Definition
of a Swiss-Type Lathe
Or,
a Swiss-Type FAQ
Our intent with this document is to attempt to explain this manufacturing process. It is our intent to provide an informative document which adequately describes the process, so that the readers might see how it may apply to their business.
Expressly, it is the *concept* this document wishes to promote, and not any particular model. Discussion of potential benefits or disadvantages of this process is welcomed and encouraged. The best place for discussion of this sort is the Internet Usenet Newsgroup below, which is regularly monitored by users of Swiss-Type machines, as well as by service and applications personnel from many different companies.
alt.machines.cnc
A "Swiss-type" lathe is unique for its "sliding headstock" style of lathe machining, which utilizes a standard single-point lathe turning tool placed very close to a "guide bushing", and by feeding the material through the bushing and past the tool, great rigidity is achieved throughout the turning process. This allows for the machining of long, slender diameters with high efficiency because the cutting conditions are kept at a maximum regardless of the part length. Chatter and taper are largely eliminated, and in general, very tight tolerances can be held with customary ground barstock. The process lends itself to long and slender parts, which are difficult to support in conventional CNC lathes, as well as short parts, for the same reason - the ability to machine up close and personal with the support.
This method means, in general, all the material must be removed in a single pass. Depths of cut can be larger than one might think practical, but the superb rigidity of the Swiss-type process not only makes it possible, but efficient.
This concept of machining was pioneered in Switzerland more than 130 years ago, by Dr. Tornos, to produce the many small shafts required by the Swiss watch industry, hence the term "Swiss-Type lathe".
Disadvantages
The biggest disadvantage of the method is seen when a long outer thread must be machined. This presents difficulties, since retracting the prepared diameter back into the guide bushing for the purpose of chasing with a threading tool eliminates or greatly reduces the support of the bushing, and in many cases the problems thus created can be quite difficult to surmount.
Another limitation is the maximum bar size - 32mm (1.260"). Experience has shown that the need to make the finish pass the only pass, combined with the fact that material larger than that lends itself more handily to conventional lathe operations, makes this the "break-even" point for the practicality of the Swiss-Type turning process.
Advantages
The biggest advantage of the machines is their ability to do a great deal in one setup, and fast. Greatly reduced cycle-times and the elimination of secondary handling are achieved through the use of live tooling and sub-spindles, providing a good degree of overlapping operation, and making available within the cycle such processes as completing both ends of the part, inside and out, performing interpolation with live tools, drilling cross holes, performing "Y-axis" milling such as flats, thread milling, and more, all in one setup. Additionally, the machines are designed to be "automatic", meaning they run from 3660mm (12') length barstock, and when a barloader, part conveyor, and chip conveyor are added, they are capable of running unattended, even "lights out", when chip control and tool life are sufficient. Fast cycle times are achieved through high-speed spindles, low idle time, and overlapping operations.
Long tool life and excellent part finishes (compared to conventional lathes) are to be expected, as a product of the high degree of support inherent to the process. For the same reason, high accuracy is achieved, as well.
In general, for parts under 32mm (1.260"), a Swiss-Type Automatic CNC Lathe may well be the manufacturing solution for you. More and more companies are discovering the value of these machines, and as the demand for an ever increasing number and complexity of small precision parts grows, there are more and more applications for which a Swiss-Type Lathe is the answer. Current industries served are medical, dental, electrical, aerospace, hydraulic, pneumatic, automotive, motor shafts, and "job shops" around the world.
While a "production" run is the norm on machines of this type, some shops also use the machines for prototype work. The high-end machines can do quite a bit in one cycle, and spending a couple day's setup on 20 identical small precision pieces, with superb accuracy on the end result, using 1 machine and 1 guy, pays off with the right jobs. However, the typical lot size is around 2000 pieces.
Some Frequently Asked Questions
How good
does the diameter of the raw barstock have to be in order
to hold tolerance on the part?
The guide bushing used in the
machine which supports the material during machining is
very precise and has very little "draw" to it.
