Construction ComponentsThe Bed and Major Components’ Material
Cast iron comes in a variety of chemical compositions and granular structures. Our structures have a “spring memory” which is the ability to flex repeatedly to the same position under load and return to the original position when unloaded. Iron repeatability assures process predictability. Further, Mitsui Seiki’s higher cast iron grades are treated with gas emulsion processes for controlling material composition. This method has reduced the aged cast iron procedure that builders used to do in the old days when they would store beds outside for a year or more. The gas emulsion method that we use at Mitsui Seiki provides much better precision characteristics. Yes, this process is more costly, but if you are machining critical parts, the result is a very solid, thermally-stable platform.
The materials we use have been thoroughly engineered and proven to provide the qualities expected by the most demanding customers in the world (ourselves included).
There are differences in guide way systems. In their simplest form they might be integral ways, which are cast into the machine component parent material, and subsequently machined, perhaps surface hardened, and then ground. The downside is as the machine slightly shifts and bends, which it does constantly, the hardened surface of the cast material flakes off. Those miniscule, chipped pieces continually rub into the ways as the machine operates. This definitely wears out the machine more quickly. In the highest end machines, such as a Mitsui Seiki, the ways are made of tool steels, which are always hardened and ground. Tools steels are inherently tougher, stronger, and do not flake, crack, or break. They are abrasive-resistant, so if by slight chance a cutting chip, or debris, gets underneath the sliding component and scrapes back and forth during operation, the damage is certainly minimal if any.
The Age-Old Art of Hand Scraping
One of the simplest ways to build a machine is to finish the axes’ slide ways on a surface grinder in order to fit the subsequent components. The grinder, depending how it was constructed, has certain geometric accuracy parameters. The grinder’s inaccuracies will be transferred to the ways being machined, and could be detrimental to the ultimate geometrical integrity of the machine under construction. Conversely, our high quality, more expensive machine tools have hand-scraped way mounting surfaces. Hand scraping is a century-old adjustment technique that allows a builder to fit components by hand to meet very specific geometric tolerances. Further, it allows the planned creation of, as an example, a slight convex shape in the bed, so that when the column is placed on the machine and compresses the bed, it will straighten and flatten to the desired tolerance. It’s tough to do, but at Mitsui Seiki, we can achieve less than 2 arc seconds of straightness in all axes of a large machine, which would be impossible to achieve on surfaces that are only machined or ground.
Spindles are susceptible to longevity and stability cycles based on the materials used. For example, the taper section is often hardened to withstand all the tool changes over the life of the machine. Case hardening and grinding are typical. However, with a very hard surface and a softer core, future cracking fatigue and increased wear are possible. At Mitsui Seiki, our material is tough and hard. The ratio of the hardened surface and core is very close. This is a toughness characteristic that is inherent to more expensive forging materials for high quality spindle shafts. Also, tool drive keys that are integral to the spindle (not replaceable keys) provide up to 40% more stiffness in the taper, thus justifying the more costly manufacturing of this spindle configuration. Another concern is when the tool is released; there are powerful pushing forces to disengage the tool. This pushing puts tremendous reverse pressure on the front-end bearings, which can shorten bearing life. That is why we use a patented tool clamping system, which uses about one-fifth of the typical release energy, hence transferring only those reduced forces to the bearings during tool changes to significantly increase bearing and overall spindle life.
Bearing design also plays a large role in spindle cost and durability. Bearing life and bearing load directly relate to bearing alignment and bearing speed. Lubrication of the bearings is also critical. At Mitsui Seiki, we employ a patented bearing lubrication deign for all our higher speed spindles. This unique, proven design ensures the proper amount of lubrication reaches the desired location on both the inner and outer bearing races. Also, when needed, we hand-fit spindles with bearings for specific applications. For example, although we have a line up of standard spindles, which work well in a wide variety of applications, sometimes the client wants to do something more exotic. We readily adjust spindles to the customers’ specific application. For example, if a customer is mainly machining a particular high-temperature alloy, we can set their spindles up to the optimum dynamic and static characteristics for the job.
Mitsui Seiki’s ballscrews are not only inspected for diameter, pitch, and lead angle. We measure with a dedicated inspection machine for what’s known as “drunkenness” or, the theoretical ball path versus the actual ball path. The actual and the theoretical paths need to be as close together as possible, because any deviation will cause wear, and wear degrades the positional characteristics of the machine overtime.
Installation technique of the lead screw is also important. The machines’ ball screw mounting locations (for each end of the screw) are hand scraped so they are super-accurately aligned with the ways. Many builders machine those pads, but hand scraping is much more accurate to ensure that the ballscrew nut travels perfectly parallel to the guide way planes. This parallel tracking avoids stress-induced heat. Another source of heat in the axis drive is the support bearings at each end of the screw. The bearings are cooled on our machines. Remember, heat is a thermal inhibitor to accuracy. Rather than trying to correct for heat-induced errors in the ball screw drive, Mitsui Seiki prefers to prevent the heat in the screw from occurring to start with.
When building a high-end machine tool, not only the mechanical components are important. Mitsui Seiki uses the latest in PLC, and PMC design and quality. The electric cabinets are properly engineered and manufactured to permit years of trouble free life, and reliable performance. When you look inside our EC cabinet, you see well thought out, and well executed wiring, and high-end components. You will not find any crimped connections, or unlabeled wiring here. Additionally, we use as much industry standard components as possible, so familiarity and service in your local area is easy. Mitsui Seiki can build our electrics to a variety of industry-accepted standards to meet the needs of customers virtually anywhere.
Mitsui Seiki is a proud integrator of FANUC™ CNC controls and drives. Our long relationship with this prestigious control builder affords us the solid benefit of providing the latest CNC controls and options to our customers. Many new and innovative CNC features and functions for this brand of controls have been conceived through this long-term partnership.
Other Components and Peripherals
Many components may seem too simple to be important, but even they should be made well if your goal is machine uptime. Consider wire, for instance. Wire also comes in different grades. For example, SRML wire grade has each piece of copper wrapped in fiber, allowing it to flex more, which is what we use at Mitsui Seiki. Wire cost is directly related to flexibility. With machine tools, wire flexes all the time. Wire breakage is a down machine condition. Pushbuttons are a similar case. The better the quality, the more you can push a button before it has to be replaced. The same goes for most small, miscellaneous items.
Other considerations are those components that may cost less up front, but consume more energy. For example, cooling units and other ancillary items that are consumptive-related need to be high quality to keep energy costs down. At Mitsui Seiki, we use items that run more efficiently.
When it comes to peripheral components, or options, we are firm believers in adding value for the customer. Seemingly simple items like chip conveyors can prove to be a major item in overall productivity, and reliability. Tool changers, and other automation devices are designed and built to meet the demands of high end users. Overall quality cannot be compromised through the use if inferior peripheral equipment. We control the design and quality of all components delivered to our clients. Many industry accepted optional devices are available to enhance the productivity, and performance of the machines we offer, however we will not sacrifice any measure of overall machine integrity or quality.