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 non-productive 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

Adaptable for a range of work-piece shapes and sizes

High cost of jaw/chuck changing automation. Resulting in a more complex & higher cost machine tool

Indexing chucksFigure 28.1

Very quick loading and unloading of the workpiece can be achieved. Reasonable range of work piece sizes can be loaded automatically

Expensive optional equipment. Bar-feeders cannot be incorporated. Short/medium length parts only can be incorporated. Heavy chucks.

Pneumatic/Magnetic chucksFigure 28.3

Simple in design and relatively inexpensive. Part automation is possible. No part distortion is caused due to clamping force

Limited to a range of flat parts with little overhang. Bar-feeders cannot be incorporated. Parts on magnetic chucks must be ferrous. Heavy cuts 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.

Expanding mandrels & colletsFigure 28.2

Long & short parts of reasonably large size accommodated. Automation can be incorporated. Clamping forces do not distort part. Simple in design

Limitation on part shape. Heavy cuts should be avoided.

Dedicated Chucks

Excellent restraint & location of a wide range of individual & irregular -shaped parts can be obtained.

Expensive & can only be financially justified with either large runs or when extremely complex & accurate parts are required. Tool making facilities required. Large storage space.

( 2) Work holding for Machining Centres:

Workholding methods Advantages Disadvantages

Modular FixturesFigure 28.6

Highly adaptable. Can be purchased in stages to increase its sophistication. Reasonable accuracy. Speedily assembled. Small stores area is required. Can be set-up to a machine more than one part. Proven technology

Costly for a complete system. Difficult to automate. Skills required in kit assembly

Automatic Vices

Relatively inexpensive. Can be operated by mechanical, pneumatic, or by hydraulic control. Quick to operate with ease of set-up. Reasonable accuracy. Easily automated. Simplicity of design. Using multi-vices allows many parts to be machined. Proven Technology

Work holding limitations. Clamping force limitations. Jaws can become strained. Work location problems. Limitations on part size.

Pneumatic/Magnetic Work holding devices

Relatively inexpensive. Reasonable accuracy. Can machine large areas of the work piece. Quick setups. Easily automated. Simplicity of design. Many parts can be machined at one set up.

Large surface area is required. Swarf can be a problem. Nonferrous material limitation on magnetic devices.

4/5 axis CNC work holding devices

Allows complex geometric shapes to be machined. High accuracy. Opportunity for "one hit" machining. Easily automated.

Costly & limited part geometry clamping. Part size limitations. Usually only one part can be machined. Cannot be fitted to all machines.

Dedicated Fixturing

Large & small parts are easily accommodated. High accuracy of part location. Easily automated. Simplicity of design. Proven technology. Many parts can be machine at one setup good vibration damping capacity

Large storage space required. No part flexibility. Heavy fixtures. Tool making facilities required.

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