Various types of cylindrical grinding machines are described below:
Type # 1. Centre Type of Cylindrical Grinding Machine:
In this machine, the workpiece is supported between centres. The headstock wheel head, and tailstock (both of which may be swivelling or non-swivelling type) are mounted on a swivel table which itself is mounted on a sliding table that can move to and fro in the bed guideways. A special feature of these is to grind multi-diameter shafts and control their concentricity.
These are classified as:
(i) Plain Cylindrical Grinding Machines:
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There are basically designed for heavy repetitive simple work. These are not very versatile.
(ii) Universal Cylindrical Grinding Machines:
These are best suited for tool room applications as these are very versatile. Both work head and wheel head can be swivelled. It is possible to grind tapered surfaces and use this machine for internal, face and surface grinding by using some attachments. It is also possible to carry out plunge cylindrical grinding, taper plunge grinding on it.
The various operations possible on a universal cylindrical grinding machine are shown in Fig. 20.16.
(iii) Plunge-Centre Type Grinding Machines:
These could be either straight plunge type or angular-wheel slide plunge cut type. These are basically high volume production machines. The wheel is plunged into the work at a predetermined feed rate and is withdrawn after correct size is produced. Angular wheel slide plunge type machine is used to finish shoulder surfaces at right angles correctly in a single operation by an angular wheel. Refer Fig. 20.17.
Type # 2. Chucking Type Cylindrical Grinders:
These are used for grinding external surfaces of short workpiece having no centre holes and no convenient surface for driving.
Type # 3. Grinding Parameters in Cylindrical Grinding:
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In the case of cylindrical grinding, cutting speed V, i.e. relative motion between grinding wheel and workpiece
In view of irregular shape of abrasive particles, it is difficult to estimate value of uncut chip thickness and width of cut. Metal removal rate is nearly equal to π Dw Fr Ft (in case of transverse grinding) Fr = radial in-feed/stroke of grinding wheel and Ft = rate of transverse radial feed)
In case of plunge cut grinding, metal removal rate = π Dw bg Fp (bg= width of grinding wheel, Fp = rate of radial feed)
Type # 4. Centreless Grinding:
Under modern requirements of accuracy and the cost at which articles be produced, it would be undesirable to fix every ground component between centres. Fortunately centreless grinding has gained much popularity and this process plays an important role in the sphere of grinding.
As the name implies, the work is not supported between centres but is held against the face of grinding wheel by combination of supporting rest and a regulating wheel. Thus centreless grinding does not require centre holes, drivers and other fixtures for holding the workpiece.
During the process, the workpiece is supported on a work rest blade and the regulating wheel holds the workpiece against the horizontal force of action controlling its size and imparting the necessary rotational and longitudinal feed. The blade on the work rest gives the necessary support to the workpiece against the cutting forces.
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The thickness of the blade is usually slightly less than the diameter of the workpiece for smaller diameters and maximum thickness is around 20 mm. The slope (1/2° to 8°) of its edge with the regulating wheel provides the V-formation into which the work seats. Work guides are also used to keep the work parallel with the space between the wheels.
The speed of regulating wheel varies from 15 to 60 metres/min. (while the speed of grinding wheel is maintained at 1800 metres/minute). The workpiece is usually ground with its centre above the line of centres of the wheels by about half the diameter of workpiece, the maximum value being 12 mm. (Fig. 20.19 shows the principle of centreless grinding).
In the side view it is shown that the regulating wheel is inclined at angle α to the grinding wheel. The velocity component can be resolved into two parts, the one in horizontal direction is V sin a which gives the necessary feed, while the other one V cos α in the vertical direction gives rotational motion to the wheel.
The material of blade for various workpiece is:
The rotation of wheel is such that the workpiece rotates in downward direction as this position can be balanced by the work-rest.
Advantages of centreless grinding over cylindrical grinding:
(1) The rate of production is much more in centreless grinding than cylindrical grinding.
(2) The work is supported rigidly along the whole length ensuring better stability.
(3) This process is very suitable for long jobs as the work is supported along the whole length and thus deflection does not occur.
(4) Due to floating conditions, the centering can be eliminated and hence no time is wasted in job setting.
(5) As the centering is eliminated, less stock is needed.
(6) Cost of production is very less.
(7) The wear and tear of the machine is less and hence economical to maintain.
Scope of the Centreless Grinding:
Due to various advantages listed above, centreless grinding is playing an important role in the field of production technology. The process can be applied to the workpiece upto 15 cm in diameter and from washer to bars of 6-7 metres in length. With the in-feed and end-feed methods, tapered, headed, stepped and formed work can also be ground.
The accuracy that can be obtained from centreless grinding is of the order of 0.0025 mm and with suitably selected wheels, the finish satisfies the most exacting requirements.
The figures given below are of valve and socket which are the examples of centreless grinding:
