machining(2) part two

Notes On“Machining (2) Course 231/64”

(Part Two: Shapers, Planers & Processes) Prepared By:

Dr. Samy Oraby (Associate Professor)

Feb 2009

Department of Mechanical Production TechnologyWeb: www.mpt-cts.com Email: [email protected]

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Dept. Mech. Prod. Tech., Machining 2 Course [231/64] –

Dr. Samy Orabywww.mpt-cts.com 2

Part One: Metal Cutting Principles1. Introduction.2. Shapers:

i. Classification of Shapers.3. Planers.4. Differences between Planers and Shapers.5. Quick-Return Techniques.

i. Straight Gears and Rack.ii. Hydraulic Systems.

6. Questions and Problems.

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1. INTRODUCTION

Metals are planed by single point tools similar in shape to lathe tools. A similarity also exists in chip formation in the two processes. At the same time, planing differs from turning in that planing is an intermittent process and chips are usually removed only during the straight forward movement of the tool or work.

The conditions under which planing tools work are less favorable than in turning, (even though the tool has an opportunity to cool on the idle stroke when no cutting takes place) because a planing tool operates with impacts.

Work are planed on planers and shapers. In the first case, the primary cutting motion is obtained by the reciprocating movement of the work while, in the second case by the movement of the tool. On a planer, the tool is fed across the work while on a shaper the work is fed crosswise in reference to tool travel. Feed in planing is intermittent; the tool or work is fed at the end of each work stroke.

Each work stroke, during which the tool removes chips, is followed by an idle or return stroke whose speed is 2 to 3 times greater than the work stroke speed.

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2. SHAPERSA shaper is a machine with a reciprocating tool that takes a straight line cut. By successive movement of the work across the path of this tool, a plane surface is generated. Power is applied to the machine by motor, either through gears or belt or by the employment of a hydraulic system. As shown in Fig. 1, a horizontal shaper consisting of a base and a frame that support a horizontal ram. The ram that carries the tool is given a reciprocating motion equal to the length of the stroke desired. The quick-return mechanism driving the ram is designed so that the return stroke of the shaper is faster than the cutting stroke. A claper box toolholder attached to the ram pivots at the upper end.

The shaper can be driven by a mechanical quick-return mechanism, Fig. 2. It consists of a rotating crank driven at a uniform speed connected to an oscillating arm by a sliding block that works in the center of the rather massive oscillating arm. The crank is contained in the large gear and can be varied by a screw mechanism. The stroke length is varied by changing the length of the crank. The ratio of return to cutting speed is about 3:2.

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Cutting Speed and Machining Time in Horizontal Shapers

Cutting speed on horizontal shapers is defined as the average speed of the tool during the cutting stroke and depends primarily on the number of ram strokes per minutes (N) and the length of the stroke (L), Fig. 3. The average cutting speed may be determined by the following formula:

2 L NAverage Cutting Speed (Va) = C

where;N strokes per minutesL stroke length

cutting time 3 Vc 2C = cutting time ratio = = (when R= = )total time 5 Vr 3

Therefore; the average cutting speed is:

==

10 L N Va= 3

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The number of strokes per minute for a desired cutting speed is then:

3 V a N = 10 L

fb =

strokeper feedwork the of Width = S

To determine the number of strokes required to complete a job;

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A general expression, therefore to determine the cutting time knowing the desired speed and the length of stroke is:

10 . b . LTotal T ime (T) = 3 . f . V a

SN1 =strokes ofNumber strokeper Time= (T) Time Total ××

The total machining time is then:

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Example (1) Example (1)

Determine the cutting time to shaper machine a workpiece of dimensions 300 × 100 × 60 mm if the cutting speed is 30 m/min and the feed per stroke is 2 mm.

Solution:

10 . b . L Machining Time (T) = 3 . f . Va10 . 100 . 300= = 1.67 min.

3 . 2 . 30 . 1000

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3. PLANERSPlaners may be classified in a number of ways, but according to general construction there are three types:

Double-Housing Planers, Fig. 4.This planer consists of a long heavy base on which the table reciprocates. The upright housing near the center on the sides of the base supports the crossrail on which the tools are fed across the work.

Open-side planersThis planer has the housing on one side only. The open side permits machining the wide workpiece.

Pit-Type planersA pit-Type planer is massive in construction and differs from an ordinary planer in that the bed is stationary and the tool is moved over the work.

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Fig. 4 Double-Sided Planer

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4. Differences between Planers and Shapers

Although the planer and shaper are able to machine flat surfaces, there is not much overlapping in their fields of usefulness. They differ widely in construction and in the method of operation. When two machines are compared the following differences may be seen:1. The planer is especially adapted to large work; the shaper can do only small work.2. On the planer the work is moved against a stingray tool; on the shaper the tool moves across the work which is stationary.3. On the planer the tool is fed into the work; on the shaper the work is usually fed across the tool.4. The drive on the planer table is either by gears or by hydraulic means. The shaper ram can also be driven using a quick-return mechanism.5. Most planers differ from shapers in that they approach more constant-velocity cutting speeds.

