About rotary swaging
Most of the energy coming from the radial pressure is turned into a high-load forming-force, while the radius of dies gets bigger in comparison with the radius of the work-piece.
This way, when a swaging process starts, there's just a line of contact between the dies and the work-piece; after a few seconds, contact part becomes a surface while the forming routine progresses.

If a work-piece with original diameter do is introduced into the tapered part of the dies, each forward stroke of the dies provides - at the same time - an axial forwarding of the work-piece.
When the smallest section of the tapered part of the dies is reached - if we keep on pushing the axial way - the part will be reduced as soon as it will enter into the calibrated part of the dies.
Generally, while dies and work-piece are in contact one each other, the work-piece will be forced to rotate on its axis during in-feed phase. Due to a fast sequence of pressure impulses and to the momentum of inertia of the piece, that rotation will result in a continuous movement (see following for more specifications).
As the work-piece can slide into the chuck of the feeding device, the rotation speed of the part will be slower than rotation speed of dies. Difference between these rotation speeds will bring - as a consequence - that the work-piece will be hit by the dies every time in a different position, until a perfectly round section will be produced.
As regards hollow sections which need to be formed inside, these should have annular sections bigger than the ones included between the closed dies and the diameter of the inside mandrel.
This concept is necessary in order to allow the piece to be pressed the radial way (at the entrance of the tapered part of the dies) against the surface of the mandrel, and in order to allow the mandrel to get perfectly in touch with the internal diameter of the piece.

Working principle (see 3D drawing)
Drawing shows the working principle of an ordinary rotary-swaging machine.
In the head of the swaging shaft, there are longitudinal slots where both the segment dies, and the thrust pieces can slide into. Shims between thrust pieces and segment dies are used to limit the dies height.
The radial stroke which is necessary to form the work-piece is given by the height of the profile and by the bending radius of the thrust-pieces.
The roller-cage is placed in such a way to turn freely between the swaging-shaft head and the outer-ring (not represented). Pressure-rollers are placed in suitable slots (housings) into the rollers-cage.
When the swaging-shaft rotates, segment-dies and thrust-pieces are driven outside by the centrifugal force. The opening stroke of segment-dies is limited by the curved profile of thrust-pieces, which travel inside the roller cage between side-stoppers and mobile wedges.
During rotation of the machine shaft, the roller-cage and the thrust-pieces - travelling on pressure rollers - provide a relative movement of the roller-cage in the same direction of the shaft, but with a slower speed.
At every transit of the thrust-pieces on the rollers, a radial-impulse is provided towards the centre: this impulse turns into a forming force on the segment-dies and then on the work-piece.
Materials
In the past issue, we said almost all metallic materials can be formed through rotary-swaging.
In this case, is interesting to notice that - especially when forming expensive materials - rotary swaging can bring maximum cost savings, as the amount of necessary material is reduced.
Machines
Rotary swaging machines can be divided in different types, because of the following criteria:
- Dimensions, or capacity, of the machines
As regards machine dimensions, there are some rotary-swagers capable to operate on solid parts, or hollow sections, ranging between a few tenths of millimetre, up to 100 mm. and over.
Inside the same machine dimension, or inside the same size of segment-dies, is normally possible to get more capacity in diameter when forming tubes or hollow sections, rather than solid parts, because of the different forming forces involved with the swaging routine.
In order to select the most suitable machine type, necessary for a given operation, in some cases we should consider not only the diameter and the wall thickness of parts, but also the length of segment dies, especially when we have to make recess-swaging.
Therefore, it's not unusual to use a swaging machine-type being (apparently) oversized in comparison with the dimension of the work-piece, but with considerably long formed sections (length of dies generally increases with the size of the rotary swaging machine).
- Different working systems
We have already described the two main swaging systems: "in-feed swaging" and "recess-swaging".
A.m. systems may co-exist on the same machine, i.e. a rotary-swager prepared to work by recess-swaging can also be used though in-feed system (provided the loading device is prepared for that).
On the opposite, a very simple kind of machine, built to work in-feed, cannot be used for recess swaging, because the head-shaft is different, and the back-group to move forward- and backward- the wedges in the axial direction is missing.
The swaging system is not related to the number of segment-dies, which can be of 2 - 3 - 4 or 6 pieces.
Amount of segments normally grows according to the size of the rotary-swaging machine: generally speaking, we have more regular profile-shapes, yet with smaller loads, as the number of segments is bigger.
Other factors which make different a working-system to the other, can be found in the use of a mandrel (also operated by suitable servo-devices) which can exist or not, and in the rotation principle, which can be internal, external, or both internal-and-external.
- In the internal-rotation principle (conventional swager) the roller-cage is stationary, while the shaft head of the machine rotates, beside - as we said - the work-piece, having a controlled sliding from the loading system, used to produce rotary-symmetrical cross-sections.
- In the external-rotation principle is the roller-cage to rotate, while the shaft-head stands still.
This system is also helpful in order to get rotary-symmetrical parts (where work-pieces can spin on their axes), but it is mostly used to produce non-rounded cross-sections, as poly-angular shapes, square-sections, parallel-flats, etc.
- In the method with double-rotation (internal-and-external), both the roller-cage, and the shaft-head rotate, generally in two opposite directions, so that to greatly limit the torsion-effect on the work-piece. This method is commonly used to swage-down pieces which are sensitive to torsion loads.

