b'How is machining large parts different from machining small ones?By Christopher TateP ower generation components are enormous compared with theautomotiveandaerospace parts that I worked with before com-ing to Mitsubishi Hitachi Power Sys-tems Americas Inc. The machines shown to me during my interview there were much larger than any-thing I had used. I had seen photo-graphs on the internet of big ma-chines, 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 manufac-turing. 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 ReceiveMost of our turning is done on vertical turret lathes. Unlike smaller turning centers, large machines typically do not have self-center-ing chucks. Most of the time, we use chuck jaws mounted to a ma-chine table, which creates the same type of setup as a four-jaw chuck on a small toolroom lathe. Each jawGas turbine rotors can weigh 64 metric tons (70 tons) when fully assembled. The largest lathe at moves independently, allowing aMitsubishi Hitachi Power Systems Americas is rated at 200 metric tons (220 tons).machinist to accurately align a part before machining. Part geometryfrom holding parts on a small ma- clamps like those found with a knee occasionally does not lend itself tochining center. When we can justifymill in a toolroom.jaw use, in which case we clamp thethe expense, we make dedicatedMuch like with workholding, cut-part in place with threaded studsfixtures that mount on a table andting tools and toolholders used on and strap clamps, much like usingprovide clamping and locating ele- large machines are not different a faceplate on a manual lathe. (Onlyments for a part. In other situations,from those used on small machines. old-timers remember using a face- it is not feasible to make a fixture,Our horizontal boring machines plate with clamps.) so a part is mounted directly on aall have CAT 50 spindles, and mill-Holding parts on a large horizon- machine table and clamped withing toolholders use ER-style col-tal boring machine is no differentT-nuts, threaded studs and straplet chucks, hydraulic chucks and 70AUGUST 2020 MachiningLargeParts.indd 70 7/17/20 3:40 PM'