Cnc Design Features
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CNC SYSTEMS - MECHANICAL COMPONENTS The drive units of the carriages in NC machine tools are generally the screw & the nut mechanism. There are different types of screws and nuts used on NC machine tools which provide low wear, higher efficiency, low friction and better reliability.
(1) Recirculating ball screw The recirculating ball screw assembly shown in figure 25.1 has the flanged nut attached to the moving chamber and the screw to the fixed casting. Thus the moving member will move during rotational movement of the screw. These recirculating ball screw designs can have ball gages of internal or external return, but all of them are based upon the "Ogival" or "Gothic arc". In these types of screws, balls rotate between the screw and nut and convert the sliding friction (as in conventional nut & screw) to the rolling friction. As a consequence wear will be reduced and reliability of the system will be increased. The traditional ACME thread used in conventional machine tool has efficiency ranging from 20% to 30% whereas the efficiency of ball screws may reach up to 90%.
Figure 25.1: Recirculating ball screw assembly
Figure 25.2: Preloaded recirculating ball screw
There are two types of ball screws. In the first type, balls are returned through an external tube after few threads. In another type, the balls are returned to the start through a channel inside the nut after only one thread. To make the carriage movement bidirectional, backlash between the screw and nut should be minimum. One of the methods to achieve zero backlash is by fitting two nuts. The nuts are preloaded by an amount which exceeds the maximum operating load. These nuts are either forced apart or squeezed together, so that the balls in one of the nuts contact the opposite side of the threads. These ball screws have the problem that minimum diameter of the ball (60 to 70% of the lead screw) must be used, limiting the rate of movement of the screw.
(2) Roller screw
Figure 25.3: Roller screw
These types of screws provide backlash-free movement and their efficiency is same as that of ball screws. These are capable of providing more accurate position control. Cost of the roller screws are more compared to ball screws. The thread form is triangular with an included angle of 90 degrees. There are two types of roller screws: planetary and recirculating screws.
Planetary roller screws: Planetary roller screws are shown in figure 25.3. The rollers are threaded with a single start thread. Teeth are cut at the ends of the roller, which meshes with the internal tooth cut inside the nut. The rollers are equally spaced around and are retained in their positions by spigots or spacer rings. There is no axial movement of the rollers relative to the nut and they are capable of transmitting high load at fast speed.
Recirculating roller screws: The rollers in this case are not threaded and are provided with a circular groove and are positioned circumferentially by a cage. There is some axial movement of the rollers relative to the nut. Each roller moves by a distance equal to the pitch of the screw for each rotation of the screw or nut and moves into an axial recess cut inside the nut and disengage from the threads on the screw and the nut and the other roller provides the driving power. Rollers in the recess are moved back by an edge cam in the nut. Recirculating roller screws are slower in operation, but are capable of transmitting high loads with greater accuracy.
28. CNC WORK HOLDING DEVICES
With the advent of CNC technology, machining cycle times were drastically reduced and the desire to combine greater accuracy with higher productivity has led to the reappraisal of work holding technology. Loading or unloading of the work will be the nonproductive time which needs to be minimized. So the work is usually loaded on a special work holder away from the machine and then transferred it to the machine table. The work should be located precisely and secured properly and should be well supported.
28.1 Turning center work holding methods:
Machining operations on turning centers or CNC lathes are carried out mostly for axi-symmetrical components. Surfaces are generated by the simultaneous motions of X and Z axes. For any work holding device used on a turning centre there is a direct "trade off" between part accuracy and the flexibility of work holding device used.
Work holding methods
Advantages
Disadvantages
Automatic Jaw & chuck changing
High cost of jaw/chuck changing Adaptable for a range of workautomation. Resulting in a more piece shapes and sizes complex & higher cost machine tool
Indexing chucks Figure 28.1
Very quick loading and Expensive optional equipment. Barunloading of the workpiece can feeders cannot be incorporated. be achieved. Reasonable range Short/medium length parts only can of work piece sizes can be be incorporated. Heavy chucks. loaded automatically
Limited to a range of flat parts with Simple in design and relatively Pneumatic/Magnetic little overhang. Bar-feeders cannot be inexpensive. Part automation is chucks incorporated. Parts on magnetic possible. No part distortion is Figure 28.3 chucks must be ferrous. Heavy cuts caused due to clamping force must be avoided.
Automatic Chucks with soft jaws
Adaptable to automation. Heavy cuts can be taken. Individual parts can be small or large in diameter
Jaws must be changed manually & bared, so slow part change-overs. A range of jaw blanks required.
