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When Buying a Five-Axis Machine, Bigger Isn't Always Better

For trunnion-type five-axis machine tools, workpiece size will have an impact on what size machine you should buy. 

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During the Top Shops Conference a couple months ago in Indianapolis, Indiana, I got to sit in on a presentation about five-axis machining given by Michael Cope, an applications engineer with Hurco. With a title like “Five-Axis: It Just Ain’t That Scary,” it was clear that Mr. Cope’s talk was clearly aimed at an audience used to machining on three-axis machine tools and possibly worried about switching over to 3+2 or five-axis machining. One of Mr. Cope’s arguments to ameliorate this fear was, “You’re already performing five-sided machining; you’re just doing it manually.”

While perhaps not “scary,” five-axis machines certainly bear more variables to take into account and consider. This may be the greatest challenge to their adoption, and it also reflects their great potential for increased efficiency.

One consideration that might be counterintuitive for the buyer new to five-axis is what size of machine to get. With a three-axis machine, you likely want to get the largest that your budget allows, but Mr. Cope says that with trunnion-type five-axis machines, bigger isn’t necessarily better. The reason has to do with Z-axis clearance.

At zero degrees (origin), the trunnion shouldn’t be any encumbrance to the tool’s axis travels, but as it tilts to 90 degrees, the table’s supports and assemblage suddenly become an obstacle, causing potential interference among the table, the machine casting and the spindle head as the tool is lowered along the Z axis. Faced with this scenario, you’d have to equip the machine with extended-length tooling in order for it to reach the part from over the trunnion, which Mr. Cope says is undesirable since longer tools amplify rigidity issues and can result in less accuracy and breakage.

Palletized fixturing also plays a role in machine selection, because adding fixtures to the table increases the effective workpiece height. When the trunnion tilts toward 90 degrees, it’s no longer Z-axis travel that is encumbered, but Y-axis travel, since now the part protrudes laterally instead of vertically. Thus, ironically, the total height of the fixture and workpiece will impact the limits of the spindle head’s horizontal (Y-axis) movement.

The takeaway here, Mr. Cope says, is that trunnion-style machines should be sized for the work that they will be machining, so contemplating the purchase of a new five-axis machine should definitely take workpiece size into account.

Mr. Cope’s presentation was based on a short book he wrote for Hurco called “The Power of Five: The Definitive Guide to 5-Axis Machining.”

A version of this blog post originally appeared at techspex.com.

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