a few diameters, a chamfer, a thread, a drilled hole
- Long-running blanket orders released weekly or monthly
- Parts that tolerate ±0.05 mm (±0.002") and a standard commercial finish
When not to
- Under a few thousand pieces: a CNC lathe or Swiss machine sets up in a fraction of the time
- Tolerances below ±0.013 mm (±0.0005") or complex milled features: use Swiss or mill-turn
- Parts larger than about 65 mm (2-1/2") diameter or longer than 150 mm (6")
- Materials that do not cut freely, such as 316, titanium and Inconel, unless the volume justifies the tooling
Design tips
- Specify a free-machining grade: 12L14, 1215, C360, 303, 6061 or 2011. Changing 1018 to 12L14 can halve the cycle time.
- Keep the design to what the stations can do: diameters, chamfers, grooves, threads, a drilled hole and a parting-off. Every cross hole or flat is a secondary operation.
- Give ±0.05 mm (±0.002") or looser on non-critical features so the part runs on the cheapest machine.
- Design to a stock bar size and keep the ratio of largest to smallest diameter low to limit roughing time.
- Allow a small witness nub on the parted-off end or add a chamfer that removes it; say so if the end must be clean.
- Call out threads by class and say whether rolled threads are acceptable; they are stronger and cheaper at volume.
- Ask for price breaks at 5,000, 25,000 and 100,000 and consider a blanket order with scheduled releases.
Frequently asked questions
- What is a screw machine?
- An automatic bar-fed lathe built for volume. Single-spindle automatics make one part at a time; multi-spindle machines carry five to eight bars through a ring of tool stations so a part finishes at every index. The name comes from the screws and fasteners they were built to make, but they make any small turned part.
- Screw machining or Swiss machining?
- Screw machines are the cheapest way to make simple turned parts at ±0.05 mm (±0.002") in the tens of thousands. Swiss machines hold ±0.005 mm (±0.0002"), handle long slender parts and complex features, and are competitive from a few hundred pieces. Modern CNC multi-spindles blur the line.
- What is the minimum quantity for screw machine parts?
- A few thousand for a CNC single-spindle, and 5,000-10,000 or more for a cam-driven multi-spindle, because the setup takes hours to a day. Below that, quote a CNC lathe.
- What tolerances can a screw machine hold?
- ±0.05 mm (±0.002") on diameters and ±0.13 mm (±0.005") on lengths are standard. With shave tools, gauging and a CNC machine, ±0.013 mm (±0.0005") on diameters is achievable. Concentricity between spindle positions on a multi-spindle is 0.025-0.05 mm (0.001-0.002").
