Title: Module 3: Major Screw Machine Systems
Course: Introduction to Swiss Programming

Module 3: Major Screw Machine Systems

Module 3 - Major Screw Machine Systems

Terminology

Facing: Machining the face of a part. Usually a cutting tool will start at the outside diameter of the barstock and move towards the center of the bar.

Turning: Cutting the outside diameter of the part. In a Swiss style lathe, this is usually done by the headstock longitudinally moving the barstock under a cutting tool while the spindle is rotating.

Front turning: Turning to a shoulder.

Back turning: Turning away from a shoulder.

Face drilling: Drilling a hole in the center of a face of part. The barstock/part rotates while a drilling tool is pushed into the part or the rotating barstock is pushed over the drilling tool.

Cross drilling: Using a driven cross spindle to drill a hole radially through or partially through a part.

Boring: enlarging the inside diameter of a hole by moving a tool along one edge of the bore making the diameter larger.

Reaming: Enlarging a hole by driving a reamer into an already drilled hole. The reamer will accurately size the hole to a tight tolerance diameter. The reamer can be used in a cross or face hole.

Tapping: Generating threads in a part in a single pass using a tap. Taps can be cutting taps that cut threads and make chips or form taps that displace material forming threads in a chipless manner. Like a reamer, a tap can be used in a cross or face hole.

Single point threading: Using a cutting tool with the proper shape to cut threads into a part. The final thread depth can only be achieved by taking multiple passes.

Cross milling: Using an end mill in a rotating cross working tool spindle to cut various shapes like milled flats, couterbores, pockets, etc. Usually the part is not rotating during cross milling.

Grooving: Similar to turning but using a tooth-shaped cutting tool. Grooves can be radial grooves that are cut along the circumference of a part or circular face grooves cut into a face of a part.

Partoff: Cutting the part off of its parent bar. Usually done with a "part off" tool.

Pickoff: The backspindle moves forward and clamps on the part before it is cut off its parent bar.

Machining Orientation

The first decision to be made regarding machining a part on a Swiss machine is to determine which end of a part is going to be the "front" or the face of the part while it is still attached to the barstock. Just because a part drawing shows the part in one orientation does not mean the part needs to be machined in that same orientation. Sometimes the decision is very clear given the geometry of the part, and sometimes considerable thought must put into the decision. Generally the end of the part with a high (>3) L/D ratio or with a good location for pickoff will be the "front."

1. Machine the front face the part to establish its exact location or "Z-zero" in preparation for ID machining. When the machines' headstock is reset prior to machining the next part the exact location of the bar's face can shift a small amount. This amount depends on the amount of force the bar loader/feeder pushes on the bar when the front spindle is open and how much drag there is between the headstock and the barstock passing through it. It is common to face anywhere between .010 and maybe .030 inches of material off the front of each new part

2. Perform any ID machining. Starting with the ID leaves the od intact so the part is well supported in the guide bushing during the ID work.

3. Machine the part OD moving from the front to the back of the part. Given the length of the guide bushing lands, options may be limited to come back to a Z location. If the part's front face doesn't have a hole in it, the facing operation is immediately followed by the turning operation, usually with the same tool.

4. Position the back spindle and cut the part off the bar.

The Swiss Machine vs. the Standard Lathe

Machining parts on standard lathe often requires using one tool to add all the features it can to a part and then switching to a different tool for its features. For example, a turning tool may turn the whole length of a part, then a grooving tool would add grooves at various places along the entire length of the part, then the threading tool would thread the part in the correct area.

Machining a part on a Swiss machine requires a different overall approach (See figure 3). The guide bushing in a Swiss lathe offers huge benefits for machining large L/D ratio parts and but also presents limitations on how a part must be machined. Once you machine the OD of the barstock, the machined section of your part will no longer make contact with the guide bushing land area if you retract it back into the guide bushing. This is going to require the part to be machined in sections from the front to rear.

See Figure 5 for an example of a part. It may seem the best way to machine this part would be to turn both diameters, then plunge the 2 grooves, then part it off the bar. After all, this would require the fewest number of tool changes and thus the most efficient way to make the part on our Swiss machine. If the length of the turned sections were short, this method could work. However, if the turned sections are not short, the part will loose engagement with the guide bushing when it is retracted for groove 1. This part requires us to turn section 1, change to the groove tool and plunge groove 1, change tools back to the turning tool and turn section 2, change back to the groove tool and plunge groove 2 and finally change tools to the partoff tool and cut the part off the bar. This demonstrates the sort of front to back machining method Swiss lathes require. Sometimes tooling is available that may lessen the need for some of this front to back segmentation, but it will always be a factor in how a part is machined on a Swiss lathe.

