Making the Connection
New tool designs and proper choice of toolholder interfaces can optimize milling and turning operations on multitasking machines
By Jim Lorincz
Manufacturers' order books are brimming with a variety of parts produced in low to mid-volume. To produce these components, companies are increasingly adopting machining strategies that feature done-in-one-setup machining on multitasking mill/turn machines.
The concept of done-in-one machining offers the advantages necessary for manufacturers to compete with low-labor-cost competitors, especially from offshore.
- Workholding is simpler, easier, and less expensive for a single setup than fixturing for a number of stand-alone machines dedicated to turning or milling, respectively.
- Total machining and throughput times are reduced.
- Quality and consistency of machining complex parts are improved.
- Work in process is reduced.
Mill/turn machines have allowed us to reduce our in-process work on the shop floor and, at the same time, allow us to have some flexibility so we can respond to a customer's changing needs promptly," says Robert Sweet, president, Hartman Enterprises (Oneida, NY), a high-production contract manufacturer.
Multitasking machines achieve these objectives by integrating the functionality of a machining center and a CNC lathe so that a turn/mill spindle(s) can be manipulated off the centerline (the Y axis) and at an angle (the B axis).
"The combination of these advances has caused many manufacturers to re-think their shop-floor strategy," Richard Sullivan, OEM manager, Iscar Metals Inc. (Arlington, TX) explains. "Previously, these machines were looked upon as great machines for a particular job. Recent advances have allowed manufacturers to look at multitask machines in a completely new light.
"Some companies now look at these machines as production machinery with flexible capabilities. Automotive suppliers are adopting this technology because these machines can deliver cycle time, quality, and complex machining for a wide array of complex part geometries," Sullivan says.
The proper tooling for multitasking machines requires a secure interface with the machine, because of the very different forces that are exerted on rotating and stationary tools. End-users must pay careful attention to the selection process to maximize performance and minimize the investment required.
Machine tool builders typically offer their machines with varying options for the principal tool-system interfaces. These tool systems include Coromant Capto from Sandvik Coromant (Fair Lawn, NJ), KM from Kennametal Inc. (Latrobe, PA), BIG Plus from BIG Kaiser Precision Tooling Inc. (Elk Grove Village, IL), and HSK, the DIN standard.
These interfaces are typically characterized by multiple-contact designs that provide more secure connection, and should be evaluated for their respective strengths, tooling programs, and applicability to both rotating and stationary machining operations.
To prepare for their multitasking machine and tooling choices, manufacturers should:
- Consider the mix (percentage) of milling and turning operations performed in their shops.
- Understand the implications of choosing the spindle interface for tooling requirements.
- Evaluate the compatibility of their existing tooling with the system chosen, and estimate the cost of new tooling.
- Look for multifunction and multistation tooling and modular extensions to satisfy reach and interference considerations in multitasking machining.
"It's important for the end user to determine which function, milling or turning, will predominate, as some parts will sit better on the horizontal bed of a lathe while others will sit better on the spinning table of a VMC," explains Jack Burley, VP, BIG Kaiser Precision Tooling Inc. (Elk Grove Village, IL).
"Turn/mill machining capability requires that tooling systems satisfy operations that were typically done on two different machines," Burley points out. "For turning on multitask mill/turn machines, how closely the insert will repeat to centerline after tool change will determine accuracy from workpiece to workpiece.
"For milling, the most important variable is runout, the ability of the toolholder to maintain very close proximity to the centerline for operations such as milling, drilling, and reaming, all of which impact accuracy and tool life," Burley says.
BIG Kaiser offers both Coromant Capto (as one of Sandvik Coromant's 50 or so licensees), and its dual-contact BIG Plus. HSK for turning is available upon special request. "For maximum rigidity, holders must have both taper and face contact," says Burley. The BIG Plus combined spindle taper and face contact nose design, which is used extensively by Japanese machine tool builders, can accept a standard CAT shank toolholder or a dual-contact BIG Plus toolholder. "For turning operations, we have developed a new standard which we call BCV40Y for Y-axis machines, which holds the drive key to a much closer tolerance for turning operations," Burley explains.
The three critical factors for multitasking tooling are rigidity, multi-functionality of tools, and total tool usage, explains Timothy E. Fara of Kennametal Inc.
"First and foremost is rigidity. When you have a spindle that is going to rotate for milling and also act as a stationary tool for turning, it requires a coupling mechanism like KM that has three-point surface contact; face contact, gageline contact, and taper contact," says Fara.
"So whether I'm using a large face mill or operating in the turning or boring mode, I have the strength to hold my tool rigidly. The reason is that I'm no longer clamped by a bolt. I'm clamped by a holding mechanism that is pulling back on my tool or a drawbar, so I've got to have maximum rigidity to get the true functionality of the machine.
"Secondly, you want your tools to be multifunctional; for example two, three, four, five, six different tools on a single head to make it a miniturret such as our Multi-Station-Tool. I can simply index it using the orientation of the B-axis spindle. I can do turning operations or boring operations. I can go from a rough turn to a finish turn, index over to a grooving insert and come back with a turning insert, and be able to do all that without going back to do a tool change, eliminating the travel time required to get a tool.
"We have also proven through laboratory testing and experience in the field that, if I can take that head, the B-axis spindle, and turn it at a 45° angle and drive the tool straight across the workpiece, all of my cutting forces go straight up the centerline of that spindle. This produces the maximum possible rigidity, so that I have the highest metal removal rate compared with the spindle being at a 90° angle, where a lot of my radial motion is pushing across the part." Using a centerline cutting unit eliminates the torque component on the connection joint and spindle and lessens the effective feed forces on the tool coupling.
