A machine tool's construction affects capital costs and the quality of the parts made on it.
BY SCOTT WALKER
Hand scraping of linear way surfaces fits components to meet specific geometric tolerances.
A completely assembled Mitsui Seiki HY80A features hand-scraped ways, a tool-release systems that relieves bearing forces during toolchanges, and a hand-fitted spindle.
Mitsui Seiki builds its own spindles with a clamping system that significantly reduces front-bearing pressure.
In tough economic times, it's tempting to compare bottom-line figures for machine tools and choose the lowest quotation. But for shops — especially those producing high-precision parts – the question, "How much is it?" should involve more than just the sticker price or even the cost of machine repairs down the road. Shops should examine how differences in machine tool construction methods and components impact the quality and cost of the parts made on their machines.
Take a machine bed, for example. It should be made from a heavy,strong, and wear-resistant material that provides a solid, thermally stable foundation for machining processes. Many builders use cast iron, which comes in a variety of chemical compositions and granular structures. The best structures have iron repeatability, which ensures process predictability, and "spring memory," which lets them flex repeatedly to the same position under load and return to the original position when unloaded.
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 once used —storing the beds outside for over a year. The gas-emulsion method provides much better precision qualities, but it's more expensive.
Granular structure also provides stability. Castings that are poured and quickly removed from a mold tend to have a larger grain structure than those cast at lower temperatures (often called a Mechanite casting). With these larger grain structures, the casting becomes harder but not as stable as a finer grain for less flex and bend.
The design of the casting also influences stiffness, rigidity, and thermal characteristics. Traditionally, double-walled, multi-ribbed castings are more structurally stable than those without these features. However, they are also more difficult to design and manufacture. And more costly.
The ways and the means
Way material has a lot to do with a machine's ability to maintain good geometrical characteristics over several thousand hours of operation. The least expensive is the integral way, which is cast into the machine bed and subsequently machines, hardened, and ground. The result is a substrate of cast iron that is softer than the hardened surface on top of it.
The downside to integral ways is that, as the machine shifts and bends, which it does constantly, the hardened material flakes off. As those minuscule, chipped pieces continually rub into the way as the machine operates, they prematurely wear out the machine.
Higher-end machines, on the other hand, generally use ways made from tool steels, which are through-hardened. Tool steels are tough, strong, and do not flake, crack, bend, or break. They are abrasive-resistant, so if a chip gets underneath the saddle and scrapes back and forth during operation, the damage is minimal.
Some builders grind the X, Y, Z, A, and B-axis ways on a surface grinder, depending on how it was constructed, has certain accuracy parameters. The grinder's inaccuracies will be transferred to the ways and could be detrimental to the geometrical integrity of the machine under construction.
Higher-end machines, conversely, have hand-scraped way mounting surfaces. Hand scraping is a century-old construction technique that allows a builder to fit components by hand to meet specific geometric tolerance. Further, it allows the creation of a slight bow in the Z axis, so that when the columns are placed on the machine, compressing the bed, they straighten and flatten the horizontal Z way.
Hand scraping is hard to do, but Mitsui Seiki uses it to achieve under 2 arc sec of straightness in all axes. This would be impossible to do on surfaces that are only machined and ground.
Spindles and ballscrews
Spindle materials should be considered in machine construction. For example, the taper section is often hardened to withstand all the toolchanges over the life of the machine. Case hardening and grinding are typical. However, a hard surface and a softer core open possibilities for future fatigue cracking and increased wear.
The optimum spindle material should be tough and hard, and the ratio of the hardened surface and core should be close. With more expensive forging materials, toughness is inherent. Shops should also look for tool-drive keys that are integral to the spindle (not replaceable keys). Although more costly, this spindle configuration provides up to 40% more stiffness in that taper.
Another issue to think about is releasing the tool. There are powerful pushing forces to disengage the tool, and this pushing puts tremendous pressure on the front-end bearings, which can shorten bearing lift. That's why Mitsui Seiki uses a tool-release system that uses one-quarter of the typical release energy. It relieves bearing forces during toolchanges to increase bearing and 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 speed. High-quality builders are hand-fitting spindles with bearings for specific applications. For example, Mitsui Seiki does not build a standard spindle that runs at a particular rpm for all applications. Instead, it adjusts spindles to the application. For example, a shop machining high-temperature alloys uses stiffer spindles than one cutting aluminum.
Ballscrews are another key to machine performance. Builders should measure them with a dedicated gage for "drunkenness" (the theoretical hall path versus the actual ball path). The actual and the theoretical paths need to be as close together as possible, because deviation causes wear, and wear degrades the positional characteristics of which increase maintenance costs over the life of the machine.
Ballscrew mounting pads should also be hand scraped. That's because these pads must be accurately aligned with the ways. Although many builders machine those pads, hand-scraping is more accurate. The ballscrew nut should follow exactly in the ways' parallel plane, and if it veers off course, the nut preload change creates heat variations. Heat is a thermal inhibitor to accuracy.
Little things matter
Many components may seem too simple to be important, but even they should be made well if the goal is machine uptime. Consider wire. It comes in different grades, including SRMI wire, which has each piece of copper wrapped in fiber, giving it more flex. This is important because wire cost is directly related to flexibility. WIth machine tools, wire flexes all the time. WIre breakage is a down-time condition.
Pushbuttons are a similar case. The better the quality, the more operators can push a button before it has to be replaced. The same goes for most small, miscellaneous items.
In addition, components that cost less up front may 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. High-end builders generally use items that run more efficiently.