It is well known that the main trend in CNC machining centers used in job shops over the past decade has been faster, smarter, and with lighter, less power-hungry spindles. And rising energy costs are accelerating this process. This trend is completely contrary to the use of powerful machine tools that can achieve deep cuts in a single pass. High-speed machining (HSM) also necessarily means low-power machining (LPH), which requires different cutting tools and a different understanding of machining tools.
In response to the trend towards high speed/low energy machining, many leading tool suppliers are developing special tool lines for high speed machining. Or adding rated spindle speed markings to their tool products. Some tool suppliers are even more advanced. The reason for this is that although the current situation of high speed machining or rated spindle speed is good and necessary for the safety of high speed spindles, it has not been fully developed. High speed machining or rated speed itself only means that the drill or tool is well balanced when it actually runs at a speed of 12,000 rpm or 40,000 rpm, and the insert is firmly installed in the tool. However, this does not indicate the machining efficiency of the tool, which is the key factor in saving energy and protecting the structure of lightweight machine tools.
Of course, high speed machining or rated speed needs to be emphasized, but the vision should also look further. You will find that there are huge differences in machining efficiency and energy efficiency among the various milling cutters and drill bits currently used for high speed machining. These differences are particularly important for roughing, one-pass milling, and large diameter hole machining.
Features and Limitations of CNC High-speed Machining Centers
Let’s first dissect a typical CNC high-speed machining center to see how it differs from traditional machine tools. Of course, it is fast, with a rated spindle speed of up to 40,000r/min and can achieve extremely high feed rates. It is also highly intelligent, and its control system can usually achieve interpolation processing, tool path optimization, and 3-6 axis linkage processing.
However, it also has disadvantages. First, the rated power of the spindle motor may be only 20 horsepower (25kW) or less. Second, the machine tool structure is very light, so it is more prone to deflection and vibration (this is often overlooked). In fact, it is usually the structural rigidity rather than the spindle power that limits the improvement of material removal rate. Not only the spindle motor, but the machine tool as a whole is designed for light-load, fast multiple-pass cutting (rather than deep-cut, few-pass cutting).
New Thinking on Tool Design for High-speed Machining
From the perspective of cutting tools, the key to efficient and low-cost machining is to instantly heat the cutting zone to soften the metal being cut and transfer the heat to the chips so that the heat leaves the cutting zone along with the chips. Obviously, the softer the metal, the less power is required to remove the metal. This is a completely different way of thinking compared to the past when every possible method was used to reduce the machining temperature, which requires us to take a different approach in the tool design stage and the user in the tool selection stage.
Critical Features of Tool Substrates, Coatings and Geometry
Cutting heat may still be the enemy of inserts, but it can be turned into a positive factor at the cutting point on the workpiece and in the chip. Today’s high-speed milling cutters are designed to perform better, and in conjunction with high-speed spindles (and their high surface cutting speeds), they can cause plastic softening of the metal at the cutting point. If such a tool can be found, it can use the heat generated by the deformation of the chip to soften the metal being cut. Getting it just in a state where it is easier to cut can achieve faster, more energy-efficient cutting, while extending tool and machine life.
Inserts developed for low-power machining should have two other important features: First, the substrate and coating should be able to withstand high temperatures and impacts. Second, the cutting edge geometry should be designed to fully achieve free cutting (high-speed, unsupported cutting). The substrate and coating need to withstand the high temperature environment associated with the plastic changes in the material in the cutting zone. They also have to withstand the frequent impacts of repeatedly hitting the workpiece at high surface speeds, and these impact forces increase proportionally with the increase in spindle speed.
Regarding the rake face geometry of low-power milling tools, the inserts should have at least a double positive rake angle, which is positive in both the radial and axial directions. This ensures a smooth plunging cut in both directions, which generates less cutting force and consumes less power than the scraping effect produced by a duller 0° rake angle tool. However, not all inserts have a double positive rake angle, so you need to be careful when selecting.
Advantages and Selection Suggestions of Spiral Cutting Edge Tools
Also look for end mills (of which there are few) with helical cutting edges that significantly reduce power requirements and impact forces, and whose curved cutting edges make it easier for the blade to cut into the workpiece. On a microscopic level, it’s more like sheet metal machining with an angled blade shearing away a portion of material at a time, rather than punching an entire sheet of metal in one go. The use of a helix angle of 20° to 45° for the milling cutter can also reduce the impact of the tool when cutting in and suppress the generation of burrs when cutting out.
Comparison of Round Nose Milling Cutters and Ball Nose Milling Cutters
In mold processing, milling with ball nose end mills wastes a lot of power because only a small part of the cutting surface (the area around the mid-line) is involved in cutting at the optimal surface speed and efficiency. A better choice is to use a round nose end mill with more straight teeth.
