Oil circulation is the most common lubricating method for VTL table bearings because it helps to maintain a lower and more uniform bearing temperature, especially at higher speeds. Closed loop circulation systems are used to filter and cool the oil to maintain the temperature around 75°F (25°C.) Oil viscosities range from ISO VG 46 to 68. It is customary to inject the oil in multiple locations around the post/shaft in between the inner rings, typically 8 to 16 injectors depending on machine size (see Figure 7.)

While there are many types of grooving operations that can be performed on miniature parts, an end user first needs to define grooving. “HORN defines a grooving operation as any operation removing material between two flanks,” Drape said. “Keeping your mind open, this means:

Space provided behind the cutting edges to prevent rubbing. Sometimes called primary relief. Secondary relief provides additional space behind primary relief. Relief on end teeth is axial relief; relief on side teeth is peripheral relief.

One way to reduce setup and machining time when grooving miniature parts is to use multifunction tools, such as one that performs slotting, endmilling, OD grooving and thread milling. One such tool is the Iscar Multi-Master. “There are applications where we can use one tool to do multiple operations, mainly on an OD or face groove,” said Andy Kelling of Iscar Metals Inc. “If you’re end slotting, you can also groove if you orient the tool a different way, pick the part off on a Swiss-style machine and machine with an auxiliary spindle. The goal is to minimize the number of tools used in setup.” He noted that the Multi-Master can groove full radiuses and create flat-bottom grooves, notch grooves and other intricate grooves.

In addition, the repeatability of an insert below 0.300 " in diameter will not be as accurate as a solid-carbide tool, said Drape. “We’re not saying that inserts are bad in these kinds of applications,” he said. “However, shops using them might have to fight these kinds of problems. That’s why we stay with solid-carbide tools for grooving applications requiring tools below 0.300 " in diameter.”

The carrier assembly in Figure 6 consists of the carrier (housing), post (shaft), TXR bearing, outer ring clamping plate/bull gear, preload clamping plate (spigot), preload (shim) spacers, and the table. Because the table bolts to the carrier on most VTLs, the outer ring of the bearing rotates with the carrier and the inner rings (cones) are mounted to the post (shaft), which is attached to the machine frame and stationary; this is referred to as a “dead shaft”.

Speed capability is an important characteristic of the table bearing. Maximum table speeds range from 800 rpm on smaller machines to 50 rpm on larger machines. The limiting speeds of TXR bearings align nicely with VTL speed requirements.

VTL table bearing options can be divided into two categories, roller element bearing and hydrostatic bearings. Hydrostatic bearings are typically used on larger tables when roller bearings of sufficient accuracy and/or load-carrying capacity are not available (see Figure 10.) Like any other bearing type, hydrostatic bearings have advantages and disadvantages.

Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.

OD grooving is easiest to generate by side turning or plunging, starting at around 0.020 " wide, according to Iscar’s Kelling. For this application, Iscar recommends its PENTACUT insert with five cutting edges and a molded chipformer.

Most VTL tables mount to a carrier assembly that houses the bearing, as shown in Figure 5. The table is typically slotted to accommodate jaws or chucks for clamping down the workpiece to be cut. Carriers may also incorporate pallet changers to allow entire tables to be automatically loaded and unloaded onto the machine, to reduce setup time. The type of table and maximum workpiece size determine how much weight the TXR bearing is required to support.

There are various grooves and grooving techniques required for miniature parts. According to Ken King, COO of Thinbit/Kaiser Tool Co. Inc., Fort Wayne, Ind., grooving miniature parts requires determining what the grooves will be used for, what types of grooves are needed and what types of geometries the grooves must have. In miniature part production, grooves are typically for connecting parts (snap rings), sealing mating surfaces (O-rings), thread relief, holding lubrication and reducing surface contact. Types of grooves include OD and ID grooves, face grooves and angled grooves. “Grooves can blend into other features and multifunction tools can produce more than grooves, so creating a definition can be tricky, but grooves can have chamfers, radii and angles,” said King.

19.2 or ISO 492 do not meet all the requirements for this application. (See Selecting TRBs for Machine Tools: Understanding Tolerance Standards, Part 1.)