The guide bushing land length is not the only factor here. You must always be conscious of the L/D ratio of features on your part as it is being machined. Once you machine material off the OD of your bar and retract it back into the guide bushing for a finish cut or to use a different cutting tool, you have now lost the rigidity benefits of the machine's guide bushing. Machining a high L/D part is relatively easy with a Swiss lathe, but only if you can machine the part with one cut. If your part requires multiple cuts then you need to be aware of the L/D ratio and take appropriate steps to process your part.

As your part is being processed, it is often necessary to retract the bar back into the guide bushing. This presents an issue with burrs created on the bar during the turning process. If the cutting edge of your tool is dull, these burrs can be significant. If the barstock needs to retract back inside the guide bushing, it is very good practice and often vital to actively remove the burr with a cutting tool before your program moves on to the next tool. Sometimes these burrs are large enough to cause the collet to slip on the barstock if the burr hits the face of the guide bushing. The proper way to exit a cut is to move radially up to or close to the bar diameter then either cut a small chamfer or just turn a small length of cut at the bar diameter a little longer than the tool's nose radius.

Tool shanks for Swiss machines often look like small versions of shanks that are available for standard lathes. However, Swiss tool shanks have an important difference: Swiss shanks almost always need the cutting edge of an insert to be in line with the edge of its shank. Standard lathe tooling often has the edge of the insert protruding from the edge of the shank. If you tried to put a small lathe tool in a Swiss machine, the insert would interfere with the face of the guide bushing.

Standard ISO cutting inserts are often used in Swiss machining. These inserts are made by many companies and they all adhere to design standards that allow the insert from one manufacturer to properly fit a shank made by a different manufacturer. ISO inserts are often made in large quantities and are generally lower cost per tip compared to more esoteric special made screw machine cutting inserts. If you are performing stardard turning, grooving or threading, choosing an ISO insert that has the correct properties for Swiss machining can often work well.

All insert manufacturers have recommendations for surface speeds and feed rates, or how fast the spindle should spin and how hard the insert should be fed into the material, for their inserts running in various materials. Be very cautious of these recommendations if they come from tooling manufacturers whose main focus is lathe turning. Their recommended speeds and feeds will be for lathes, not screw machines, and their speeds and feeds will be much higher than what will work in screw machines. Lathe insert manufacturers often show how well their insert's chipbreaker works by picturing examples of small broken chips. Getting chips to break as shown in catalogs often requires feeding at very high feed rates. Good chip control in a screw machine is where the chip is curled into a small spiral that is heavy enough for gravity to drag down into the chip pan. Bad chip control in screw machines is a little looser spiral that gets out of hand and forms a 'bird nest' of chips wrapped around the part or a tool shank.

Cutting inserts for Swiss lathes usually have a very free cutting design that lets the insert slice through the material. Cutting inserts for lathes are often intended to be very durable and tend to "plow" through material with brute force. Lathe inserts usually are intentionally "dull" so their cutting edges hold up to lots of metal removal. The very sharp free cutting inserts used by Swiss lathes usually don't hold up very well in lathes unless they are used for finish cuts.

Swiss lathe tooling is available from many sources Most of the well-known tooling companies out there make tooling intended for Swiss machining. Some of these companies seem to be very knowledgeable about Swiss machining and make very well thought out tools. Others seem to think Swiss machining is just lathe machining using small shanks. Tooling is also available from companies that specialize in Swiss tooling. These companies often offer more variety and options than the mainstream tooling companies. Sometimes using a standard ISO insert works well and is the most economical choice. Sometimes, however, having the exact right insert with the exact right shape with the exact right grind with the exact right coating makes a huge difference. An ISO insert will typically cost between $5 and $20. Inserts from the specialists start at $20 and go up to maybe $50. That $50 insert that just 'works' when nothing else will could very well be the cheapest option available.

There are 5 basic types of inserts for general Swiss machining:

Front turning/facing, back turning, grooving, threading and partoff. While separate inserts are available for these operations, sometimes one or more of these tasks can be combined into one insert. The ability to combine functions is very dependent on the material being machined. If the material is easily machined (free machining brass, for example), you could perform front turning/facing, back turning, grooving and maybe even part off with one insert. That is not possible with nastier materials.