The pitch radius of the tool and the related surface cutting speed are quite consistent across the entire cutting surface. At both ends of the cutting surface, the surface speed does not approach zero because it must be close to the nose of the ball nose milling cutter. Secondly, in straight-line cutting, a large scraping radius can be used to take advantage of the chip thinning effect to achieve faster material removal. The large tip radius combined with the reverse taper makes it easier to clean corners and minimize cutting forces. All cutting surfaces use large positive rake angles to reduce cutting forces and power consumption.
High-speed Dry Cutting and Tool Parameter Optimization
Once the right end mill is selected, it is important to make full use of it. The rule for cutting most steels is: fast cutting, hot cutting, dry cutting. Increasing the spindle speed and feed rate can cause the material to plasticize and also improve productivity. Use the cutting parameters (feed and cutting speed) recommended by the tool manufacturer as a starting point only and improve on them. The most important point is not to use coolant. In addition to protecting the end mill tool from thermal shock, the milling tool must also generate the heat required to soften the workpiece material. Cutting fluid may not be needed during machining to flush the chips, and high-speed machining with positive rake tools can evacuate chips well. Adding a jet with dehumidification function will help.
Here are some guidelines, some of which can be applied to rough milling operations in general, but all of them are important for low-power milling.
- Use down milling when possible, which allows the cutting edge to cut into the workpiece more smoothly, protects the lighter machine tool structure, and prolongs tool life.
- Studying the color of the chips can reveal clues about cutting efficiency. When milling steel, don’t worry about dark blue chips, as they indicate good milling and material softening, and that the heat of the cut is being carried away by the chips in the right way. When milling stainless steel, light straw-yellow chips are also a sign of good milling.
- Narrow shoulders are more energy efficient than wide shoulders, and the contact width of each cut should not exceed 75% of the tool. For the same reason, do not use more than two inserts in the cut at the same time, otherwise it will only create more friction and consume more power, which is not worth the effort. If chatter occurs during processing, you can change the tool geometry parameters (such as the rake angle, setting angle or lead angle), increase the chip load and/or reduce (not increase) the insert rake angle.
A Low-power Alternative to Holemaking: Spiral Milling
Holemaking is often considered the most energy-intensive process per unit of material removed. Even with modern twist drills, only a small portion of the cutting surface is being cut at the ideal surface speed. Even under the best machining conditions, friction between the chips and the chip pockets consumes cutting energy. In addition, the pump that delivers the cutting fluid to the cutting surface consumes energy. The larger and deeper the hole being machined, the greater the energy consumption.
For large holes over 25.4mm in diameter, a better alternative is spiral milling. Of course, this requires interpolation control on the machine tool. This dry, energy-saving cutting method has a good effect instead of wet, energy-consuming traditional machining processes. Large diameter holes produced with single-tooth or multi-tooth end mills require less machine power and system rigidity than any drill.
Users of spiral milling technology have reported cycle times for dowel pin holes reduced by 3/4 and milling power consumption of only 40%. Most modern CNC machines would not be able to provide the power required to produce such large diameter holes with flat drills. For this reason, many moldmakers have had to move the tooling to a jig boring machine or heavy-duty drill press just to machine the dowel holes. With spiral milling, they can machine all the dowel holes in one clamping on a low-power milling machine used for cavity machining. Believe it or not, spiral milling allows large holes to be machined directly on workpieces without pre-drilling, without wasting a lot of time and energy on the increasing number of large hole drilling operations.
When using spiral milling to machine holes of various depths or blind holes, pay attention to chip removal. The geometry of the milling cutter can generate fine chips, but the chip removal does not necessarily have to be done by the tool itself. In vertical milling and some horizontal milling operations, air chip blowing may be required.
Application of Replaceable Crown Drill Bits in Large Hole Machining
Replaceable crown drills can also be used for large hole machining and require less power than fluted drills. Due to their unique geometry, the crowns are very efficient in cutting and the diameter of the drill rod is smaller than the crown diameter, which makes it easier to remove chips and reduce friction. In addition, the tough alloy steel drill rod can withstand the vibration and tool deformation common on light machine tools, while solid carbide drills are easily damaged by vibration and deformation.
Replaceable crown drills are mainly used in large-scale production to avoid re-adjusting machine tools and large stocks of various solid carbide drills. Due to their more efficient cutting edge geometry and tougher drill rod, they increase the added value of low-power drilling operations.
To improve the efficiency of high-speed/low-power machining, when selecting milling and holemaking tools, pay attention not only to high-speed machining (spindle speed) but also to make the tools suitable for the actual processing. In addition to the safety of high-speed machining, there are more gains. When milling, choose a tool geometry with “complete free cutting” (double positive rake angle milling cutter with spiral cutting edge) and use tools with good thermal hardness. When drilling, consider spiral milling for machining large holes. For general drilling, try a replaceable crown drill with a high-toughness alloy steel drill rod to avoid the breakage of solid carbide drill bits in unstable installation conditions. Replaceable crown drills can increase metal removal rates and protect light machine tools and tools while saving processing power.