Cutting Tool Considerations For High Speed Machining

Fast CNC processing and high pressure coolant all contribute to removing metal at dramatic rates. But, what should a shop know about cutting tools in high speed applications? By Chris Koepfer, Senior Editor

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's High Speed? 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. Material Differences 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: HSS, Tungsten carbide (uncoated and coated), Cermets, 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. Hold It 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 simplecentrifugal 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. Cutting Dry 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. MMS Compiled by Rahul KP

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Milling Operation

Milling

The Product Realization Lab has several different types of milling machines, including manual Bridgeports, 2-axis CNC Anilam Crusaders, 3-axis HAAS vertical machining centers, and the 3-axis Roland.  Each of them has specific guidelines for how it can be used, but this section documents the fundamentals that are common to all of them.

General Mill Information

Standard Mill Tools:

Endmills

·        Both the side and the end of an endmill can cut

·        They are available in different styles

-         2 flute

-         3 flute

-         4 flute

-         6 flute

-         Ball end, for cutting rounds and doing 3D CNC surfacing

-         Corner-rounding, for rounding corners

·        End mills are available in different lengths

-         Stub, a good idea when using a small diameter cutter if the workpiece is not very thick

-         Standard

-         Long, for reaching into deep cavities

-         Extra long, for reaching into extra deep cavities

You should always use the shortest endmill available that will cut the geometry you need.  Long and extra-long end mills may leave a relatively poor surface finish because they are prone to chatter and can break easily.   Ask a TA for advice when a long end mill seems necessary.

·        Available endmill sizes depend on style and length

-         2 flute standard end mills are manufactured in 1/64" increments down to 1/16" diameter

-         Ball end mills available in  1/32" increments down to 1/16" dia

-         Because small end mills break so easily, the machine shop will typically stock down to 5/16" diameter endmills.  We sell 1/4", 3/16”, 1/8”, 3/32”, and 1/16” in standard and long lengths.

·        Endmills are either center cutting or non-center cutting.  Beore plunging into a part with an end mill, check the end teeth to see if it is center cutting.  If the teeth meet at the center, it is OK to plunge.  If they don't meet, don't plunge with it.

Slitting Saws

·        Used for general slotting or cut-off operations

·        Available down to  0.006"  thick (thin)

·        Because slitting saws break easily, the machine shop has a limited stock.  Find out what's available.  Get assistance before cutting.

Single Lip Cutters

·        Custom made cutters for special applications:  in our shop they are typically ground for putting draft angles onto casting patterns

·        If you need something specific, you may need to make your own, so ask the staff for advice and refer to the Single Lip Cutter Grinder document in the TA Handbook.

Fly Cutters

·        Used for taking light face cuts on large surfaces

·        Max depth of cut depends on material being cut

steel : don't flycut

aluminum, brass : 0.025" max

plastic : 0.010" - 0.050"  (be careful with plexiglass, the edges  will chip with too heavy a cut)

·        CAUTION- A spinning flycutter is often difficult to see.  Keep your hands well away from the workpiece and tool when performing flycutting operations.

Drilling, Reaming, Boring

When drilling or reaming holes on the mill, straight-shank tools with diameters less than 1/2" are mounted in a mill drill chuck.  Large straight-shank tools mount directly into collets.  Large tapered drill bits are mounted into tapered sockets. 

NOTE: Do not mount taper shank drills in chucks. Chucks cannot hold tapered shanks securely.

NOTE: Do not mount end mills in drill chucks.  Drill chucks cannot withstand the forces induced on an end mill in use.

Center Drilling

·        A center drill is used for starting holes to assure accurate location.  Center drills are short and stubby to maximize their stiffness and keep them from deflecting under load.

·        Cut to depth illustrated - halfway up the larger taper.  This creates a funnel to guide the drill bit accurately.

Drilling

·        Twist drill sizes less than 1/2"  diameter are called out by

Fraction:  in 1/64" increments

Number:  to fill in the spaces between fractional sizes <1/4"

Letter:  to fill in the spaces from 3/16" - 7/16"

Metric:  ask about availability

·        Our drill chucks will hold twist drills  <1/2".

·        The chuck should grab onto the drill's shank, not its flutes.

·        Larger drill bits are available in fractions at  1/32" increments

·        Most of the larger bits have tapers that fit into tapered sockets.

Measuring Depth

·        Hole depth is usually measured from the surface of the part to the shoulder made by the drill bit, as shown at right.

·      When designing parts with blind holes, keep in mind the length of the tip of the drill bit.  Allow adequate wall thickness so the tip does not poke through the bottom of the part.  In depth critical applications, it makes sense to measure to the tip.

·      Twist drills do not make flat-bottomed holes.

Drilling to Depth

·        Set the quill stop to allow adequate spindle travel to drill hole.                     

·        Lower quill to the stop and lock in place.

·        Raise table until the shoulder of the drill bit just enters the part as illustrated.  This is the zero point.

·        Raise the quill to retract the drill bit.

·        Raise the table the distance equal to the depth of the hole to be drilled.

·        Now, using the quill, drill into the part.  Periodically, back the drill bit out to clear chips.

·        When you hit the quill stop, the shoulder of the drill bit is at the desired depth.

Making Large or More Accurate Holes

·      When drilling large holes, you will need to start with a smaller drill bit and gradually increase to larger drill bits.

