Power generation components are enormous compared with the automotive and aerospace parts that I worked with before coming to Mitsubishi Hitachi Power Systems Americas Inc. The machines shown to me during my interview there were much larger than anything I had used. I had seen photographs on the internet of big machines, but photos mask the true scale of the components and the machines used to make them.

It is easy to be intimidated by the size of power generation manufacturing. Several questions entered my mind as I realized what I faced: How do I hold these parts? Do they act differently from small parts? How do I estimate cycle times? What cutting tools do I use? The answers that came surprised me.

Ask and Receive

Most of our turning is done on vertical turret lathes. Unlike smaller turning centers, large machines typically do not have self-centering chucks. Most of the time, we use chuck jaws mounted to a machine table, which creates the same type of setup as a four-jaw chuck on a small toolroom lathe. Each jaw moves independently, allowing a machinist to accurately align a part before machining. Part geometry occasionally does not lend itself to jaw use, in which case we clamp the part in place with threaded studs and strap clamps, much like using a faceplate on a manual lathe. (Only old-timers remember using a faceplate with clamps.)

Gas turbine rotors can weigh 64 metric tons (70 tons) when fully assembled. The largest lathe at Mitsubishi Hitachi Power Systems Americas is rated at 200 metric tons (220 tons). Image courtesy of C. Tate

Holding parts on a large horizontal boring machine is no different from holding parts on a small machining center. When we can justify the expense, we make dedicated fixtures that mount on a table and provide clamping and locating elements for a part. In other situations, it is not feasible to make a fixture, so a part is mounted directly on a machine table and clamped with T-nuts, threaded studs and strap clamps like those found with a knee mill in a toolroom.

Much like with workholding, cutting tools and toolholders used on large machines are not different from those used on small machines. Our horizontal boring machines all have CAT 50 spindles, and milling toolholders use ER-style collet chucks, hydraulic chucks and some shrink-fit holders. Many cutting tools surprisingly are small relative to the size of parts; most of our tools are 25 mm (1") or less in diameter. However, 500 mm (20") boring heads, 100 mm (4") drills and 75 mm (3") taps are not uncommon with horizontal boring machines.

Much like with milling, turning tools used on big lathes are small relative to part size. All lathes at Mitsubishi Hitachi Power Systems Americas utilize Capto C8 toolholders with various types of inserts in common shapes and sizes. With many of our parts, chatter can be a problem because lathe tools frequently have long extensions. Capto tools provide the option to use vibration damping tools, and the Capto interface gives support in multiple planes. Both features help eliminate chatter. Capto also supplies an exceptionally repeatable connection, so we do not need to remeasure tools each time they are changed.

Big parts probably have big tolerances, right? Not really. On the loose side, we are allowed ±0.15 mm (0.006"), which may sound like a lot until considering that some parts can be 3.7 m (12') tall with features 3 m (9.8') apart. On the tight side, we hold some 850 mm (33.5") shaft diameters to +0/-0.01 mm (+0/-0.0004"), which can be difficult for small parts. To achieve these tolerances, we spend large sums of money on calibration to ensure that machines are repeatable and accurate.

Casings for gas turbines are enormous. The overhead cranes are rated at 226,796 kg (500,000 lbs.) each. Image courtesy of C. Tate

Small parts are subjected to distortion from clamping, stress relieving and aggressive machining. Big parts are worse. They are subjected to the same issues but have an added challenge: weight. Large parts can distort under their own weight, causing problems like bowed shafts or out-of-round rings. Some machining processes start with a "slow roll" in which we allow a sizable shaft to rotate for an entire shift to remove a bow before we machine. In some cases, like with a ring that has a thin section relative to the diameter, we must perform stress-relieving heat-treat operations before taking a part to a machine. This prevents movement during machining operations that require big ovens. Our largest oven is 10 m (33') deep and 4 m (13') tall — almost spacious enough for a semitrailer.

Estimating cycle times is similar to estimating for small parts. To calculate cutting times, we use our CAM software the same way that a user would for small parts, but that is where similarities end. Small parts are loaded by hand, and handling times are only a few seconds. Our handling times for big parts are measured in hours. It can take two hours to flip a large cylinder and three to four hours to move a gas turbine rotor from one end of the shop to the other. It is necessary to account for all material handling or else cycle time and lead time estimates can be wrong by multiple shifts.

All in all, machining large parts on large machines is not very different from machining small parts as long as the few distinct differences are recognized. The primary difference is simply size.