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Vibratory finishing

There are a number of valid reasons for wanting to improve the surface finish of race engine components. Aesthetics, friction reduction and durability can all be enhanced by reducing surface roughness, although the aesthetic aspect will always be subject to the opinion of the beholder. Controlled surface roughness though is very important on some components – unless you are very sure about what you are doing, mirror-finishing a piston skirt or cylinder liner for example can lead to disaster.

In terms of friction, components with lower roughness surfaces require thinner oil films to separate them completely; once separated, the friction coefficient is a function of the lubricant and the film geometry only – there is no component of solid-to-solid (coulomb) friction. The durability aspect of surface finish is well documented in textbooks on engineering design. For example, Shigley and Mischke* give detailed coefficients for different surface finishes to allow calculation of more accurate fatigue strengths of components.

One way to achieve a very high level of polish on a surface is through vibratory finishing, which can be applied to a wide range of engine and transmission components, from tiny ‘widgets’ to crankshafts and gears. Components are placed into a vibrating ‘bath’ of media, which may be hard or relatively soft and which may be dry or contain fluids. Relatively soft media can be something like finely divided walnut shell, maize or ground corn cob, and this can work well, even for some hard materials. I have seen very highly polished press tools, engine components and transmission gears which have been finished using walnut shells.

The same technique can be used with success to de-burr components while polishing them.

The action of the media continuously rubbing against the part removes asperities and improves the surface finish further by removing the top layer of material, increasing the ‘flat’ area on the metal surface and decreasing the depth of the valleys in the surface.

There are some advantages over other surface finishing techniques using ‘loose media’ such as rumbling, where the polishing media is much larger. Owing to the size of the polishing media, the improvement in surface finish works well on internal surfaces such as holes. The limit on the size of an internal feature that is able to benefit from the process is a function of how large the media is. For example, if the media size is 0.5 mm, it is unlikely to polish a 0.4 mm ‘internal’ radius effectively, such as might be found on many turned parts. Another advantage is that the process is can be run with the top of the machine open, so it is very easy to see when the parts have been polished to the desired finish, in the case where the finishing is done for cosmetic reasons.

The media is constantly cycled around the machine, and as it becomes ground to ever finer sizes, the media which is too fine to work properly is screened out.

There is a wide range of media sizes available, and other process variables are vibration amplitude, frequency and processing time. Large amplitudes and frequencies imply high energy, and unsurprisingly the higher energy processes remove material more quickly and can also remove sharp edges too, which may not be what you are looking for. As ever, it is important to work with a finishing company in order to get the correct result. So, if you want to retain sharp edges, you will probably need a different process from someone who wants high rates of material removal, even for the same material.

* Shigley, J.E., and Mischke, C., “Mechanical Engineering Design”, McGraw Hill, 2001, ISBN 0-0736-5939-8 

Written by Wayne Ward

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