effects of cnc on machine

8/13/2019 Effects of Cnc on Machine

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5. Effects of CNC on Machine Components

CNC has brought about lasting changes to major components of

machine tools, leading to new machine configurations and types of

automation equipment.

5.1. Machine Configuration

The principal reason for numerical control's influence on machine

configurations is the fact that, as with conventional automation, it becomes

possible to do without continuous tending and observation of the work

sequence by a worker. At the same time, the continued development of

cutting tools has led to significantly higher cutting speeds, feed rates, and

cutting depths, which, in practice, can only be implemented without a human

operator. These improvements offer greatly increased performance but at the

same time place new requirements on the machines.

Especially machines used to process smaller workpieces offer many options

in this regard. With large workpieces, the weight of the workpiece places

tight limits on the feeding and load-bearing characteristics of the machine, as

well as on the dimensions, clamping, and large working space.

Thus, for small and medium-sized turning machines, it has become common to

place the bed at the rear, whether inclined or vertical, as had already been

introduced for copying lathes. In this way, the chip clearance is not

obstructed by the bed. This also has the advantage that it is possible to

swivel the transverse axis away from the horizontal, significantly improving

the access to tools and the workpiece. Space is created under the workpiece

for installation of a chip conveyor.

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One new machine design was prompted by the development of variable-

speed three-phase motors. Here, there is a vertically suspended spindle, with

the longitudinal and transverse motions being executed not by the tool but

by the spindle. As a result, not only is there good chip clearance even during

internal machining, but the motion of the spindle also allows easy tool

changing using the pickup principle. Here, too, there is good access to tools

and the workpiece and space for a chip conveyor (Figure 5.1 ).

In large turning machines, the configuration of conventional lathes has been

retained, that is, a horizontal bed in machines for long workpieces and a

vertical design for short workpieces. For longitudinal and shaft turning

machines, it may be possible to install a chip conveyor inside the bed. In

vertical lathes, the horizontal clamping surface interferes with chip removal.

Automatic chip removal is almost impossible, especially during internalmachining.

Free program design and the practically unlimited amount of data that can

Figure 5.1. Vertical turning machine using the pickup principle for

automatic loading and unloading of workpieces.
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be entered have lead to lathes with two or even three heads that work

independently but in coordination with each other. This shortens the

production time per piece and increases the number of tools available.

Even lathes developed especially for automated production, in which the tool

motions are generated in a conventional manner using cam discs (e.g.,

multiple-spindle automatic lathes), have in the meantime been converted to

numerical control. The fundamental machine configuration was retained,

however.

As for drilling machines, the radial drilling machine, with its non-Cartesian

motion directions, has been discarded entirely. The standard type is now a

design with a vertical spindle and a horizontal workpiece table. The division

of the axes among the table and pedestal follows the working space of its

conventional predecessors. As a result of automation, however, automatic

tool changing becomes almost obligatory, with a corresponding influence on

the machine configuration.

In milling machines as well, it is possible to find machine configurations that

already existed among conventional machines. However, the freedom of

programming in numerical automation set loose a trend toward complete

machining and thus a new type of machine: the machining center, a machine

that makes possible all types of machining that use recirculating tools.

Naturally, this makes it necessary to have an appropriately dimensioned tool

changer and one or even two rotational axes in addition to the three

translational axes. The configurations of these machines also mainly

correspond to those of conventional machines, especially to those of boring

mills. To expand the automatic sequence, these machines generally are also

equipped with an automatic tool-changing system. This additional equipment,of course, also influences the structure of the basic machine.

In all these machines with a horizontal clamping surface, there is a problem

with chip removal and installation of a chip conveyor. For this reason, in a few

cases in machines for processing smaller workpieces, the workpiece is

machined with a vertical clamping surface after being clamped onto the

surface in the horizontal position in the clamping station.

The configuration of the original conventional machines generally also was

retained in the various types of grinding machines. The reason for this was

that priority had to be given to the requirements of the grinding process.


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Only with smaller external surface grinding machines, in which the

longitudinal motion traditionally is assigned to the workpiece, was a different

configuration occasionally adopted: that of a lathe with an inclined bed at the

rear. Thus the longitudinal motion is executed by the grinding head.

