Title: Today's Machines
Course: Introduction to Swiss Programming

Module 1 - Swiss Screw Machine Principles

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Today's Machines

Swiss style screw machines are very compact so all the tools are very close to one another. Tool changes are often fractions of a second whereas standard larger machines may take some number of seconds to switch to another tool. This compactness also means there is not typically a generous amount of room in these machines for various attachments. Often they require the use of special –screw machine-only attachments which are smaller and more compact compared to the standard versions.

Layout:

Front spindle:

The spindle that holds the barstock. A barloader or bar feeder is on the front spindle side of the machine. The front spindle's tools are located very close to the face of the guide bushing in the machining zone of the machine.

Back spindle:

The spindle that will clamp on the part at the pickoff/partoff operation.

Class note: For purposes of this class, we will refer to these spindles as 'front' and 'back.' It is common to refer to the 'front' spindle as a 'main' spindle and the 'back' spindle as a 'sub' spindle. The terms 'main' and 'sub' will be used for machine programs, not spindles. Referring to spindles as 'main' and 'sub' will potentially cause confusion when we start to learn about 'main' and 'sub' programs.

Guide bushing:

The barstock slides through the guide bushing. The guide bushing housing usually rotates on larger Swiss type machines found most machine shops.  It is common for smaller bar size machines (4-6mm or smaller) to use fixed guide bushings. The stock just rotates inside the guide bushing with oil introduced for lubrication. Surprisingly tighter tolerances can often be achieved on machines with fixed guide bushings. While the guide bushing and its housing are very accurately made and the housings use very high quality bearings, there is still a little bit of error in these assemblies as they rotate. A fixed guide bushing does not have any error since it is stationary. With small stock, even rotating at high rpm's will not produce much friction and heat if the guide bushing is stationary as long as sufficient lubrication is supplied to the guide bushing bore. Unfortunately, as bar sizes increase the friction and heat produced by a stationary guide bushing will increase to the point where the guide bushing must rotate with the stock or the spindle speed must be kept low. There are guide bushings that fit in fixed guide bushing machines where the guide bushing uses needle rollers that enable higher spindle rpm's even with larger stock.

A standard guide bushing has a hardened steel body with carbide "lands" for round barstock. Hardened steel for non-round. Meehanite cast iron also available for use in materials that tend to gall. Carbide lined guide bushings will last much longer than most other materials. Options include extended nose guide bushings and 'max land" versions.

Extended nose guide bushing:  Generally used when your part requires whirled threads or when you have a small diameter part that requires milled features. The extended nose gets your part support closer to the live tools.

Max. land: the length of the carbide lands is increased.

Guide bushings can be ordered in "extra precision" where the manufacturer tightens their manufacturing tolerances for diameter and runout.  Guide bushings only have a very limited (maybe .002") size adjustability range. Using the correct size works best.

Figure 4, too small/large drawing.

Collets:

(figure 3 "collets and guide bushings" drawing)

Standard length standard material (hardened steel). Often the both the front and back spindles use the same collet series (A-20=yes, S&M TF25). Collets produced by American manufacturers (Southwick and Meister, Hardinge, KEB and NAS) generally come standard in smooth bore for diameters up to ½" and grooved for diameters over ½". Smooth bore collets have enough grip strength for smaller bars but grooved ones have more grip strength when looking at how much Z-direction force the collet can resist before the material slips. Since a larger diameter bar could require a larger depth of cut (takes more Z-direction force to cut a large cut depth compared to a shallower cut) larger collets come standard with grooves. Smooth bore collets are available for any diameter but must be specified for diameters over ½". Likewise, grooved collets are available for ½" and under sizes but both these non-standard configurations require a custom order.

Collets are also available in "serrated" (longitudinal grooves) to offer more torque capacity, "serrated and grooved" both standard and longitudinal grooves to offer the most torque and Z-thrust capacity.   Grooved collets can be used for a machine's back spindle but often leave tell tailed grooves on the part. Collets are available with a rough sand-paper like carbide coating applied to them to increase their gripping force. This process may require sending the collet to a specialized vendor.

Extended nose collets can be used for clearance reasons for the back spindle. Different nose lengths can be specified. These can be tapered or straight and are oten used to pickoff a part with a right hand part off tool.  The grip force of extended nose collets is not as great as with standard length collets due to collet flex. Extended nose collets do not last as long before they need replacement.

Standard and extended nose collets are available with carbide lined bores. These are useful where you are chucking on a grooved or threaded area of a part or where you want to avoid damage to a part. A steel collet, even hardened steel will eventually start to get grooves in it if you clamp onto a threaded section of a steel part. The carbide lining will not deform. Steel collets also will also embed a chip in them if you happen to pickoff a part with a chip on it. That chip will often fuse to a steel collet so that chip will damage part after part after part until the chip and collet damage is remove or the collet replaced.

All collets are available in non-round shapes.

