Planetary Transmission

Figure 2.46 shows a planetary transmission with bevel gears that is widely used in machine tools. Accordingly, any two members may be the driving members, while the third one is the driven member. The differential contains central gears Z, and Z₄

FIGURE 2.46 Planetary transmission.

and satellites Z₂ and Z, (an additional wheel) rotated by worm gear 2. The differential can operate as follows (Chernov, 1975):

• 1. Z₄ is a driving member, the carrier is a driven member, and worm gear 2 is stationary.
• 2. The carrier is a driving member, gear Z₄ is a driven member, and worm gear 2 is stationary.
• 3. Gear wheel Z, is a driving member (rotated by worm gear 2), gear wheel Z₄ is a driven member, and the carrier is fixed.
• 4. The carrier is a driving member, so is gear Z„ and gear wheel Z₄ is a driven member.
• 5. Gear wheels Z, and Z₄ are driving members, and the carrier is a driven member.

The principal relationship between axis speeds is described by Willis formula, with Z₂ = Z₃ and Z, = Z₄, as follows:

where

i = conversion ratio

«о = speed of carrier rotation

и,, n₂ = rotational speeds of Z, and Z₄, respectively

The minus sign in this equation indicates that gear wheels Z, and Z₄ rotate in opposite direction when the carrier is stationary. Willis also suggested the following relations:

The plus sign in Equation 2.22 indicates opposite rotational directions, and the minus sign indicates the same direction of the differential driving members.

Machine-Tool Motors

Most machine-tool drives operate on standard three-phase 50 Hz, 400/440 V ac supply. The selection of motors for machine tools depends on the following:

• 1. Motor power
• 2. The power supply used (ac/dc)
• 3. Electrical characteristics of the motor

TABLE 2.8

Machine Tool Motors

From Nagpal. G.R., in Machine Tool Engineering, Khanna Publishers, Delhi, India, 1999.

FIGURE 2.47 Reversing mechanisms: (a) tumbler yoke gear, (b) spur gear with clutch, and (c) bevel gear with clutch.

valve, and electrical reversal is achieved by changing the direction of the drive motor rotation.

Couplings and Brakes

Shaft couplings are used to fasten together the ends of two coaxial shafts. Permanent couplings cannot be disengaged while clutches engage and disengage shafts in operation. Safety clutches avoid the breakdown of the engaging mechanisms due to a sharp increase in load, while overrunning clutches transmit the motion in only one direction. Figure 2.48 shows permanent couplings. Figure 2.49 shows a typical claw clutch (a) and a toothed clutch (b). These two clutches cannot be engaged when the difference between the speeds of shafts is high. However, a friction clutch (c) can be engaged regardless of the speeds of its two members. Additionally, they can slip in the case of overloading. Other types of clutch include friction multidisk, contactless magnetic, and hydraulic clutch (Chernov, 1984). Brakes are used in machine tools to quickly slow or completely stop their moving parts. This step can be performed using mechanical, electrical, or hydraulic (or a combination of these) devices. Figure 2.50

FIGURE 2.48 (a) Flanged coupling and (b) Oldham coupling.

FIGURE 2.49 (a) Claw clutch, (b) toothed clutch, and (c) friction clutch. (From Chernov, N., Machine Tools, Mir Publishers, Moscow, 1975. With permission.)

FIGURE 2.50 Shoe brake. (From Chernov, N.. Machine Tools, Mir Publishers, Moscow, 1975. With permission.)

FIGURE 2.51 Friction brake. (From Chernov, N.. Machine Tools, Mir Publishers, Moscow, 1975. With permission.)

shows the shoe brake, in which shoes (1 and 6) are connected by a rod (3), whose length is controlled by a nut (2) that controls the clearance between the shoes and the pulley (7). Braking is achieved by pressing the shoe against the pulley by an arm (4) driven by the brake actuator (5). Band brakes operate frequently by electromagnetic or solenoid actuators. In a multiple-disk friction brake, shown in Figure 2.51, when the shaft sleeve (3) is moved to the left, it engages with its lever (2), which in turn compresses the clutch disks, thereby engaging the clutch. For braking, the sliding sleeve (3) is moved to the right, disengaging the clutch (1) and engaging the friction brake (4).