Gear-cutting machines are in their nature single-purpose machines with a

fully automatic sequence. Therefore, their configuration was kept completely

the same during the transition to numerical control.

No entirely new configuration for machine tools with a completely different

structure appeared until after CNC control systems with high-performance

computers became available: machines in which the tool was positioned via a

parallel kinematic system ( Part 3 , Section 1.6). Here the command position

values specified in the Cartesian coordinate system had to be convertedquickly into the command values for the lengths of the individual jointed

arms. Because of the low masses being moved, this kinematic system allows

very rapid responses but has serious disadvantages when it comes to the

range of motion that is possible, especially for swivel motions.

5.2. Machine Frames

The requirements placed on machine frames are essentially identical to those

for conventional machine tools. However, the higher precision required

means that optimization of the static and dynamic stiffness is necessary. For

trouble-free automatic production, maximum thermal stability and low

thermal drift are also important. Temperature changes from the environment

or owing to heat sources in the machine should not lead to gradually

increasing positional deviations in the machine. The heat sources in the

machine can cause particularly noticeable negative effects as a result of high

power conversion. These include hot chips that heat up the machine frame

locally, or high loads on the main drive motor, or the heat generated by the

bearings of a rapidly rotating main spindle. Machine frames made of cast

mineral composites are useful here owing to their large mass and the poor

thermal conduction of the concrete (Figure 5.2).
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A certain scope for free design of function-related machine frames comes

from the fact that today CNC machines, especially small and medium-sized

ones, generally have a machine enclosure on all sides. Thus the visual effect

can be ignored in design of the frame.

Machine parts that are moved in closed-loop position control are subject to

particular requirements with regard to their weight, especially those

positioned by a linear drive. For machine slides, optimization of stiffness

using finite-element method (FEM) analysis and weight reduction by means

of topology optimization can produce considerable benefits not only in cast

constructions but also most notably in welded designs. Because of their cost,

fiber composite materials have not achieved much success so far in series

production (Figure 5.3).

Figure 5.2. Machine frame made of cast material composites.


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5.3. Guides ( Figures 5.4 and 5.5)

Figure 5.3. Cast-metal machine frame.

Figure 5.4. Hydrostatic guide with identical dimensions to a linear

roller guide in order to ensure interchangeability. (Image courtesy

of INA.)
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As a general rule, guides, especially motion guides that are moved during

machine operation, are subject to the following requirements:

Low friction and no stick-slip effect so as to allow precise positioning

High stiffness, in order to absorb operating loads without excessive

displacement

High damping to suppress vibrations

Low wear to ensure precision over a long time period

Low costs.

In conventional machine tools, these requirements were met adequately by

sliding guides with a wide variety of designs. They could handle heavy loads,were reliable under operational conditions, and provided good damping. Low

friction and the no-stick/slip effect could be achieved by lining them with

plastic low-friction liners.

In contrast, closed-loop position control requires particularly low friction and

freedom from stick/slip effect in order to achieve high positioning accuracy.

Therefore, today, diverse types of rolling guides are used in numerous

controlled machines. These guides are supplied by specialist manufacturers

and have become quite inexpensive. This trend is supported by the high

rapid traverse speeds that are being introduced to save time. Here the lower

Figure 5.5. Damping test: On the left side, a roller guide; on the

right side, a hydrostatic guide. (Image courtesy of INA.)


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friction allows lighter feed drives. Additional improvements have been

achieved using hydrostatic guides, especially with regard to damping

characteristics. Some manufacturers of rolling-contact guides offer these as

mass-produced products.

5.4. Main Drives

When planning main drives with closed-loop control, a fundamental decision

has to be made as to whether to use synchronous or asynchronous motors.

The decisive factor here is whether the motor will be operated only with

closed-loop speed control (e.g., as a spindle drive for drilling and milling

tools) or with closed-loop position control (e.g., with turning machines with

an additional C-axis drive).

