by Kip Hanson
Industry experts offer advice on high speed machining
Good machines, good software, good cutting tools. High speed machining (HSM) is like a three-legged stool. You might think you can buy the right chair and the rest is gravy. Not so fast. While it’s clear that HSM needs top-notch technology, dropping a bunch of cash on the latest and greatest equipment won’t make you a high speed master. It also takes a good deal of know-how.
But what is HSM? The truth is there’s no industry standard. Most experts agree, however, that HSM removes material faster than conventional methods. It utilizes feedrates two to three times higher, together with light depths of cut, often less than 10 per cent of the tool diameter. And as the name implies, HSM uses high spindle speeds—40,000 rpm or more is not uncommon.
The first leg of the stool
Ironically, many in the industry think this last point—spindle speed—is the defining factor for HSM. Far more important is a spindle with broad torque capability across a wide rpm range. HSM can be used in a variety of machining scenarios—mould/die work with its intricate surfaces and tool steels up to 62 Rc is a prime candidate. Another is aluminum wing spars, struts and other aerospace components. Buying a machine with the proper rpm range is critical.
Steve Bond, national sales manager for Fanuc RoboDrill, RoboCut and EDM Products at Methods Machine Tools Inc., Sudbury, MA, says a 40,000 rpm spindle by itself won’t help you machine parts any faster. “Spindle rpm is relevant to the type of part you want to cut.” For example, a 20,000 rpm spindle might be fast enough for hardened tool steel, whereas 50,000 rpm or more is warranted when machining with cutters the size of a human hair. “It really comes down to tool load. If the cutter can’t remove any more material given a higher rpm spindle, then you won’t gain anything by buying one.”
More important is the machine tool itself. With HSM’s extreme feedrates, and cornering faster than the Canadian AutoSlalom, vibration and machine harmonics can be a concern. Good dampening capability is a must—some builders opt for a polymer composite base or even one made of granite, lending rigidity and thermal stability to the machine tool.
Bond explains that a high speed machine must be capable of executing programmed axial movements rapidly, accurately and consistently. To do this, a fine pitch ballscrew is needed. Some commodity machine tool builders achieve high rapid traverse rates though the use of wide pitch screws. While this certainly makes the machine axes move farther per rotation of the servo motor, thus achieving faster rapids, it also reduces the accuracy of the motion control system overall. “When you look at higher end machines, you’ll find the class of the ballscrew goes up. Without this, you won’t have the ability to make small movements accurately.”
Another consideration is acceleration and deceleration. In our racecar example, HSM is no different than Jacques Villeneuve slowing down around the curves and punching it on the straightaways. If you can’t do that, you’ll lose the race. And like a racecar, machine tools have a great deal of inertia, which must be controlled by responsive, powerful servo drives that take commands from the machine controller and execute them in microseconds.
A large part of this depends on good look ahead—machine controllers “look through the corner” by buffering large blocks of motion, calculating where they’re going well in advance of arrival. Bond says 200-block look-ahead is standard on many controls, but that upgrading to 1000 blocks is advisable for HSM. “This is very typical for any of the surfacing done in medical and aerospace shops.”
Good tools, good holders
Greg Pozzo, VMC group leader at Makino Inc.’s die/mould technology centre in Auburn Hills, MI, agrees quick program execution and good servo control is a critical piece of HSM. “The ability of the control to process information is very important, but you have to weigh that against how fast your machine can make those moves accurately. Electronics are great, but if you don’t have a good mechanical system first, then you’re going to have workpiece inaccuracies and bad surface finish. You must have all the necessary pieces for HSM, otherwise you’ll overshoot or undershoot the programmed position, and end up gouging the workpiece.”
One of those pieces is the cutting tool. Cutter runout is like an out of balance tire on the Autobahn—if you’re running 30,000 rpm, a low quality tool will surely put you in the ditch. This can mean poor surface finish, struggles with part tolerance, and rotten tool life. Look for a supplier that guarantees form and flute dimensions. Ask for a copy of their tooling specs and a drawing of the tool, and verify they are meeting those specs. Whatever else, don’t cheap out on end mills and drills, or the $250K you just invested on a high end machine tool will be wasted.
