Tool Considerations for High Speed Cutting
Fast CNC processing and high-pressure coolant contribute to removing metal at dramatic rates. But what should a shop know about cutting tools in high speed machining?
Metal removal rates are faster today than ever before. What was considered high speed machining just a few years ago is regarded as conventional today. Many factors are driving shops to faster metal cutting rates. These include better and more capable machine tools and CNC processors that allow the machine to accurately cut at increasingly higher speeds and feeds.
Commercial considerations are also driving shops toward higher rates of productivity. The need to put more work across machine tools has shops looking constantly to improve metalcutting processes.
While much of the discussion about high speed machining tends to focus on the role played by the machine tool, the cutter is its partner in high speed machining. And, that's the focus of this article. We're going to look at what a shop needs to know about specifying cutting tools for their high speed applications.
To get perspective on tooling considerations for high speed machining, we contacted Kennametal (Raleigh, North Carolina) to discuss high speed cutting tools.
What Counts as High Speed Cutting?
A generic definition of high speed machining is elusive. High speed is relative. What's very fast for one industry segment seems glacial to another.
Machining speed is very application specific. Calling a machining process high speed draws a comparison between its current and previous performance capabilities. For example, high speed might mean changing from an HSS tool to solid carbide, which allows you to bump up machine feeds and speeds. Because carbide cutters, in many applications, can remove metal faster than HSS, a shop using carbide is machining faster compared with HSS rates. But it's relative because another shop, using cermet or ceramic cutters, can cut faster than carbide.
So, we're not going to assign a definitive value to high speed. Suffice it to say high speed machining means cutting metal faster that is customary for your operation.
Some Basics: SFM and IPT
Elusive as high speed machining is to define, there are measurements used for machining speed. These allow comparison between different rates and help a shop determine its place in the machining speed continuum.
To quantify how fast a machine actually cuts metal, spindle rpm needs to be converted into something more useful. According to Dan Spanovich, application specialist for Kennametal, this figure is expressed as surface feet per minute (sfm).
Likewise, feed rate for the machine tool is usually measured in inches per minute (ipm). But for cutting tools, it is expressed as inches per tooth (ipt). Working from the optimum sfm and ipt for workpiece material, a machine tool's rpm and ipm can be determined and programmed.
These two measurements are dependent on each other to determine the speed at which a workpiece can be maintained. For example, titanium can be cut effectively at about 250 sfm. That's using a chip load of 0.005 ipt. However, some shops report machining titanium at close to 500 sfm, but to do that, lighter chip loads are taken.
Depending on the cutter material, chip load and surface speeds can be adjusted to deliver the best combination for a shop's application. If heavy metal removal is the goal, cranking up the chip load and sfm will maximize the cutting efficiency. For better finish, backing off the chip load while keeping sfm up will give good surface finish.
There are no specific formulas to determine the best combinations and results. It takes a little experimentation to find optimum feeds and speeds for a specific application.
A cutting tool material has specific attributes that make it usable in a metalcutting application. Because applications vary so widely, there are many cutting material combinations from which to choose.
But in general, only two performance criteria are used to determine the applicability of a cutter. These are toughness or resistance to fracture (ductility) and thermal hardness (resistance to heat). A myriad combination of coatings, substrates and base materials can be created to deliver specific proportions of toughness and thermal hardness to fit various applications.
Cutting tool materials can be classified into five general categories. The materials are arranged from best toughness characteristics to best thermal hardness:
- Tungsten carbide (uncoated and coated),
- Ceramics, and
- Diamond and CBN.
Starting with HSS and progressing to diamond and CBN coatings, a scale can be built progressively from best toughness characteristics to best thermal hardness. High speed steels take a pounding but can't take much heat. Ceramics and diamond coated cutters can take the heat but fracture easily.
Generally, tungsten carbide cutters have a working range of 100 to 1200 sfm, according to Mr. Spanovich (HSS goes up to approximately 100 sfm). Ceramics, including silicon nitride, push the envelope up to 4,000 sfm. Polycrystalline diamond and CBN coated tools push sfm above 4,000. These rates are at chip loads of 0.003 to 0.030 ipt.