Today, all spindle drives in machine tools without exception use electric

motors with closed-loop speed control. These have two main tasks:

1. To provide the torqueand speedneeded for the work process

2. To allow interpolation of the speed of the main spindle with the feed

drives, if operation as a C axis is required in turning or machining centers

The automatic work sequence in CNC machines means that the main drives

have to satisfy some additional requirements that exceed the requirements

placed on drives for conventional machines. These are in particular

Automated speed changing.The automatic work sequence also requires

programmable, automatic changing of the speed.

Speed changing in very small steps, preferably continuously

adjustable.CNC machines are capital-intensive items of equipment with a

high cost per hour. Therefore, it is important to make optimal use of the

performance of state-of-the-art tools. For example, to keep the cutting

speed constant during face turning or taper turning, continuously

adjustable speed changing will be necessary for technological reasons.

Large speed adjustment range.CNC machines are universal machines

that are intended to process various types of workpieces using various

tools. To do this, the main spindle has to cover a large range of speeds

without any intermediate transmission; that is, the entire required


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adjustment range is provided by the motor alone.

Very fast speed changes.Every change in speed represents a loss of

time. This is especially noticeable when there are frequent tool changes on

machines with recirculating tools, such as in machining centers. These are

equipped with an automatic tool changer, and the spindle has to be

stopped for each tool-changing operation.

As a result, a spindle motor with the shortest possible run-up and braking

times is required.

High drive output.The automatic execution of the machining process in

CNC machines makes them independent of manual control and reaction

speeds. This allows a completely enclosed working space. It is thus

possible to achieve working speeds that make full use of modern tools'

performance potential. This requires drive outputs that exceed those of

conventional machines by a large factor.

Large range with constant output.The high drive output should be

available over as large a range of speeds as possible.

High torque at low speeds.As high torque as possible should be

available at lower speeds.

Compact dimensions and low weight.In many CNC machines, the main

drive motor is part of a larger mechanical assembly and constantly moves

along with it. There is thus a requirement for motors that are as light and

compact as possible so as not to compromise the acceleration that can be

achieved by the entire assembly.

Low heat generation.The adverse effects that localized heating of themachine has on precision were already been mentioned earlier (Part 2 ,

Section 1.3).

5.4.1. Types of Main Drives (Figure 5.6 )
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In principle, the same types of motors are available as for feed drives. Speed-

controlled asynchronous motorsare preferred for use as standard main

drives owing to their positive features, such as low price, their simple,

rugged design, and their low maintenance requirements. For speed

adjustment, they receive their power from a frequency converter. By now

they have supplanted the previously dominant direct-current (dc) motors.

They appear in various designs. Depending on the nature of the main drive

( Figure 5.6), they can take the form of housed motors or kit motors with a

hollow shaft (Figure 5.7).

Figure 5.6. Types of main drives.
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In order to ensure the full torque at low speeds and even down to a speed of

zero, housed motors for main spindle drives are always designed with

external ventilation or liquid cooling. Kit motors for direct installation in thespindle are as a rule always liquid-cooled because it is necessary to ensure

both high power density in a reasonably small space and a motor with a

thermally neutral signature.

5.4.2. Designs for Main Drives

The classic design of the main drive involves the coupling of a housed motor

to the tool spindle via a geared or belt transmission, in some cases with

multiple stages. This arrangement has the advantage that the motor is

thermally decoupled from the machining space and from the spindle. The

motor can be installed in a location outside the machining space, meaning

that main spindle motors with standardized installation dimensions can be

used. However, the belt drive limits the speed, stiffness, and dynamic

characteristics of the drive and thus the productivity of the entire machine

tool.

These disadvantages have led to the use of directly driven spindles. The belt

or gear drive is eliminatedthe torque is transmitted via the rotor of the

drive motor directly to the spindle shaft. This makes the speed of the system

very stable and allows high amplification factors and short acceleration and

braking times. In order to clamp the workpiece, the motor is equipped with a

hollow shaft. Because the heat input from the motor does not go directly intothe spindle, the motor can have external ventilation. Liquid cooling is

possible as an option; this can be used to further increase the motor's use.

This arrangement is especially beneficial in machining centers.