Myers added that guiding a continuous grooving chip from the cutting zone is important because a broken chip can lodge in the workpiece and fracture the insert. “This process is critical on small bores where the cutting tool is smaller, the room for chip evacuation is limited and the grooves are deep inside a bored hole,” he said.

One of the key decisions in grooving miniature parts is whether to use indexable or single-point inserts or solid-carbide tools. Duane Drape of HORN USA Inc. has some specific guidelines for shops to consider.

Groove or other tool geometry that breaks chips into small fragments as they come off the workpiece. Designed to prevent chips from becoming so long that they are difficult to control, catch in turning parts and cause safety problems.

“Some people are put off by the fact that a solid-carbide tool in some situations can cost $70, while an insert with three cutting edges costs just $12,” said Drape. “But that $12 insert goes into a $300 to $400 toolholder, and since the holder is very small, it is relatively weak and has to eventually be thrown away, possibly after using 10 inserts. When you add up those costs, your price is getting very comparable to solid carbide, and if you don’t get 10 uses out of a holder, and you get just one or two, the total cost is a lot more.”

Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.

According to Drape, the main challenge for ID grooving in miniature parts is the tiny tools’ relative lack of strength. “When you are going into a 1mm bore, you need a tool that has to fit axially into that 1mm bore and then apply radial force to [the bore]. This cutting pressure typically tries to bend the tool dramatically, causing form and chatter problems as well as tool breakage and chip evacuation issues,” he said.

While many shops purchase off-the-shelf grooving tools for miniature part applications, larger manufacturers that use high volumes of these tools may purchase blanks and grind their own tools. “We offer our Micro-Mini solid-carbide grooving tools that can be made into specials,” said Chris Wills, product specialist, Mitsubishi Materials USA Corp., Schaumburg, Ill. “Our customers do the finishing work themselves. Higher-end users will have a high-precision grinder, or they will send it to a supplier and have them custom made,” he said.

The advantages of the TXR bearing over other bearings of similar design and other bearing configurations are why it has become the standard for VTL table bearings. Their compactness and small cross-section provide a simplified table design together with a maximum running accuracy. Their wide, effective spread ensures high stability and maximum rigidity. Low starting and running torque reduce heat generation to yield good speed capability.

Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.

Myers of Denitool agreed that indexable grooving inserts should always be considered when the part or bore size permits access. He noted that Denitool’s smallest insert grooving system can enter a 0.150 " bore. “Insert inscribed circle sizes produced by Denitool have decreased over the past 5 years,” said Myers. “Our partnerships with key customers worldwide have indicated a clear need for development in this area.”

Any machining process used to part metal or other material or give a workpiece a new configuration. Conventionally applies to machining operations in which a cutting tool mechanically removes material in the form of chips; applies to any process in which metal or material is removed to create new shapes. See metalforming.

Before focusing on the TXR bearing, let’s review the bearing options for supporting VTL tables found in machining operations today, along with their advantages and disadvantages.

Measure of the relative efficiency with which a cutting fluid or lubricant reduces friction between surfaces.

Image

Turning is how material is removed from the workpiece on all lathes, horizontal or vertical. What makes turning fundamentally different from milling and grinding (which removes material from a stationary workpiece with a rotating tool) is that turning removes material from a rotating workpiece with a stationary tool (insert), as shown in Figure 8.

One parts manufacturer used the Multi-Master to make grooves in a 3 "-long dental probe on a Tsugami Swiss-style machine. The part is made of 17-4 PH heat-treated stainless steel and its largest diameter is 0.160 ". The instrument point tapers to a 0.0008 " tool-generated radius. The OD perpendicular groove and an OD parallel slot are both made with the same MM GRIT Multi-Master 0.020 "-wide insert. The parallel slot is about 0.750 " in length along two diameters. The 0.020 "-wide perpendicular groove on the rear OD was generated by the same rotating MM GRIT Multi-Master insert after picking off on the machine’s subspindle end. Grooving and slotting time decreased by 60 percent compared to the original process developed by another supplier, which used three tools.

Hydrostatic bearings accommodate larger tables than even the largest rolling element bearings available will do. They also have higher load ratings than rolling element bearings, and slightly higher accuracy.