These rates represent optimum cutting potential for the right combination of workpiece material and cutter material. However, there are other factors that must be considered before a shop can hope to approach these kinds of cutting speeds.
Workholding for High Speed Machining
The importance of rigid fixturing cannot be overemphasized in high speed machining applications. While the goal of any fixturing or clamping setup is to hold a workpiece securely and allow for repeatable location of subsequent parts, high speed requirements magnify any imperfections in a workholding setup.
In high speed machining applications, the fixture should support the workpiece on a solid base and have enough mass to help damp cutter-induced vibrations. Fixtures for high speed need not be overly complex but should follow good shop practice.
For example, a good vise is adequate if it supports the workpiece securely. It is recommended that positive stops be used to prevent torquing or movement of the workpiece in response to cutter motion.
The Force Is With You
Proper selection of a cutting tool, especially an indexable cutter that is rated to spin at elevated speeds, is important. Not to put too fine a point on it, but we're talking potentially serious or even fatal accidents if a shop tries to exceed tooling speed ratings.
The reason is simple centrifugal force. For the same reason tire manufacturers have speed ratings for radials, tooling manufacturers put a "not to exceed" rpm on cutters. The force created by rotating a body is exponential to the speed of rotation. That force is trying to rip the inserts away from their seats. Any part of a cutter flying off at 10,000, 15,000 or 20,000 rpm poses a risk to life and limb.
Indexable insert tools for high rpm are different than tools for conventional rotating speeds. Inserts are secured differently to the cutter body for high speed indexable tools. According to Mr. Spanovich, a simple setscrew clamp is not adequate for high rotation. Inserts are secured to the cutter body with a pin that fits into a detent molded in the insert. It is anchored on the cutter body in a direction perpendicular to the centrifugal forces generated by rotation.
At elevated cutting speeds and feeds, coolant may be less necessary than at conventional speeds. Heat is the by-product of machining. Generally it's the enemy of metalworkers. Increasingly however, heat is being used to help the cutting process.
In an ideal cut, workpiece material, machine feeds, spindle speeds and cutter are all making their respective contributions in optimum fashion. As the cutter creates a chip, the heat generated by that action is transferred to the chip. When the chip breaks and leaves the cutting zone, the heat is carried away with it.
A big advantage of high speed machining is that at elevated rates of speed and feed, the chip is cut and evacuated so fast it tends to transfer little or no heat to the green workpiece. At conventional machining speeds, there is time for heat to move from chip to uncut metal and create a work-hardening condition. This increases the force needed to create a chip, which creates more heat, and on it goes. Coolant mitigates the cycle by reducing the temperature in the cut zone and flushing away the chips.
But at very high rpm, the tool rotation throws coolant away from the cut zone so without very high pressure or through-the-tool piping, it never reaches the cutting zone. "In some cases," says Mr. Spanovich, "trapped chips can remain in the cut, allowing them to be recut by the tool. We've found an air blast is very efficient for evacuating chips in high speed applications."
Thermal shock is another consideration for users of high speed tools--especially ceramic and harder cutting edges. Irregular distribution of coolant in the cut can create an unstable heat zone for these cutters. Designed to operate at elevated temperatures, the cutter material can undergo successive heat and chill cycles in the cutting zone that can create premature failure from thermal shock.
The Right Angle
Cutter speed is the major influence in creating heat at the cutting edge of the tool. Maintaining a high chip load or feed is how heat is dissipated. Correct ipt, combined with the right cutter rake angle for the material being machined, produces a chip of sufficient density to carry heat from the cutting zone so work hardening can be avoided.
Chip load is feed rate for each cutting edge of the tool. For indexable insert tools, it's the load against each insert. On solid body cutters, chip load is rated against each tooth. According to Mr. Spanovich, a good working range of chip load is generally between a minimum of 0.003 ipt to a maximum of 0.012 ipt.
The angle of attack for the cutter edge, its rake angle, influences the chip load for a cutter. Rake angles vary from positive through neutral to negative. Positive rake angles present a sharper edge to the workpiece. It's also a weaker edge. Positive rake tools tend to pull the workpiece toward them during the cut. They also tend to push chips up and away from the cutting zone.