- Number of stations and automation
Rotary-swaging machines can be supplied with different degrees of automation and consequently - in most complex systems - with different machines in-line to produce sometimes a finished component, i.e. in different combinations among dimensions, working-systems (see above) and even linked with several working systems.
The simplest machine has a single station, it can be eventually supplied with the only head, to be later positioned on a pedestal, or existing frame (this case is quite common, especially for small-dimension heads, or when integrating more complex machines, or systems).
Feeding of work-piece can be either manual (through operator), or with a simple feeding-device: mechanic, hydraulic, pneumatic, or servo-assisted via motors.

This is the common solution suitable for limited series, prototypes, etc., especially for small-dimension parts and were operational-costs and workers-involvment can be born by the company.
A typical "stand-alone" rotary-swaging machine is provided with a rack-feeder, or step-feeder, also linked to an axial loader / extractor, which makes a working-cell with good operational flexibility.
This is a classic solution for medium- to large-production batches, and a good starting-base for further integration of more complex automation, as pick-up feeders, or gantry-feeders.
Number of working-stations can be virtually unlimited, whenever more machines are placed in a transfer-line, thus maximizing the concept of automation, in case production volumes are very important.
In manufacturing of complex components, like shock-absorber rods or cylinders, steering shafts, spindles for electrical appliances, etc., it is not difficult to find swaging lines consisting of 15-20 machines side-by-side, including turning units, milling stations, rolling heads, axial presses, etc.

Practical examples
Components represented in the picture number 1-2 are normally manufactured using conventional rotary-swaging machines with internal rotation of the shaft (in-feed system), where finished parts made out of tubes can be formed even without mandrel or internal profile.
In hollow pieces, it is also proved how is possible to get completely air-tight end-closures, formed as a pin.

Parts represented in the picture number 3 are also manufactured with conventional rotary-swagers.
At the same time with the reduction of the external diameter, tubes are formed on a internal mandrel, to get an hexagonal cross-section, or a spline-profile as per DIN 5481.

Parts shown in the picture number 4 and similar are manufactured through machines with stationary shaft.
Tubes do not rotates, thanks to the external rotation of the external roller-cage, therefore even non-symmetrical sections can be obtained, as parallel flats, etc.

Parts represented in the pictures number 5 and 6 are produced through the recess-swaging system.
In this kind of machines, at the back-side of segment-dies there are some wedges which can open and close axially, in order to allow introduction and extraction of the work-piece.
The forming section is made during the forward-stoke of the segment-dies, when the work-piece stands still in the axial direction. When it takes to make longer recesses, the work-piece can be axially shifted-through, while segment-dies are still in the working phase.

A further possibility of use of rotary-swagers consists in making mechanical-assemblies by interference of metal to metal (without welding, or chemical products in between).
For instance, in the picture number 7, a spur-wheel with helical-spline is represented as inserted and mechanically locked in to a hub, through a rotary-swaging process. This mechanic-connection is stable, axially precise, concentric, quick and un-expensive in comparison with other joining methods.
Here, in the pictures number 8, is shown how is possible to get - in two working phases - the production of a tube with cylindrical external diameter and wall-thickening of the internal section, at both ends.
At the end, it will be also possible to make further rotary-swaging operations, or tube reductions, profile rolling, or localised recesses without chips removal.

Final remarks
When manufacturing a new product in hi-volumes, cost of raw-material is more and more important.
Due to this reason, it is convenient to consider all manufacturing techniques allowing to optimise the product, starting from the engineering-phase, even under the point-of-view of materials-cost.
Among the other advantages, technology of rotary-swaging allows:

- Material savings
- Serial production
- High repeatability
- Excellent surface-finish

The experience have also shown that in case a heat treatment is necessary, at the end of the production process, the amount of distortion of materials is very reduced in comparison with parts made by chip removal.