Integrating the drive motor directly into the spindle created the so-called

motor spindle. This direct installation generally requires liquid cooling. This

type of main spindle-drive design is increasingly becoming the standard in

the modern machine tool industry.

In both these direct-drive designs, the lack of speed adaptation means that

the following requirements become particularly important:

Figure 5.7. Three-phase asynchronous motor as a kit motor with

hollow shaft.


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High power density

Large speed-adjustment range

Large range with constant output

High torque at low speeds

High maximum speed

5.4.3. Three-Phase Asynchronous Motors

Speed-controlled asynchronous motors have developed into standard main

drives owing to their positive features, such as low price, simple and rugged

design, and low maintenance requirements. The speed of three-phase

asynchronous motors can be changed over a broad adjustment range by

changing the output frequency and output voltage of the converter that is

supplying the power. The speed/torque characteristics of three-phase

asynchronous motors operated with converters are provided in the form of

characteristic curves (Figure 5.8).


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Using purely mathematical calculations, the speed-adjustment range then

would be "infinite" and up to 1:12 in the field-weakening range, that is, at

constant power. The behavior of the characteristic curves is determined by

the strength of the intermediate-circuit voltage and by the corresponding

motor-specific data, such as inductivity, resistance, motor constants, and

breakdown torque. Figure 5.8provides a comparison of the two different

motor principles.

In the basic speed range, the voltage and frequency increase proportionally

up to the rated speed. If externally cooled, the motor generates a constant

torque. Once the voltage reaches its maximum value at the rated speed, it is

only possible to increase the frequency. From that point onward is the

beginning of the so-called field-weakening range. The field-weakening range

Figure 5.8. Speed-adjustment ranges of synchronous and

asynchronous motors with the same output and torque.


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begins with a range of constant output, in which the torque decreases

hyperbolically, that is, inversely proportional to the frequency/speed (rpm). If

the speed/supply frequency is increased further, the breakdown torque or

stability limit of the motor will be reached. The breakdown torque of an

asynchronous motor decreases as the square of the frequency/speed (1/n ).

In contrast to operation from power mains, with modern converters and an

appropriate control system, the stability limit of the motor initially does not

represent any real limit because loss of stability of the motor (a drastic

reduction in torque and even motor standstill) is prevented. The maximum

speed thus is limited only by mechanical components, such as bearing,

rotors, rotor mountings, and so on.

The characteristic curves generally are indicated for continuous operation

(operating mode S1) with various duty cycles, frequently 25, 40, or 60

percent.

The closed-loop control and control structure of main-drive motors are

largely the same as those of a modern feed drive. The main-drive controllers

encountered today merely have a few supplementary functions, for example,

for special field regulation. Thus C-axis operationthe interpolation of the

main spindle with the feed drive, as is often required today, especially in

turning machinesrepresents no problem.

5.4.4. Three-Phase Synchronous Motors

In machine tools, synchronous motors are as a rule used primarily as feed

drives( Part 2 , Chapter 1 : Implementation of Dimensional Data ).

In special cases, demands for greater power densities and temperature

stability have led to the use of permanent-field three-phase synchronous

motorseven in main drives. In the meantime, state-of-the-art practical motor

principles and closed-loop control methods have made it possible to

implement such drives in series-produced machines. More extensive use is

unlikely at present, however, owing to the higher costs. At present, speed-

controlled synchronous motorsare used primarily as main driveswhen

the following requirements have to be met:

Extremely high requirements as to machining quality, precision, and

smooth operation

2
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Extremely short times for run-up or speed changes

Standstill torque

Limited installation space

This is especially the case in high-quality turning machines and

turning/milling centers when high-quality drilling and milling operations have

to be executed on the end faces or lateral surfaces of the workpieces.

Two main types of synchronous motors are available:

High-speed motors. These primarily involve four-pole synchronous motors

that are used for milling applications. These motors have been optimized

for high maximum speeds of up to 40,000 rpm and a wide speed-adjustment

range. These motors are used mainly with closed-loop speed control via

frequency converters. The speed-adjustment range is about 1:3 in the field-

weakening range. Using purely mathematical calculations, the overall

speed-adjustment range would be "infinite."

