Swiss screw CNC lathes are highly productive machines, but running them at less than maximum efficiency can mean a significant loss of output. This blog explains the kind of precision machining work they are best suited to and how to optimize utilization and productivity. First, though, is an explanation of what a Swiss screw machine is.

Basics of Swiss Screw Machines

A Swiss screw machine is a lathe, but with multiple spindles arranged in a circle. Held in a circular drum, these spindles index around, moving from one tooling station to another. Most machines have either six or eight stations with one or two cutting tools at each station.

Swiss machines are typically loaded with bar stock fed through a collet from the side of the machine. A guide bushing supports the bar as it emerges from the drum into the cutting area. Unlike a conventional lathe where the cutting tool traverses along the rotating Z axis of the workpiece, in a Swiss machine the bar is pushed through the guide bushing to provide Z axis motion.

Advantages of Swiss Screw Machining

The Swiss machine performs simultaneous turning at each of the indexing stations. This results in a throughput rate several times higher than a conventional, single spindle lathe. Machines are extremely accurate and repeatable but limited in the diameter of the bar they can handle: the largest is around 3.5” (90mm).

Tooling at each station can turn and face, but it’s also possible to drill cross holes, mill flats, cut threads and center drill. Some Swiss-style machines are equipped with secondary spindles that work on the back of the part after it’s parted-off from the bar. Machines like these are capable of “one-and-done’ machining, meaning the part is finished in a single operation.

Good candidates for Swiss machining are smaller diameter parts like valve spools, sleeves, couplings and fasteners. Fuel injection components are typical of the type of precision parts produced this way.

Optimizing Swiss Screw Machining Efficiency

As with any precision machining operation, it’s important to consider both what happens when the machine is cutting metal, and how to minimize non-productive time.

During cutting, points to review include:

• Balancing machining time between spindles – the drum can’t index until the longest machining step is complete
• Using appropriate chip breakers on cutting tools – otherwise long, stringy chips will require frequent machine stoppages to clear
• Use high quality bar stock – ideally, ground for roundness, straightness and a consistent diameter
• Adopting a ‘segmented’ approach to turning to minimize deflection – feed a small length through the guide bushing and work on that, then feed more through and machine that region
• Using cutting tools with small nose radii to minimize cutting forces and deflection
• Explore whether it’s faster to use round bar and mill hexagons than to turn hexagonal bar into cylindrical shapes – a good CAD/CAM package should permit this comparison

The biggest cause of non-productive time, besides essential maintenance and poor production scheduling, is setup or changeover. The complexity of Swiss-style machines can make for long setup times, but there are ways to minimize this.

• Sequence orders to minimize bar size changes – dedicate machines to specific bar diameters, if capacity allows
• Invest in high quality tool presetting equipment for accurate tool offset values that can go straight into the CNC controller
• Use machines equipped with modular toolholder/slide systems for quick changeovers

Impro: For Your Precision Machining Needs

As an efficient means of producing precision-turned parts, it’s hard to beat the throughput of a CNC Swiss screw lathe. These are complex machines however, and maximizing output rate and quality requires expert knowledge and close attention to detail.

Impro makes extensive use of Swiss machining to satisfy the needs of automotive and industrial customers as well as those from other industries. For smaller quantity production we also maintain significant conventional turning capacity. If you’re looking for a source of precision machined parts in large or small quantities, we invite you to talk to us.