Chapter 3

Exact alignment of the guided parts

Availability of means to compensate wear

Minimum friction

Ease of assembly and economy of manufacture

Allow freedom from restraint

Prevention of chip and dirt accumulation

Effective lubrication

High stiffness

Based upon the nature of friction between bearing surface:

Guideways with sliding friction: slideways

Guideways with rolling friction: anti-friction ways

a. Flat

b. Vee or Prism

c. Dovetail

d. Cylindrical

Advantage

Simple to manufacture

Self adjusting in case of wear

Saddle is well supported

Disadvantages

Tendency to accumulate dirt and chips

Need adjustment for clearance

Poor in lubricant retention

Use

High supporting force on long slideway

Bigger machine tool with high force

Two types: Symmetrical and Unsymmetrical

Symmetrical if load is evenly distributed

Usually combined with flat slideway for better

guidance and load bearing

Symmetrical Vee

Employed when the load on the slideway is vertical, which is rare.

Unsymmetrical Vee

More commonly used in machine tool

Longer face should be placed perpendicular to the direction of the resultant force.

Advantages

Self adjusting capability

Surrounded vee prevents dirt accumulation

Easy traverse and accurate guiding

Disadvantages

Surrounded type cannot retain lubricant

Surrounded type weakens the saddle

Wears rapidly

Use

Medium and small lathe

Planer machine

Advantages

Require small space

Simple clearance adjustment

Can stand vertical upward force

Disadvantage

Relatively costly to manufacture

Poor retention of lubricant

Use

Cross slides in lathe

Milling machine tables

Saddles and knees

Theoretically fully constrained slideway

Easy to manufacture

Disadvantages

Low rigidity as fixed at two ends

Clearance is difficult to adjust

Use

Column drilling machine

Over-arm of milling machine

Tailstock sleeve

Life and smooth functioning of slideways depends on

clearance between sliding surface

Too small clearance give rise to excessive frictional

force

Too large clearance adversely affect the uniformity

of travel

Initial clearance alters in the course of slideway

operation due to its wear

Necessary to provide devices for periodic

adjustments of clearances in the slideway

Flat slideways

Flat strip (adjusted by laterally arranged screws)

Flat slideways

Taper strip

Vee slideways

Dovetail Slideways

Flat strip

Dovetail Slideways Taper strip

Dovetail Slideways

Wedge Strip

Cylindrical slideways

Selection of Materials depends on:

wear resistance

strength, specially bearing and bending strength,

damping capacity

heat capacity (It should not build up much heat.),

environmentally neutral (It must stand hostile

environment like new coolant, lubricant etc.)

the cost of production

Gray cast iron is the most commonly used material

for slideways.

Nickel and chromium is added to increase the wear

resistance and the bending strength

The surface hardness can be increased by flame or

induction hardening.

Hardness attained is 40-52 RC for ordinary gray cost

iron and 45-55 RC in nodular cast iron.

Hardness upto 60-62 RC can be attained by Hardened

steel

Laminated plastics strips backed by cost iron or steel

pieces can be used.

Advantages :

Uniform pressure

Less seizure

Reduced friction

Reduction of wear

Easy to fabricate

Lower cost

Disadvantages:

lower strength and hardness, usually designed

for pressure l0 kg/cm2

tendency to-swell when they absorb oil.

poor thermal conductivity, resulting in thermal

distortion.

low speed range, v< 40m/min for satisfactory

running.

Wear is caused by factors such as:

the properties of the material of the mating parts

the surface roughness of the mating parts

the pressure on the slideways

the sliding velocity

the total working life

the dirt and chip accumulation on the slideways.

Any shape of slideways can be hydro-statically

lubricated.

High load capacity and high rigidity are independent

of the sliding velocity.

There is no stick slip, because there is no metal to

metal contact.

Little wear of the sliding elements and thus longer

life.

Oil forces dust and chips out of the bearing area.

Oil leakage also facilitates heat dissipation.

High machining precision.

Good damping property.

The aperture diameter varies from 0.2 to 0.5 mm

humidity in air is to be reduced to a minimum value

Compressed air is passed through a filter and a pressure

regulating valve. It then enters the pocket at a pressure of 3-

4 kg/cm2

ADVANTAGES:

low friction, coefficient of friction is approximately

0.001 for ground steel slideways and 0.0025 for

scrapped cast iron slideways.

Less stick slip effect.

highly accurate movement.

Uniform slow motion is possible

less wear and considerably long service life

operation without external lubrication.

DRAWBACKS :

The cost of a such slideway system is high, because it

is highly finished and hardened.

Because the contact is at a point or along a line, the

waviness of the surface impairs accuracy of motion.

The rolling elements (balls, rollers etc.) lags behind the

sliding member (Fig.3.28). Hence recirculation of the

rolling elements is a must, if the travelling distance is

long

The motion of tool or job in NC machine tool faces the problem of friction an backlash.

To eliminate and compensate these problem, ball-nut screw assembly is used.

The balls roll in the helical channel of the thread from.