Limited Engagement: Drilling Performance

Courtesy of B. Kennedy

Advances in machine tool technology, tooling and software facilitate high-productivity milling strategies.

In milling, metal-removal rate is calculated by multiplying the radial WOC times the axial DOC times the feed rate (ipm). Advances in machine tool technology, tooling and software enable manipulation of those parameters to maximize productivity.

A traditional approach to maximizing mrr when milling involves combining radial engagement (WOC or step-over) of 25 to 50 percent of the cutter diameter with axial engagement or DOC of 25 to 50 percent. According to Craig Segerlin, owner and president of consulting firm Performance Tooling Solutions, Browns-town, Mich., that approach has historically produced acceptable results, but can generate heavy and uneven radial cutting forces. This can produce unpredictable tool wear and stress on the machine tool spindle and other components.

An alternative and more productive strategy involves reducing the radial engagement of the milling cutter while simultaneously increasing both the feed rate and cutting speed. The resulting cut is lighter and faster, producing a higher mrr and reducing forces on the cutting tool and machine, according to Segerlin. Despite the higher cutting speeds, heat is reduced because the engagement time for the cutter edge is short and it spends more time cooling in the air.

However, reduced radial engagement also reduces the thickness of the chips produced when milling. Excessive chip thinning can be a problem. When a cut is too light, the milling tooth or endmill flute rubs instead of shears the workpiece, creating additional heat and reducing tool life. A too-thin chip also loses the ability to act as a heat sink that carries damaging heat from the tool and workpiece.

Courtesy of Methods Machine Tools

A double-face-contact spindle that locates the tool both on the taper and the face of the spindle, such as this one on a Feeler machining center from Methods Machine Tools, provides added radial stiffness for operations that generate significant side loads, such as milling.

To regain sufficient chip thickness after reducing radial engagement, it is necessary to increase the chip load on the milling tool’s cutting edges. Segerlin provided an example: when a 0.500 “-dia. cutter is taking a 0.250 ” radial WOC, the cutter engagement is 50 percent and the arc of the cut is 90°. Assuming the cutter is taking a 0.002 ” chip load, the chip thickness at 50 percent cutter engagement is also 0.002 “. If the radial WOC is reduced to 0.010 ” (1⁄25 of 0.250 “), the arc of cut becomes 3.6º (1⁄25 of 90º). To maintain the chip thickness of 0.002 ” at a 0.010 ” radial WOC, the chip load can be increased 3.5 times, to 0.007 “.

That can be accomplished by increasing the feed rate. Then, because cutting speed is also a factor in determining chip load (chip load per tooth × No. teeth × rpm = feed rate [ipm]), cutting speed can be increased as well. Segerlin said reduction in radial WOC to 5 to 10 percent of the cutter diameter can be accompanied by a cutting speed two to three times faster and a feed five to seven times faster to achieve a three to five times greater chip load.

Limited radial engagement also greatly reduces side loads on the cutter, permitting use of the endmill’s entire cutting edge (axial DOC), Segerlin said. Because mrr is determined by the product of the radial WOC, axial DOC and feed rate, the increased DOC directly boosts the volume of metal removed. Segerlin calls this approach “high-efficiency productive machining,” or HEPM. (See sidebar below for details on an application of this strategy.)

Machine Tool Considerations

According to Segerlin, in some cases the low-radial-engagement strategy can enable a less-powerful machine tool to produce results rivaling those of a more powerful one, depending on the workpiece material. Even though a typical 50-taper machine may have more horsepower than a 40-taper machine, the smaller machine may be capable of higher spindle speeds and therefore is better able to achieve the lighter, faster cuts characteristic of the strategy.

Michael Minton, national application engineering manager for Methods Machine Tools Inc., Sudbury, Mass., said taking a smaller radial WOC at higher spindle speeds and feed rates “provides an opportunity to look at alternative strategies as opposed to traditional methods where you might bury the tool and take a big, heavy cut.” He agreed that this method reduces the load on the machine tool, and could allow a lighter-duty machine tool to achieve higher productivity.

However, he said, the existence of such strategies shouldn’t be the only influence on machine tool purchasing decisions. “My belief is you buy the right machine for the job,” he said, noting that each application is unique, and machine buyers should consider machine construction features that facilitate execution of their particular operations.

For example, he said, “A double-face-contact spindle, locating the tool on both the taper and the face of the spindle, gives you better radial stiffness.” That quality may be less important when drilling, but is critical when side loads like those generated in milling are involved. Minton said double-face contact spindles are standard even on Method’s commodity-based, value-priced Feeler machining centers.

Adopting new strategies also involves open-mindedness on the part of machine users, Minton said. “There are some progressive thinkers who routinely think outside the box,” he said, “but many just want to continue doing things the way they always have.”

Minton described an application “where we were cutting nickel-base alloy with a cobalt endmill at low machining parameters. We were running about 115 rpm and 1 ipm,” he said. “We offered to demonstrate something different: how to take that same cut in less time for less money. We might take that cut in eight passes at 40 ipm or even 16 passes at 40 ipm and would still be significantly faster, while putting a lot less wear on the spindle and the tool, and a lot less stress on the part. The best way to sell that technology is by showing customers how it applies to their end products.”

