Microfinishing 101

QPAC Quality Products and Concept

The ever-increasing needs of mankind have placed a constant stresson quality improvement and cost reduction. This has necessitated the need for high dimensional accuracy and smooth surfaces on parts made of conventional metals such as steel, as well as on wear resistant and difficult to machine materials such as Titanium, Super Alloys, etc. The process of micropolishing can be cost effectively used to meet these needs.

As an example, let us evaluate a steel piston used in a piston type hydraulic pump. Under traditional use, grinding would be the final step in the process. However, given the severity of many new applications (higher load, pressure, heat, etc.), the micropolishing process has been added to impart the properties that allow the product to meet new demands. We will start by evaluating the part in as-ground condition.

Examination of the ground surface under a metallurgical microscope (figure 1) reveals, a surface with numerous flaws, pits, jagged edges and a fragmented unstable surface called the amorphous layer. This layer created by the heat generated on the surface during grinding, not only possesses lower hardness than the base metal, but also possess a different and non-homogeneous microstructure. The existence of this layer can lead to undesirable conditions for wear resistance.

Evaluation of the surface finish of the ground surface using a Surface Analyzer reveals an average surface roughness of 8.036 (in Ra-figure 2).

Analyzing the part with a Roundness Instrument (figure 3) reveals an average out-of-roundness of 45.4 in with both high frequency form error (often called chatter) and low frequency form error (often called lobing)(figure 4).

In addition to the existence of unstable amorphous layer described above, high surface finish values typically in the range of 8 to 30 (in Ra, a lack of straightness, out-of-roundness as illustrated above can breakdown the lubrication film between the bearing surfaces, permitting metal to metal contact, which results in increased bearing loads and can eventually lead to premature failure.

Insufficient performance of certain products and related part analysis similar to the piston shown above yielded the development of the micropolishing process (also known as superfinishing process).

For the sake of simplicity this discussion is limited to cylindrical OD micropolishing only. Internal diameters, flat surfaces, spherical surfaces and other shapes can also be micropolished. The basic principles and the benefits of the process remains the same for all different part shapes.

Micropolishing consists of rotating a workpiece at relatively slow speeds. An abrasive cutting media, is then applied to the part while oscillating at lower amplitudes and higher frequencies. The cutting medium can be traditional vitrified abrasive stones or Mylar-backed coated abrasive tape. The tape can be either be supported by a hard backing that is designed to mirror the ideal part geometry, or a semi hard contact roll that conforms to the part being finished. Unlike stones, where fresh abrasive crystals are exposed as stones wear, the tape approach always introduces fresh abrasive crystals either indexing between each workpiece or through continuous tape feed. The rotational speed of the workpiece, amplitude and frequency of the oscillation, micropolishing time, grit size of the cutting medium, pressure applied on the workpiece and type of coolant are all used as process control parameters.

Tape finishing process have become more popular due to the many advantages of the tape media, but certain applications such as internal and external bearing races can only be finished using stones. It should also be noted that micropolishing process almost always requires the use of coolant mineral seal oils for stones and either mineral seal oils or water soluble synthetics for tape.

Now let us examine the pistons analyzed earlier in the as-ground state, after tape micropolishing. It can be observed from figure 5 that the surface finish has improved and has an average Ra of 1.86 (in which is down from an average Ra 8) in the as-ground condition. It can be observed from figure 6 that the average out of roundness has improved from 46 (in to 18 in an impressive 61% improvement). Also, from the roundness polar chart in figure 7 it can be observed all the high frequency chatter has been removed and the low frequency lobing is reduced.

It can be observed from the photomicrograph in figure 8 that the surface flaws, jagged edges, fragmented unstable layer is removed exposing the base metal, and higher peaks are removed leaving the surface predominantly with valleys for oil retention capability.

From the illustrations above it is apparent that parts become rounder, straighter, smoother and benefit from micropolishing. These benefits translate into reduced wear, increased load-bearing capacity, quieter operation, elimination of break-in periods, etc.

Some of the parts types that can benefit from this process are:
Load bearing journals, seal surfaces, shafts with bearing or seal failure problems, piston pins, crankshafts, camshafts, transmission shafts, gear sets for hydraulic pump, valve spools, armature shafts, compressor shafts, bushings, etc.

It should be noted that it is not enough to simply micropolish, parts and make them as smooth, round and straight as possible. Smoother is not always better. Ideal surface conditions must be developed based on a thorough understanding of the conditions under which the component function, coupled with an iterative approach using competent micropolishing source possessing metrology laboratory equipped with surface analyzers, roundness gauges, metallurgical microscopes and precise size measuring equipment.

In the example of the pistons discussed above, the manufacturer was experiencing premature field failures. Figures 5 through 8 illustrate the ideal conditions developed for the part. To determine the ideal conditions numerous batches of pistons were finished to various finish ranges and were put to accelerated wear tests.

As expected it was found that parts with rougher finishes had maximum wear rate, parts with 1.5 to 3 (in Ra had the least amount of wear, and surprisingly pistons with < 1 in Ra wore more than those with 1.5 to 3 in Ra).

Microscopic analysis of the component showed that by finishing the part so smooth, the process had not only removed the peaks, but also greatly reduced the valley depth and effectively reduced the oil retention capability, thus creating a metal to metal contact and eventually premature failure.

In conclusion it can be state that Micropolishing has now emerged as precise and cost effective finishing process that can add immense value to the component.

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