Piston Maker Boosts Quality With Special Geometry Software

A leading manufacturer of pistons for high performance engines has improved its manufacturing accuracy and quality control by using a software package developed especially to measure the unusual geometry of pistons. The Formscan circular geometry gage with optional piston software from Mahr Federal, Inc. (Providence, Rhode Island), is the only way to reliably and efficiently analyze features such as cam drop (that is piston ovality) and skirt taper, says Eric Nessa, vice president at JE Pistons Inc. (Huntington Beach, California).

JE, which develops and manufactures automotive and marine racing pistons, found it difficult to measure these features using other technologies. "For years we used conventional tools, and you can only get so close," Mr. Nessa says, adding that CMMs, which are extremely awkward to use in this application, generated results that conflicted with those of the company's customers.

The problem stems from the unusual design geometries of pistons, which are engineered intentionally out-of-round and barrel-shaped. Most geometry software assumes that parts are nominally round and straight-sided, so any amount of ovality or barrel shape is normally reported as error. Piston manufacturers, however, want to be able to define ovals and barrel shapes as nominal specifications and measure parts for conformance to those specifications.

As Mr. Nessa learned, Federal's optional piston software, used with a Formscan 3300 or 3600 circular geometry gage, allows the user to specify and measure a piston's change in radius at a selected angular increment. While the longest radius is usually oriented 90 degrees from the wrist pin bore centerline, the Federal software allows users to program in other "timing" specifications (for example, select an angular offset for the high point) and to specify tolerances for asymmetrical pistons, which have differing front and rear profiles.

JE installed a Formscan 3600 system with optional piston software and uses it for new product development and as a quality control tool. Concerning the former application, the company develops new cam and skirt profile designs in CAD and downloads data directly to CNC lathe controllers. According to Mr. Nessa, even though the lathes are designed specifically to turn pistons, the ability to cut an oval cross section on a lathe has an element of uncertainty. After test parts are cut, they are measured on the Formscan gage and analyzed with the piston software to assess conformance to the design. Often, the process is reiterative, and either the CAD file or the CNC programming will be fine-tuned to achieve the desired piston shape.

JE also uses the Formscan system as a quality inspection tool, particularly on its ultra-high-end product line, where it checks cam drop, skirt profile and timing. This last feature, says Mr. Nessa, is "absolutely critical." Formscan equipment is also used to check the plane flatness of ring grooves. The Formscan gage and software, says Mr. Nessa, are a practical means to inspect these features on a production basis. "CMMs just don't work on this application," he adds.

Optional piston software is available as an add-on to Federal's Formscan metrology software package. Formscan gages feature easy-to-use touchscreen controllers and offer radial accuracy to ±1 microinch. Systems are in use by major automotive and off-road engine manufacturers, as well as by manufacturers of bearings, fuel injectors and in many other applications.