"The last thing is total tool usage. I have turning tools as well as boring tools going into the B-axis spindle, but I'm going to have rotating tools, be it a drill, tap, end mill, or face mill, and I've got to able to handle that face mill. If I have a 6" [152.4 mm] or 8" [203.2-mm] diam face mill, I need to be able to travel over center at least 4" [101.6 mm], so that my face mill clears the piece part when I'm milling. That's what multitasking machines are doing today. They're allowing a lathe to go over the centerline in machining," Fara concludes.
Sandvik Coromant (Fair Lawn, NJ) works with all the major builders of B-axis multitasking machines to design spindle solutions that integrate its Coromant Capto modular tooling system. To this end, Sandvik Coromant has developed a nitrogen-charged gas spring clamping design that is lighter and smaller than its existing spring pack design. In addition, the units have improved balancing capability when used in high spindle speed applications.
James Grimes, product specialist for Coromant Capto spindle integration, describes some of the strengths of the Coromant Capto system. "Since its introduction in 1991, every Capto tool has the same coupling. Tools purchased over 15 years ago will still fit on today's multitask machines. The same rigid, polygon shape coupling is used throughout the wide assortment of tools, ranging from collet chucks and adapters to boring bar holders and drills.
Sandvik Coromant has developed a group of combination tools called CoroPlex which are specifically designed for B-axis multitasking machines. "The Coroplex TT (Twin Tool), for example, is actually two turning tools in one, that can be indexed 180° for a fresh edge. Another version of the CoroPlex TT has two different inserts for performing two different operations simply by indexing 180°. "With these tools, tool change time can be greatly reduced, resulting in lower total cycle times.
The Coroplex MT is another example of a tool designed specifically for multitasking multifunction use. "It's a two-flute, square shoulder end mill with two turning inserts. The two milling inserts are 180° apart on a larger diameter than the turning inserts. This allows it to be rotated and used as a milling tool or locked in a stationary position and used as a turning tool," Grimes says.
Grimes says, "The new Coroplex combination tools provide end users the benefits of completing parts in one setup, reductions in tool change times on simple turning operations and reducing the number of positions required in the tool magazine. That's an especially big consideration for customers who have a limited number of tooling positions available, when considering the large number of different tools that can be used in the multitask machines."
Sandvik Coromant has also introduced a longer milling tool, the 390 end mill with an antivibration dampening device inside. This tool is also well-suited for B-axis machines that usually require longer tools to avoid interference between chucks and holding devices.
Challenges for cutting tool manufacturers arise from two basic issues unique to multitasking machines— cutting tool force and reach/interference issues, Iscar's Sullivan says. "Multitasking machines typically will use the spindle and/or subspindle to hold the workpiece. The versatility of the B axis combined with the orientation of the C axis and some intricate part geometry can often create compromised part rigidity, as well as interference issues," he notes.
Iscar has developed several products to address the cutting-tool-force issues. Three of the most popular "problem solvers" are Finish-Shred, MillShred, and FeedMill. Finish-Shred is a solid-carbide end-mill that incorporates offsetting flutes of rough and finish design, generally reducing cutting forces (due to the two roughing flutes). Iscar says this design also tends to eliminate vibration, delivering a finished-part-quality surface in one pass. MillShred, which is available in face and end-mill styles, incorporates a serrated button-style insert to reduce cutting pressure while producing large metal removal rates. FeedMill is a tool that uses an exaggerated radii incorporating high feed and shallow depth of cut to produce high metal removal rates. The typical result is to reduce the overall cutting forces, and send most of the residual forces in an axial direction instead of a radial direction.
"Multitasking machining provides end users with more degrees of freedom in terms of orientation of the axes and how tools can be brought to bear on the workpiece," says Gregory A. Hyatt, VP and chief technical officer, Mori Seiki USA Inc. (Rolling Meadows, IL).
Mori Seiki has a vested interest in developing optimized tooling for its multitasking mill/turn machines. "In some cases we are struggling with what to call them because they're not turning tools as we know lathe tools and they're not milling tools as we know milling tools. They're completely new tools and we are going to have to coin new names for them and write new manuals for guidelines for cutting conditions and how to apply them," he says.
Mori Seiki and Kennametal are jointly developing a lathe tool for performing turning operations that rotates like a drill or an end mill so that both the workpiece and the cutting tool are rotating. This spinning tool is in the latter stages of development and is in field trials, though not yet commercially available.
"Using the spinning tool is a continuous chip-making operation like conventional turning but because the tool is turning, a different portion of the button insert cutter is engaged with the workpiece, millisecond by millisecond. Any one point on the circumference of the tool is only engaged and making a chip for 1 to 3 milliseconds. Heat is distributed around the circumference of the tool so existing cutting tool materials and grades can be used and run at two to three times the recommend cutting conditions without compromising tool life," Hyatt explains.
Mori Seiki is also developing its NitroMilling compound machining tool that looks like a milling tool but performs turning operations. "The turning operation is performed with a multi-insert tool on a workpiece that may be spinning one or two thousand sfm underneath the milling tool. The result is absolutely positive chip control as with milling and none of the birds nesting associated with some of the tough stringy materials. The result is longer untended machine operation because tool life is extended in proportion to the number of inserts in the tools," he says.
This article was first published in the April 2007 edition of Manufacturing Engineering magazine.