Reciprocating Mechanisms

Quick-Return Mechanism

Ruled flat surfaces are machined on the shaping or planing machines by the combined reciprocating motion and the side feed of the tool and WR Figure 2.52 shows the quick-return mechanism of the shaper machine. Accordingly, the length of the stroke is controlled by the radial position of the crank pin and sliders A and B. The time taken for the crank pin to move through the angle corresponding to the cutting stroke a is less than that of the noncutting stroke p (the usual ratio is 2:1). Velocity curves for the cutting and reverse strokes are shown in Figure 2.52. The maximum speed occurs when the link is vertical.

FIGURE 2.52 Quick-return mechanism.

The speed of the link at point P for a given stroke length L will be that at the corresponding crank radius r; hence, the cutting speed v_c at point P, is

where

n = number of strokes per minute / = length of crank arm (constant)

Similarly, the maximum reverse speed v_r is given by the following equation:

In terms of the stroke length for maximum radius using similar triangles OBA and OCD,

hence

and

therefore, the speed ratio, Q Illustrative Example 3

In the slotted arm quick-return mechanism of the shaping machine, the maximum quick-return ratio is 3/2, and the stroke length is 400 mm. Calculate the length of the slotted arm. Calculate the maximum quick-return ratio if the stroke length is 180 mm.

SOLUTION

The quick-return ratio Q

The quick-return ratio Q for L = 180 mm

Whitworth Mechanism

This arrangement is shown in Figure 2.53; when AB rotates, it drives CE about D by means of the slider F so that G moves horizontally along MN. AB moves through an angle (360° - a), while CE moves through 180°, which is less than 360° - a. Also, the crank moves through a while CE moves through 180°, which is greater than a. Hence, with a uniformly rotating crank, the link moves through one-half of its revolution more quickly than the other. The angle a is used for the return stroke. Hence,

The stroke can be changed by altering the radius DE, with the angle a being unchanged. Provided that the fixed center D lies on the line of movement of G, the ratio of the cutting speed to the return speed lies between 1:2 and 1:2.5.

Hydraulic Reciprocating Mechanism

As shown in Figure 2.54, the electrically driven pump supplies the fluid under pressure to the operating cylinder through the solenoid-operated valve. The piston is connected to the machine table. At the end of the forward stroke, the direction control

FIGURE 2.53 Whitworth quick-return mechanism.

FIGURE 2.54 Reciprocating mechanism (a) and velocity diagram (b) of hydraulic shaper.

valve reverses the direction of the flow through limit switches set at the stroke limits, and the table moves backward.

Material Selection and Heat Treatment of Machine-Tool Components

The operating characteristics of a machine-tool component depend on the proper choice of the material of each component. The most extensively used materials in machine-tool components include Cl and steels.

Cast Iron

In the majority of cases, machine-tool beds and frames are made of gray Cl (see Table 2.9) because of its good damping characteristics. If the guideways are cast as an integral part of the bed, frame, column, and so on, high-wear resistance grade Cl (GG22 or A48-30B) with pearlitic matrix is recommended for medium-size machine-tool beds and frames for a wall thickness of 10-30 mm and the grade GG26 or A48-40B for a wall thickness of 20-60 mm. High-strength, wear-resistant special gray Cl of the grade GG30 or A48-50B with a pearlitic structure can be used for heavy machine-tool beds with a wall thickness of more than 20 mm.

Due to the drawbacks associated with the manufacture of beds and frames by casting, beds and frames are made by welding rolled steel sheets. The elastic limit and the mechanical properties of such steel are higher than those of Cl. Therefore, much less material (50-75%) is required for welded steel structures or beds than Cl ones to be subjected to the same forces and torques, if the rigidity and stiffness of the two structures are made equal. Cl beds are more often used in large-lot production, while welded steel beds and frames are preferable in job or small-lot production.