Emergency collets (soft interior, sort of hard exterior) are available. The end user of these must machine the bore to the required size and shape. Since these collets are not as hard as regular ones, they can only be used for a limited time before their working surfaces become worn.

Overgrip collets. The amount these "open" is increased so the collet can pass over a large diameter section of a part and clamp on a smaller diameter section.  Not everyone makes these and only certain collet series are available. (Hardinge=yes, TF25=yes). These collets concentrate more force on the leading edge of a spindle's "collet sleeve" and will wear this expensive part out faster.

Collets have a small size adjustability built into them. It works best to have the exact size collet for every application. Depending on part and application details, a collet can grip a part slightly larger or slightly smaller than its size. This size variance is not exactly a published number. Some people are comfortable with gripping a part .005 or .010 smaller than the collet size, others are not. Having the correct size collet always works better than "making something work".  However, sometimes you may need to "make something work".

Collets and guide bushings are also available with "inclined slots". Inclined slots guide bushings are slightly less likely to mark the barstock as it slides through. Inclined slot collets are useful when you are picking off a hex or non-round part with a round collet. The inclined slots insures the corners of a hex (square, octagon or ??) don't line up along the entire length of a slot.

Other options ar available if you are really, really motivated or have a very specific need where the easy to acquire tooling just doesn't meet your part's needs are available. These options pretty much require you to get on the internet and start looking. Swiss manufacturers; where all this started and to this day offer more expertise and options than pretty much anywhere else, offer plastic guide bushings, bronze guide bushings,  ceramic lined guide bushings, coated guide bushings, collets with replaceable brass pads, guide bushings and collets with wavy slots and other options.

Barstock Precision

Many Swiss screw machine shops use only 'ground stock.' Ground stock is the same material as standard extruded or rolled stock but has gone through a straightening and centerless grinding process that yields a very straight and accurate bar. All barstock materials, regardless of material type have a manufacturing tolerance for the bar straightness and diameter. Some materials like brass are known to be more accurate than others like plastic. Since a collet and especially guide bushings are very size dependent, using standard non-ground stock sometimes presents problems. These problems range from difficulty in maintaining tight tolerances during a part run to part length variance or in the worst case, a machine crash.

If you are using non-ground stock, then the guide bushing must be adjusted to accommodate the largest diameter section of bar you may encounter. If a guide bushing is set too loose, the part you machine will tend to be more 'out of round' compared to the same part made on the machine with the guide bushing perfectly adjusted to the bar size.  If a guide bushing is set too tight, then the force required to move the barstock through the guide bushing increases. Sometimes with small diameter stock, the force can "bow" the stock. The headstock will move the exact amount your program requires, but the bar may not move that same distance as the tool. As it turns out, the headstock can push a too-large section of bar through the guide bushing since the direction of the movement will tend to open the guide bushing; upon retracting, that same too-large section of bar, the bar may grab the guide bushing or have the front face of the carbide land dig into the bar a bit and the headstock may actually pull the guide bushing back a little, causing it to close in and lock on the bar. The headstock will have enough pulling force make the front spindle collet slip on the bar. If this happens a machine crash is about to occur.

If the front spindle collet slips, then the front of the bar is not where it is supposed to be and as tools move into position, they may hit the bar and destroy themselves. These tools are very hard and wear resistant but they can't withstand impacts. A chain reaction may happen, damaging every tool in the machine.

Sometimes the only way to get a piece of barstock unjammed is to drill as large of a hole as possible in the barstock while it is stuck in the guide bushing.  The hole allows the bar to collapse a bit so it can be removed.

Ground stock tends to cure all these problems and also lets the machine produce more consistent parts. Everyone would use ground stock all the time if it cost the same as standard stock. Unfortunately it is more expensive. There is always pressure for a company to save money. Anyone can look at a quote for X number of bars of Y material and see that the purchase price of ground stock is more than the purchase price of standard stock. What is much harder to determine is if the "total cost" of standard stock is less expensive than the "total cost" of ground stock once all the costs associated with it are considered. Very few people will ever know the "total cost."

Some material manufacturers are better than others at controlling quality. You may find a certain material from one manufacturer works well whereas the "same" material from somewhere else presents problems. Some shops also segregate their material into size bands. They will sort through all the bars they receive with ring gages to separate the bars into groups.  They will adjust the guide bushing for one group and run those bars, then readjust for a different group and run those bars.

Some forms of materials aremore consistent than others. In aluminums, the cold finished versions of a particular stock are often much better in diameter tolerance compared to extruded versions.

Some materials are sold "oversize" so they can be machined down to the nominal size. This is very common in plastic rod, especially in diameters larger than ½". Your shop orders and expects 1" round bars of nylon to show up and what you get are 1.030" bars of nylon.  If you have the collets and guide bushings for this oversize size, no problem. If you only have the nominal size collet and guide bushing then big problem.

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Date last modified: March 19, 2015.
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