Big parts, big questions: Turning Performance

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.