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4. Quick-Return TECHNIQUES

4.1 Using Straight Gears and RackAs shown in Fig. 5, the motion is primarily supplied by the motor

to the loose pulley (b). This motion can be transferred either to the pulley (c) or the pulley (a) using a special mechanical switch designed according to the length of the machining stroke.

To obtain the cutting stroke, motion is transferred to pulley (a) which it is moving on the shaft (c1) freely. Then motion is transferred to gear (G2) across gear (G1) which is rigidly connected to pulley (a). Since gear (G2) is bolted to axis (c2), motion is transferred to gear (G6) and, then, to the machine table through the rack (R).

To obtain motion for the return stroke, the motion is shifted from pulley (b) to pulley (c) which is bolted to the axis (c1). As axis (c1) rotates, motion is transferred from gear (G3) to gear (G5) through an intermediate gear (G1). The motion then is transferred to gear (G6) and then to the machine table.The ratio (R) may be determined as the inverse ratio between theteeth of gears (G2) and (G5).

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Fig. 5 Rapid Return Speed using Straight Gears and Rack

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Example (2)Example (2)

Determine the cutting speed in a planer working using rack and gears combination if motor speed is 1000 rpm and a reduction ratio of 4. Assume suitable number of gear teeth.

SolutionSpeed on axis (c1) = 1000/4 = 250 rpm.Assume that:

Z for G1 = 30 teeth Z for G2 = 68 teethZ for G3 = 30 teeth Z for G4 = 20 teethZ for G5 = 30 teeth Z for G6 = 40 teethD for G6 = 100 mm.

Cutting Speed (Vc) =π × 100 × (1000/4) (30/68) = 34.65 m/min.Return Speed (Vr)= π × 100 ×(1000/4)(30/20)(20/30)=78.54 m/min.R = Vc / Vr = 0.44.

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4.2 Hydraulic System

As shown in Fig. 6, this technique operates by pumping hydraulic oil from reservoir (K) through the pump (P) to a cylinder (C) from two opposite sides (C1) or, (C2). The machine table is connected to the rod of the cylinder (R).To obtain the cutting stroke, the valve (V1) is opened while the other valve (V2) is closed as described by the dotted position of the lower piston. The pumped oil is fed to the cylinder through its right valve (V1). This pushes the piston (S) causing the cutting action in the direction described by the figure. This motion continues until socket (d1) hits the lever (L) at the right position making it to rotate around its center (O). This pushes the piston rod (r) to the new position described in the figure by the continuos line. This allows the oil to be pumped through the valve (V2) through the other end (C2) causing the return stroke to start.This continues until the lever (L) hits the left socket (d2) opening the right valve (V1) and closing the left one allowing another cutting stroke.The rapid return speed is obtained since there is a reduction in the cylinder area due to the existing of the piston rod.

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Fig. 6 Hydraulic Rapid Return Speed Technique

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Example (3)Example (3)

Determine the speed ratio (q) in a planing operation using a hydraulic system.

The oil pressure is 1.1 kp/mm2 and flow of 0.1 m3/min. The piston diameter is

50 mm with a rod of a 30 mm diameter.

Solution

Flow (Q) = Flow Speed × Area

Cutting Speed (VC) = 0.1/[(π/4)(50/1000)2]= 50.93 m/min.

Return Speed (Vr) = 0.1/[(π/4)((50/1000)2)-(30/1000)2)]=79.58 m/min.

Speed Ratio (q) = (Vr) / (Vc) = 1.56

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5 QUESTIONS AND PROBLEMS1. In shaping process it is required to remove a 4 mm thickness from a 200

× 80 × 50 mm steel workpiece using average cutting speed of 60 m/min and 0.1 mm feed and 2mm depth of cut. Calculate:i) Number of strokes per minutes (N); ii) Number of strokes to finish the job (S); iii) Time per stroke; and iv) Total time.

2. It is required to remove a 10 mm from a 300 × 110 × 40 mm steel workpiece. Maximum depth of cut in one pass is 4 mm. Range of shaper strokes/minutes is between 100 and 500, and range of machine traverse feed is between 0.1 and 2.0 mm. Calculate the range of machining time.

3. Explain by a sketch the different parts of double sided planer?4. What are the main differences between planers and shapers?5. In a planer with a hydraulic driving system the oil pressure was 2.2

kp/mm2 with a flow rate of 0.15 m3/min. The piston and rod diameters are 80 and 30 mm respectively. i) Draw a sketch indicating the case.ii) Determine the speed ratio (q) and the system power.

6. In a planer with rack and gears driving system the motor rotates at 2000 rpm. Motion is transferred with (1:5) ratio.i) Suggest a suitable gears size of the system and draw a sketch indicating the main elements. ii) Determine the speed ratio (q).

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End of Part (Two)

BREAK