Shaper Machine Mechanism
In this article, we will learn about the shaper machine mechanism and their types.
In a shaper, the rotary movement of the drive is converted into reciprocating movement. It is done by the mechanism contained within the column of the machine.
The ram holding the tool makes the reciprocating movement. In a standard shaper, metal is cut in the forward cutting stroke, while the return stroke goes idle and no metal is cut during this period. To reduce the full machining time it is important to reduce the time taken by the return stroke.
The shaper mechanism should be so designed that it can allow the ram to move at a slower speed during the forward cutting stroke. The cutting speed depending upon the type of material and machining condition whereas during the return idle return time. This mechanism is known as the quick return mechanism.
The reciprocating movement of the ram and the quick return mechanism of the machine. These are usually obtained by following the four types of shaper machine mechanisms.
- Crank and slotted link mechanism.
- Whitworth quick mechanism.
- Hydraulic shaper mechanism.
- Automatic table feed mechanism.
1. Crank and Slotted Link Mechanism
In this type of shaper machine mechanism. The power is transmitted to the bull gear through a pinion which receives its motion from an individual motor. Or overhead line shaft through speed control mechanism.
The speed of the bull gear may be changed by a different combination of gearing or by shifting the belt on the step cone pulley.
Arrangement and working of parts
The bull gear is a large gear mounted within the column. The radial slide is bolted to the centre of the bull gear it carries a sliding block into which the crank pin is fitted.
Rotation of the bull gear will make the crank pin to revolve at a uniform speed.
The sliding block which is mounted upon the crank pin is fitted within the slotted link. The slotted link which is also known as the rocker arm is pivoted at its bottom end attached to the frame of the column.
The upper end of the rocker arm is forked and attached to the ram block by a pin.
As the bull gear rotates causing the crank pin to rotate, the sliding block fastened to the crank pin will rotate on the crank pin circle. And at the same time will run up and down the slot in the slotted link providing it with a rocking movement which is communicated to the ram. Thus the rotary motion of the bull gear is converted to reciprocating movement of the ram.
2. Whitworth Quick Return Mechanism
This mechanism is also used in slotting machines other than a shaper. In this type of shaper machine mechanism, the link CD (link 2) forming the turning pair is fixed. Shown in the figure.
Arrangement of parts
The link 2 matches to a crank in a reciprocating steam engine. The driving crank “CA” (link 3) rotates at a similar angular speed. The slider (link 4) connected to the crank pin at “A” moves along the slotted bar “PA” (link 1) which oscillates at a pivoted point D.
The connecting rod PR carries the ram at R to which the cutting tool is fixed. The movement of the tool is constrained along the line RD produced. i.e. along a line passing through D and perpendicular to CD.
When the driving crank CA moves from the point CA1 to CA2. (or the link DP from the point DP1 to DP2) through an angle α in the clockwise direction, the tool moves from the left hand end of its stroke to the right hand end by a distance 2 PD.
Now when the driving crank moves from the point CA2 to CA1. (or the link DP from DP2 to DP1) through an angle β in the clockwise direction, the tool moves back from the right-hand end of its stroke to the left-hand end.
A little consideration will show that the time taken during the right movement of the ram (i.e. during forward stroke) will be equal to the time taken by the driving crank to move from CA1 to CA2.
Similarly, the time needed during the right to left movement of the ram (or during the idle or return stroke) will be equal to the time taken by the driving crank to move from CA2 to CA1.
Since the crank link CA rotates at uniform angular velocity, thus, the time taken during the cutting stroke (or forward stroke) is more than the time taken during the return stroke.
In other words, the mean speed of the ram during a cutting stroke is less than the mean speed during the return stroke. The ratio between the time taken during the cutting and return stroke is given by
3. Hydraulic shaper mechanism
In this type of shaper machine mechanism, the ram is moved forwards and backwards by a piston moving in cylindrical placed under the ram.
The machine is consists of a constant discharge oil pump, a cylinder, a valve chamber and a piston. The Piston-rod is bolted to the ram body. As shown in the figure.
Arrangement and working
The oil under high pressure is drawn from the reservoir. The oil is passed through the valve chamber to the right side of the oil cylinder exerting pressure on the piston.
At the end of a forward stroke, the shaper dog hit against the reversing lever causing the valves to alter their positions within the valve Chamber.
Oil under high pressure is now pumped to the left side of the piston causing the ram to perform return stroke. Oil present on the right side of the piston is now discharged to the reservoir.
At the end of the return stroke, another shaper dog hits against the reversing lever altering the direction of stroke of the piston and the cycle is thus repeated.
The quick return is affected due to the difference in the stroke volume of the cylinder at both ends. The left-hand end being smaller due to the presence of the piston rod.
As the pump is a constant discharge one, within a fixed period, the same amount of oil will be a pump into the right or to the left-hand side of the cylinder. This will mean that the same amount of oil will be packed within a smaller stroke volume causing the oil pressure to rise automatically and increasing the speed during the return stroke.
The length and position of stroke are adjusted by shifting the position of reversing dogs.
The cutting speed is changed by controlling the throttle valve which controls the flow of oil. When the throttle valve is lost the excess oil flow cut through the relief valve to the reservoir maintaining uniform pressure during the cutting stroke.
4. Automatic Table feeding mechanism of shaper
The automatic feed mechanism of the table is very simple. This is done by rotating a ratchet wheel, mounted at the crossfeed screw. This enables a corresponding equal rotation of the crossfeed screw after each stroke.
Arrangement of parts
It consists of a slotted disc, which carries a T-slot, as shown in the figure. In this slot is fitted an adjustable pin and to this is attached a connecting rod. The other end of the connecting rod is attached to the lower end of the rocker arm of the pawl mechanism.
The rocker arm swings about the screw C, and at its upper end carries a spring loaded pawl, as shown.
Note, that the lower end of the pawl is bevelled on one side.
This arrangement helps the power feed to operate in either direction, but the same should be set to operate during the return stroke only. If otherwise, the mechanism will be subjected to a severe stress. In some latest types of shapers, can driven feed mechanisms are provided which are more efficient and provide a wider range of feed.
Variation in the feed can be provided by varying the distance R between the disc centre and the centre of the adjustable pin. Larger the said distance greater will be the feed and vice versa. The amount of feed to be given depends upon the type of finish required on the job.
For rough machining, heavier cuts are employed, and thus, a coarse feed is needed. Against this, a finer feed is employed in finishing operations.
The slotted disc at its back carries a spur gear which is driven by the bull gear. As the disc rotates through this gear the adjustable pin, being eccentric with the disc centre.
This causes the connecting rod to reciprocate. This, in turn, makes the rocker arm to swing about the screw C to move the pawl over one or more teeth. Thus transmit an intermittent motion to the crossfeed screw which moves the table.
That’s it. If you have any question about shaper machine mechanism leave a comment. Or ask anything about this topic on our Facebook Page.