Reciprocating grinding is the oldest form of surface grinding. It is characterized by low depths of cut between 0.005 and 0.03mm (0.0002 to 0.0012inch) and fast reciprocating table movements of around 15 to 30m/min (45 to 90ft/min).
Reciprocating is a very cost effective method for easy-to-grind materials, low number of work pieces and low stock removals. Also, the initial investment for such a grinder is relatively low. These machines are typically found in tool mould making shops.
There are, however, some serious disadvantages inherent in the reciprocating process. The low depths of cut prevent proper chip formation and rapidly lead to wear flats on the abrasive grit resulting in wheel glazing and a high level of friction. This generates excessive levels of heat often causing thermal damage and micro cracking. Low depths of cut result in short arcs of contact. This, in combination with hard grades of grinding wheels and low machine rigidity, may cause detrimental chatter and vibrations. Furthermore, with each reciprocating movement, the grinding wheel impacts on the edges of the work piece and suffers and increasing loss of form. Valuable production time is subsequently wasted on redressing. Additional time is wasted in air-grinding at either end of each reciprocating table stroke.
Creep-feed grinding, on the other hand, eliminates the disadvantages listed above. It requires, however, a considerable investment in a sophisticated machine tool and its ancillary equipment such as rotary dressing units, large capacity coolant supply and filtering units.
Creep-feed grinding is characterized by increased depths of cut ae, the length of arc contact, lk increases exponentially, following a square root function ƒ (lk) = lk x (ae) 1/2. (Doubling the depth of cut ae will increase the length of arc lk by 2.)
"Length of arc lk" refers to the distance an abrasive grit must travel while in contact with the work piece. On a 0 400mm wheel (16 inch), a depth of cut ae of 0.025mm (0.001 inch) will generate an arc of contact lk between the work piece and the grinding wheel of 3.16mm (0.125 inch). Increasing the depth of cut ae a hundredfold to 2.5mm (0.1 inch) will increase the arc of contact lk tenfold to 31.65mm (1.25 inch).
By multiplying the two arcs of contact lk by the same given grinding wheel thickness (width) bs, we will get the actual areas of contact Ak between the grinding wheels and the work pieces during the grinding process, with the second area of contact again being ten times larger. Within these areas of contact Ak, based on abrasive size and structure, there will be a given amount of active abrasives particles in contact with the material. Applying the same given grinding force to both areas will result in much smaller unit pressure per abrasive particle in the case of the larger area. A closed structure grinding wheel, therefore, which may have worked well with a low depth of cut ae, would result in excessive friction in deep cut operations due to a very high number of active particles, and to make the wheel self-sharpening, the abrasive grits have to be spaced further apart. This is achieved by manufacturing open structure wheels with induced porosity. To further assist the self-sharpening of open structure wheels, only the more friable abrasives such as white aluminum oxide are used.
There are many advantages of creep-feed grinding over reciprocating grinding such as higher material removal rates, better holding, etc. These, however, come at a price. There are much higher forces and temperatures at play than in reciprocating grinding. A creep-feed machine must be more rigid and powerful with spindle powers between 40 and 80kW (55 to 110hp) and higher. The high resulting temperatures must be counteracted with huge coolant supply units delivering volumes between 250 and 500l/min (65 to 135 gallons/min) at 15bar (230 psi.) pressures, or higher.
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