Main body of a tool; the portion of a drill or similar end-held tool that fits into a collet, chuck or similar mounting device.

Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.

He added that the most commonly used operations on miniature parts include ID and OD grooving of seals, ID and OD threads, relief grooves for mating components and broaching of keyways, hexes, squares and Torx features.

Enlarging a hole that already has been drilled or cored. Generally, it is an operation of truing the previously drilled hole with a single-point, lathe-type tool. Boring is essentially internal turning, in that usually a single-point cutting tool forms the internal shape. Some tools are available with two cutting edges to balance cutting forces.

Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.

Operation in which the cutter is mounted on the machine’s spindle rather than on an arbor. Commonly associated with facing operations on a milling machine.

“That being said, you still can use a replaceable insert tool for grooving in bores as small as 0.300 " in diameter,” Drape said. “These are usually single-edge replaceable inserts, but they provide a greater axial depth into the component and a more reasonable cost for longer reaches than a solid-carbide tool could provide.”

Dimension that defines the exterior diameter of a cylindrical or round part. See ID, inner diameter.

Basically there are five VTL table roller bearing configurations that have been used to support VTL tables, including the TXR bearing, thrust TRB, thrust cylindrical roller bearing (CRB), ball bearing (BB), and a cross-roller CRB (CXR.) Comparing the characteristics of each of these bearing types makes it obvious why the tapered roller XR is the most popular choice for this application (see Table 1.)

Applying small grooving tools requires a different machining approach, including much lower feed rates than with larger grooving tools, said Drape. “You need to slow the feed rate down so as not to put too much pressure on the tool and break it,” he said. Typical feed rates for miniature grooving tools with 2mm to 6mm minimum bore sizes are from 0.0002 to 0.0008 ipr. Drape added that tool monitoring is essential when using tiny grooving tools because it is hard to visually or audibly detect a broken tool when the cutting edge is so small.

“One of the biggest changes we’ve seen is in the minimum bore sizes [the bore size that a grooving tool can fit into],” said Duane Drape, national sales manager, HORN USA Inc., Franklin, Tenn. “Four years ago, our minimum bore for grooving was 4mm. Now we have off-the-shelf 2mm minimum-bore grooving tools, and some customers are requesting off-the-shelf tools for grooving down to 1mm. Our smallest standard minimum-diameter tools are 0.2mm, but they are for boring, not grooving. We are constantly getting calls to reduce diameters even further.”

The Micro-Mini tool line is offered in widths of 1mm and 2mm with radii down to 0.05mm. Shank sizes vary depending on the round shank holders on the machine they are used for. Standard groove widths for Mitsubishi’s indexable GTAT grooving insert line are 0.3mm to 3.0mm (0.012 " to 0.118 ").

VTLs are used in a wide variety of industries to machine an extensive range of parts, with tolerances and surface finish requirements that vary greatly. Even parts with lower precision requirements still need a table bearing with precision-class tolerance. Unfortunately, the bearing features controlled in the ABMA

Other miniature grooving applications include fittings for automotive airbag components, fuel injection components, aerospace components and spray nozzles.

Still, he agreed that it is generally best to use inserts whenever possible due to cost concerns. “Most [grooving tool] manufacturers go down to 5/16 ", or an 8mm bore diameter, with inserts,” said King. “Regarding groove width, we go to 0.004 " as a standard size and offer every 0.001 " increment to 0.250 ". The trend is to smaller parts that require smaller grooves.”

With products such as medical implants, high-precision fuel injectors and electronics like the iPod growing in popularity, the growth trajectory for miniature parts will continue. These devices are not only getting smaller, they are also packed with more components. Grooving is one of the many metalcutting applications that will enable manufacturers to meet the growing demand for high-precision parts that make these devices work. CTE

Rollers have free-floating polymer separators situated between them to prevent direct contact during operation, as seen in Figure 15. Because they are free-floating and lightweight, they help to minimize the starting and running torque of the bearing. This is part of the reason that TXR bearings have the speed ratings for their size, capacity, and stiffness. The TXR bearing is “separable,” meaning the components are shipped separately and the bearing is assembled during the table installation (see Figure 14.)

