Keeping accuracy within reach: part quality is at risk if a machining center cannot hold tolerances at the farthest reaches of its work envelope. This makes volumetric accuracy a key indicator of a machine's performance. One machine tool builder discusses the implications

Modern Machine Shop, March, 2005 by Russ Olexa

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For more information from Mitsui Seiki, call (201) 337-1300, or enter MMS Direct code 279AX at www.mmsonline.com

For more information from Optodyne, call (310) 635-7481, or enter MMS Direct code 279MH at www.mmsonline.com

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From The Ground Up

Mitsui Seiki has organized its new machine tool production facility in Tokyo, Japan, for building high-accuracy machine tools. This plant represents a 15-billion Yen investment.

At this facility, hand scraping and hand fit parts are the norm. Often machines are scraped, assembled, measured, and disassembled and re- scraped a number of times to achieve the sought-after level of volumetric accuracy. This type of assembly also necessitates numerous large cranes, which were installed in the new facility.

The company starts with an atmosphere-controlled environment in which the temperature in the machining and assembly areas is held at 20[degrees]C to within [ or -] 0.2[degrees]C. Humidity is also tightly controlled. By regulating the ambient conditions, tighter accuracies are achieved on machined surfaces, reducing the amount of scraping.

The machine assembly area has a concrete base 1,200 mm thick with 40-meter deep pilings every 10 square meters throughout the plant. This base provides a stable platform for benchmarking the capabilities of a newly built machine.

Part machining capability is customized for each machine by design. For example, on a machine that's designed to hold a 10,000-pound part, workers scrape a crown in the X axis so that when the part is loaded, the weight causes axis travel to be flat. However, the machine has a very specific volumetric capability with this 10,000-pound part. If a user wants to load a 50,000-pound part with equally tight tolerances, the bow in the X axis would have to be proportionately greater.

After machine assembly, the company uses straight edges, cylindrical squares and granite blocks to check for perpendicularity and axis straightness. Once the machine is built, it is run off and must meet the stringent standards that the new owner demands before it's delivered.