High-torque motors. Six-pole/eight-pole synchronous motors are available

that have been developed for turning and grinding machines with

moderate maximum speeds. These motors are characterized by very hightorque utilization.

High-speed synchronous motors are much more important and widespread

than torque motors, which have very high prices, if nothing else, owing to

their low production quantities.

Speed control of synchronous motors is likewise implemented via the voltage

and frequency of the three-phase current that is fed in. For continuouslyadjustable closed-loop speed control of a synchronous motor, a frequency

converterhas to be connected upstream of the motor. Both the speed and

the rotor position are measured and reported to the converter by a rotary

encoder. From this the control electronics determine the necessary

"electronic commutation" to advance the rotating field and also the actual

speed.

5.4.5. Benefits

Compared with the lower-priced asynchronous motors, synchronous


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spindle drives offer the following benefits:

High efficiency

Low mass moment of inertia and thus good dynamic characteristics

Low maintenance (in the case of rotors without slip-ring rotors)

Speed independent of load

No electrical power necessary for field excitation

Up to 60 percent higher torque and thus more compact machine designs

Extremely short run-up and braking times (50 percent) thanks to the

torque

High standstill torque

High torque even during standstill and change of direction of rotation

Compact construction (e.g., for turning machines and vertical milling

machines) thanks to the elimination of mechanical components such as

pivoting motor bases, belt drives, gearboxes, and spindle encoders

High power density when water-cooled

Maximum speeds of up to 40,000 rpm, torques of up to 820 Nm or greater

Low rotor heating owing to equipment with permanent magnets (The

result: Significantly less power loss in the rotor in the lower speed range

and thus less bearing heating and spindle expansion.)

Extremely high precision on the workpiece thanks to smooth, even spindle

operation even at extremely low speeds, because there are no transverse

drive forces

Interpolating C-axis operation with the feed drives, for example, in turning

machines

Larger internal rotor bores than cage rotors of asynchronous motors with

the same external diameter (This is an advantage for the bar capacity of

automatic lathes and for greater spindle stiffness owing to the greater

shaft diameter for milling spindles.)


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Increased stiffness of the spindle drive thanks to mounting of the motor

components between the main spindle bearings

Less cooling output necessary for the same power output compared with

asynchronous motors, that is, greater efficiency

Only one encoder (hollow-shaft measuring system) for detecting the motor

speed and spindle position

Easy servicing by exchanging complete motor spindles

Further potential for improved efficiency, such as faster part machining times

and smaller footprints, can be achieved through the optimized combination of

synchronous spindle motors, drive controls, and CNC controllers.

On the other hand, there are a few disadvantages:

Expensive magnet materials, therefore high procurement costs for

permanent magnet motors

Elaborate closed-loop control requirements (frequency converters)

Possibly an annoying whistling noise from the motor

5.5. Machine Enclosures

During our discussion of main drives, we have already noted that automatic

work sequences are necessary for full utilization of the high-performance

characteristics of state-of-the-art cutting tools. One result of this is that chips

are thrown outward at high speeds, presenting a risk of injury to persons in

the vicinity. This fact must be addressed through the use of appropriate

enclosures and by securing the working space.

On small and medium-sized machines, such enclosures generally are a sheet-

metal construction attached to the body of the machine, often combined with

the electrical cabinet and even enclosed on the top. Thus they form a single

transport unit together with the machine, a so-called machine suitable for

single-point lifting. In such cases, the machine enclosure not only serves as

protection against chips but also retains the vaporized coolant and provides

good protection against process noise. These are a key means of preventing

accidents and thus are subject to a large number of regulations. On the other


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hand, they are also intended to make operation, servicing, and maintenance

of the machine easier. All this means that they have to satisfy a number of

extensive requirements that are often mutually conflicting.

