Tooling for Numerical Control Machines

The most important points to be considered are:

Tool materials. Although high-speed steel (HSS) tools are used for small- diameter drills, taps, reamers, end mills, and slot drills, the bulk of tooling for NC machining involves the use of cemented carbides. Hardness and toughness are necessary requirements for a tool material. In this regard, HSS tools possess high toughness but are not hard and therefore cannot be used for high material removal rates. The hardness of cemented carbides is almost equal to that of diamond. However, lack of toughness presents a major problem, w'hich can be improved by the addition of cobalt to the tungsten carbides (WCs). Titanium and tantalum carbides are also used. Coated and nanocoated tools provide high wear resistance and thus increase the tool life by up to five times.

Solid carbide tools. These are used when the WP material is difficult to machine using HSS tools. Solid carbide milling cutters of 1.5 mm diameter, small drills of 0.4 mm diameter, and reamers as small as 2.4 mm diameter are available. Such tools should be short, mounted with minimum overhang, and used on vibration-free NC machine tools.

Indexable inserts. These have the correct cutting geometry and precise dimensions and are located in special holders or cartridges. Such inserts do not require resharpening and ensure rapid replacement. The inserts are indexable; that is, as the cutting edge becomes blunt, the insert is moved to a new position to present a new edge to the machining process. A facility for the control of swarf is ensured by forming a groove in the insert that works as a chip breaker.

Tool turrets. Automatically indexable turrets, shown in Figure 9.23, are used to accommodate cutting tools. These turrets are programmed to rotate to a

Indexable tool turrets

FIGURE 9.23 Indexable tool turrets.

new position so that a different tool can be presented at work. Indexable turrets are used in the majority of turning centers as well as some NC milling and drilling machines. Turrets are now available that can accommodate 8 to 10 tools. Some machines have two turrets; one is in use, while the other is loaded with tools for a particular job and attached to the machine when required. Turrets fitted to NC drilling and milling machines have to rotate their tools at a predetermined speed, as they act as a spindle. Tool stations are numbered according to the tooling stations available. When writing the part programs, the programmer provides each tool with a corresponding number in the form of a letter T followed by the corresponding numerical identity in two digits: T01, T02, and so on.

Tool magazines. A tool magazine, shown in Figures 9.24 and 9.25, is indexable storage used on a machining center to store tools not in use. They are available as rotary drum and chain types. When the tool is called into use, the magazine indexes by the shortest route to bring the tool to a position where it is accessible to a mechanical handling device. At the end of use, the tool is returned to its slotted position in the magazine before calling the next tool. Rotary drums with 12-24 stations are available, and 24-180 stations are available for the chain type.

Tool replacement. Cutting tools should be replaced when affected by wear or breakage. Tool changes must be made rapidly. The replaced tool must be of identical dimensions to the original one, which is achieved by using a qualified or preset tooling. Temporary modifications could also be achieved by offsetting the tool from its original datum. The preset tooling concept, shown in Figure 9.26a, is used for both turret and spindle-type machines. For NC machines, the cutting tool is preset to a specific length and diameter while it is off the machine using special fixtures and gauges. The tool length

Chain-type magazine with automatic tool changer

FIGURE 9.24 Chain-type magazine with automatic tool changer.

Rotary-type magazine with automatic tool changer

FIGURE 9.25 Rotary-type magazine with automatic tool changer.

is used by the part programmer to develop the Z-axis coordinate. Preset tool holders and boring bars are available for many NC turning machines. Once these are preset to the appropriate dimensions, inserts can be changed, and WP tolerances are maintained by minor adjustment of the tool offset switches.

Qualified and preset NC tools

FIGURE 9.26 Qualified and preset NC tools: (a) preset tool and (b) qualified tool.

Qualified tool holders for NC lathes are ground to standardized dimensions at close tolerances, and no presetting is required (Figure 9.26b). The qualified tool holder is usually inserted into the turret tool block and tightened in position. The dimensions provided by the manufacturers of the qualified tool holder are used by the part programmer, and minor adjustments are easily made by tool offset switches.

Types of Numerical Control Machine Tools

The most important types of NC machine tools are:

  • 1. NC drilling machines. An NC drilling machine holds, rotates, and feeds the drilling tool into the WP. They are available in a wide range of types and sizes that are built with single spindle or multiple spindles. Some machines are equipped with turrets and others with tool-changing mechanisms. Either two or three axes are available, and some drilling machines are even capable of performing milling operations.
  • 2. NC milling machines. These machines (Figure 9.27) are used to machine flat surfaces and produce contours and curved surfaces. The orientation of the spindle may be horizontal or vertical and provided with a single spindle or multiple spindles. Milling machines with two perpendicular spindles allow machining a hole and a vertical surface simultaneously. Such a facility is useful when machining large components, as shown in Figure 9.28.
Typical CNC milling machine. (From Hardinge Inc.)

FIGURE 9.27 Typical CNC milling machine. (From Hardinge Inc.)

On the other hand, simultaneous machining of the vertical and horizontal planes could be achieved, in this setup, by replacing the boring tool in the vertical spindle by a face milling cutter. NC milling machines may have from two to five axes under tape control. NC milling machines can do some of the work normally performed on NC drilling machines, such as drilling, boring, and tapping.