Steels

The majority of machine-tool components, such as spindles, guides, shafts, springs, keys, forks, and levers, are generally made of steels. Since the Young’s modulus of various types of steels cannot vary by more than ±3%, the use of the alloy steels for machine-tool components does not provide any advantages unless their application is mandated by other requirements. Tables 2.10 and 2.11 show the different

TABLE 2.9

Grades of Gray Cl According to DIN 1691, American Iron and Steel Institute (AISI), Society of Automotive Engineers/American Society for Testing and Materials (SAE/ASTM)

T. Thomas; M. Martin.

CK 10* and CK 15* are carbon steels of quality better than CIO and CI5 due to smaller contents of S and P: Cy, cyaniding.

types of structural and alloy steels frequently used in machine tools. Structural steels are used when no special requirements are needed. Case-hardening steels of carbon content <0.25% and phosphorus (P) or sulfur (S) not exceeding 0.40% are used when the surface hardness of the component needs to be very high while the core remains tough. Typical applications of case-hardening steels are in gears, shafts, and spindles. Tempered steels, shown in Table 2.12, have a higher carbon content than case-hardened steels. They are used when high strength and toughness are required. Nonalloy tempered steels are used for machine components that are not heavily loaded. For components that are heavily loaded, such as gears, spindles, and shafts, the alloy type is recommended. Nitriding steels (see Table 2.13) contain aluminum as the main alloying element. After nitriding, the components possess an extraordinary surface hardness and therefore are used for machine parts subjected to wear, such as spindles, guideways, and gears. The main advantage of the nitriding steel is minimum distortion after nitriding.

Testing of Machine Tools

After the manufacture or repair of any machine tool, a machine-tool test (usually called an acceptance test) should be performed according to the approved general specification. Such tests are essential, because the accuracy and surface quality of the parts produced depend on the performance of the machine tool used. Testing machine tools has the following general advantages:

• 1. Determines the precision class and the accuracy capabilities of the machine tool
• 2. Prepares plans for preventive maintenance
• 3. Determines the actual condition and hence the expected life of the machine tool

Machine-tool tests are classified into two categories: geometrical alignment tests and performance tests.

Geometrical tests cover the manufactured accuracy of machine tools. These tests are carried out to determine the various relationships between the various machine- tool elements when idle and unloaded (static test). They include checking parallelism of the spindle and a lathe bed, squareness of the table movement to the milling machine spindle, straightness of guideways, and so on. Static tests are inadequate to judge the machine-tool performance, because they do not reveal the machine-tool rigidity or the accuracy of machining. The normal procedure for acceptance tests is made through the following steps: ^{[1] [2] [3]}

TABLE 2.13 Nitriding Steels

• 4. Testing the spindle concentricity, axial slip, and accuracy of axis
• 5. Conducting working tests to check whether the accuracy of machined parts is within the specified limits
• 6. Preparing acceptance charts for the machine tool that specify the type of test and the range of allowable limits of deformation, deflection, error in squareness, flatness eccentricity, parallelism, and amplitude of vibrations

In contrast, dynamic tests are used to check the working accuracy of machine tools through the following steps:

• 1. Performing an idle run test and operation check mechanisms
• 2. Checking for geometrical accuracy and surface roughness of the machined parts
• 3. Performing rigidity and vibration tests

Standards for testing machine tools are covered by Schlesinger (1961).

Maintenance of Machine Tools

Machine tools cannot produce accurate parts throughout their working life if there is excessive wear in their moving parts. Machine-tool maintenance delays the possible deterioration in machine tools and avoids the machine stoppage time that leads to lower productivity and higher production cost. Maintenance is classified under the following schemes.

