Quality improved on tire mold details engraved with built-in high speed electric spindle systems.

Mold makers often must engrave lettering and other fine details into the molds they produce. High speed electric spindle systems added to existing machine tools or incorporated into special machinery are one way to achieve the accuracy required. The accuracy and rigidity of these spindles can help limit variations and maintain tight tolerances to ensure consistent results from mold to mold, in new production and also when repairs are needed.

One such application is on molds used to make tire sidewalls, which include a substantial variety of letters, numbers, logos and other characters. To improve precision and repeatability when engraving these features, the leading tire mold manufacturer designed and built several special CNC machines that incorporate high speed electric spindle systems. The company, Quality Mold of Akron. OH, converts tire designs into precision molds for automobile, light truck, off-the-road, motorcycle, bicycle, agricultural and aircraft tires. Since its founding in 1978, Quality Mold has grown to become the world's largest independent supplier of tire molds.

Engraving's key role

The names, identification numbers, designs, company logos and other information that must be added to the molds range from small to large and from simple to complex. All finished details must be sharp and clear so they effectively convey important product information and complement the finished tire's appearance. Accuracy and repeatability have become more important than ever, because tire manufacturers want all tires lo look exactly the same, no matter what mold they came from. Traditional methods of engraving these details are now either too slow or not accurate enough to meet higher customer expectations and the changing nature of tire sidewall artwork itself. Also, when molds are repaired. CNC accuracy is needed to maintain consistency between molds.

Traditionally, these details were either stamped into the molds or engraved by using a pantograph-type machine and a template. Neither process was satisfactory to maintain the companies" continually tighter quality standards. Stamping left impressions that were not crisp and clear enough for some of the characters, especially as the molds wore, and it was difficult to obtain consistently accurate depth results. Pantograph engravers use a mechanical linkage to guide a rotating cutter as the operator follows a template that replicates the desired design. As tolerances become tighter, the templates lacked the necessary accuracy, and the mechanical linkage could not provide the precision and repeatability that customers were demanding. Also, the manual nature of the pantograph operation made production too slow for the demanding volume of molds being produced. Accuracy and consistency, especially important with foreign language character displays, were difficult to achieve.

Improving accuracy and productivity

To meet customers' increasing production needs and improve the expected accuracy of the engraving operation, the company first converted an existing CNC machining center fl)r the specialized task, but soon found it more beneficial to have its own people design the machines it needed. Since then, the firm has built seven new CNC machines, all of which use the same type of high speed spindle systems.

Each of the machines is equipped with a motorized spindle that has a speed range of 10,000 to 40,000 rpm. The 3 kW (4 hp) spindles each are powered by a high frequency converter and are equipped with a liquid cooling system that dissipates the heat generated by the spindle bearings and motor. The spindles are built around a dynamically balanced quill shaft that turns in pre-loaded high precision ceramic ball bearings and is driven by an integral motor. Spindle accuracy of 2 m (.00008") maximum T.I.R. at the spindle nose allows the cutting tool to work at high speeds and achieve excellent surface finish. The light chip load and low tool pressure inherent in high speed machining make it possible to achieve highly accurate results when engraving even the finest characters.

By combining the spindle accuracy with the inherently tighter tolerances and greater repeatability of the CNC machines, Quality Mold is able to meet its customers' expectations for consistently crisp, precise mold engraving. Machines are said to be in operation about 20 hours per day, engraving tire mold sidewalls made out of either aluminum or steel. Carbide cutting tools are used, with size depending on the configuration being engraved. It has been noted that some of the logos with extremely small details may require cutting tools approaching .002" diameter, while others can range up to about 1/4".

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