Rack Planing Process

Rack Planing Process

Rack Planing Process - Gears such as spur and helical can b planed using the sunderland process or the Maag process which are identical in principle but different in the machine configuration and Referring to Fig.  (a), 0 is the center of the gear blank and C is the rack type cutter. The blank is rotated uniformly about its center and simultaneously the cutter is caused to advance along its length, the speed of advance being equal to the speed of the pitch line of the blank.

(a)

(b)

(C)

(d)

Rack planing Process

The relative movement of the blank and cutter is thus one of rolling, and teeth of true involute shape would be formed in the blank if it were made of a soft or plastic material. Since this is not the case, therefore, the rack cutter is provided with relieved cutting edges and is caused to reciprocate (just like the gear shaper cutter) across the face of the blank also (as shown in Fig, b).

The operation eycle is :

(i) Rack cutter fed into full tool depth with cutter recipro­cating and blank stationary,

(ii) Blank rotates and cutter feeds longitudinally. An involute shape is generated on the gear teeth flanks by the involute rack cutter, (Fig. d). The figure indicates successive positions of the gear blank relative to the cutter, and shows how the straight-sided rack teeth would generate the involute tooth curves.

(iii) When the cutter and blank have thus rolled a distance equal to one pitch the blank ceases its rotation? and the cutter (while continuing to reciprocate) is withdrawn and returned, in the direction of its picth line, to its starting point.

(iv) The cutter is then again fed to depth and the cycle of operations is repeated in the manner described above until every tooth is cut (refer Fig. c). The rack type cutter serves a double purpose, as it gashes and finishes simultaneously.

The front teeth of the cutter take the heaviest cuts and do all the hard work, while the rear teeth only perform light duty and give a fine finish I to tbe gear. In rack planing, the rack cutter controls tbe pitch and pressure angle, and the machine gearing controls the number of teeth produced on the gear.

The first of a batch of blanks is always set up, and a cut, just 3 craping the surface, is taken in order to verify that the machine has been set up to cut the correct number of teeth. For cutting helical gears, the rack type cutter is set at the helix angle of the gear to be machined.

When cutting double helical gears, the right and left hand sides of the gear are cut simultaneously by opposite hand cutters reciprocating in directions parallel to the tooth spirals. As one cutter approaches the centre on the cutting stroke the other is withdrawn in readiness for its next stroke.

The cutting edges come to rest when they just pass the central plane dividing the two helices, and in this way the space formed by one cutter is clearance for the other. As regards its applications, the rack planing process is ideally suited for cutting large, double helical (herring bone) gears which do not have a clearance recess between the two sets of teeth.