The term ‘media’ as used in mass finishing / vibratory finishing refers to the aggregate which carries out the work required on the component.

The media carries out a number of important functions apart from actually working on the parts.  These functions are as follows: -

1. It holds parts in suspension in the mass.
2. It keeps parts separate thus avoiding damage.
3. It acts as a carrier for the compound.
4. It carries out the required work i.e. grinding, deburring, cleaning, polishing.
5. It transmits the machine energy in terms of vibration or centrifugal force to the parts.
 
In order to choose the right media the most important things to consider are the following: -
1. Composition and type
2. Shape
3. Size
4. Density and weight

Composition and Type

In the early days of barrel and vibratory finishing natural media were used and were primarily of granite, limestone and aluminium oxide from the foundry industry.  The main problem with each of these materials was that they were of a random shape and size range and this often resulted in the media getting lodged in holes and recesses.   This invariably led to a widespread use of masking devices on many components.
These products are not now used having given way to performed media which is manufactured in a comprehensive range of shapes and sizes. The two principal types of media now in use are ceramic and low-density plastic bonded materials.
Non abrasive media for polishing are also available for high finishing operations.  These include steel balls, pins and shapes, ceramic balls and porcelain pins.

Performed Ceramic Media

This forms the greatest part of the media used in finishing operations.  It is produced by firing in kilns such as those used in the pottery industry.  They are manufactured from ceramic clays which are mixed with solid abrasives.
Variations in the firing process and in the type and size of abrasive determine the hardness and the cutting rate of this type of media.
Ceramic media is produced in a range of shapes and sizes and for each shape you will find a number of grades extending from rapid cutting to fine finishing applications.  The most common shapes are triangles and angle cut triangles, straight cut and angle cut cylinders, elipse, star shapes, tetrahedrons and balls.

Ceramic media is normally used on the harder metals such as steel, iron, bronzes etc though they can, and often are, used on softer metals.

Plastic Bonded Media

These media have considerably increased the scope of finishing, particularly on non-ferrous materials.  They can be used in either vibratory or centrifugal force machines.

For this type of media polyester/styrene resins and used as a base and are mixed with abrasives eg silica, silicon carbides, aluminium oxides or quartz.

Because of the comparatively low density of this material approximately (0.9 kg/ltr) and hence its light weight it gives a finer finish on aluminium, zinc based and other light metal components, though it can also be very effective in the fine surface finishing of other metals.  The low density of the media can also be an economical factor in filling a machine.
The media is moulded rather than extruded as are the ceramic materials are therefore a new range of shapes are available e.g. cones, pyramids, tetrahedrons, rhomboids in addition to the more orthodox triangles.  These media are useful in situations where lodging is a problem and for components with a large variety of holes of various shapes and size.
 
Other Media

For the burnishing or polishing of components it has been necessary to find other media.  These are usually of steel or stainless steel which are themselves highly polished.   These media are supplied as balls or ball cones but other shapes such as steel pins and angle cut crop wire are also available.
For lower density work media have been produced in porcelain and these can be effectively used in either centrifugal force or vibratory machines.

Guidelines of the selection of media for a specific application

When considering which media to use there are a number of points to be considered.

The size and shape of media
When considering a media for a specific component there are a number of factors to be considered.  These are: -
1. The amount of work to be done. e.g are you looking for a heavy deburring or cutdown or do you want a fine finish.  A general rule for this is ‘the larger the chip – the faster the cutting action. – the poorer the finish’.  Conversely ‘ the smaller the chip size – the slower the cutting action – the better the finish’.
2. Where the work is to be done? – will the media reach the area to be finished?
3. What standard of finish is to be achieved? What surface finish?
4. Lodging – is there a risk of media sticking in holes or recesses?
 
The size should also be considered for ease of separation of the parts from the media if screening is used.

Other points to note when selecting media:

1. Try to keep the number of chips and types stocked to a minimum.
2. A media size mixture can sometimes give a better surface finish than a single size.
3. If lodging is a problem media should be regularly re-graded. This can be dome in the machine by using a special media grading screen.
4. Low-density media produces better surface finishes in vibratory or centrifugal machines but there are exceptions. eg Keramo® process.

Good Vibrations
by Paul Rawlinson, Contributing Editor MFN and General Manager Rösler UK
Tel: +44.151.482 0444
Fax: +44.151.482 4400
PRawlinson@rosleruk.com