Preventive Maintenance

Preventive maintenance is mainly carried out to reduce wear and prevent disruption of the production program. Lubrication of all the moving parts that are subjected to sliding or rolling friction is essential. A regular planned preventive maintenance consists of minor and medium repairs as well as major overhaul. The features of a well-conceived preventive maintenance scheme include

• 1. Adequate records covering the volume of work
• 2. Inspection frequency schedule
• 3. Identification of all items to be included in the maintenance program
• 4. Well-qualified personnel

Preventive maintenance of machine tools ensures reliability, safety, and the availability of the right machine at the right time. Figure 2.55 shows preventive maintenance of a machine tool.

Corrective Maintenance

When a machine tool is in use, it should be regularly checked to determine whether wear has reached the level when corrective maintenance should be carried out to

FIGURE 2.55 Preventive maintenance scheme.

avoid machine-tool failure. A record of all previous repairs shows those elements of the machine tool that need frequent inspection. Additionally, such records are used for decisions regarding the need for machine-tool reconditioning and replacement.

Reconditioning

The need for machine-tool recondition is determined by the frequency of the corrective maintenance repairs. Every machine-tool component has a certain life span beyond which it becomes unserviceable despite the best preventive maintenance. A major overhaul or reconditioning is required.

Inspection reports of the machine indicate the components to be replaced, labor time, and the cost estimate. As a general rule, it is undesirable to recondition the machine if the cost exceeds 50% of the cost of buying new equipment.

Review Questions

• 2.14.1 State the main requirements of a machine tool.
• 2.14.2 Give examples of open and closed machine-tool structures.
• 2.14.3 Explain why closed-box elements are best suited for machine-tool structures.
• 2.14.4 Sketch the different types of ribbing systems used in machine-tool frames.
• 2.14.5 Explain what is meant by light- and heavyweight construction in machine tools.
• 2.14.6 Sketch the different types of machine-tool guideways.

• 2.14.7 Show how wear is compensated for in machine-tool guideways.
• 2.14.8 Differentiate between cast and welded structures.
• 2.14.9 Distinguish among the kinematic, structural, and speed diagrams of gearboxes.
• 2.14.10 Show an example of externally pressurized and rolling friction guideways.
• 2.14.11 Show the different schemes of spindle mounting in machine tools.
• 2.14.12 What are the main applications of pick-off gears, feed gearboxes with a sliding gear, and Norton gearboxes?
• 2.14.13 Compare toroidal and disk-type stepless speed mechanisms.
• 2.14.14 Give examples of speed-reversing mechanisms in machine tools.
• 2.14.15 Derive the relationship between the cutting and the reverse speeds of the quick-return mechanism used in the mechanical shaper.
• 2.14.16 State the main objectives behind machine-tool testing.
• 2.14.17 Compare corrective and preventive maintenance of machine tools.

References

Browne. JW 1965, The theory of machine tools, book-1, Cassell and Co. Ltd.. London. Chernov, N. 1975, Machine tools, Mir Publishers, Moscow.

DIN 1691—Grades of gray cast iron.

DIN 17100—Tempered and structural steels.

DIN 17210—Case hardened steels.

DIN 323—Standard values of progression ratio.

DIN 803—Standard feeds.

DIN 804—Standard speeds.

ISO/R229—Standard feeds and speeds.

ISO/R229—Standard values of progression ratio.

Koenigsberger, F 1961, Berechnungen, Konstruktionsgrundlagen unci Bauelemente spanender Werkzeugmaschinen, Springer, Berlin.

Nagpal, GR 1996, Machine tool engineering, Khanna Publishers, Delhi, India.

Schlessinger, G 1961, Testing machine tools, The Machine Publishing Company, London. Youssef, H, Ragab, H & Issa, S 1976, Design and construction of machine tool elements, Dar Al-Maaref Publishing Company, Alexandria.

• [1] Checking the principal horizontal and vertical planes and axes using a spiritlevel
• [2] Checking the guiding and bearing surfaces for parallelism, flatness, andstraightness, using dial gauge, test mandrel, straight edge, and squares
• [3] Checking the various movements in different directions using dial gauges,mandrels, straight edges, and squares