MAY 2017

Techspex provides metalworkers free research and analysis tools to help them find the right machine for their job.

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I S I T T I M E T O M O V E U P T O A N H M C ? 4 cutting time if it runs another shift a day, and more yet if operates six days a week.) Running only one shift, the HMC's total overall added chip-cutting time can exceed 1,200 hours a year. The added cost of the horizontal almost becomes insignificant, provided you have the work to keep the machine fully loaded. FLIPPING PARTS Whenever the operator must open the door of a VMC to load or unload the part, remove chips, perform in-process quality checks, or flip the part, the spindle must be stopped. If this part requires machining on six sides, the operator must move the part a total of seven times (load, reposition five times, unload). When the same part is being machined on an HMC, the operator touches the part only three times (load, reposition, unload). Productivity and efficiency is obviously improved. In addition, with an automatic pallet changer, the operator can handle these steps while the spindle continues cutting the part on the pallet inside the workzone. Improved quality may also result, because the HMC requires less operator intervention, which reduces the chances of loading errors or other missteps. MACHINE DESIGN CONSIDERATIONS There has been a long-standing debate about whether box ways or linear guideways are better. When discussing VMCs, one could argue there isn't enough difference to make that choice matter. It's a different situation with HMCs. On HMCs, linear guideways have proven to be better for higher axis-travel speeds on materials such as aluminum because the acceleration and deceleration rates can be faster. As a result, cycle times can be reduced significantly. Some linear-guide HMCs reach 1G acceleration without sacrificing machine rigidity and without generating excess heat. However, box ways can be better for certain HMC applications. Although HMCs with box ways have slower maximum programmable feed rates, performance is improved for operations in which high cutting forces are likely to be encountered. Box ways are recommended for heavy cuts in tough materials such as titanium, super alloys and tool steel. HMCs with box ways will deliver superior finishes, even in the heavier cuts in tough materials. The way the spindle head is constructed on an HMC is generally very rigid. This design resists deflection from cutting forces, thus enabling a heavier chip load and promoting longer tool life. Some HMCs with larger work envelopes have dual Y-axis ballscrews to support the proportionately greater weight of the Y and X axes.

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