For example, machine enclosures have to make the working space easily

accessible for setup work and tool changing. For this purpose, they generally

have large doors to allow access to the working space. These doors, however,

must be locked when the machine is operating, or at least the work sequence

has to be interrupted immediately when they are opened. The doors and

often other parts of the fixed enclosure are provided with windows to allow

safe observation of the work sequence. These windows have to withstand

"bombardment" by chips without becoming obscured. This is only possible

with silicate glass. Then again, in the event of collisions that cannot be

avoided completely, they have to be able to survive impacts by large parts,

such as flying workpieces, clamping equipment, or tools. This works better

with elastic plastic windows. Therefore, composite windows are often used in

such cases. It is also important for the window to be firmly anchored in its

frame to prevent it from being pressed out too easily.

Coverings that enclose the entire machine often obstruct access to areas that

have to be accessible for cleaning and maintenance work. They thus have to

be easy to remove or open.

Frequently, the operator panel with the keyboard, equipment for the CNC,

and additional control elements for manual actuation of the machine motions

for setup and maintenance work are also integrated into the machine

enclosure. After all, the coverings are decisive in determining the external

appearance of the machine and therefore display an important starting point

for design. Like all components of the machine, they are naturally alsosubject to heavy pressures to reduce costs, all the more so because it is easy

to view them more as a necessary evil and not as an important functional

component.

5.6. Coolant Supply

Because in CNC machines the tool can move freely in the working space, the

coolant has to be coupled with the tool. In turning machines, this is done by

feeding the coolant over the turret to the tool holder that is located in the

working position, which, in turn, feeds it to the cutting edge via a


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preadjusted tube. For recirculating tools, the coolant is fed to the tool via the

main spindle. Owing to heavy misting, however, there is at present a trend

toward dry machining.

5.7. Chip Removal

As a consequence of the high productivity of numerically controlled machine

tools, large quantities of chips are produced in any given time interval. These

chips have to be removed from the machine without adversely affecting the

work sequence. The issues associated with free chip clearance and

installation of the associated chip conveyors in the machine have already

been discussed in connection with the machine configuration.

Various types of conveyors are used depending on the shape of the chips.The most common and widely distributed ones are hinged belt conveyors.

Scraper conveyors are more suitable for very small and friable chips.

Magnetic conveyors can only be used for steel chips.

5.8. Summary

The automatic work sequences made possible by numerical control have

extensive effects on the design of machines because there is no need for

continuous tending and observation by workers. This makes it possible to

focus the design of the machine on optimal execution of machining, making

full use of the increased performance of modern tools. This requires a main

drive with correspondingly high output and a completely enclosed working

space. The high productivity of such machines results in a high level of chip

generation, and the chips have to be removed automatically.

Automatic work sequences require a higher level of precision, which has to

be taken into account, especially in the design of machine bodies and guides.

CNC closed-loop position-control systems also place requirements on machine

designs. Moving parts have to be as light as possible, especially when driven

by linear motors. The guides should have low friction and be free from stick-

slip effects. Machine designs also must take into account automatic tool

changing and workpiece changing, which are often associated withconsiderable space requirements.

5.9. Effects of CNC on Machine Components


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Important points to remember:

1. CNC machines are machines that operate automatically. They do not have

to be tended by a worker. For this reason, they often have a different

configuration than conventional machines. In particular, their working

space should be completely enclosed on all sides. In part, this is due to

their very high cutting speeds.

2. Machine frames should have high static, dynamic, and thermal stability in

order to achieve trouble-free automatic production.

3. The demanding requirements on guides have resulted in increased use of

rolling guides.

4. Main drives are primarily frequency-controlled three-phase asynchronous

motors.

5. Increasing use is being made of directly driven motor spindles, especially

when positioning tasks or synchronization with feed motions is demanded

at the same time.

6. Machine enclosures have a very wide range of functions:

Protection against flying chips

Containing the coolant vapor

To allow observation of the work sequence

To allow machine setup and clamping of the workpiece

To contain flying parts resulting from collisions

7. The coolant in-feed has to be coupled with the tool; with recirculating

tools, it must be fed through the spindle.

8. The chips have to be removed in such a way that they do not interfere with

the work sequence and so that they do not cause local heating in the

machine frame.

Citation

Hans B. Kief; Helmut A. Roschiwal: CNC Handbook. Effects of CNC on MachineEXPORT
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