Since this is a bushing, it is not clamped on the
material, but is tensioned to a precise fit. Therefore,
if the barstock diameter varies, the amount of support
varies. For this reason the diameter of
the barstock must be "pretty good" along the
entire length of the bar. In general, this means that the
diameter of a bar should not vary more than .012 - .025mm
(.0005 - .001") along its length. Problems also
arise in some cases when, in a given bundle of material
that was ordered at a certain size, the individual bar
sizes may vary as much as .1mm (.004") or more,
while the actual size of the bar along its length changes
very little. In these cases, the bars should be
individually measured and segregated into lots of a
preferred increment of .025mm (.001"), and the
bushing may require retensioning between lots. Other
problems can occur when a bar is "out of round",
since a Swiss-Type lathe does not establish a new center
when turning the part as do conventional lathes.
What
it all boils down to is that in cases where close
tolerances must be held, as with general tolerances below
.018mm (.0007") total on a diameter or length, the
barstock may have to be centerless-ground to a tolerance
below that required on the part to be produced. This is
not true in all cases, but will be painfully evident in
others. The actual material to be machined and the
process used to form the barstock at the mill have much
to do with whethor or not it will need to be ground prior
to running, as do the actual machining processes and the
desired result.
How good
does the straightness of the raw barstock have to be in
order to hold tolerance on the part?
The
straightness of the barstock has a direct impact on how
"smooth" it will run and how easy it will load.
Keeping in mind that these machines commonly run at
spindle speeds of 6000 RPM or more, and that the raw
material starts out as a 3660mm (12') length, the
straightness of the bar may have a significant input on
the quality of the product, in addition to the
reliability of barchanges. There are many different types
of barfeeds, whose principle job is to contain the bar
and dampen vibration, and their construction has a lot to
do with how "crooked" a bar can be. The spindle
speeds to be run are also a factor. The barfeed
manufacturer should be consulted as to the required
degree of bar straightness for your application.
Barstock that is not straight enough may cause excessive
vibration when it spins. This vibration may, in some
cases, be transmitted to the part in the form of
deteriorated finishes, tolerances, and tool life.
If a long OD
thread must be machined after it is turned, how does the
machine compensate for the retraction of the prepared
diameter into the bushing support?
This
is one of the more difficult situations you may run into
with these types of machines - making a long OD thread or
going back to make a groove on a turned shaft. The
concept of the machine is to work up close to the
support, but when a turned diameter must be retracted
back into the support *after turning* in order to machine
an additional feature, in many cases, the tool to be used
on that feature must be offset away from the bushing
support. This is so that the turned part need not be
retracted so far back into the bushing, and is usually
accomplished by the use of an offset tool holder or a
left-handed blade.
What kind of
part lends itself to Swiss-Type machining methods?
Small
parts under 32mm (1.250") whose length to diameter
ratio causes support difficulties on conventional lathes.
Short or tiny parts that are hard to handle with other
methods. Parts needing work done on both ends. Parts with
operations like milled features, cross-holes, or off-center
face holes that would otherwise require secondary
operations.
Can the
machines accomodate tooling such as rotary broaching
heads, threading heads, air spindles, etc?
The
heads and add-on spindles must be of a "low-profile"
nature, as the clearances in these machines are generally
minimal. When wishing to use tooling of this sort,
careful planning is a must, and some modifications to the
heads, holder, or machine may be required. Sometimes,
adjacent tool stations are eclipsed for clearance
reasons, and this must be taken into account when laying
out a job. Try to get as small of a tool as is practical
- for example, a Fette F3 thread rolling head may be an
extremely tight fit, where an F2 may be comfortable.
Typically, the type of threading and broaching heads and
other attachments one might use in a Brown & Sharp
automatic work well in CNC Swiss-Type lathes.
What is the
typical setup time of a fairly complex part that includes
cross-working and overlapping back work?
Assuming
a skilled person, everything needed is on hand, and the
part has been previously run and proven, somewhere around
4 to 6 hours. If it is a new job, with an unproven
process, the time may increase to 8 hours or more. On the
higher end machines, where the part is new and the
operations are complex, the time may be significantly
longer. Conversely, on the smaller, simpler machines, the
time may be significantly less. Consideration should also
be taken for applications where minimal tooling changes
are required.
When it comes to setup time on these machines, there is
no substitute for experience.
