A surface finishing technique that has not been surpassed Most engineers involved in the manufacturing of mass produced components are familiar with the term vibratory finishing and will be aware that the most common method for surface finishing is to process the components loose in a vibratory or centrifugal disc type machine either in batches or feed them continuously.

This method has been extremely successful for many years and for the majority of applications will remain so for the foreseeable future at least whether it be deburring, grinding, polishing or surface enhancement. However, it is a matter of fact that when using conventional methods the components will come into contact with one another during the process cycle and although for most applications this is not an important factor there are some high value and/or sensitive components that will damage if processed this way.

Another category of components not suitable for conventional finishing methods are parts with small holes or recesses. The energy and forces generated in barrels, vibratory machines and even centrifugal disc type machines are often not enough to effectively contact these restrictive areas, at best one or two pieces of media may enter or contact the area but the effect is negligible.

Drag Finishing, the concept...

A modern drag finishing machine incorporates a normally stationary process chamber which holds the process media.  The media is relatively small because the forces generated in this type of machine are enormous compared to other methods, so large media is not required and in some cases would damage the components purely because of the huge forces created. So small precision media is used to penetrate recesses and holes whilst still removing stock material should that be a requirement of the particular application in question.

Positioned above the process chamber is a rotating turret onto which a number of further rotating spindles are attached. The number of spindles and size of machine is determined by the component size and production through-put requirement of the customer but typically a machine would have eight or ten (See Picture 2).

The main turret, whilst rotating, is lowered into the stationary media, and at the same time the individual spindles rotate submerging the components into the finishing mass.  The “dragging” action begins, the rotation speed of the turret and individual spindles are variable as are the rotation directions and depth for which the component are lowered into the finishing mass; these factors together with correct media and compound selection will provide a process result that is unsurpassed both in the finish achieved and process time, which is typically measured in minutes rather than hours.

Extremely low Ra values are achieved, very important when finishing performance critical components such as aero-engine and motor sport parts.
It is true to say that a certain amount of component handling is required when compared with conventional processing, however, in reality the drag/plunge type machine is mainly suited for components with a relatively high value that are often delicate. Such components cannot be successfully finished in conventional machines.
With PLC control it is possible to have 100% repeatability and consistency of the finish.  Options include:  a plunge facility that is important for components with a certain geometry, and multi-process stations (See Picture 3).

Occasionally components do not require finishing all over, so in these cases the component holding fixture is designed in such a way that it will completely mask and protect these areas. Such was the finish criteria with a recent application to process the under-crown of a Formula 1 piston where no abrasive contact was allowed on the crown of the piston, likewise an aerospace application required the super polishing of a turbine blade airfoil but the blade root was to be protected from the process.

Plunge Finishing

In some ways similar to the drag finishing concept but normally used for relatively larger components, although not exclusively.

A typical application is the finishing of aero-engine turbine discs that require very consistent deburring and a very uniform radius on the profiles of the dovetails. The plunge finishing machine replaces the labour-intensive hand deburring and finishing that not only requires many man hours but was also possible for the person carrying out the manual task to make a mistake and miss one of the many complex areas, the TSA – R plunge machine ensures repeatability time after time.
The normally single spindle movement travels approximately 69o across the process chamber constantly oscillating from right to left changing direction every 15 seconds ensuring uniform component coverage and thoroughly mixing the media so the wear rate is uniform throughout the process chamber.

The variable programmable parameters such as – orbital movement, spindle speed and vertical pendulum movements are all preset into the PLC and would normally not require adjustment unless the component type was changed.

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