The most commonly used shapes of guiding elements of slide ways are:
(i) V-type
(ii) Flat type
(iii) Dovetail type
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(iv) 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 and 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 sometimes it becomes desirable to use a combination of these slideways on the machine tools.
The V-type slideways are advantageous because of the automatic adjustment due to gravity action which always keeps the surfaces 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.
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The disadvantage of V-type guides is that it wears away rapidly due to lack of bearing surface, and is difficult to manufacture.
Flat type-slideways are simple in construction and offer quite a large bearing area to the moving parts. Therefore, to overcome the disadvantage of V-type slideways by the advantages of the flat type slideways, generally these two together are used for guides as in lathes.
Separate guides are provided for the saddle and tail-stock movement so that any wear caused by the saddle movement does not affect the height of tail stock centre. The flat type bed guides the saddle by the side guides and in offer to prevent lifting movement the retaining pieces are fitted below the flat bed.
Roller slideways (refer Fig. 11.15 (b)) are used to move heavy loads positively and accurately. In this system rollers retained in a brass cage are interposed between the sliding members. This arrangement reduces the friction between the two members of the slide because rolling friction is substituted for sliding friction.
The advantages of flat slideways are that they require devices for adjusting the clearances, have tendency to accumulate dust, and their retention of lubricating oil is poor for lathe bed, both flat and inverted slideways, which are simple to manufacture and easily escaped when wear has taken place, are used (Refer Fig. 11.16).
Dove tail type slideways are preferred when the location of the moving parts is considered essential. These are generally used for milling machine tables, saddles and knees. In dovetail type slideways, adjustment for wear can be made by parallel blocks pushed by set screws.
Dovetail slideways occupy small space. A gib strip is provided as shown in Fig. 11.15 (a) so that it can be adjusted to ensure that the two elements of the slide are a good sliding fit.
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Cylindrical slide fulfils the conditions of kinematic principles being fully constrained. These guides are used in the pillar or radial drilling machines.
Clearance between mating surfaces of slideways are inevitable because of wear and manufacturing imperfections. Clearance between flat ways may be controlled periodically by means of three gibs or strips.
Slip-stick motion of slideways caused by frictional resistance between the sliding surfaces can be controlled by using antifriction ways. In this way sliding friction is substituted by rolling friction. Fig. 11.19 shows use of ball bearings and Fig. 11.20 shows how rollers can be used between hardened guiding strips to minimize friction.
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The travel length of non-circulating ball or roller bearing guide ways is limited to about twice the length covered by the balls. However with circulating-type guide ways this limitation is overcome.
