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Desenco Inc. (Akron, Ohio) has been producing high-quality rubber molds for injection and transfer molding of precision parts for more than 20 years. Until recently, most of these molds were manufactured using CNC vertical machining centers. When a new horizontal machining center did not yield the expected results, Desenco installed a laser toolsetter from Marposs (Auburn Hills, Michigan) to optimize performance.
Prior to toolsetter installation, Desenco personnel report that it was "almost impossible" to attain accurate Z depths and repeatability in the Z axis. Setting tool lengths was extremely difficult, and the machine did not cut to required depths, with shallow or deep variances as high as 0.0025 inch.
According to Craig Jorstad, CNC coordinator at Desenco, these problems were caused by the machining center's high speed (15,000 rpm) spindle and Cat shank. When the machine was run at high speeds, centrifugal force opened the spindle and the tool physically drew back into the taper, resulting in unpredictable variations in depth-of-cut.
Despite the machine builder's involvement, there were no easy solutions. Desenco employees made attempts to manage the problem by measuring tools in the static condition with a conventional contact-type toolsetter. But in reality, depth accuracy depended on operator intervention with the tool rotating.
Operators were making shallow trial cuts and then making adjustments to attain required depths. This made setup time-consuming and the results unpredictable. Operators were never certain of the depths they would achieve, nor could they count on a repeatable pattern. Depending on the properties of various tools, variances ranged between +0.0005 inch and -0.0025 inch. "You were never certain of what you were going to get in the trial cut from the first tool to the last," Mr. Jorstad says. "And it takes time to dial those things down and bring yourself to where your depths are all the same."
The resulting step variances were unacceptable, as Desenco adheres to strict tolerances of ±0.0005 inch for most of the features cut for its molds.
For example, accurate core pin orientation is particularly critical to successful mold production. Core pins are used to create holes in parts, and during the machining process, core pin height is maintained by a counterbore. "The core pin comes up from underneath the cavity and its orientation in that cavity in terms of its height is established by a counterbore on the bottom plate," Mr. Jorstad explains. "That counterbore has to be held to a very tight tolerance in terms of its depth if we are going to be able to maintain pin height. We try to hold counterbores at ±0.0005 inch."
The Marposs laser compensates for the dynamic errors of the machine tool, spindle and toolholder, and is said to greatly improve the performance of the machine. Other reported benefits include faster setup, improved part quality and greater consistency between operators. These advantages also apply when using a broad range of tool sizes, from 0.024 inch to1 inch in diameter.