manufacturing processes cutting tool

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Manufacturing Processes II 3rd Year Lecture Notes

Manufacturing Processes II

3rd Year Materials Engineering

By

Dr. Eng. Alaa A. Ateia

PhD. Metallurgical Eng.

Materials Engineering Department

University of Technology

2008-2009

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CHAPTER ONE

CUTTING ENGINEERING

1. Basic cutting tools

1.1 Cutting speeds, feeds, tools and times • Cutting is a balance between a number of factors,

1. Cutting slowly will add costly time to manufacturing operations.

2. Cutting faster will lead to decreased tool life, and extra time will be required to

repair tools.

• Some reasonable speeds and feeds for a single cutting point tool are given below.

1.2 High speed machining • Usually spindle speeds above 10000 RPM, but this is highly relative to the cutting

tool and work.

• The cutting velocity is higher, but the feed/depth of the cut is reduced, the resulting

mrr is still higher.

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• Higher spindle speeds call for new low inertia spindle and tolerances as well.

Small tolerance problems can result in unacceptable vibrations at these speeds.

2. Cutting theory • When we cut metal, the severed pieces are cast off; these are referred to as chips.

2.1 CHIP FORMATION • There are three types of chips that are commonly produced in cutting,

1. discontinuous chips

2. continuous chips

3. continuous with built up edge

• A discontinuous chip comes off as small chunks or particles. When we get this chip it

may indicate,

- Brittle work material

- Small rake angles

- Coarse feeds and low speeds

• A continuous chip looks like a long ribbon with a smooth shining surface. This chip

type may indicate,

- Ductile work materials

- Large rake angles

- Fine feeds and high speeds

- Use of coolant and good chip flow

• Continuous chips with a built up edge still look like a long ribbon, but the surface is no

longer, smooth and shining. This type of chip tends to indicate,

- High friction between work and tool causes high temperatures that will occasionally

weld the chip to the tool. This will break free, but the effect is a rough cutting action.

• Continuous chips, and subsequently continuous cutting action is generally desired.

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5.2 THE MECHANISM OF CUTTING • Assuming that the cutting action is continuous we can develop a continuous model of

cutting conditions.

• Orthogonal Cutting - assumes that the cutting edge of the tool is set in a position that is

perpendicular to the direction of relative work or tool motion. This allows us to deal with

forces that act only in one plane.

• We can obtain orthogonal cutting by turning a thin walled tube, and setting the lath bit

cutting edge perpendicular to the tube axis.

• Next, we can begin to consider cutting forces, chip thicknesses, etc.

• First, consider the physical geometry of cutting,

• We can obtain orthogonal cutting by turning a thin walled tube, and setting the lath bit

cutting edge perpendicular to the tube axis.

• Next, we can begin to consider cutting forces, chip thicknesses, etc.

• First, consider the physical geometry of cutting,

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• Next, we assume that we are also measuring two perpendicular cutting forces that are

horizontal, and perpendicular to the figure above. This then allows us to examine specific

forces involved with the cutting. The cutting forces in the figure below (Fc and Ft) are

measured using a tool force dynamometer mounted on the lathe.

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5.2.1 Force Calculations

5.2.1.1 - Force Calculations • The forces and angles involved in cutting are drawn below,

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• Having seen the vector based determination of the cutting forces, we can now look at

equivalent calculations

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5.2.1.2 - Merchant’s Force Circle with Drafting (Optional) • Merchant’s Force circle is a method for calculating the various forces involved in the

cutting process. This will first be explained with vector diagrams, these in turn will be

followed by a few formulas.

• The procedure to construct a merchant’s force circle diagram (using drafting

techniques/instruments) is,

1. Set up x-y axis labeled with forces, and the origin in the centre of the page. The

scale

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1. Should be enough to include both the measured forces. The cutting force (Fc) is

drawn horizontally, and the tangential force (Ft) is drawn vertically. (These forces

2. will all be in the lower left hand quadrant) (Note: square graph paper and equal x

& y scales are essential)

3. Draw in the resultant (R) of Fc and Ft.

4. Locate the centre of R, and draw a circle that encloses vector R. If done correctly,

the

5. Heads and tails of all 3 vectors will lie on this circle.

4. Draw in the cutting tool in the upper right hand quadrant, taking care to draw the

correct rake angle (⟨) from the vertical axis.

5. Extend the line that is the cutting face of the tool (at the same rake angle) through

the

6. Circle. This now gives the friction vector (F).

7. A line can now be drawn from the head of the friction vector, to the head of the

resultant vector (R). This gives the normal vector (N). Also add a friction angle

() between vectors R and N. As a side note recall that any vector can be broken down into

8. Components. Therefore, mathematically, R = Fc + Ft = F + N.

6. We next use the chip thickness, compared to the cut depth to find the shear force.

To do this, the chip is drawn on before and after cut. Before drawing, select some

magnification factor (e.g., 200 times) to multiply both values by. Draw a feed

thickness

9. Line (t1) parallel to the horizontal axis. Next draw a chip thickness line parallel to

the tool cutting face.

7. Draw a vector from the origin (tool point) towards the intersection of the two chip

lines, stopping at the circle. The result will be a shear force vector (Fs). Also

measure the

10. Shear force angle between Fs and Fc.

8. Finally add the shear force normal (Fn) from the head of Fs to the head of R.

9. Use a scale and protractor to measure off all distances (forces) and angles.

11. • The resulting diagram is pictured below,

5.3 Power consumed in cutting • There are a number of reasons for wanting to calculate the power consumed in cutting.

These numbers can tell us how fast we can cut, or how large the motor on a machine

must be.

• Having both the forces and velocities found with the Merchant for Circle, we are able to

calculations:

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