Title: Module 1 - Swiss Screw Principles
Course: Introduction to Swiss Programming

Module 1 - Swiss Screw Machine Principles

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- Module 1 - Swiss Screw Principles
- History
- From Then to Now

Module 1 - Swiss Screw Principles

History

Before there were Swiss-style lathes, Switzerland and to some extent the areas around Switzerland in surrounding countries developed to be the manufacturing hub of precise, small mechanisms: watches, clocks, wind-up toys, music boxes, timers, etc. These sorts of products require many small, accurate parts. Just as it happens today, the physical size of these products was ever-shrinking and thus the parts that made them "tick" needed to shrink in size also.

Since there was demand for machining small parts, the people is this area naturally developed expertise in the manufacture of both tooling and machines.  As the parts became smaller, they often tended to become very long and skinny. Machining a long and skinny part or one with a large "Length to Diameter" ratio (L/D), is difficult. See figure 1.

When a part has a low L/D ratio, the material itself has sufficient rigidity to resist the bending force a cutting tool exerts on the material during the cutting operation. As the L/D ratio of a part increases, this bending force causes increased deflection. It is not just the L/D ratio of the barstock diameter compared to the part's overall length that is important; the L/D for every section of the part as it is being machined that is important. Every time a cutting pass is made to the part, its diameter or length will change and the difficulty machining it may change too. Consider the following:

- L/D of up to 2.5 or 3 = standard practice.
- L/D up to 5-ish. Still possible on a standard lathe but light cuts and sharp free cutting tools generally must be used.
- L/D >5-ish then supports must be used during the machining process. (Steady rests, follow rests, tailstock w/center).
- Long parts are also possible with special tooling but often this tooling is limited in its flexibility (bar shave tool)

As with any sort of manufacturing, if you improve the efficiency of how you make a part, you will be more prosperous. If you could make one hundred parts in a day using one method but can make one hundred and fifty parts in a day using a new method, you would have incentive to develop and use that new method. Since many people were involved in the manufacture of lots of small often large L/D parts, over time there was lots of development of tooling, methods and machines for production of those parts.  Eventually someone had the idea to use a sliding headstock machine combined with a steady rest (guide bushing) located very near the cutting tools--and a Swiss-style screw machine is born! These types of machines were very effective at machining small precise parts. Think of all those watches and clocks that lots of people in Switzerland were making. The effectiveness of these machines started to be realized in other parts of Europe and later in America, and people referred to them as "Swiss Style Lathes" and "Swiss-type Lathes" because of where the major development of them occurred.

Those early "Swiss style" machines were very different from the machines we have here today. They started out as machines with a few tools arranged in a radial pattern all controlled by cams. The instructions or "program" the lathe used to machine a part would be a shaft rotating with a number of cams mounted to it. The rotation of this cam shaft would be synchronized to the rotation of the spindle. Each tool would have its own cam on the cam shaft and its own cam-follower mechanism following the shape of the cam to transmit the cam motions into tool motions. Coarse adjustments would be made by physically moving a cutting tool in its "slide" and fine adjustments would be made by turning an adjustment limit screw.  Programming a new part meant making new cams or modifying some old cams to suit the new tool path requirements. Many features were created using "form" tools. (sketch a form tool on the white board).  As one might imagine, programming and setting for a new part could take quite a while and involve a significant cost for new tooling.

From Then to Now

Red Rocks Community College uses a Citizen A-20 machine. The "A" is Citizen's most basic model, but it is still capable of machining a medium complexity part. It can machine the front and back faces of a part as well as its OD and can machine barstock up to 20mm diameter. Here in America, given the barstock is generally available, the largest bar these machines will work with is typically ¾". The machine has "driven or live tools" to enable it to drill and tap a cross hole or perform milling-wrench flats for example.  (Show some sample parts)

The "Citizen" of this screw machine is part of the Citizen Watch Company of Japan.  They started making screw machines because they needed them to make parts for their watches. The story goes that Switzerland was reluctant to sell their best machines to Japan in support of the Swiss watch industry. At first, Citizen's machines were only used for their own internal needs, but then they decided there was a viable market in the general machining world for larger versions. The machines Citizen offers for sale are generally not the ones they use for their own internal watch production with one exception. The only model they use in their watch production is their the R4 model (4mm max. 5/32" capacity) for larger turned components. Most of their turned parts are made on smaller proprietary machines.  RRCC's Citizen A-20 machine does have a direct connection to the watch business which started the whole Swiss machining sector in the first place.

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Date last modified: March 19, 2015.
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