No less important is the toolholder itself. Pozzo says HSK toolholders are the only way to go. “All of our die/mould machines come with HSK spindles. In our opinion, it is a far superior solution.” Pozzo cites a host of advantages compared to conventional CAT or BT toolholders, including less mass, better runout characteristics and a two-point interface that provides greater rigidity than other systems. “The retention knob on a CAT holder is held with a set of fingers external to the toolholder. The faster you turn the spindle, the greater the centrifugal force, which tends to pull those fingers apart. Depending on the cutting forces at that moment, this will actually draw the tool farther into the spindle, or allow it to pull out and possibly damage the workpiece.”
Aside from the HSK interface, Pozzo recommends hydraulic toolholders for HSM roughing operations, and shrinkfit for everything else. “Shrinkfit has superior runout characteristics and radial stiffness. This is going to make your tools last much longer and provide better surface finish. The downside to shrink fit is that, on roughing operations, or even heavy semi-finishing cuts, the interface is so rigid that it can transmit vibration back to the spindle. In this case, a hydraulic mill chuck would be a better option. It has the damping characteristics needed for aggressive HSM cuts, but still holds the tool without the risk of pullout.”
Lastly, Pozzo says you might need a friend. “Without a reliable and experienced engineering or supplier partner to guide in the application of this technology, the transition to HSM could be a long and winding road, one that’s filled with a lot of choices or testing to prove out a process. Having a partner with application resources and training classes in proven processes can be key to a successful implementation of high speed machining technologies.”
The right path
CAM software is the third leg of HSM. When driving cutters through complex geometries at high rates of speed, you need smooth and consistent G-code. This is especially true with microcutters—a 0.5 mm tool won’t tolerate abrupt directional changes and varying chip loads without giving up the ghost. Accurate toolpaths are another must-have. Eric Ostini, product manager for GF Machining Solutions, Lincolnshire, IL, says HSM lays bare the good, bad and ugly of CAM systems.
“One of the things that people see when they switch to HSM is that any little hiccup in their programming manifests itself on the workpiece. On older or less capable machines, they could get away with less accuracy in their G-code because the machine tool would wash small variances away. The control can’t handle the speeds, so it just takes the two points and says, ‘okay, I’m just going to take a shortcut here.’ So the customer would never see that dimple or sharp corner present in the CAD model. But with HSM, the superior structure of the machine tool and servo systems means you get exactly what was programmed.”
Another consideration is step-over. HSM is perfect for shops looking to reduce secondary polishing and EDM operations. Ostini said this is achieved through toolpaths with very small step-over amounts, typically 0.00254 mm (0.0001 in.) less. “You can actually eliminate polishing with this approach, even in the high Rockwell materials used in mould and die work, medical and surgical tools, and so on.” The take away here is that you need a CAM system that generates smooth, accurate toolpaths, and is capable of the fine resolution required for complex geometries and close tolerance work.
The Game Changer
Ostini says HSM is changing the face of machining. For starters, EDM die sinking is rapidly becoming a niche market. “You still can’t do square corners or very deep ribs without EDM. You can’t machine carbide. But for everything else, HSM competes very well in terms of surface finish and tolerance, and is certainly more productive than EDM.”
If you’re ready to pursue a high speed machining strategy, consider this—properly executed, HSM puts more parts on the bench in less time than conventional machining methods. Tool life is better and costs go down. But you’re going to need a few things to get started:
•A high quality machine tool capable of lightening fast interpolation and rigid enough to take demanding cuts.
•Top-notch toolholders, preferably HSK or equivalent, loaded up with premium grade cutting tools. Don’t cut costs on this critical link.
•CAM software to support the two. Smooth, accurate toolpaths are a must, as well as ultra-fine programming resolution for micro work.
Then prepare yourself to climb the HSM learning curve—it might be a steep one. HSM defines a new paradigm in machining technology, one that requires more than good tools, good machines, and good code. It needs common sense and a willingness to embrace new ideas. Are you up to the challenge? SMT
Kip Hanson is a contributing editor. [email protected]