Tooling for Low-HP Machining
Choose the right tool to optimize operations
By Konrad Forman
National Milling Applications Manager
Ingersoll Cutting Tools
It's no secret that the main trend in job shops over the past decade has been toward CNC machining centers that are faster and smarter, but are also lighter and have lower-horsepower spindles. Today's rising energy costs have simply accelerated this trend. This is a complete departure from the use of muscular machines that are able to take very deep cuts in a single pass. Inevitably, High-Speed Machining (HSM) also means Low-Power Machining (LPM), which requires different tooling—and different thinking about that tooling.
More recently, rising energy costs and concern about global climate change have given the trend new urgency. Many manufacturers have seen their utility bills rise by 25% or more in a single year. And in response, we're beginning to hear of company-wide policy commitments to "green" manufacturing and "low-energy" manufacturing. While not saying so publicly, many more manufacturers are quietly but deliberately seeking process changes on an enterprise-wide basis that expend less energy to do metal removal. Tooled properly, low-power machining can make a meaningful contribution to this effort.
Most leading tooling providers have responded to the HSM/LPM trend by developing lines of HSM-rated tooling, or adding spindle rpm ratings to the labels of their tools. Some tooling suppliers have moved beyond this step. Reason: the HSM or spindle-speed rating is good as far as it goes, and necessary for safety in high-speed spindles, but it doesn't go far enough. By itself, the HSM or RPM rating simply means that the drill or cutter is balanced well enough to run true in a 12,000 or 40,000-rpm spindle, and that the inserts will stay put in the cutter. It says nothing about the tool's machining efficiency, which is the key to saving energy and protecting a lightweight machine frame.
Insist on the HSM or rpm rating, certainly, but look beyond it. You will find wide differences in the efficiency—and the power efficiency—among the mills and drills available today for HSM work. Those differences are especially important in rough or one-cut milling and large holemaking.
Let's look first at the anatomy of a typical high-speed CNC machining center and how it differs from a conventional machine. Certainly it is fast, with rated spindle speeds to 40,000 rpm or more, and with extremely high feed rate capabilities. It's smart, usually with controls able to execute interpolations, toolpath optimization, and varying degrees of three to six-axis machining. One of the downsides, though, is that the spindle motor may be rated at only 20 hp (25 kW) or less. The other issue, often overlooked, is that the frame is very light, thus more prone to deflection and vibration. Often, in fact, the limitation on material-removal rate is frame rigidity, not spindle horsepower. The machine as a whole, not just the spindle motor, is designed to make many light, quick cuts, rather than a few deep passes.
From a tooling standpoint, one key to efficient low-power machining is to instantaneously heat up the cutting area to soften the metal being cut and put the heat into the chip, so both leave the cutting area together. Obviously, the softer the metal, the less power is needed to cut it. This is a different way of thinking from the old days when we did everything possible to keep the operation cool, and it requires a different approach to tooling for us at the design stage and users at the selection stage.
Heat may still be an enemy to the insert, but at the cutting point in the workpiece and in the chip, it can definitely be a friend. Today's more advanced HSM milling tools are designed to perform best in conjunction with a high-rpm spindle and its attendant high surface speed, and actually plasticize the metal at the cutting point. Look for tooling that uses the heat of chip deformation to soften the metal behind it "just enough" to cut more easily. You'll be able to cut faster, conserve power, and help your tooling and machine last longer.
The two other principal features to look for in inserts for low-power machining are: high heat and impact resistance in the substrate and coatings, and sufficiently free-cutting geometry at the cutting edge. Both the substrate and coatings need to withstand the combination of hotter conditions involved in plasticizing the cutting area, and the higher impact of repeatedly hitting the workpiece at higher surface speeds. Those impact forces rise in proportion to rising spindle rpm.
As to top-face geometry for low-power milling, inserts should at least have double-positive rake—positive both radial and axial. This ensures a smoother, cleaving-type cutting action in both directions, which generates lower cutting forces and uses less horsepower than the scraping action produced by moreblunt zero-rake cutters. Not all inserts are double-positive, however, so you'll need to look.
Look also for helical edges on milling cutters, found on very few brands, which markedly reduce power requirements and impact forces. Their curved edge eases the insert into the workpiece. On the micro level, it's much like the angled blade on a sheetmetal shear cuts just a portion at a time rather than slamming into the sheet all at once. Lead angles of 20–45° also reduce the impact of tool entry and inhibit burrs at exit.
In moldmaking, a lot of power can be wasted milling with ball nose cutters because only a small portion of their cutting surface—the area around the equator—is working at optimal surface speed and efficiency. A better alternative is a more straight-sided, toroidal cutter like our company's Chip Surfer, which puts more of the surface area to work.