What is a
minimum lot size that would justify such a machine?
On
average, a couple thousand pieces is a decent run.
However, many shops use the machines for prototyping as
well, running only 30 or 50 pieces, or for running
families of parts where the same tooling makes a variety
of different part numbers in various lot sizes. On the
other hand, many machines are dedicated to a single part
for months on end. Every application is different - each
one must be looked at individually.
How
difficult is it to learn to program such a machine?
Of
course, that depends on the person's prior experience
with programming, the ability of the trainer, the
complexity of the machine, and the tolerances of the part
to be made, as well as the programming system the machine
uses. One of the things many wrestle with at first is
remembering that in most cases, Z+ motion cuts the part,
and Z- motion goes away from it. This is opposite from
the majority of machines, and is due to the fact that in
a Swiss-Type lathe, the raw material moves along the
length, rather than the tool. The second thing to get
used to in many cases is the fact that 2 or more
different sides of the machines are running
simultaneously, and they must be appropriately
synchronized. Every machine has it's own peculiarities,
but in general, the programming of a Swiss-Type lathe can
be quite a bit more complex than that of a conventional
lathe.
Is standard,
off-the-shelf tooling available for these machines?
Yes!
Many tooling manufacturers make standard "qualified"
tooling for these lathes.
Can I expect
the machine to run "un-attended" or "lights-out"?
On
certain parts, when the chip flow and tool life is
adequate, most of the machines can be run "lights
out", meaning the plant is closed and no personnel
are present. The machine should be equipped with the
following features where available to ensure safe and
efficient "lights out" operation:
Auto Power Off" (any alarm generated by any part of
the system will result in a shunt of the main breaker,
killing power to the entire system)
Broken Cutoff Tool Detection
Part Pickup and Ejection Detection
Coolant Level and Flow Detection
Parts Conveyor (to deliver parts out of the machine -
large parts may fill the "basket" of the
machine quickly)
Chip Conveyor (both to assure constant coolant flow and
steady evacuation of chips)
Automatic Barloader with Remnant Retraction (to assure
uninterrupted supply of barstock and the removal of
remnants which can be harmful to chip conveying systems)
What kinds
of barfeeds do the machines require?
Due
to the loss of the remnant from each bar machined, these
types of machines almost always run from 3660mm (12')
barstock. The type of barfeeder largely depends on your
application. If you want continuous automatic cycling,
with fast cycle times or long parts, you may want to
consider an Automatic Bar Loader. If your cycle times are
usually long, or the parts are usually short, you may
only need a "single bar" barfeeder. The
decision comes in both the downtime of the machine during
barloading, and the need for manpower to manually change
the bar in the event of a "single bar"
barfeeder. In any event, the barfeeder should be
hydrostatic, meaning it utilizes an oil-filled channel in
which the bar rides, to dampen vibration.
What kind of
"learning curve" would be expected when
acclimating to our first Swiss-Type lathe?
Since
many of these lathes incorporate the use of live tooling
and sub-spindles and simultaneous operations, the
programming and setup can be complex at times. With a
person skilled with both CNC lathes and CNC mills, the
learning curve will be short - a couple-three weeks. With
a person skilled in one or the other, the curve will be
slightly longer. In order to realize the maximum benefits
and efficiency from a Swiss-Type lathe, it is advisable
to put your best, brightest, most skilled people to the
task of learning the machine and making it work.
In a 3660mm
(12') bar, how much is lost to the "remnant"?
Typically,
on larger machines, one may expect to lose 150mm (6")
plus the part length plus about 5mm (.250"). This is
due to the necessity of bridging the gap between the
headstock collet, where the bar is gripped, to the front
of the guide bushing, where the bar is machined. As a
"rule of thumb", figure 3350mm out of 3660mm (132"
out of 144") usable bar, unless the part is longer
than 280mm (11").
For more information on the process and machines of this nature, check out the links below:
Star CNC Machine
Tool Corp.
Official Web Site: www.starcnc.com
Email Inquiries to: ohio@starcnc.com
Last updated Sunday, January 28, 2001.
The original Swiss-Type Lathe FAQ is here.
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