Different Shape of Guide Ways
Most commonly used shape of building elements of slide ways are
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 Guide Ways:
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 Guide Ways:
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 Guideways:
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 Guideways:-
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.
Anti-friction
Different Types of Anti-Friction Guide-ways are
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.
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.