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Cutting Tool Engineering April Issue
Author: Kip Hanson
Published April 1, 2020
“Compared to the typical 40"×20" (1,016 mm × 508 mm) single-spindle machining center you can find in any job shop, twin spindles are purchased with one thing in mind: maximum spindle utilization. For that, you want either a pallet changer or robotic part handling.”
Bakers use double ovens to whip up twice as many cakes and cookies each day. Double martinis get the job done in half the time. A bicycle is safer than a unicycle. There are many other examples in which two of something is better than one, but possibly nowhere is this more quantifiable than with twin-spindle machining centers. After all, for a slight increase in floor space and additional tooling expenses, having two spindles can effectively halve production costs for a variety of parts compared with a single-spindle machine. So why don’t twin-spindle machining centers rule the production floor at machine shops everywhere?
In some situations, twin-spindle machining centers do rule. Graham Roeder, national product manager at JTEKT Machinery Americas Corporation, Arlington Heights, Illinois, said primary users of the company’s Gemini XL line of twin-spindle machining centers make automotive parts, such as steering knuckles and brake caliper components.
“But twin spindles present significant advantages to pretty much any high-volume shop, automotive or not,” he said.
Roeder said the number of sales inquiries for twin spindles has been rising. Even companies that replace antiquated twin-spindle machines can look forward to dramatic performance improvements. Today’s twin spindles are better in every respect, with larger tool magazines, higher accuracy, faster spindle and traverse speeds and, perhaps most importantly, integrated automation.
There’s just one problem: These new and improved machine tools are no easier to set up than those of yesteryear. The tool length and diameter on the right-hand spindle must match those of the left-hand spindle, and fixture placement and construction must be equally precise. He said accomplishing all this requires enhanced planning and a high level of engineering skill, which is why many customers purchase machines as turnkey systems — tooled up, programmed and ready to get to work.
“We’re typically building complete packages to order, with most going out with rotary tables and some level of automation,” Roeder said. “Compared to the typical 40"×20" (1,016 mm × 508 mm) single-spindle machining center you can find in any job shop, twin spindles are purchased with one thing in mind: maximum spindle utilization. For that, you want either a pallet changer or robotic part handling.”
An additional difference in the world of twin spindles is the presence of a w-axis. In JTEKT Machinery’s case, this provides up to 20 mm (0.787") of independent z-axis travel to compensate for any differences in tool length. A w-axis is a nice feature, though it’s probably less important than it once was.
The people interviewed for this article said offline tool presetters have made setting up a twin spindle easier. So has zero-point workholding, which allows shops to tool up a range of parts and quickly swap fixtures and vises as needed. For these two reasons above all others, twin spindles deserve a second look from those who’ve discounted them as “only automotive” machine tools.