- March 30, 2015
Turn mill operators are faced with a bewildering number of tooling choices
Tooling up a CNC lathe was once a straightforward exercise. Mount and touch off an 80° diamond for roughing, along with whatever profile of finishing tool you fancy. A groover and threader might be needed, and since most turned parts have holes, a drilling station is called for, along with a boring bar or two to finish the hole. And if the machine has a barfeed, you’d best grab a cutoff tool. Allowing for differences in hole size, groove widths and so on, this basic tool assortment once covered the majority of all turning jobs. Lathe life was simple. One day some big-brain machine designer decided live tools on a lathe would be just the thing. Suddenly all of us turning guys (and gals) had to start thinking like those mill folks. Strange new cutting tools such as slotting cutters and face mills appeared on the bench. Newfangled taper shank toolholders and complex milling routines became known to us, along with the feedrate and rpm conversions needed to program those spinning tools. When machine builders started putting automatic tool changers (ATCs) on the backs of lathes, and added multiple turrets and sub-spindles, along with the A, B, C, and Y axes to operate them, the simplicity of two axis turning became a nightmare of flexibility. Welcome to the world of multi-tasking.
John Winter, product application specialist for Sandvik Coromant Co., says there’s no need to be scared, but adds that a little common sense goes a long way when tooling up a multi-tasker. “Some shops get so excited about multi-tasking capability that they don’t stop and ask what the machine is for. If it’s lights out production machining, the tool magazine might be loaded up with redundant and multi-function tools. Low volume and prototyping work, however, may require a broad collection of general purpose tools to handle whatever comes through the door.”
Clearing the bar
Regardless of production needs, clearance is often a defining factor when choosing a holder. When tools are no longer fixed in one location, banging into the chuck or bumping a turret becomes a real concern. Machines such as DMG MORI’s NTX series, the Okuma Multus and Mazak’s Integrex have replaced the tool turret—a mainstay of NC lathes since the days of punch cards and high speed steel tool bits—with a swiveling, multi-purpose toolhead that holds both turning tools and milling cutters..
Tooling manufacturers have done an admirable job of addressing the unique toolholder needs of these machines. For example, Sandvik Coromant has an entire section of its turning catalog devoted to multi-task machining. Its CoroPlex MT is a combination milling/turning tool that can mill a face or shoulder as well turn or even bore. The CoroPlex TT offers two turning positions, and takes advantage of a multi-tasking machine’s ability to shift tool centreline and rotate the cutting tool 180°, thus allowing for roughing on one face of the toolholder and finishing on the other. Similarly, the CoroPlex SL mini-turret has four cutting positions, making turning, threading and grooving possible from a single toolholder.
If you’re having a tough time visualizing how these tools work, picture yourself standing in front of a two axis lathe. Now remove the turret and replace it with a milling spindle that moves left and right (the Z axis), up and down (the X axis), and in and out (the Y axis). Now, reach out and tip that milling spindle like the hands of a clock up to 100° or so in either direction—that’s the B axis.
Here’s the cool part—switching from a milling operation to turning is simple: call up a tool from the magazine, orient the spindle to one of a handful of fixed positions, and start cutting. Using one of the multi-purpose toolholders mentioned earlier, you might rough the part, rotate the tool 180° to finish turn, then re-engage the spindle and mill a shoulder, all without the same tool.
And since these B axis machines can rotate the toolhead to one position, take a cut, then rotate to a different position and take additional cuts at a different angle, a little clever programming can make cutting tools more versatile. Winter gives a recent hard turning application as an example: “We were using a round insert to rough a very large shaft for a steel mill, where the insert only lasted for half the cut. Using the B axis, we were able to pull back, index the head to get a fresh edge on the insert, and then continue turning. It extended tool life considerably.”
BIG Kaiser Precision Tooling Inc., Hoffman Estates, IL, is another company serving the needs of multi-tasking machines. Engineering manager Alan Miller says some unique things can be done with these machines that weren’t possible before. Aside from the ability to mill or drill at practically any angle, multi-taskers are often equipped with a lower turret and a second turning spindle, opening the door to multiple simultaneous operations.
One of these is pinch milling. “This is where you use a lower turret milling head together with the B axis main spindle to machine two sides of the part at the same time,” says Miller. “This works especially well for long, thin parts—the cutters actually support the workpiece from both sides simultaneously. It’s pretty impressive to watch, even though it’s a bit difficult to program.”
BIG Kaiser offers a wide range of tooling for mill/turn machines, including its BIG Capto system, designed specifically for B axis multi-taskers. This system is available in Type F and Type S toolholders, which, according to Miller, have a distinct difference. “Type F right angle toolholders are similar to traditional turret style tooling. They’re designed to be oriented at 90°, or perpendicular to the Z axis. You can even use stick tooling as you would with a conventional turret. However, this is not ideal for a B axis machine.”
Because a B axis milling head relies on the spindle motor to orient and hold the tool in place during turning, off-centre cutting forces tend to fight this orientation. With Type F holders, which often position the tool tip two inches or more (50 mm) off spindle centreline, there may be enough torque to deflect the tool during heavy cutting. Also, a Type F toolholder’s right angle alignment creates more potential for interference. This can be offset somewhat by extending the tool enough to clear the workpiece and chuck, but this in turn leads to a less rigid setup. In either case, Miller suggests that light cuts are the norm with Type F holders.
A better approach is a Type S 45° tilt style toolholder. As the name implies, these are designed for use with the B axis rotated to 45° although other angles are possible, clearance permitting. Regardless of the B axis angle, Type S holders maintain perfect alignment between the cutting edge and the centreline of the milling head, minimizing deflection. “The S type is actually the new version designed specifically for these machines. Since the upper toolhead is also a rotating spindle, it’s not as rigid as the old turrets were. Type S keeps the cutting edge on center, directing all forces against the spindle bearings. That’s where these machines really start to perform at their highest potential", says Miller.
These are just a few examples of the tooling possibilities on a multi-function lathe. Each tooling provider has its own solution, including traditional stick tools, quick change Capto and KM style toolholders, HSK and a variety of other short taper holders. Considering the many adapters used between these different systems, literally hundreds of tooling combinations are possible. It’s a far cry from the simple and straightforward tools used on a tape controlled two axis lathe, but with complexity comes flexibility, less part handling, and greater part accuracy. To an old lathe guy, it seems like a good tradeoff. SMT
Kip Hanson is a contributing editor.