Chasing the Microns of Accuracy

Although Mori Seiki already produces a wide-range of machinery, the addition of DIXI Machines adds a nano-precise category to their line-up. The manner in which these machines are built – from the preparation of each mating surface through the assembly – is a decades-old tradition.

"Built-in accuracy and rigidity means that 4-axis DIXI machines achieve volumetric accuracy of 15µ (0.0005") while the 5-axis machine volumetric accuracy is 25µ (0.0009")," says Bob Courville, team leader – LPP/CPP, Mori Seiki U.S.A., Inc. "DIXI delivers volumetric accuracy – meaning anywhere within the work envelope the stated accuracy is achieved by eliminating any variables that can affect a machine."

Hand-scraping is one of the ways that nano-accuracy is achieved. With more than 500 hours, on average, this pull-scraping technique of all mating surfaces – to achieve perfect match – offers the high degree of volumetric accuracy and rigidity inherent to all DIXI machines. In order to continue this learned trade, the five to six full-time scrapers work with apprentices in order for all Journeymen to scrape in the exact same way.

The process is to scrape – in a pull-motion with a ¼" wide tool – then blue ink is applied to a master piece. That master piece is then slid along the surface that was scraped, and anywhere blue ink remains is an indication of high areas of the surface, so the scraping process begins again. Each scrape is removing less than 1µ of the surface, so each part is scraped and blued a minimum of a half-dozen times.

"In addition to achieving a perfect match, they hand-scrape planned curvature into the machine by taking into account sub-micron levels of accuracies. In this manner, when the bed, column, linear guides, rotary table, spindle head, etc. are all compiled on the machine, the machine will straighten out from the built-in, sub-micron curvature to perfect straightness," explains Courville.

Controlled accuracy

Uninfluenced structures mean everything is taken away from the casting of the machine – machines are assembled in a way so that nothing will impact the machine geometry – because everything is thermallycontrolled; the microns of machining can consistently be obtained. Items that can influence the machine's accuracy – such as ATC, APC or electrical case – are removed (separate from the actual machine), or addressed in such a manner, that they won't negatively influence precision.

One step involved in hand scraping is applying bluing ink to show where high areas remain that require additional scraping.

"Anything in the machine that generates heat – ballscrew support bearings and nuts; spindle ball bearings and motors; direct drive motors on the B and C axes; electrical cabinets; flushing coolant; hydraulic units – all are maintained at an ambient temperature," says Courville. "DIXI runs fluid through lines around these heat sources in order to maintain that constant temperature so there will be no negative affect on the machine of anything that could create heat. Anywhere there is a coolant line, it is leading into sources that would create heat, such as the electrical cabinet, which even though it's separate from the machine, a degree or two can matter when you are chasing microns of accuracy."

Another example is where there are chips, there is heat. However, stainless steel liners with built-in augers make sure that the chips do not flow along the casting, but rather they are delivered out the back of the machine to the chip conveyor.

Some people are under the impression that a DIXI machine is only for finishing. "But that's not true," states Randy Harland, vice president, national sales manager, Mori Seiki U.S.A., Inc. "What we do is we put a high-torque, highhorsepower motor on so we have the ability to do roughing, semifinishing and finishing operations all in one setup so you don't have to take a part and put it on a high horsepower machine, rough it, and then bring it to the DIXI for finishing. It would be counterintuitive because you would never want to move a part that is already on a high-accuracy machine."

Heat-generating parts are thermally-controlled on all DIXI machines. The stainless steel liner for chips, with the augers already built in, is designed so there's appropriate air between the liner and the casting.

In order to effectively perform roughing, semi-finishing and finishing operations, the spindle bearings are under variable preload, so it automatically adjusts based on rpm – it's always maintaining a constant preload. At a low rpm, theoretically it is adding pressure to have a rigid, accurate spindle. At very high rpm, theoretically it is reducing the preload so heat is not generated. This constant, automatic adjustment of the spindle based on rpm helps avoid heat in the spindle, which produces growth, reducing accuracy.

The basic structure of a DIXI machine is a spheroidal cast iron base.

Dynamic balancing of each spindle is initially done prior to installation on a machine. Dynamic balancing is then done once the spindle is installed on the machine in order to ensure balancing, harmonics and frequency are all kept within a certain range – because this too can impact the overall accuracy of a machine.

Other areas contributing to accuracy are the box-in-box concept, which eliminates the possibility of column rock; Driven at the Center of Gravity (DCG) – the use of double ballscrews to reduce vibration; the number of bolts used to attach the column to the base, making the base almost a unicasting; and the control of all thermal effects. In addition, DIXI utilizes a three-point leveling system that "benefits the installation of the machine, allowing it to maintain its geometric accuracy much more easily without having to readjust it because there is less settling with three-point leveling," explains Courville.

Direction headed

Since the purchase of DIXI by Mori Seiki, nothing has changed as far as DIXI's quest for precision. What has changed is that they are now manufacturing bases in advance of order – bases that are interchangeable with DHP's or JIG 700. DIXI plans to have a ready stock for orders that come in for either machine in order to reduce delivery time from its previous one year time frame down to around six months.

The range of machines available from DIXI are the JIG 700 and JIG 1200 – high-precision fixed table horizontal machining center – and the DHP 50 and DHP 80 – high-precision horizontal machining centers.

Mori Seiki U.S.A. Inc.
Rolling Meadows, IL
moriseiki.com