·      For more accurately sized holes, you will want to drill the hole undersized and finish with a reamer.  Ask the TA for assistance.

·      Use a boring bar mounted in the boring head to enlarge or true an existing hole or to make an accurately sized hole that is not available with our twist drills or reamers (in-between sizes or >1")

                                             

Using an Edge Finder

An edge finder is used to locate the machined edge of a workpiece.

The edge finder is made of a shank with a floating tip retained by an internal spring.

·        Mount edge finder in collet or drill chuck

·        Set spindle speed to  >1000 RPM

·        Slide tip off-center

·        Turn spindle on.  Raise the knee or lower the quill to bring the edge finder tip adjacent to the workpiece.

·        SLOWLY move along the x-axis or y-axis to make contact between the edge finder tip and the edge of the workpiece.

·        Continue to slowly advance the workpiece toward the edge finder tip.  As soon as the tip jumps sideways, STOP feed or you will break the edge finder.

·        The spindle is now positioned 1/2 tip diameter off of the edge of the workpiece.  (diameter = 0.200", so offset = 0.100”)

·        Lower the workpiece or raise the quill until the tip of the edge finder is above the workpiece.

·        Now advance the table 1/2 tip diameter  (0.100") farther to bring the spindle directly over the edge of the part.  Set the DRO to zero.

  

Tapping

Taps break very easily and are expensive, so read on ...

Tapping is used to thread a drilled hole.  The tap drill size is found on the chart near the drill index.

·      To accurately locate a hole to be tapped, start with a center drill.

·      Next, drill the pilot hole with the correct tap drill.

·      Tapping is done by hand with the power OFF.

·      Put tap in tap handle.

·      To align tap... Do not move workpiece in  X or Y axes after drilling.  Use a dead center in the milling machine spindle to align tap with pilot hole as illustrated.  Taps break easily if bending is applied.  Proper alignment eliminates bending.

·      Use cutting oil or tapping fluid on metals, and kerosene on plexiglas

·      Turn tap handle by hand 3-4 turns while applying light pressure into the hole.

·      Now, after each 1/2 turn, back the tap out of the hole to break the chip.  Continue advancing the tap until you feel the bottom of the part, or until the through hole has been tapped.  CAREFUL... stop when you feel the bottom of the hole or the tap will break.

IMPORTANT - Use a light touch.  It is VERY difficult  (impossible or expensive)  to remove a  broken tap.  Often a part must be re-started due to a broken tap.

Chatter

Chatter is the rattle or vibration between a workpiece and a tool due to the lack of rigid support for the workpiece or tool.  You may hear screeching or notice a wavy surface finish on the workpiece.

To Prevent Chatter

·        Check spindle speed.  When there is a lot of line contact between the tool and workpiece, keep the spindle speed low.        

·        Provide more support of the workpiece near the cut to reduce vibration of the workpiece.

If these suggestions don't help, ask a TA for assistance.

Other Workholding Devices

You can often hold parts in the vise on the mill, but some projects require other fixturing methods.

Strap Clamps

·        Use at least two strap clamps to hold a workpiece.

·        The parts of a strapclamp are pictured at right

·        To prevent the strap from marring the surface of the workpiece, use a small piece of scrap material, such as plexiglas, as a pressure pad at the point of contact.

·        To get maximum clamping force, the flanged nut should be tightened on the strap as near to the workpiece as possible.

·        Parts should be completely disassembled when you are finished.  If a nut gets stuck, ask for help with removing it.

Backing Sheets 

·        Use a backing sheet whenever there is the slightest chance of cutting through the workpiece and into a fixture or (heaven forbid) the milling machine table.

·        Use a piece of plexiglass or other scrap material.

·        Do not use wood.  Wood chips gum up the slides on the machines.  Wood will also continue to compress when clamped allowing a "loose" set up.

·        When drilling, make sure the backing sheet is thicker than the length of the pointed tip of the drill bit.

·        Set the quill stop to insure that you will not drill through the backing sheet and into the table.

·        DO NOT MACHINE INTO THE TABLE !!!!

Double stick tape

·        Double stick tape works only on thin, plastic sheet stock with lots of surface area to tape to.

·        Use double stick tape when other work holding devices get in the way of machining operations.

·        Do not use cutting oil or kerosene when fixturing with double stick tape as it may weaken the adhesive bond.

·        If you are using a backing sheet, tape your workpiece solid  (no gaps between the strips of tape), and "set" the tape by squishing the pieces together in a vise.  Whenever possible, use an oversized backing sheet and clamp the backing sheet to the table.

·        If you must tape to the table ...

-         Clean the table or fixture with solvent and clean paper towels to remove oil.

-         Apply tape to the workpiece

-         Mount workpiece and backing sheet onto the table or fixture.

-         To set the tape, clamp workpiece to table using a bar or thick plate and strap clamps.

-         Remove clamps and start machining.

-         When you are finished machining, removing your part from the backing sheet will be difficult.  Release it with ethanol, heat it with the heat gun, or get assistance.

Collet Block

·        Used to hold round parts in a collet (<1" diameter) during milling operations

·        4-sided and 6-sided blocks are available

·        Clean outside of collet and inside of collet block before assembly

·        Hold collet block in milling machine vise while tightening collar

How to change a milling machine vise

·        Do not change a vise or other workholding device without first getting approval AND assistance from the TA.

·        The table and vise can be damaged if not treated properly.