Runout is the displacement of the surface of a rotating object (see Figure 16), while error motion is the displacement of the axis of rotation (Figure 17.) Although error motion is what matters most in this application, assembled bearing runout is the traditional method of validating whether all the component tolerances are met when the bearing comes together. Error motion is typically much less than the bearing runout.

Using an indexable insert for grooving is limited by bore diameter. In most cases, the smallest bore diameter for these types of tools is 0.708 ", according to Drape. There may be indexable tools for smaller bores but the radial DOC achievable with such tools is typically less than 0.040 ".

The roller small ends are drilled at the centerline and a polymer insert is pressed into the hole. The insert is referred to as a “roller extension” and prevents the roller from falling away from the outer race (rib) during assembly (see Figure 15.) When the bearing is in operation, there is an air gap between roller extension and the inner race when the large end is seated properly.

Another issue to consider is price. Solid-carbide tools typically have a higher unit price, but Drape argues that shops should consider the total application cost for grooving a miniature part before making a decision.

Eric Faust is an Application Engineering Specialist at The Timken Company, providing sales and customer support with technical bearing expertise and helping to resolve customer performance concerns.

A body cut turns the OD or bore of a workpiece along the Z axis. A face cut turns the top of the workpiece in the X-Y plane. The three primary turning parameters are the cut depth, surface speed, and feed rate. The cut depth is how deep the insert nose is plunged into the workpiece; the surface speed is the linear speed of the cut, or how fast the tool is moving through the workpiece; and the feed rate is how far the tool indexes for every rotation of the workpiece, and it is specified in inches/rev or mm/rev.

A typical solid-carbide grooving tool looks like an L, and the leg (noncutting) portion has to fit within the bore diameter. For a 2mm bore, that means the most the noncutting edge can be is 2mm minus a clearance. The foot portion of the L-shape has to be even smaller than the leg portion, which weakens it dramatically. Small-diameter bores also limit groove depth. “If you want to put a groove in a 2mm-dia. bore, the most tmax (radial DOC) is 0.2mm,” said Drape. Consider the machining of a 2mm bore. A grooving tool with a 1mm leg would require a safety factor of 0.2mm between the leg and the component. Theoretically, then, the tool could cut a 0.8mm-deep groove. However, with only 1mm of leg, the tool will probably be too weak to actually cut a groove of that depth, and would be limited instead to a 0.2mm groove depth, said Drape.

Denitool Inc. (615) 867-8882 www.denitool.com HORN USA Inc. (888) 818-4676 www.hornusa.com Iscar Metals Inc. (877) BY-ISCAR www.iscarmetals.com Thinbit/Kaiser Tool Co. Inc. (888) THINBIT www.thinbit.com Mitsubishi Materials U.S.A. Corp. (800) 523-0800 www.mitsubishicarbide.com

Replaceable tool that clamps into a tool body, drill, mill or other cutter body designed to accommodate inserts. Most inserts are made of cemented carbide. Often they are coated with a hard material. Other insert materials are ceramic, cermet, polycrystalline cubic boron nitride and polycrystalline diamond. The insert is used until dull, then indexed, or turned, to expose a fresh cutting edge. When the entire insert is dull, it is usually discarded. Some inserts can be resharpened.

Small parts require even smaller grooves. That simple idea is driving one of the ongoing trends in grooving—the need to design and apply tools that can cut a variety of grooves in a growing number of miniature parts. The challenge is to design tiny tools and develop cutting strategies that resist tool breakage and manage chip generation in tight spaces.

The rollers are designed with an L/D less than 1.0, meaning that their length is less than their diameter. This is required to achieve the 45° included angle and helps to keep the bearing section compact with large, effective rolling centers. The greatest advantage of this design is the high tilting stiffness that the effective spread yields, as illustrated in Figure 13.

Strip or block of precision-ground stock used to elevate a workpiece, while keeping it parallel to the worktable, to prevent cutter/table contact.

Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.

Machining, normally milling, that creates slots, grooves and similar recesses in workpieces, including T-slots and dovetails.

