If machine tools were dinosaurs, horizontal boring mills would be brontosauruses. Even the smallest of these brutes have axis travels best measured in meters, and spindle horsepower sufficient to drive a small car. But what is a boring mill? Isn’t it just a gigantic horizontal machining center?
Boring mills do share many of the same characteristics as HMCs. They have X, Y and Z axes, are frequently equipped with rotary tables for multiple-sided machining and generally have large-capacity tool magazines. But the defining characteristic of a boring mill is its bar spindle, or W-axis.
Similar to a quill, the bar spindle is an extension of the Z-axis. With conventional machining centers, the ability to reach deep into a workpiece is limited by the size of the spindle housing and the Z-axis travel. Manufacturers can sometimes get around this by using an extended-reach toolholder, but this can cause chatter, tool wear and loss of accuracy.
On a boring mill, however, once the Z-axis, or ram, is in position, the W-axis can extend to machine features that would otherwise be inaccessible—without the need for long-reach toolholders.
Also, a bar spindle is inherently more rigid and accurate, according to Bob Conners, vice president of sales and marketing for United Precision Services Inc., Cincinnati. “The W-axis allows for better access into tight areas, greater rigidity as well as overall parallelism of the spindle to the machine axes,” he said.
Parallelism is key. Conners noted that the fixed spindles on traditional machining centers are relatively short. Of course, builders strive to perfectly align those spindles, but depending on a number of factors, including less than perfect installation, improper machine maintenance and wear, slight misalignment may occur. “But in a boring mill, a spindle might be 10 ‘ long,” Conners said. “Compared to a traditional machining center, the spindle on an HBM has wider support and a shorter lever arm. This makes it easier for the builder to dial-in and maintain proper alignment.”
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