Machines Evolve in One Setup Processing
As technology advances, the distinction between mills and lathes has become clouded
By Jim Lorincz
The evolution of CNC machining over the years has blurred the distinction between multifunction CNC lathes and mills. Today, instead of being called turning equipment, these versatile multifunction machines are more commonly referred to as mill/turn centers or multitasking machines. Whatever the nomenclature that is adopted, however, the various styles of machine are designed to produce parts in single setup in volumes that can range from high-mix, low-volume to low-mix, higher-volume production.
Walt Bachmann of Exsys Tool Inc. (San Antonio, FL), explains how different configurations of machine and tooling connections enable milling machines and lathes to increasingly resemble one another’s capabilities:
"Constant innovations from single spindle and one turret to main and subspindle with multiple turrets have clouded the difference between a lathe and a milling machine. Older CNC lathes used only the main spindle requiring a second operation for the unmachined other end. Today the subspindle picks up the part from the main spindle where it can be machined complete in one operation. On newer equipment both main and subspindles have C-axis control, programmable to a fraction of a degree for machining hole patterns or contouring. But this required many tools and, therefore, the turrets had to accommodate several tool stations, or the machine needed more turrets, so the front end of the part could be machined with one turret, while the other end was machined by the second turret.
"Today, we still have single-turret machines with slots for fixed turning tools, but the trend has been to new turrets which accommodate many live tools, which use rotating drills, end mills, and taps, similar to the older two-axis milling machine. New turret designs incorporate up to 24 or more different tool stations, where all of the tool stations accept live tooling. Multiple tools attached to one station increase capabilities of single turret machines. Many machines incorporate several turrets which can be used simultaneously for both main and subspindle work. Tool carousels are incorporated to accept up to 80 tools, reducing tool change time and setups," Bachmann says.
Just as turret configurations are changing, so too is the turret/tool holder interface. Bachmann explains: "At the end of the 1980s, VDI tooling became the quickest method to change toolholders, although it still took quite some time to accurately indicate a radial holder to be on centerline. The trend is bolt-on-mount, BMT, in various sizes, BMT 45 to BMT 85. It allows for more precise mounting, repeatability in the low microns, and more rigidity for heavier cuts. The shaft diameter of the toolholder does not act as locator anymore. Four adjustable keys are used in a patented solution by my company."
Bachmann points out trends that will continue to increase machine capability all without increasing manpower at the machine:
"These turning centers with live tooling capability are similar to milling machines by using X and Z axes for standard turning, C and Y axis for milling and contouring, and a B axis for angular drilling. The latest machines incorporate simultaneous five-axis milling capabilities.
"New turret designs incorporate up to 24 or more different tool stations, where all of the tool stations accept live tooling."
"Spindle speeds of main and subspindles have increased to over 12,000 rpm and turrets can drive live tooling to 18,000 rpm. For heavy-duty milling up to 25 hp [18.6 kW] is available at the turret. Feeds and speeds are increasingly getting faster. Linear drive motors allow rapid feeds up to 120 m/min. Three turrets may be simultaneously in the cut on B and Y-axis machining. Live and fixed tooling incorporate multiple tool pockets to increase the amount of tool stations available. Double tools allow for rear and front machining.
"Newer machines allow the use of half stations on the tool indexer, i.e., the tool stations double on a 12-station turret by indexing half between stations. In other applications, the Y axis can move a quadruple toolholder up or down, so the stations can be used for four separate applications, i.e., four separate tools, live or fixed.
"Gear hobbing has come online, as some machine tool builders have improved their synchronized software between spindle and turret to accommodate multi-teeth hobs at various pitches for cutting spur or helical gears, and splines. The B axis on a turret has taken the place of angular adjustable mill/drill units on some machines.
"Vertical and horizontal mill/turn
centers, others now call them
production centers, will incorporate
more uses for gantry loaders,
and integration of robots into
"As the mill/turn centers become ever more sophisticated, we will see more machines with special features, some of which are already available, such as vibration control, heat displacement control, voice programming, and patented machine interference protection. Vertical and horizontal mill/turn centers, others now call them production centers, will incorporate more uses for gantry loaders, and integration of robots into flexible work cells.
"With the advent of finite element analysis programs, better machine beds can be developed to increase rigidity, accuracy, and repeatability. Diagnostic programs will alert the user to perform maintenance or error detection. Sensor technology will identify best machining conditions, reduce heat and vibration for increased tool life.
"The trend to improve and enhance machining operations will become increasingly faster and more sophisticated. Today’s single-turret machine will have two turrets tomorrow, the two-turret machine will have three or four, or a carousel with a milling machine type toolchanger. A fixed turret will become flexible in two or three axes. More machine tool builders will incorporate the BMT interface in place of the older VDI type. Higher feeds and speeds will reduce machine cycles and unit cost. Robots will load and unload parts, and controls will diagnose machine conditions and alert the operator or self correct." Bachmann concludes.
Exsys/Eppinger designs and manufactures fixed and live tools for almost all available mill/turn centers. Dual station holders and quadruple tool stations have been developed for various lathe tool manufacturers. Special BMT turret interfaces, and compensation couplings have been designed and patented by Exsys/Eppinger. The patented PreciFlex system, an adapter system that reduces setup times, changeover times and through its rigidity allows faster cycle times, has been incorporated as standard equipment by some of the world’s largest machine tool builders. ME
For more information from Exsys Tool Inc., go to www.exsys-tool.com,
or phone 352-588-4345.
This article was first published in the September 2012 edition of Manufacturing Engineering magazine. Click here for PDF.