Screw Machine vs. CNC Lathe: Choosing a Turning Process for Round Parts

Choosing between a screw machine and a CNC lathe is a process decision that affects setup cost, lead time, and how easily a design can change later. Both turn round parts from bar stock, and the line between them has blurred, so the choice rests on volume, part complexity, and how often the part is likely to be revised.

Screw machines belong to the family of automatic lathes. According to this overview of automatic lathes, small and medium cam-operated automatics are usually the machines called screw machines, and numerical control has been used on lathes since the 1950s. This guide compares the two on setup, part features, volume, and repeatability. For the CNC side of the comparison, see the CNC lathe turning page.

What is a screw machine, and how does it differ from a CNC lathe?

A screw machine is an automatic lathe that feeds bar stock through the spindle, cuts a part, parts it off, and feeds the bar forward to repeat the cycle. Traditional screw machines are controlled by cams that move the tools through a fixed sequence, and some have several spindles that index so that each station works on a different part at the same time. A single-spindle machine performs its operations one after another.

A CNC lathe follows a stored program instead of cams. The tool path, speeds, and feeds live in software, so a change to the part is a change to the program. Many CNC lathes also run from bar stock with a bar feeder, which is why the two machine types can compete for the same parts. The term screw machine is also used loosely, and some shops apply it to CNC machines that run bar-fed production, so it is worth confirming what a supplier means when the term appears on a quote.

How do setup and changeover compare?

On a cam machine, changing to a new part means building or fitting new cams and arranging the tooling. That work takes skill and time, and it pays back when the run is long. Once the machine is running, a single operator can oversee several machines, which is why mechanical screw machines stay cost-effective on large runs.

A CNC lathe changes over by loading a program and the tools for that part. Programs can be stored and recalled for repeat orders, so the second run of a part starts from a proven program. That usually makes CNC changeover shorter on short and mid-size runs, where preparing the job is a larger share of the price per part. The factors that add time on a CNC lathe, such as tool count and second operations, are covered in the guide to CNC turning cost.

Which part features suit each process?

Screw machines suit parts with a modest number of operations that run in a fixed sequence: turning, facing, drilling, threading, and cutoff. Simple, high-volume small parts such as pins, fittings, and fasteners are the classic fit.

CNC lathes handle the same work and extend further. Contoured profiles, features that change from one lot to the next, and parts that carry design revisions suit a programmable machine. Some CNC lathes also add live tooling and extra axes for milling and cross-drilling in the same setup, though that depends on the machine configuration.

Small-diameter parts with long length-to-diameter ratios sit in the overlap with Swiss-type machines, which are CNC machines with a guide bushing. The comparison between Swiss and conventional turning is in the guide to Swiss machining for small precision parts.

How does order volume change the choice?

Volume is the main lever. A cam machine carries a heavy preparation cost and a low cost per part once it is running, so it favors long runs of an unchanging part. A CNC lathe carries a lighter preparation cost, so it favors short and mid-size runs and parts likely to change. Cam machines can also beat CNC machines on cycle time for some parts, which is part of why they remain in service.

Where the crossover falls depends on the part, the material, and the shop, so no single quantity separates the two. Asking for pricing at two quantities shows where a given part sits on that curve.

How do repeatability and documentation compare?

Both process types hold repeatable dimensions once set up. A cam machine repeats the motion its cams define, and the operator adjusts tooling to hold size as inserts wear. A CNC lathe repeats its program and can apply tool offsets to correct size without changing the machine’s motion, and the stored program serves as a record of how the part was made. Buyers who need inspection reports, first article documentation, or a controlled process record may find a programmed machine easier to support, though documentation comes from the shop’s quality system rather than from the machine type.

Screw Machine vs. CNC Lathe at a Glance

The table below summarizes the differences discussed above. Treat the entries as general tendencies, since individual machines and shops vary.

Questions to Settle Before Choosing a Process

A few questions narrow the choice before a quote request goes out. The list below applies to either process, and the design details behind it are covered in the guide to design for CNC turning.

• How many parts will be ordered at once, and how often will the order repeat?
• Is the design final, or likely to change?
• Does the part have contours, cross holes, or features beyond simple turning?
• What tolerances, finishes, and thread requirements apply?
• What inspection documentation does the buyer require?
• Is the part small enough to fall in the range where Swiss-type machines compete?

Send This With Your Quote Request

A print with the material, expected quantity per order, repeat order forecast, tolerances on critical features, and finish and thread requirements lets a shop recommend a process instead of guessing. Say whether the design is expected to change. For the full range of machining capabilities, see precision CNC machining services.

Submit a quote request with your print and quantity, and the FM Machine team will review it and respond with a quote.