Turning is one of the oldest material processing processes in mankind. Evidence of lathe work has been found in Egyptian tombs and other ancient archaeological sites. Interestingly, some of the most compelling innovations have been applied to the turning process of metal processing in the past few years. Real-time tools, C-axis, Y-axis, tool changers, vertical spindles, etc. can be found on many machines currently available.
In some cases, it is difficult for the industry to define some of these new machine tools. Are they a lathe that can mill, drill and tap, or a machining center that can rotate? In many shops, these advanced technology machines have been successfully applied to their workpieces.
There are also many applications where what appears to be old-fashioned technology is the best way to produce parts. An example of this class of technology is the traditionally designed VTL (vertical lathe). While its basic form has been around for some time, VTL hasn't stopped evolving. The new versions of these machines incorporate many of the productive technology innovations found in horizontal turning centers.
Most VTL workpieces are discs of varying sizes that will ultimately hold the turbine blades on gas turbine engines. Many different major components from one of the world's largest turbine engine suppliers rely on VTL.
One of the main applications for gas turbine engines is the back-up application for generating electricity. As the peak demand for electricity reaches higher and higher levels, US power plants are looking for auxiliary energy sources to meet the additional demand.
Like most turbines, a gas turbine is functionally a relatively simple machine. The fuel is burned in a combustion chamber and the expanding gas passes through a series of blades, rotating them like air in a fan. Speeds and temperatures are very high in these engines.
VTL machines may have fabrication of shafts and discs used in turbine engines. The material is chrome molybdenum steel. The blanks are pre-machined forgings.
The order goes to the store and is processed as a set. All essential parts for your gas turbine are processed in-store right away. Scheduling requires coordination between different machining departments so that all work goes through the entire shop at the same time.
What do you call a modern VTL? The historical difference between vertical boring machines (VBM) and vertical turret lathes has been blurred. Traditionally, VBMs have had one or two slider heads to deliver the tool to the workpiece. The indexable tool turret distinguished traditional VTLs from VBMs.
In addition to the turret or ram head, both machine designs use a turntable to support the workpiece and use a bridge-type design that carries the X axis. The piston passes through the X axis guides and supplies the cutter.
Numerous modern machine designs combine towers with actuators, and some have a boring ram mounted next to the turret head. The traditional distinction between the two types has lost its meaning.
An important advantage of the VTL machine is the ease with which large heavy workpieces can be positioned and held in place for processing. The advantage of any store using VTL is that gravity is an important factor in holding these parts. Generally only minimal hard fixation is required. Some blanks weigh up to 20,000 pounds.
Parts are held in place with relatively light clamps. This is preferred in applications where too tight fastening could warp the turbine disks. There is no need for flat face tables on machines due to the runout requirements of the workpieces. Instead, they are able to get independent four jaw tables on VTL machines.
Like many shops looking for high turning performance, some have switched to the used machine market in the past. Rebuilt and reconditioned VTL can be of good value.
The prices of the new VTL have become competitive with some used and rebuilt machines. In part, this was due to Asia's economic problems. Also, the collapse of the Soviet Union made castings and other basic materials available to builders at very discounted prices. Both of these factors contributed to lowering the prices of this class of VTL machines.
But buying inexpensive components is not enough to make a good machine. The design considerations sought are of course in line with the company's requirements for rotating turbine discs and other major gas turbine engine components.
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