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 guide ways
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
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
Different Shape of Guide Ways
Most commonly used shape of building elements of slide ways are
Dove Tail Type
Circular or cylindrical type
The selection of the proper shape is based on the magnitude and direction of the load to be carried, position of transmission, wear characteristics ad provisions for wear adjustments, ease of assembly and economy in fabrication, effective lubrication and the easy chip disposal.
All the four types of shapes have their own merits and demerits and thus some times it becomes desirable to use a combination of these slide ways on the machine tools.
1) V-Type: Slide ways are advantageous because of the automatic adjustment the to gravity action which always keeps the surface in contact, thus the possibility of any play is eliminated. In addition, the wedging action due to cross winding is also prevented and there is no possibility of chips falling on the V-surface and getting entrapped into it. It has no tendency to accumulate dust. The Disadvantages of V-Type guides is that it wears away rapidly due to lack of bearing surface and is difficult to manufacture.
2) Flat Type: Slide ways are simple in construction and offer quite a large bearing area to the moving parts. Therefore to overcome the disadvantage of V-type slide ways by the advantages of the flat type slide ways. Separate guides are provided for the saddle and tail stock movements so that any wear caused by the saddle movement does not affect the height of tail stock centre.
The flat bed guides the saddle by the side guides and in order to prevent lifting movement the retaining pieces are fitted below the flat bed.
3) Dove Tail Type: This type slide ways are preferred when location of the moving parts is considered essential. These are generally used for milling machine tables, saddles and knees. In dove tail type slide ways, adjustment for wear can be made by parallel blocks pushed by set screws.
DOVE tail slide ways occupy small space. A Gib is provided. So that it can adjusted to ensure that the two element of the slide are a good sliding fit.
4) Cylindrical Type:- This slide fulfills the condition of kinematic principles being fully constrained. These guides are used in the pillar or radials drilling machines.
Clearance between mating surfaces of slide ways are inevitable because of wear and manufacturing imperfections. Clearance between flat ways may be controlled periodically by means of there gibs or strips.
Stick-slip motion of slide ways caused by frictional resistances between the sliding surfaces can be controlled by using anti-friction ways.
1) Hydro-static Guide ways:- In hydro-static guide ways, air or oil is pumped into the small pockets or cavities machined into the carriage or slides which are in contact with the guide way. The pressure of fluid gradually reduces to atmospheric pressure as it sweeps out from the pockets, through the gap between slide and guide ways. The hydro-static guide way provides almost a friction-less condition for the movement of the slide. For the efficient operation it is very important that the fluids and guide ways are kept clean and also the hydro-static guide ways need a very large surface area to provide adequate support.
2) Aerostatic GuideWays:- In aerostatic guide ways, the slide is raised on a cushion of compressed air which entirely separates the slide and the guide way surface. The major limitation of this type of guide ways as a low stiffness which limits it use for positioning application only e.g. CMM and other measuring instrument. The selection of guide ways for a particular application basically depends upon the requirements of the load carrying capacity, damping property and the traverse speed. In order to get maximum benefit, most of the machine tool manufacturers make use of a combination of anti-friction and friction guide ways with Turcite/PTFE lining. Such combination improves load carrying capacity through the use of anti-friction guide ways and damping property by using guide ways.