Figure 4 is a typical table drivetrain that consists of a variable-speed motor connected to a gear reducer that drives the table carrier assembly via a precision rack-and-pinion gear set. Note that the bull gear is fixed to the table carrier housing that is supporting the bearing outer ring, thus rotating the bearing outer ring.

Most VTLs have four degrees of freedom, three linear (X, Y and Z axes) and the table rotation (C axis.) The cross slide provides the “X” and “Y” motions, the ram provides the “Z” motion, and the table performs the “C” rotation. The reason these lathes are referred to as “vertical” machines are because the workpiece rotates about the vertical “C” axis when material is removed by the turning process.

Solid-carbide tools are preferable for the smallest bores, but shops should keep in mind that some manufacturers, HORN included, call small solid-carbide tools inserts because they typically require a specific toolholder, said Drape.

Sizing and tuning hydrostatic bearings properly is very difficult, because friction power grows exponentially with speed that is limited by heat. Typically, hydrostatic bearings have speed ratings 50% less than will be possible on a table of the same size supported by a TXR bearing.

The values of these parameters are based on the material, insert, and the desired quality of the cut (rough vs finish.) The geometry of the insert and whether cutting fluid is used also factor into determining these values. The quality of the cut is judged by the runout, surface finish roughness, and visual appearance of the workpiece (see Figure 9.) Cutting parameters are not addressed here, but assume that they are properly selected for workpiece material and cut type.

TXR bearings range in size from 11 in. OD (279 mm) to 111 in. OD (2819 mm.) Sizes above 111 in. (2819 mm) are possible, up to 3.4-meter OD currently. With table ODs that are 40 to 60% larger than the bearing OD, it is possible to use a TXR bearing in a VTL with a 28-foot (8.5-meter) table.

Because of the unique requirements of this application, the VTL table bearing manufacturer is compelled to control internal geometry to minimize the assembled bearing error motion, not the runout. (See Selecting TRBs for Machine Tools: Going Beyond Tolerance Standards, Part 2.)

It is important to understand that when measuring the tilt error motion of the table there are two components to the error motion: 1) the synchronous (repeatable) and the 2) asynchronous (non- repeatable), as shown in Figure 19. The synchronous tilt error motion contributes mostly to the runout of the workpiece where the asynchronous is primarily responsible for the surface roughness. The visual appearance of the surface finish is a function of frequency/modulation of the tilt error motion.

Manufacturers of miniature parts typically use Swiss-type and multiple-spindle machines, but any machine—including smaller CNC lathes—that can hold the part and the tooling can produce such parts.

Grooving miniature parts requires a conservative cutting approach and a choice between solid-carbide and insertable tools.

Although there are other options for VTL table bearings, the TXR is by far the most popular and common for these reasons. It ranks highest in almost every category, as shown in Table 1. With ongoing efforts to continually improve its performance, it will most likely remain the leading choice for bearings to support tables in VTL applications for years to come.

For ID grooving and undercutting, the company recommends a coolant-feed, solid-carbide insert, starting at a 0.002 " radius and a 0.024 " ID.

When making this choice, “first define an insert,” said HORN’s Drape. “There are many types of inserts available. Some are indexable [with multiple cutting edges] and others are single use.”

“We don’t go below 0.300 " in diameter with an insertable grooving tool,” he said. “There are insertable tools on the market below 0.300 ", but consider that an insert tool requires a pocket, so now you have a piece of carbide to be held down by a setscrew, and that piece of carbide has to have a certain amount of thickness to it. Once you machine that pocket, you have a very weak area in the front of the tool where the insert seats. You can’t necessarily apply the proper geometries to that insert. So, will tools like that work on a specific miniature application? Absolutely. But there could be a quality issue.”

It is worth mentioning that the size of a VTL is based on the table diameter and/or the maximum size workpiece that can be turned on the machine. Typically, the VTL model number is based on the table OD in centimeters, millimeters, or inches. Examples are the Phoenix VTL 144 seen in Fig. 1, the Honor model VL-160CM VTL in Fig. 2, and the You Ji Model VTC1000ATC+C in Fig. 3.

OD grooving, such as parting off, can be performed with a PENTACUT tool with five cutting edges and a molded chipformer.

