Manufacturing technology - Quick return mechanisms used in planner and shaping machines

TABLE OF CONTENTS

1. Quick return mechanism

2. Quick return mechanisms used in a planer

2.1 Open belt and cross belt drive 

2.2 Electrical drive

2.3 Hydraulic drive

3. Quick return mechanisms used in a shaper

3.1 Crank and slotted link mechanism 

3.2 Whitworth mechanism

1. Quick return mechanism

It is a mechanism which is used to produce machine table in faster during the return stroke.

2. Quick return mechanisms used in a planer

Belt drive, Electric drive and Hydraulic drive.

2.1 Open belt and cross belt drive 

• This drive is used for smaller planers. 
• An electric motor is used to drive a counter shaft, it is placed under the table.
• The shaft has two larger pulleys of same diameter and one smaller pulley. Open belt is connected to one pulley and cross belt is connected to another pulley.
• Driving pinion is fitted to the one end of the shaft. One of the smaller pulley and one larger pulley is keyed to the shaft. They are called as fast pulleys.
• The other two pulleys rotate freely on the shaft. They are called loose pulleys.
• Driving shaft speed can be reduced by using a speed reduction gear box. The driving pinion meshes with the rack provided at the bottom of the table.
• The table reciprocates when the pinion rotates. The position of the open belt and cross belt is changed by a belt shifter.
• During the return stroke, cross belt is connected to the larger loose pulley and open belt is connected to the smaller fast pulleys.
• At the end of the return stroke, the trip dog pushes the belt shifter and both the belts are shifted to the right. Now cross belt connects the larger fast pulley and the open belt connects the smaller loose pulley.
• Next the drive is transmitted to the main shaft through the cross belt on the larger fast pulley. Due to this, the direction of rotation of main shaft is reversed.
• Due to the larger diameter of the pulley, the speed of main shaft is reduced. So, that the table moves slowly during the cutting stroke.

2.2 Electrical drive

• Reverse switch is used to change the current direction. DC motor transmit power to bull gear through gear box.
• Bull gear meshed with rack provided on table. Thus the table moves when the bull gear rotates slowly for forward cutting stroke.
• When power supply is given to AC motor, it rotates the DC generator and DC generator supplies current to DC motor. Then the table moves slowly for forward cutting stroke.
• After end of cutting stroke, the trip dog actuate reversing switch and faster return stroke.
• Due to this, DC generator supplies more current in opposite direction. Then DC motor rotates at high speed in opposite direction and table returns at faster speed.

2.3 Hydraulic drive

• Trip dogs change the direction of the oil flow in 4-way valve and table is reversed.
• During the cutting stroke, the oil enters in to the left side of the cylinder and pushes the piston to move to the right. 
• Now the table moves slowly in forward direction. 
• At the same time, oil in the right side flows to the oil sump through the valve.
• At the end of this stroke, the trip dog actuates the 4 way valve and intensity of pressure will be more compared with the intensity of pressure during forward stroke. 
• Hence the return stroke is occurred at faster rate. 
• Thus the quick return of the table is obtained. 
• The stroke length can be adjusted by adjusting the distance between the trip dogs.

3. Quick return mechanisms used in a shaper

Belt drive, Electric drive and Hydraulic drive.

3.1 Crank and slotted link mechanism 

• Motor power is given to the pinion gear.
• Pinion rotates the bull gear.
• Lead screw is attached in bull gear. 
• It is rotated by the bull gear. By this motion rocker arm is oscillate left to right and vice versa.
• Due to this motion ram will be moved left to right and vice versa.
• Stroke length can be adjust by the clamping handle.
• First the handle will be loosed. And the adjustment screw is rotated as required.
• And finally the clamping handle is tightened.

3.2 Whitworth mechanism

• The whit worth quick return mechanism generally used in slotter is shown in figure. 
• An electric motor drives a pinion at constant speed. 
• The pinion drives a bull gear at uniform speed. 
• The bull gear has a crank pin fitted with a sliding block.
• A crank plate is pivoted eccentrically on the bull gear at the point O by a pin. 
• The crank plate has a slot along its length. 
• The crank pin with sliding block slides along the slot of the crank plate. 
• The other end of the crank plate is corrected to a connecting rod through a pin P. the ram is fitted to this connecting rod.
• When the bull gear rotates, the crank pin rotates about its centre. 
• At the same time, the sliding block slides along the slot of the crank plate.
• This makes the crank plate to rotate about the point O. 
• Thus the rotary motion of the crank plate is converted into the reciprocating motion of the ram to the connecting rod.