The pitch radius, and therefore the surface cutting speed, is more uniform over the entire cutting surface. Surface speed doesn't approach zero on the extreme ends of the cutting surface as it must near the nose of a ball nose mill. Next, a robust sweeping radius capitalizes on chip thinning for faster removal on straight cuts. A generous corner radius and backdraft combine to facilitate cleaning out corners and minimize cutting forces. And all the cutting surfaces are high-positive-rake to reduce cutting forces and power consumption.
Once you've selected the right milling tool, be sure to make the best of it. And the rule for most steels is: "Run it fast, run it hot, run it dry." Push the rpm and feed rate, to create the plasticizing effect as well as to gain throughput. Use the manufacturer's recommendations on feeds and speeds only as a starting point, and ramp them up from there. And above all, turn off the coolant. Besides protecting the tool from thermal shock, you'll be letting the tool generate the heat necessary for material softening. You probably won't need any cutting fluid for flushing reasons either. High-speed machining combined with high-positive-rake tools clears the chips out well enough. Adding an air blast (with moisture removed) should help.
Here are a couple more guidelines. Some may apply to rough milling in general, but they're all the more important in low-power milling.
Use the climb mode whenever possible. It introduces the cutting edge to the workpiece more smoothly, protecting the lighter-weight machine frame as well as extending tool life.
Study chip color for clues about cutting efficiency. In steel, don't be concerned about a rich blue color. It means you're getting good cutting and softening action, and the heat is leaving in the chips as it should. In stainless steel milling, a light straw color is a good sign for the same reasons.
Narrow shoulder cuts will be more energy-efficient than wider ones. Engage no more than 75% of the cutter in each cut. By the same token, engage no more than two inserts at a time in the cut. Any more and you're just creating more friction, wasting power, and getting little back. If you're getting chatter, change the geometry of the cutter (rake, presentation angle, or lead angle), increase chip load and/or decrease rather than increase insert pitch.
Holemaking is generally regarded as the biggest energy hog per unit of material removal. Even with a brand-new twist drill, only a small part of the cutting face is cutting at the ideal surface speed. And even in the best circumstances, friction between chips and flute takes energy away from cutting. Finally, there's the pump energy needed to deliver cutting fluid to the cutting face. The larger or deeper the hole, the more extreme these conditions become.
For larger holes—say 1.000" (25.4 mm) and up—a better alternative can be corkscrew milling, provided your machine controls can do interpolation. In effect, you're substituting a wet, energy-intensive operation with a dry, energy-efficient one. A single or multitooth end mill does the cutting, and requires much less horsepower and system rigidity than any drill.
Users of the corkscrew milling technique report cutting cycle time for guide-pin holes by four to one, and power consumption for the operation by 40%. The horsepower required to spade-drill such large holes can stall most modern CNC machines. For that reason, we've seen many moldmakers move the toolset to a jig borer or heavy-duty drill press just for the guide-pin holes. With corkscrew milling, they can do it all on the same low-power mill they use to create the cavities. They grab the part once and finish it. And believe it or not, you can open holes right from the solid with corkscrew milling, so you don't waste time and electricity running progressively larger drills.
One caution when corkscrew-milling holes of any depth or closed-end holes: pay attention to chip clearance. The cutter geometry makes small chips, but doesn't necessarily clear them by itself. On vertical and some horizontal cuts, you may need to blow some air.
Replaceable-tip drills like our company's Quick-Twist drill can also open holes using less power than fluted drills. Cutting action at the tip is more efficient because of geometry, and the shaft is smaller than the tip for freer chip clearance and less friction. Moreover, the ductile alloy steel shaft withstands vibration and tool deflection, common in lightweight machines, which can shatter solid-carbide drills.
The main niche for replaceable-tip twist drills has been in high-volume operations, to eliminate the reconditioning "merry-go-round" and big tool inventoriesinvolved with solid-carbide drills. But they add value in low-power drilling with their more efficient cutting geometry and forgiving shafts.
For more efficiency in high-speed/low-power machining environments, look beyond the HSM or spindlespeed rating when selecting milling cutters and holemaking tools, and putting the tool to work. You'll get more than the safety that comes with the HSM rating. Choose the most-free-cutting geometries for milling (double-positive rake with helical edge), and run the tools hot and hard. For drilling, consider corkscrew milling for big holes. For general drilling, look into replaceable-tip drills with forgiving alloy shafts, to avoid snapping off solid carbide drills in unstable setups. You'll make more chips faster, protect the lightweight machine and the tool, and use less energy as well.
This article was first published in the January 2008 edition of Manufacturing Engineering magazine.