The list of the characteristics that make TXR bearings very attractive for VTL table bearings includes:

Vertical lathes (VTLs), especially large ones, are used to manufacture critical parts in most heavy industries including aerospace, wind, oil-and-gas, and many more. The bearing that supports the table and workpiece has a direct impact on the tolerance and surface finishes that a VTL can produce. This report will help readers understand how precision tapered cross-roller bearings (TXRs) used to support the VTL tables impact cut quality of the parts processed on these machines. (See Figure 1.)

According to Andy Kelling, national product manager, screw machines, Iscar Metals Inc., Arlington, Texas, the overall length of most miniature parts that require grooving ranges from 1mm to 25.4mm, with diameters starting at 0.2mm. These parts are used in dental implants, stents, probes, instruments, electrodes, injectors, small fasteners and internal valve body components.

“Small parts usually have tight tolerances, so machine accuracy is important,” said Thinbit’s King. Small parts also rotate at a high rpm. The formula is (sfm × 12)/(π × diameter). That means a ½ "-dia. part has to rotate at twice the speed as a 1 " part.

Image

For face grooving miniature parts, Iscar recommends its standard solid-carbide inserts, which start at 1mm wide and an ID of 0.236 ".

Small grooving operations also present chip-form and chip-flow challenges, according to Larry Myers, vice president of operations for Denitool Inc., Murfreesboro, Tenn. “It is desirable to compress the width of the chip to insure that it does not lodge in the groove being produced, which can stop chip flow and cause catastrophic failure,” he said.

When used in lathe or screw-machine operations, this process separates a completed part from chuck-held or collet-fed stock by means of a very narrow, flat-end cutting, or parting, tool.

Special application methods recommended by Iscar use face forming and trepanning inserts to produce intricate forms containing grooves on the ends of some parts. Also, a multifunction tool that performs slotting and grooving has been used in several applications, according to Kelling. “When applications are reviewed, the focus is on using the fewest cutting tools,” said Kelling. “This decreases the required number of cutting tools, tool stations and tool change times, which reduces tooling costs along with machining cycle time.”

Machining grooves and shallow channels. Example: grooving ball-bearing raceways. Typically performed by tools that are capable of light cuts at high feed rates. Imparts high-quality finish.

Preloading of the bearing is accomplished with a shim pack that is captured between the shaft face and the preload clamping plate, as shown in Figure 6. This is referred to as a “spigot design” because it looks like a water spigot, with the “nozzle” protruding and seating on the lower inner ring (cone) backface. The shim pack also may be positioned between the lower inner ring backface and the clamping plate. The location is a matter of preference by the machine builder, either configuration is acceptable.

Image

Turning inserts are made from very hard materials (such as tungsten carbide) with the goal of cutting cleanly through the workpiece material. Both the insert and the workpiece material have a profound impact on the roughness (Ra) and visual appearance of the finished surface of turned parts, but for the scope of this report it is assumed that the insert is correct for the workpiece material.

Figure 2 highlights the major components and sub-systems of a VTL that includes a cross slide, headstock, ram, table and CNC controls, and chip removal system see. Under the guarding and sheet metal covers there is a table drive, lube oil circulation system, tool changer, and the base-frame that supports and integrates all these components. (See Figure 2.)

Imaginary circle that touches all sides of an insert. Used to establish size. Measurements are in fractions of an inch and describe the diameter of the circle.

About the Author: Alan Rooks is editorial director for Cutting Tool Engineering. He has been a business journalist for the past 27 years. Contact him at (847) 714-0174 or by e-mail at arooks@jwr.com.

The thickness of the shim pack determines the preload setting. TXR bearings are preloaded to achieve the speed, stiffness, and runout requirements of this application. The outer ring clamping plate ensures that the double outer ring is securely fixed to the carrier, and it is not uncommon to the have the bull gear integrated into this plate.

Space provided behind a tool’s land or relief to prevent rubbing and subsequent premature deterioration of the tool. See land; relief.

