CNC GuideWays

CNC Machine Guideways

Precise positioning and repeatability of CNC machine tool slides are the major functional requirements of CNC machines. The inaccuracies that are caused is mainly due to the stick slip motion when plain slide ways are used [metal to metal contact] therefore.

• Guide ways are used in machine tools to:
• Control the direction of the table on which tool or work is held
• To absorb all static and dynamic forces
• Size and shape of the work produced depends on accuracy of movements and geometric and kinematics accuracies of guide ways.
• Straightness, parallelism and flatness errors in guide ways result in tracking errors
•  Alignment of guide ways as to each other and to the axis of spindle is critical
• Wear in guide ways reduces accuracy of movement
• Sped vary from 20mm/min to 30mm/min

TYPES OF GUIDE WAYS 

1) Friction guide ways

2) Anti-friction guideways

1. Friction guideways

CNC machine tools has made necessary the rapid response of slides to a command signal and constant frictional resistance to slide movement is desirable. The phenomenon gives rise of the familiar stick of oil is lubricated sliding surfaces when the adjustment is needed or jerky action when movement takes place at low velocities, the term stick-slip is used to describe these situation.

To overcome these problem of stick-slip CNC machine use

• Anti-friction bearing elements like re-circulating roller packs, re-circulating ball bushing or
• Non metallic liners like Pol Tetra fluro Ethylene [PTEF], Turcite-B and Ruion being very commonly used or
•  A combination of anti-friction bearing elements and nonmetallic liners which provide low friction and necessary damping or
• Hydro-static or Aerostatic slide ways

Advantages of friction guideways

• Used on conventional machines
• Low manufacturing cost
• Good damping properties
• Do not possess constant coefficient of friction
• Problem of stick slip occurs
• Can not be Preloaded
• Guide ways are integral part of structure and hence can not be replaced
• Turcite  /  PTFE lining to reduce stick slip.

Examples of friction guideways

V guide way : self aligning

Flat guide way                         : better load bearing capability

Dovetail guide way                 : compact

Cylindrical guide way              : ideal for shorter traverses

2. Anti-friction Linear motion guide ways

Metal to metal contact has a relatively high coefficient of friction and results in higher wear and heat generation. To minimize this, anti-friction linear motion guide ways are used on the CNC machine tools to:

• Reduce to amount of wear
• Reduce friction
• Reduce heat generation
• Improve the smoothness of the movement

Advantages of Anti-Friction guideways

• Low frictional resistance
• No stick slip
• Assembly time is minimized
• Ease of availability
• High load carrying capacity
• Heavier preloading possibility
• High traverse speeds

Disadvantages of Anti-Friction guideways

• Lower damping capacity

Examples of Anti-Friction guideways

• Non re-circulating ball or roller cages
• Re-circulating ball bushings
• Re-circlulating roller bearings
• Cross roller bearings
• Linear motion guides