Long & short parts of reasonably large size Expanding mandrels accommodated. Automation Limitation on part shape. Heavy cuts & collets can be incorporated. Clamping should be avoided. Figure 28.2 forces do not distort part. Simple in design
Dedicated Chucks
Expensive & can only be financially Excellent restraint & location justified with either large runs or of a wide range of individual & when extremely complex & accurate irregular -shaped parts can be parts are required. Tool making obtained. facilities required. Large storage space.
( 2) Work holding for Machining Centres:
Workholding methods Advantages Disadvantages Highly adaptable. Can be purchased in stages to increase its sophistication. Costly for a complete Modular Fixtures Reasonable accuracy. Speedily assembled. system. Difficult to Figure 28.6 Small stores area is required. Can be set-up automate. Skills required in to a machine more than one part. Proven kit assembly technology Relatively inexpensive. Can be operated by mechanical, pneumatic, or by hydraulic Work holding limitations. control. Quick to operate with ease of set- Clamping force limitations. Automatic Vices up. Reasonable accuracy. Easily automated. Jaws can become strained. Simplicity of design. Using multi-vices Work location problems. allows many parts to be machined. Proven Limitations on part size. Technology Relatively inexpensive. Reasonable Large surface area is accuracy. Can machine large areas of the required. Swarf can be a Pneumatic/Magnetic work piece. Quick setups. Easily problem. Nonferrous Work holding devices automated. Simplicity of design. Many material limitation on parts can be machined at one set up. magnetic devices. Costly & limited part geometry clamping. Part Allows complex geometric shapes to be 4/5 axis CNC work size limitations. Usually machined. High accuracy. Opportunity for holding devices only one part can be "one hit" machining. Easily automated. machined. Cannot be fitted to all machines. Dedicated Fixturing Large & small parts are easily Large storage space accommodated. High accuracy of part required. No part flexibility.
location. Easily automated. Simplicity of design. Proven technology. Many parts can Heavy fixtures. Tool be machine at one setup good vibration making facilities required. damping capacity
Figure 28.1: Indexing chucks
Figure 28.2: Mandrels
Figure 28.3: Magnetic chucks
Figure 28.4: Vise
Figure 28.5(a): Pallets
Figure 28.5(b) Figure 28.5(c)
Figure 28.6 : Modular fixture
Figure 28.7 : Chucks
(1) Recirculating ball screw The recirculating ball screw assembly shown in figure 25.1 has the flanged nut attached to the moving chamber and the screw to the fixed casting. Thus the moving member will move during rotational movement of the screw. These recirculating ball screw designs can have ball gages of internal or external return, but all of them are based upon the "Ogival" or "Gothic arc". In these types of screws, balls rotate between the screw and nut and convert the sliding friction (as in conventional nut & screw) to the rolling friction. As a consequence wear will be reduced and reliability of the system will be increased. The traditional ACME thread used in conventional machine tool has efficiency ranging from 20% to 30% whereas the efficiency of ball screws may reach up to 90%.
Figure 25.1: Recirculating ball screw assembly
Figure 25.2: Preloaded recirculating ball screw
There are two types of ball screws. In the first type, balls are returned through an external tube after few threads. In another type, the balls are returned to the start through a channel inside the nut after only one thread. To make the carriage movement bidirectional, backlash between the screw and nut should be minimum. One of the methods to achieve zero backlash is by fitting two nuts. The nuts are preloaded by an amount which exceeds the maximum operating load. These nuts are either forced apart or squeezed together, so that the balls in one of the nuts contact the opposite side of the threads. These ball screws have the problem that minimum diameter of the ball (60 to 70% of the lead screw) must be used, limiting the rate of movement of the screw.
(2) Roller screw
Figure 25.3: Roller screw
These types of screws provide backlash-free movement and their efficiency is same as that of ball screws. These are capable of providing more accurate position control. Cost of the roller screws are more compared to ball screws. The thread form is triangular with an included angle of 90 degrees. There are two types of roller screws: planetary and recirculating screws.
Planetary roller screws: Planetary roller screws are shown in figure 25.3. The rollers are threaded with a single start thread. Teeth are cut at the ends of the roller, which meshes with the internal tooth cut inside the nut. The rollers are equally spaced around and are retained in their positions by spigots or spacer rings. There is no axial movement of the rollers relative to the nut and they are capable of transmitting high load at fast speed.
