Closingin

IMPCO.MACHINE.TOOLS

Closing in on Machined Perfection

Tooling and Production Magazine

Machining has come a long way in the last 100 years. Machine tools have become increasingly accurate, routinely achieving workpiece tolerances not even imagined 50 years ago.

The surface texture or roughness imparted by shaping and planning is generally a roughness average (Ra) from 32 to nearly 150 micro inches (mu), a very rough and unusable bearing surface. Surfaces produced by milling, drawing and extruding are not much better. Drilling is worse. Boring and turning produce inadequate bearing surfaces. Grinding, honing and polishing are a step in the right direction, with the latter resulting in surfaces ranging between 4 and 16 mu. Lapped surfaces com in at 2 to 16 mu. Lapping is no longer adequate in the manufacturing of more fuel-efficient automobile and truck engines. The top of the line is microfinishing (superfinishing), a process that consistently produces parts with surfaces ranging from 2 to 8 mu.

While the perfect bearing surface will never be achieved, it is possible to come very close with microfinishing (superfinishing). Automobile companies from Audi to Volvo demand crankshafts and camshafts with bearing surfaces that are consistently in this 2 to 8 mu range. IMPCO Machine Tools, Lansing, Michigan USA has spent more than a half-century developing the metal removal process needed to produce reliable and fuel-efficient engines.

Surface Measure, Ra

Ra surface measurement is nearly always shown on engineering drawings where surface finish is specified, but it is not the only surface measurement that indicates a good bearing surface. When measuring the surface for Ra values only, both Figure 1 and 2 will have the same Ra values, even though they appear to be quite different. While the upper surface shown on Figure 1 is basically flat with deep grooves, it could be a good bearing surface that features good oil film retention. The surface of the bearing in Figure 2 would be quite unsuitable for such a bearing application.

Nodular iron surfaces leave ferrite caps protruding above the part surface. Unless removed with microfinishing, these can cause bearing or part failure.

Bearing Ratio, Tp

This parameter is a percentage of the value of the sum of flat areas at a specified slice level over its length. This level would be more representative of the surface after a normal break-in period when the highest peaks have been worn away in a short time, leaving a larger supporting surface. The slice level can be specified to a depth (measuring below the highest peak) or a height (measuring from the Ra mean line). The bearing ratio distance, Htp, is the distance between tow specified bearing ratio slice levels.

More fuel-efficient smaller engines produce higher bearing loads than engines produced a decade ago. Tests have shown that some bearing surfaces with cross-hatch finish specifications help engines maintain maximum hydrodynamic oil film on journals, but oil seal surfaces generally do not require cross-hatch finishing. A cross-hatch finish on an oil seal may cause a vane-type pumping action to pump oil past the seal. An oil seal with a straight-line finish is much more effective.

Grinding operations on crankshaft main bearings and connecting rod journals, camshaft lobes and bearing and transmission shafts will produce bearing journals with many different types of cylindricity errors, including errors in roundness, waviness, chatter, taper, barrel and hourglass. The hydrodynamic oil film cannot be maintained if any of these conditions exist.

The ideal bearing journal is a right circular cylinder with a finish that is as smooth as possible. Some wearing surfaces may require a small amount of texture to retain lubrication, since a journal-to-bearing interface requires a cushion of oil under pressure to keep the two mating surfaces separated. The smoother the surface, the thinner the lubricant cushion can be before rough spots come into contact, and the higher the load capacity of the bearing.

These two surfaces have identical Ra values, but will perform quite differently as a bearing surface. That is why auto makers are looking at other surface finish parameters to characterize a "good" part.

The Solution

Microfinishing (superfinishing), as developed by IMPCO, removes what is best described as unstable surface material from a dimensionally finished part. This amorphous (non-crystalline) material layer exists on every ground or turned part. That is not a problem with previous machining operations but simply an undeniable physical fact. Left on bearing journals, this layer can, at best, cause poor performance and, at worst, bearing failure.

When a precision ground part is examines under a microscope, fragmented material will be seen. This material loosens and the erodes the mating component. This type of surface also breaks up the supporting film of lubricant, and will not support the loads of high-performance engines and transmissions. This layer is like the snow on a frozen lake. A person's weight produces footprints in the snow, but the ice beneath supports even the pressure of an ice skater.

The IMPCO process enables manufacturers to remove the amorphous material layer accurately and control surface finish, bearing ratio, geometry, and even size bearing journals. The process uses a non-compressive, coated abrasive film backed by patented, precision-shaped non-resilient tooling for full, consistent part-to-abrasive contact through the finishing cycle. The film is automatically indexed after each cycle to present fresh abrasive to each workpiece and thereby help to achieve consistent finish specifications. Used with consistently applied coolant, pressure, part rotational speed and tooling oscillation, the GBQ process reliably improves shaft surface texture, roundness and straightness to predetermined specifications, as well as the tightest tolerances.

To accommodate varying production requirements, IMPCO GBQ process parameters - including shoe pressure, oscillation frequency, stroke, part rotation speed, cycle duration and amount of abrasive tape index - are fully adjustable for optimum performance and productivity.

Many automobile engines use nodular iron crankshafts, which include ferrite caps protruding above the material surface. Other finishing processes do not adequately remove the ferrite caps, presenting a serious component durability problem. IMPCO GBQ microfinishing (superfinishing) removes the hard ferrite caps, presenting a serious component durability problem. IMPCO GBQ microfinishing (superfinishing) removes the hard ferrite nodules protruding above the bearing surface on iron bearing journals. It is a capability that engine manufacturers worldwide require. Although the ferrite caps or modules may be no more than 0.000315" (0.008mm) above the bearing surface, they can cause bearing failure, resulting in expensive warranty claims.

Charting Success

IMPCO microfinishers (superfinishers) have tooling design capability to process virtually any portion of bearing journal areas. This includes improving surface texture, geometry, and even size.

Taper. With IMPCO's Axially Variable Taper Compensation Process, microfinishing (superfinishing) tooling permits manufacturers to control the big three bearing journal conditions - surface finish, geometry, size - plus taper. In-process gages monitor the presence of taper and are used to control right and left shoe pressure, removing unwanted taper.

Size Control. When single or even multiple grinders feed one microfinisher (superfinisher), the incoming diameter sizes may vary greatly. This is due to the many differences among grinding machines, including wheel wear, dressing sequences and machine wear. Each bearing journal on a given shaft may, therefore, be different. The result is that stock is removed at different rates, causing a variation in journal sizes.

Return to What's New page

IMPCO Machine Tools

3417 West St. Joseph Street, Lansing, Michigan USA 48917

Phone: (517)484-9411 Fax: (517)484-0502

Email: Sales @IMPCO.com