The error motion of the VTL table bearing contributes directly to the runout and surface finish of the workpiece. It manifests as a table tilting error, a characteristic that directly impacts workpiece runout and face flatness (see Figure 18.) The tilt amplitude is the bearing error multiplied by the height of the cut/measurement above the bearing centerline. Tilt error is minimized with TXR bearings because the roller and raceway geometry are controlled beyond ISO and ABMA standards. It is important to remember that the altitude of the cut test does have an impact on the results.

The drawbacks for hydrostatic bearings are their cost, complexity, and speed. Hydrostatic tables are significantly more expensive, both up front and to operate, because they require a hydraulic power unit and a chiller to manage the heat generated.

The thrust TRB design is a close second because it has comparable tilting stiffness and typically a higher load-carrying capacity. But it requires a second set-up bearing, which make the design more complicated and the overall machine cost higher. Minimizing the height of the table is one of the greatest advantages of the TXR because it makes for a less complex and more compact design than a table with a 2-TS or a thrust bearing (TTHDFL or CRB) with a TS set-up bearing (see Figure 11.)

There are two turning operations that are common to all lathes: the face cut and the body cut (see Figure 8.)

Eric Faust is an Application Engineering Specialist with The Timken Company. Contact him at [email protected], or via LinkedIn.

Drilling deep holes that are too large to be drilled by high-pressure coolant drills or gundrills. Trepanning generates a solid core and normally requires a big, powerful machine. Shallow trepanning operations can be performed on modified engine or turret lathes or on boring machines. See boring; drilling; spade drilling.

According to King of Thinbit, solid-carbide tools are typically preferred in most ID grooving applications under 0.500 ". While these tools are more expensive, they are stronger. “With inserts, you must reduce the strength of the toolholder to have a clamping mechanism for the insert and a place to mount the insert,” said King. This is not required for small solid-carbide grooving tools.

A TXR is effectively double row bearing where both rows reside in the same axial location. TXR’s are referred to as “cross-roller bearings” because alternating roller’s apex is oriented in the opposite direction of its two adjacent neighboring rollers forming a cross or “X” pattern. The most common available configuration is the TXRDO and consists of a double cup (outer ring with two raceways), two inner rings, a roller set, roller separators, and roller extensions (see Figure 12.)

Another issue when grooving miniature parts is the choice between insertable and solid-carbide tools. Many shops like to use inserts whenever possible due to their lower unit costs, but there are size limitations and other considerations that factor into the decision (see sidebar on page 61).

Chip management factors include insert and chipbreaker geometry as well as the feeds, speeds and toolpath. “In all grooving operations [including threading], cutting pressure is very high compared to other cutting [operations] because of the length of the cutting edge required to make the groove and the confined chip-removal space,” said Myers. “The reduction of this cutting pressure is critical to successful grooving and can be accomplished mainly by using proper feeds and speeds, tool design and lubricants.” The recommended speeds and feeds depend on the type and hardness of the workpiece material, according to Myers. He recommends lubricants with high lubricity for miniature part applications.

The bearing is one of many components on a VTL that must work nearly flawlessly to performance requirements. It is important to note that although the bearing is one of the primary contributors to the runout, surface finish, and visual appearance of the workpiece there are other factors, outside the bearing, that also can impact the quality of the cut. The rigidity and natural frequency of the ram, headstock, cross slide and base need to be taken into consideration as well. The workpiece material inserts and cutting parameter along with the workpiece material also impacts the quality. Finally, any external vibrations that can be transmitted to the table post such as motors, pumps, compressors, gear drives, etc. will also show up in the table error motion signal.

Operation in which a cutter progressively enlarges a slot or hole or shapes a workpiece exterior. Low teeth start the cut, intermediate teeth remove the majority of the material and high teeth finish the task. Broaching can be a one-step operation, as opposed to milling and slotting, which require repeated passes. Typically, however, broaching also involves multiple passes.

Having detailed where the TXR bearing is located in the VTL, and how it is mounted, let’s address the basics of turning – which is the primary operation of all vertical lathes.

2-D or 3-D path generated by program code or a CAM system and followed by tool when machining a part.

To best explain how TXR bearings influence the performance of a VTL let’s start by providing a brief overview of the application, including a rundown of the VTL’s major components and sub-systems, along with a basic explanation of “turning,” the primary operation performed on these machines.