The term Mass Finishing is applied to a variety of mechanical surface treatments in which the articles are processed in bulk rather than piece by piece as in typical manual and mechanical polishing operations. The equipment employed for mass finishing includes conventional barrel finishing machines, vibratory finishing machines and high energy finishing equipment.
The choice of equipment will be influenced by the:
- Type and purpose of the surface finish required and in particular the degree of smoothness and the dimensional limits.
- Initial surface condition.
- Scale of production.
Conventional barrelling still has applications where a general surface improvement is required and dimensional tolerances are fairly wide. Vibratory finishing has a broad field of application ranging from the production of decorative finishes to the deburring and surface improvement of precision components. The high energy techniques are predominantly used for surface engineering applications where components have to be finished to close tolerances and relatively short processing times are required.
Applications of Mass Finishing
Mass finishing operation and the surface effects which it produces are controlled by:
- The type of machine employed;
- The media and the compound used in the machine;
- The operating cycle and operating conditions.
Where articles are processed in mass, i.e. are free to move in the abrasive media, it is not usually possible to limit the operation of the process to particular areas as with mechanical polishing, i.e. it is not selective. With plunge polishing and with spindle finishing, however, where the articles are mounted on spindles the treatment can be limited to specific areas if required.
Applications of mass finishing may be classified under the following headings:
- Grinding, including deburring, deflashing, radiusing and blending
- Polishing and surface treatment
- Colouring or burnishing
The use of a barrel or vibrator for the removal of rust or heat treatment scales provides an effective alternative to conventional pickling with strong acid solutions. A mild acid solution is used to loosen the scale that is then removed by the mechanical action of the abrasive media working upon the loosened scale. The mild acid compounds employed for barrel or vibratory descaling are safer to use than sulphuric or hydrochloric acid pickles and there is also very much less risk of hydrogen embrittlement occurring.
In grinding there is mechanical removal of metal from the surface of the articles being processed for the purpose of removing imperfections and improving the surface condition. On components which are suitable for mass finishing these techniques are generally more effective and also more economical than the use of abrasive wheels, linishers, hand filing or scraping.
Grinding operations can be sub-divided into deburring, deflashing, radiusing and blending.
The removal of burrs, fraze and sharp edges from machined components without affecting adversely critical dimensions, for example of diameters or faces. This is possible in mass finishing as the media tends to flow over the article surface attacking edges and removing burrs before there is any noticeable change in the faces or diameters which may be subject to close tolerances.
To secure effective deburring it is important that burrs are kept to a minimum and of a shape which permits their effective removal. On correctly machined components the root of the burr will be thin, thus easily removable by the mass finishing operation. If the tool edge is allowed to become dull or too high a cutting speed/feed is used the burrs may have thick roots. Such burrs will require extended process times for their removal with the risk of excess metal removal in other critical areas or the need for an initial hand preparation before mass finishing. It should be noted that it is easier to remove burrs on external faces of components than those formed in recesses or on internal faces. This factor should be considered when planning machining operations.
The removal of the flash from castings or die castings. Where the amount of flash is well controlled it may be removed by the mass finishing operation. Where, however, there is a heavy flash due to old or ill-fitting moulds, it will generally be necessary to employ an initial abrasive banding operation if the casting surface is to be subjected to a minimum of metal removal in the mass finishing operation and the parting lines are to be removed completely.
Where a cylindrical section gate is used to feed the metal to the mould, this must be clipped off before mass finishing. Where, however, the gate is in the form of a narrow slot clipping can be avoided as the gate will be broken off during the mass finishing operation.
An extension of the deburring operation, in that after the removal of the sharp edges a radius is formed on the edges and corners of the component. The controlled radiusing of edges and corners is an important requirement on many components reducing the possbility of stress cracking and increasing the life of moving parts.
Blending or cutting down
Operations give an improvement in the component surface prior to subsequent machining or coating, and is particularly applicable to castings and forgings. It should be noted that with heavy cutting down operations on article surfaces there will also be heavy radiusing of edges. If sharp edges are required the cut down operation should be undertaken before the production of such edges. Stock removal by mass finishing in a Harperizer machine can provide an economical alternative to grinding for metal removal from ball or roller bearings.
Polishing or Surface Refinement
In mass finishing these terms refer to functional as well as decorative finishing. Improving the surface condition or smoothness is an important requirement on many functional components including gears, roller and ball bearings, switch components and disc brakes. Surface smoothness is generally expressed as a centre line average value (C.L.A.).
Brightening is a term applied to the production of a semi-bright but very smooth surface on zinc base die castings and other metals by barrelling or now more frequently vibratory finishing. It is a true honing operation and provides an effective basis for a subsequent electrodeposit.
Colouring or Burnishing
This operation, using steel balls or shapes in a barrel or vibrator, gives a bright lustrous surface finish to semi-polished surfaces. The peening action of the burnishing operation will also tend to seal the surfaces and to reduce surface stress.