Negative rake tools have a much stronger leading edge and tend to push against the workpiece in the direction of the cutter feed. This geometry is less free cutting than positive rakes and so consumes more horsepower to cut.
High speed tooling geometry, in general, mirrors the geometry of conventional machining. "What you know about tool geometry for conventional machining transfers to higher speed applications," says Mr. Spanovich. "If there is a trend in high speed, it is toward a positive lead angle tooling. This lead angle effect allows greater ipt, by lifting the chip, while maintaining the same chip thickness. This greater feed rate results in higher speed machining.
"The formation of a sufficiently thick chip is the goal," says Mr. Spanovich. "The idea is to use chips as a heat sink. Faster speeds make more heat, so directing that heat into chips becomes critical in high speed machining applications."
Watch for Wobble
Successful high speed machining is dependent on static and dynamic rigidity among the many components that bring together the tool and the workpiece. Critical to this is a highly rigid connection between the tool, toolholder and the machine tool spindle.
Tool balance becomes a big issue at high spindle speeds. "We recommend smooth shank tools, for end mills and drills, held by a hydraulic or roll-lock collet chuck for high speeds," says Mr. Spanovich. "Balance becomes an issue at 5,000 rpm and up. At those speeds, a notch shank with setscrew can move the tool enough off-center to induce vibration--hence chatter." For speeds of 20,000 rpm and up, a custom balance of tools and toolholder combination is recommended.
The V-flange taper connection is a potential source for high speed vibration. Until recently, the V-flange taper and measurement gages used by cutting tool manufacturers were made to ANSI/ASME B5.10 standards. "Until high speed applications came along, the ANSI/ASME standard worked well," says David Lewis, staff engineer for Kennametal and vice chairman of the ANSI/ASME B5 standard committee.
Taper fit between a tool body and the machine spindle can be in tolerance (per ANSI/ASME B5.10) and still cause runout and eccentricity problems for a high speed cutter. Mr. Lewis and others representing U.S. tooling manufacturers recommend application of the European ISO 1947 AT3 standard in place of ANSI/ASME B5.10. The ISO standard has twice the accuracy requirement of ANSI/ASME and results in a better connection between the spindle taper and the V-flange tool. To make sure the tooling you purchase for high speed applications is made to the new standards, specify ISO 1947 AT3 or equivalent from your tooling manufacturer for toolholders and collet chucks. For machine tool spindles, specify ISO 1947 AT2 (a lower AT number means a better fit). Mr. Lewis recommends gaging be acquired to check spindle and tool tapers in the shop.
A Word about HSK
Much has been written about HSK or equivalent tooling as a possible replacement for the V-flange connection in machining operations. "While there are some advantages to the design concept," says Mr. Lewis, "its widespread application is being held up in part by a lack of manufacturing standards."
The primary difference between HSK or other hollow shank, short taper toolholders is the way the tool fits into the machine tool spindle. HSK uses a simultaneous fit between the short taper and the face of the spindle. The connection is very rigid.
"The problem with HSK," says Mr. Lewis, "is no governing body has established a standard for tooling companies to manufacture to. There is a German DIN standard that's being considered by ISO but so far it has not been approved. There are also some challenges to HSK from Japan, other European countries and the United States. The question of what HSK will look like is not yet decided.
"In the mean time, shops looking to do high speed machining on their machining centers may be better off specifying AT3 or better V-flange tooling than waiting for an HSK standard tooling configuration," says Mr. Lewis. It could be a while before HSK or an equivalent standard for tool, spindle and gages comes along.
Why Higher Speed?
Implementing higher speed machining in a shop has many benefits, some obvious but others less so. Obviously, making parts faster helps satisfy customers' demands for quicker deliveries despite shorter lead times. There are also benefits derived from increased tool life. It may seem paradoxical, but machining at high speed with the right tooling matched to the application can reduce tool wear because of the diminished cutting forces at high speed.
High speed machining can help a shop manufacture more accurate parts with better surface finishes. Often, because a machine tool and workpiece setup must be very rigid for high speed machining, the results are more consistent workpieces.
A less obvious benefit of high speed machining for shops moving in that direction is derived from the exercise of implementing it. Learning to do the things necessary for successful high speed machining can simultaneously elevate other facets of an enterprise to equivalent levels of productivity.
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