Overcoming Common Challenges in Swiss Screw Machining
It’s said Swiss machining originated with the need to produce tiny screws for the Swiss watch industry. True or not, the definition has broadened over time. Today it describes a technology for precision machining small cylindrical parts quickly and efficiently.
These parts can be much larger than watch screws, and they needn’t be threaded, but machinists still refer to the process as Swiss screw machining. It’s done on sophisticated multi-spindle lathes and presents some challenges beyond those commonly seen in turning.
Here we’ll explain:
- How this process differs from conventional turning
- Its advantages and applications
- The challenges involved, and how they are addressed
Multi Spindle Vs. Single Spindle
Cylindrical parts are produced on lathes. They can be held in a rotating chuck or turned from a bar pushed through a collet from the side of the machine. Bar loading reduces cycle times, and if the lathe has a second or sub spindle it can produce a finished part in a single step.
A multi spindle lathe takes the bar feeding concept further, by using multiple collets arranged on an indexing disc. Each collet holds a bar and indexing takes it to a different tool. A typical lathe could have six collets / spindles, and therefore six machining tool stations.
A true Swiss machine provides Z-axis motion, (that’s the direction of the bar axis,) through bar movement rather than the cutting tool traversing along the part surface. A guide bushing is used to hold the bar and take the cutting forces.
Advantages and Applications
Multispindle turning minimizes throughput time. It often cuts manufacturing lead time too by eliminating secondary operations.
By avoiding part handling and the need to regrip multiple times it can achieve higher quality than conventional turning. In addition, the guide bushing approach improves precision by minimizing workpiece deflection.
Maximum bar diameter is limited – 1 3/8” (32mm) is typical – but this still allows machining of many different parts. Long, cylindrical shapes are the ‘sweet spot”, making medical devices and hydraulic components good candidates.
Challenges and Solutions
Swiss-style machining poses some additional challenges to those normally present in turning, (optimizing speeds and feeds, controlling surface finish, and so on.) These are:
- Chip management
- Programming and setup
- Barstock quality
- Tool life/replacement
Chip problems occur when turning produces long strings. Undesirable in single-spindle turning, these are a bigger problem on multi-spindle machines. “Birds nests’ ‘ of long, stringy chips can tangle with other spindles and the workpieces to create surface finish and dimensional problems. They can even jam the machine, and require an operator in attendance all the time.
Chip problems are usually addressed by adjusting feed and speed to make the chips thicker so they break easier. Using cutting inserts with optimized chipbreakers helps too.
Programming and Setup
When multiple tools are working on multiple parts there’s a bigger risk of collisions. Careful programming is essential and tool setting is more of a challenge. There are also differences in the programming / machining approach.
Rather than turning the entire length to size in one pass, Swiss machining benefits from a segmented approach: turn a short length, then push more bar out through the guide pushing and turn down a second length. This reduces deflection and improves precision.
Consistent work holding requires dimensionally consistent bars. Diameter variations cause problems for the collets holding the bar, and can allow movement and deflection in the guide bushing. Most Swiss turning is done using a ground bar: it’s a little more expensive but results in higher productivity.
For extended periods of unattended running each tool should last the same number of cycles. To maximize precision tool offsets are often entered directly into the CNC controller at the machine.
Impro Are Machining Specialists
For smaller cylindrical parts, Swiss-style machining often provides the highest efficiency and lowest cost. It can also result in higher precision than single spindle turning. However, the machines themselves are complex and need considerable expertise to set up and run efficiently.
Impro maintains well-equipped CNC machine shops for producing complex metal parts. Contact us to learn how we can meet your precision machining needs.