Recirculating roller screws: The rollers in this case are not threaded and are provided with a circular groove and are positioned circumferentially by a cage. There is some axial movement of the rollers relative to the nut. Each roller moves by a distance equal to the pitch of the screw for each rotation of the screw or nut and moves into an axial recess cut inside the nut and disengage from the threads on the screw and the nut and the other roller provides the driving power. Rollers in the recess are moved back by an edge cam in the nut. Recirculating roller screws are slower in operation, but are capable of transmitting high loads with greater accuracy.
28. CNC WORK HOLDING DEVICES
With the advent of CNC technology, machining cycle times were drastically reduced and the desire to combine greater accuracy with higher productivity has led to the reappraisal of work holding technology. Loading or unloading of the work will be the nonproductive time which needs to be minimized. So the work is usually loaded on a special work holder away from the machine and then transferred it to the machine table. The work should be located precisely and secured properly and should be well supported.
28.1 Turning center work holding methods:
Machining operations on turning centers or CNC lathes are carried out mostly for axi-symmetrical components. Surfaces are generated by the simultaneous motions of X and Z axes. For any work holding device used on a turning centre there is a direct "trade off" between part accuracy and the flexibility of work holding device used.
Work holding methods
Advantages
Disadvantages
Automatic Jaw & chuck changing
High cost of jaw/chuck changing Adaptable for a range of workautomation. Resulting in a more piece shapes and sizes complex & higher cost machine tool
Indexing chucks Figure 28.1
Very quick loading and Expensive optional equipment. Barunloading of the workpiece can feeders cannot be incorporated. be achieved. Reasonable range Short/medium length parts only can of work piece sizes can be be incorporated. Heavy chucks. loaded automatically
Limited to a range of flat parts with Simple in design and relatively Pneumatic/Magnetic little overhang. Bar-feeders cannot be inexpensive. Part automation is chucks incorporated. Parts on magnetic possible. No part distortion is Figure 28.3 chucks must be ferrous. Heavy cuts caused due to clamping force must be avoided.
Automatic Chucks with soft jaws
Adaptable to automation. Heavy cuts can be taken. Individual parts can be small or large in diameter
Jaws must be changed manually & bared, so slow part change-overs. A range of jaw blanks required.
Long & short parts of reasonably large size Expanding mandrels accommodated. Automation Limitation on part shape. Heavy cuts & collets can be incorporated. Clamping should be avoided. Figure 28.2 forces do not distort part. Simple in design
Dedicated Chucks
Expensive & can only be financially Excellent restraint & location justified with either large runs or of a wide range of individual & when extremely complex & accurate irregular -shaped parts can be parts are required. Tool making obtained. facilities required. Large storage space.
( 2) Work holding for Machining Centres:
Workholding methods Advantages Disadvantages Highly adaptable. Can be purchased in stages to increase its sophistication. Costly for a complete Modular Fixtures Reasonable accuracy. Speedily assembled. system. Difficult to Figure 28.6 Small stores area is required. Can be set-up automate. Skills required in to a machine more than one part. Proven kit assembly technology Relatively inexpensive. Can be operated by mechanical, pneumatic, or by hydraulic Work holding limitations. control. Quick to operate with ease of set- Clamping force limitations. Automatic Vices up. Reasonable accuracy. Easily automated. Jaws can become strained. Simplicity of design. Using multi-vices Work location problems. allows many parts to be machined. Proven Limitations on part size. Technology Relatively inexpensive. Reasonable Large surface area is accuracy. Can machine large areas of the required. Swarf can be a Pneumatic/Magnetic work piece. Quick setups. Easily problem. Nonferrous Work holding devices automated. Simplicity of design. Many material limitation on parts can be machined at one set up. magnetic devices. Costly & limited part geometry clamping. Part Allows complex geometric shapes to be 4/5 axis CNC work size limitations. Usually machined. High accuracy. Opportunity for holding devices only one part can be "one hit" machining. Easily automated. machined. Cannot be fitted to all machines. Dedicated Fixturing Large & small parts are easily Large storage space accommodated. High accuracy of part required. No part flexibility.
location. Easily automated. Simplicity of design. Proven technology. Many parts can Heavy fixtures. Tool be machine at one setup good vibration making facilities required. damping capacity
Figure 28.1: Indexing chucks
Figure 28.2: Mandrels
Figure 28.3: Magnetic chucks
Figure 28.4: Vise
Figure 28.5(a): Pallets
Figure 28.5(b) Figure 28.5(c)
Figure 28.6 : Modular fixture
Figure 28.7 : Chucks
