An Introduction to Swiss Screw Machining - Impro Precision


An Introduction to Swiss Screw Machining

February 27th, 2024

Swiss screw machining is for turning long, cylindrical parts like rods, pins, and screws. It’s a precision process capable of maintaining tight tolerances over long production runs, and it’s often extremely cost-effective.

If you need a source for small turned metal parts, a precision machine shop with Swiss screw machining equipment may be the best option. If you’re designing such parts, knowing more about this machining process may help you consolidate components and reduce assembly work.

For both audiences, this blog explains what makes the process special and will help you decide whether it’s worth further investigation, first, though, about that name.

Why “Swiss Screw” Machining?

Switzerland is famous for watch-making, and in the 1800’s also had a big optics and microscopes industry. Products like these use a lot of small screws and similar parts. These were turned on manual lathes, a slow process with excessive part-to-part variation.

The answer arrived in the mid-19th Century. It consisted of a bar-fed lathe with a cutting tool moved in and out radially by a cam driven from the machine spindle. A key element of this machine was the mechanism used to hold and feed the bar.

Instead of using a chuck, the bar was gripped by a collet held in a sliding headstock that fed it out through a guide bushing into the work zone. Metal removal in the axial direction is accomplished by pushing the bar forward as the tool cuts. This provided a big increase in productivity and part quality.

Multi-Spindle Machines

After WW1 Swiss screw machines took another productivity leap when they adopted multiple spindles. This indexed a drum of bars through a fixed angle, taking each bar to a new station with a new tool. With this arrangement, multiple operations could be performed on each workpiece.

Eventually, to overcome the complexity and limitations of actuating each cutting tool by cam, Swiss machines adopted CNC technology. This provided a higher level of control and permitted turning of more complex parts.

Today, most Swiss screw machines are equipped with six or eight spindles and have as many as 13 axes under CNC control. Some axes may be equipped with driven tooling for precision milling work. Large machines can handle bar stock up to 1.5” (38mm) diameter.

Swiss Screw Advantages for Precision Machining

It’s primarily the guide bushing and headstock arrangement that differentiates Swiss screw machining from regular turning. By minimizing overhang, it reduces deflection due to cutting forces on the bar.

Using a “segmented machining” technique, where the bar is pushed out in increments, and each increment turned to the final diameter, it’s possible to machine slender cylindrical parts with high accuracy. A precision machine shop using Swiss machines should be able to hold tolerances of +/-0.0002” (0.005mm).

A second advantage is the productivity gain. With multiple turning and milling axes, it’s possible to machine complex parts in a single setup. This saves on handling in the factory and reduces lead time and inventory, which also cuts down on storage space.

An additional benefit is the opportunity to consolidate multiple simple components into a single, more complex part.

Bar Quality and Setup Time

Swiss screw machining has two limitations to be aware of. First, the guide bushing must be a close fit with the bar material. This often requires the use of a more expensive ground bar.

Second, the complexity of the machines means it takes longer to write and prove out a part program, and the changeover may take considerably longer to complete than that for a conventional CNC lathe.

Good Applications for Swiss Screw Machining

Screws are still one of the biggest applications, especially medical screws turned from alloys like titanium. Valve components, pins for electronic devices and parts for precision measurement equipment are other applications. By sector, the automotive industry uses a lot of Swiss machined parts, because they have the volumes to justify the setup times. Hydraulics and medical devices are other big markets for this type of machining.


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