Choosing the Right Material for Swiss Screw Machining - Impro Precision


Choosing the Right Material for Swiss Screw Machining

December 5th, 2023

Swiss screw machining is valued for its ability to produce complex parts while driving down individual piece costs. This blog explores how material selection contributes to that goal, while helping meet design objectives.

Parts Produced by Swiss Screw Machining

Swiss-style multi-spindle lathes are optimized for producing intricate cylindrical parts with a maximum diameter of 1 ¼”. They are especially effective for parts up to 24” long.

The multi-spindle configuration maximizes output rates by performing metal removal operations in parallel. Powered tools on the various stations can mill flats and drill cross holes while the guide bushing workholding mechanism results in high accuracy and excellent surface finish.

Parts made by Swiss screw machining are used in hydraulics, engines, medical devices and equipment and more. A partial list includes:

  • Valve spools
  • Couplings
  • Connectors
  • Shafts
  • Sleeves
  • Fasteners (Screws, bolts, pins…)
  • Spacers

Where corrosion resistance is needed these are usually turned from stainless steel. For weight-sensitive applications aluminum and titanium are often preferred. Parts with sliding contact may be produced from carbon steels while electrical spacers are made from either copper or nonmetallic materials.

Material Form for Swiss Machines

These multi-spindle lathes are fed with bar stock. This minimizes material waste as parting off one part creates the front face of the next. The only offcuts are bar ends too short to feed out from the guide bushing.

However, the way in which a Swiss machine holds the bar imposes a requirement for prepared stock. Specifically, imperfections or out-of-roundness in the bar will show up as concentricity or roundness faults in the machined parts. To address this, standard practice is to use only ground bar stock. Parts should be designed to use standard diameters.

Hexagonal bar stock may be used for some applications, like hydraulic or fuel system couplings that will be tightened with a wrench. A machine shop using Swiss machines will evaluate material form options using high-end CAM software for process optimization.

Machinability Considerations

Once set up, Swiss screw machines usually run unattended. The risk is that if something goes wrong it could be a while before the problem is spotted.

Examples of things that can happen are:

  • Stringy chips jamming the machine
  • Chips catching fire (when they build up and friction generates heat)
  • Tool breakage
  • Built-up edge (BUE) on the tool

Risk is reduced by reviewing the material selected for the job and where possible switching to alternatives with better machinability. In parallel check cutting tool inserts are optimized for the material chosen.

Material Selection and Machinability

Machinability relates to the cutting forces needed, the ease with which chips break into fragments, and tool wear. In general, ductile alloys are more prone to forming long, stringy chips. This includes aluminum, low carbon steels, nickel alloys and titanium.

Some alloys are known for being “gummy”, meaning they produce stringy chips and tend to stick to the cutting tool. This accelerates tool wear, which degrades surface finish and tool life. Titanium is especially notorious for this.

Switching to alloy grades with better machinability will reduce problems and lower machining costs. For a specific example, consider 304 and 303 grade stainless steel.

304 stainless is used for applications needing corrosion resistance. However, this grade tends to work-harden and can produce stringy chips. The 303 grade was developed to address this problem. It has better machinability although lower yield strength and corrosion resistance. If the application can tolerate it, switching to 303 will improve the machining robustness.

Manage Swiss Machining Costs

Swiss screw machining is often very cost-competitive for complex, high length-to-diameter aspect ratio cylindrical parts. However, material selection plays a role in the savings achieved.

The main consideration is always what the application needs. If corrosion-resistance is paramount it will rarely be acceptable to use a low carbon steel, although finishing options should be explored. However, bar stock form and alloy grade can influence process economics and should be considered during part design.

Impro has a long history with Swiss screw machines and our engineers are experienced in optimizing the machining process. If you need an economical source of high-quality turned parts, we can almost certainly help. Contact us with details of your application.


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