  • 3. NC turning machines. Lathes are primarily used for producing cylindrical shapes in addition to cutting tapers, boring, drilling, and thread cutting. NC lathes are equipped with either straight-cut or continuous-path control systems. Most NC lathes produced today are equipped with continuous-path control and circular interpolation. They are capable of tool offset so that the machine operator can make fine adjustments in the cutting tool location to achieve the required part size. Figure 9.29 shows a typical NC turning machine.
  • 4. NC machining centers. Machining centers perform a wide range of operations that include milling, drilling, boring, tapping, countersinking, facing, spot facing, and profiling. Machining centers are able to change the cutting tools automatically, which allows most of the machine time to be devoted to the cutting operation. Most NC machining centers have three axes. In a four-axes system, the fourth axis is used to rotate the table, which
Machining a hole and a vertical surface simultaneously. (From Gibs, D., An Introduction to CNC Machining, ELBS Cassell Publishers Ltd., London, 1988. With permission.)

FIGURE 9.28 Machining a hole and a vertical surface simultaneously. (From Gibs, D., An Introduction to CNC Machining, ELBS Cassell Publishers Ltd., London, 1988. With permission.)

CNC lathe QUEST 10/56 of Hardinge Inc. (From Hardinge Inc., Berwyn, PA. With permission.)

FIGURE 9.29 CNC lathe QUEST 10/56 of Hardinge Inc. (From Hardinge Inc., Berwyn, PA. With permission.)

enables the machining of four sides of a part. In many cases, it is possible to machine a part completely without removing it from the machine. Figure 9.30 shows a typical machining center.

5. NC turning centers. These machines combine the features of bar-type, chucking-type, and turret lathes. They are built with four axes of control and are also equipped with continuous-path NC systems with circular interpolation. Their design may include a slanted or vertical bed rather than the horizontal one normally used with conventional center lathes. The capabilities of turning centers can be extended by providing two turrets, such that two tools can cut simultaneously. Power-driven tool holders (which rotate when the WP is stationary) permit milling of flats, keyways, and slots in addition to the drilling of holes offset from the machine axis. Figure 9.31 shows a typical turning center. The use of tooling magazines extends the range of tooling that may be used, as shown in Figure 9.32.

Five-axes vertical machining center (5ax400) of Hardinge Incorporation. (From Hardinge Inc.)

FIGURE 9.30 Five-axes vertical machining center (5ax400) of Hardinge Incorporation. (From Hardinge Inc.)

CNC (Super Quadrex 250M) turning center. (From MAZAK Corporation.)

FIGURE 9.31 CNC (Super Quadrex 250M) turning center. (From MAZAK Corporation.)

Additional tooling facilities. (From Gibs, D., An Introduction to CNC Machining, ELBS Cassell Publishers Ltd., London, 1988. With permission.)

FIGURE 9.32 Additional tooling facilities. (From Gibs, D., An Introduction to CNC Machining, ELBS Cassell Publishers Ltd., London, 1988. With permission.)

Input Units

Data can be input into the MCU using one of the following methods:

Manual data input {MDI). This method is normally used for setting the machine and editing the program as well as writing complete simple programs. For NC machines with noncomputerized control units, data recording facilities are not often available. In the case of CNC machines, the computer retains the data so that it can be transferred to a recording medium such as magnetic tape or disk or transferred back to the machine when required.

Conversational MDI. This method involves the operator pressing appropriate keys on the control console in response to questions that appear in the visual display unit (VDU). This method is faster than methods that require the use of data codes.

Punched tape. Punched tapes are of standard 1 in. width. They use eight 0.072 in. holes across the width of the tape and one 0.046 in. sprocket/feed hole between tracks 3 and 4. Punched tapes can be read inexpensively, are less sensitive to handling, are inexpensive to purchase, and require less equipment for manufacturing and less costly space for data storage.

The binary coded decimal (BCD), shown in Figure 9.33, is used for coding digits on the tape. Accordingly, five of the eight tracks per channel are assigned the numerical values 0, 1,2, 4, and 8 so that any numerical value from 0 to 9 can be represented in one row of the tape. The combination of punched holes per bit in the tape establishes the values associated with that row.

The El A RS-244-A system and the American Standard Code for Information Interchange (ASCII) RS-358 systems are available for NC and are currently used for coding the numbers on the tape, as shown in Figure 9.33.

(a) EIA RS-244-A and (b) RS-358 (ASCII) coding systems

FIGURE 9.33 (a) EIA RS-244-A and (b) RS-358 (ASCII) coding systems.

The EIA RS-244-A system is commonly used, while ASCII-coded input is optional in many of today’s NC systems. It should be mentioned here that the RS-244-A coding system involves the use of odd parity, in which track 5 makes certain that an odd number of holes (not including the sprocket hole) appears on every row of the tape, whereas the ASCII subset uses even parity, in which an extra hole is added to track 8 in the tape to ensure an even number of holes in each row'. The BCD code format is the same in both code systems. All numerical and alphabetical codes along with some special characters and function codes are available in both systems.

Magnetic tape. Magnetic tapes, in the form of cassettes, are widely used for transmitting data. They require expensive equipment for program recording and reading. The programmer cannot see the recorded data, and therefore recording errors cannot be seen as they can on punched tapes. Magnetic tape requires special storage space and must be handled carefully.

Portable electronic storage unit. In this method, the data transferred into the storage unit away from the machine shop are carried to the machine and connected to the MCU, and data are then transferred. Data transfer is high, and the capacity of such units is high, so that a number of programs can be accommodated at a time.

Magnetic disk input via computer. In this method, it is possible to transfer data stored on a floppy disk into the computer and hence into the MCU. Similarly, data on the control unit can be extracted and recorded. The rate at which the data can be transferred or retrieved using a disk is faster than when using a tape, and the storage area is also much greater.

Master computer The prepared program stored on the memory of a master computer is transferred to the microcomputer of the MCU when required. Such an arrangement, also described earlier, is what is known as DNC.

 
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