Bill Howard, vertical machining center product manager for Makino Inc., Mason, Ohio, said with low-engagement strategies, “because you are taking basically a smaller bite and running at higher speed, the approach may require less horsepower and torque. But the machine still must be rigid and accurate.”

Courtesy of Makino

Compared to milling strategies that involve large radial engagement of a milling cutter in the workpiece, a much lighter engagement combined with simultaneous increases in feed rate and cutting speed results in a light, fast cut that produces a high mrr as well as reduced forces on the cutting tool and machine.

To assure a fine surface finish, every cutting pass must precisely match the one that preceded it. The considerations are the same at the end of each pass. “If you have a really loose or weak machine, you overshoot, then the cut is not going to end where it is supposed to,” Howard said. He compared the situation to a racecar rounding a curve: “If there is too much play in the steering, you are constantly fighting to keep control of the car, at speed, through the turn.”

Speaking of machines for high-speed, accurate operation, Howard cited Makino’s F-series VMCs. Among the VMCs’ features are ballscrews with an 8mm pitch rather the more common 16mm pitch. The finer-pitch screw provides 8mm of axial linear movement per rotation, compared to 16mm per rotation with the more coarse pitch. The finer pitch ballscrews combined with digital servos allow the smaller axial movement to be divided into much finer increments and “help that racecar get around the corner at higher speeds,” Howard said.

Paired with the mechanical elements of the machines are the “brains behind that movement,” according to Howard, namely the super geometric intelligence (SGI.4) technology that Makino reports is engineered for high-feed, tight-tolerance machining of complex, 3-D, contoured shapes. SGI.4 anticipates changes in axis motion, servo lag or following error in rapid toolpath changes and compensates in advance to maximize toolpath accuracy. According to the company, the 3-D compensation lets the machine follow toolpaths on mold contours and complex geometries, even at feed rates five or more times higher than conventional machines.

High-Feed Tools

To take advantage of high-productivity milling strategies, tool manufacturers have developed various high-feed cutters. Bill Fiorenza, die and mold line product manager for Ingersoll Cutting Tools, Rockford, Ill., said the purpose of a high-feed cutter is to increase the feed rate and take a lighter DOC, leveraging the chip thinning process and driving the cutting forces up through the center of the spindle. In general, the tools generate consistent chip loads and minimize chatter, protecting cutting edges and extending tool life. Specific high-feed geometries vary, Fiorenza said, “but, in essence, the high-feed cutter typically is a high-lead-angle, straight-edge cutter or a rounded triangular shape.”

On the high-lead-angle side, Ingersoll offers SP6H/SP6N S-Max facemills with inserts inclined at an almost-horizontal 80° lead angle. As the lead angle increases, chip thickness shrinks because the chip is spread over a greater length of the cutting edge. To regain sufficient chip thickness, the feed rate must increase. The aggressive lead angle on the S-Max mills results in chip thinning that requires feed rates as much as five times higher than 0° lead angle cutters.

Tom Noble, MAXline product manager, described the benefits of high lead angles in terms of mrr. He used an example of a 0° lead angle tool running in P-20 mold steel at a 0.150 ” to 0.200 ” DOC and a chip load of 0.010 ” to 0.012 “. Switching to an 80° lead angle tool, “You are going to be taking a little bit lighter depth of cut, from 0.060 ” to 0.070 “, but you may be running at a feed rate of 0.060 ” per tooth, the result of using that five times chip thinning factor. Now, when you look at the cubic inches per minute that you are removing, they are four, five or six times what they were with a 0° lead tool.” Noble emphasized that the feed rate multiplier will not be as large for tools operating at greater than three times length-to-tool-diameter ratios in order to minimize side loads.

Courtesy of Ingersoll Cutting Tools

As the milling tool’s cutting edge lead angle increases, chip thickness shrinks. Ingersoll Cutting Tool’s SP6H/SP6N S-Max facemills have inserts inclined at an almost-horizontal 80º lead angle, which results in chip thinning that requires feed rates as much as five times higher than 0º lead angle cutters.

Fiorenza said high-feed strategies can also be used with traditional button cutters, where the chip volume is spread over the longer, round cutting edge, producing a thinner chip at lighter depths of cut (in the range of 0.020 ” to 0.050 “). “Basically, you can run a button cutter as a high-feed mill; just take a lighter DOC,” he said. But, Fiorenza added, high-feed cutters generally will do a better job than button cutters because they typically have a small corner radius and lower radial contact. The complicating factor with round inserts is that the chip load varies in relation to the DOC.

To calculate how much of a button cutter should be radially engaged in the cut, Fiorenza advises subtracting the insert inscribed circle dimension from the cutter diameter measurement, then multiplying that value times by 60 or 65 percent. For example, using ½ ” IC inserts in a 4 “-dia. cutter, the cutter diameter (4 “) minus the insert IC (½ “) equals 3½ “. Sixty percent of 3½ ” yields a radial engagement value of 2.1 “. Fiorenza said the formula “generally works very well.” He added that that centerline cutting (engaging exactly half the cutter diameter) is not recommended because the cutting edges tend to slap the workpiece material instead of arcing into it, but approaching it closely, as in this example, is usually acceptable and can significantly aid in achieving higher feed rates.