4. Forces and Temperatures in Cutting

• There are three main sources of heat when cutting,

1. Heat is produced as the tool deforms (works) the metal

2. Friction on the cutting face

3. Friction on the tool flank

 

• Heat is mostly dissipated by,

1. The discarded chip carries away heat

2. Coolant will help draw away heat

3. The workpiece acts as a heat sink

4. The cutting tool will also draw away heat.

** factors 1 & 2 dissipate 75 to 80%, factors 3 and 4 dissipate 10% each [Krar, ]

4.1 Tool Wear

• Tool wear is still a significant problem in cutting.

• Typical types of tool wear include,

Flank wear

Crater wear

• Flank wear: the point of the tool degrades

 

• Crater wear also decreases tool life

 

• Tool failure can typically grouped under one of the following categories,

Complete Failure: the tool is unusable

Flank Failure: this can be estimated with maximum lw values,

Roughing Cuts

0.03” for carbide tools

0.06” for high speed steel

Finishing Cuts

0.010” for carbides

0.015” for high speed steel

Work surface finish is inadequate

Work dimension outside tolerance

• Flank wear can be discussed as a function of time,

 

• General notes of concern are,

The main factor in tool wear is temperature

The main factor in tool life is cutting speed

Critical temperatures for High Speed Steels are 1150°F and for carbides it is 1600°F

A higher velocity will increase temperature more than an increase in feed for the same mrr

A higher feed will increase the tool forces

4.2 Cutting Tool Materials

• These materials generally need to withstand high temperatures, high forces, resist corrosion, etc.

• The names used for certain materials will be brand names, and so various manufacturers may be calling the same material, different names.

• The List below shows some commercial tool materials

CBN: Cubic Boron Nitride

ceramic:

HSS: High Speed Steel

PCD: PolyCrystalline Diamond

sialon:

WC: Tungsten Carbide

coated WC: Tools coated with Tungsten Carbide

4.2.1 A Short List of Tool Materials

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• Carbon Steels

Limited tool life. Therefore, not suited to mass production

Can be formed into complex shapes for small production runs

low cost

suited to hand tools, and wood working

Carbon content about 0.9 to 1.35% with a hardness ABOUT 62°C Rockwell

Maximum cutting speeds about 26 ft/min. dry

The hot hardness value is low. This is the major factor in tool life.

• High Speed Steel

an alloyed steel with 14-22% tungsten, as well as cobalt, molybdenum and chromium, vanadium.

Appropriate heat treating will improve the tool properties significantly (makers of these steels often provide instructions)

can cut materials with tensile strengths up to 75 tons/sq.in. at speeds of 50-60 fpm

Hardness is in the range of 63-65°C Rockwell

The cobalt component give the material a hot hardness value much greater than Carbon Steels

Used in all type of cutters, single/multiple point tools, and rotary tools

• Stellite

a family of alloys made of cobalt, chromium, tungsten and carbon

The material is formed using electric furnaces, and casting technique, and it cannot be rolled, or worked.

The material has a hardness of 60-62°C Rockwell without heat treating, and the material has good hot hardness properties

Cutting speed of up to 80-100 fpm can be used on mild steels

The tools that use this method either use inserts in special holders, or tips brazed to carbon steel shanks

• Tungsten Carbide

Produced by sintering grains of tungsten carbide in a cobalt matrix (it provides toughness).

Other materials are often included to increase hardness, such as titanium, chrome, molybdenum, etc.

Compressive strength is high compared to tensile strength, therefore the bits are often brazed to steel shanks, or used as inserts in holders

These inserts may often have negative rake angles

Speeds up to 300 fpm are common on mild steels

Hot hardness properties are very good

coolants and lubricants can be used to increase tool life, but are not required.

special alloys are needed to cut steel

• Ceramics

sintered or cemented ceramic oxides, such as aluminum oxides sintered at 1800°F

Can be used for turning and facing most metals, except for nimonic alloys and titanium. Mild steels can be cut at speeds up to 1500 fpm.

These tools are best used in continuous cutting operations

There is no occurrence of welding, or built up edges

coolants are not needed to cool the workpiece

Very high hot hardness properties

often used as inserts in special holders

• Diamonds

a very hard material with high resistance to abrasion

very good for turning and boring, producing very good surface finish

operations must minimize vibration to prolong diamond life

also used as diamond dust in a metal matrix for grinding and lapping. For example, this is used to finish tungsten carbide tools

• Cemented Oxides

produced using powder metallurgy techniques

suited to high speed finishing

cutting speeds from 300 to 7500 fpm

coolants are not required

high resistance to abrasive wear and cratering

4.3 Tool Life

• Tool life is the time a tool can be reliably be used for cutting before it must be discarded/repaired.

• Some tools, such as lathe bits are regularly reground after use.

• A tool life equation was developed by Taylor, and is outlined below,

 

• An important relationship to be considered is the relationship between cutting speed and tool life,

 

• Although the previous equation is fairly accurate, we can use a more complete form of Taylor’s tool life equation to include a wider range of cuts.

 

4.3.1 The Economics of Metal Cutting

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• As with most engineering problems we want to get the highest return, with the minimum investment. In this case we want to minimize costs, while increasing cutting speeds.

• EFFICIENCY will be the key term: it suggests that good quality parts are produced at reasonable cost.

• Cost is a primarily affected by,

tool life

power consumed

• The production throughput is primarily affected by,

accuracy including dimensions and surface finish

mrr (metal removal rate)

• The factors that can be modified to optimize the process are,

cutting velocity (biggest effect)

feed and depth

work material

tool material

tool shape

cutting fluid

• We previously considered the log-log scale graph of Taylor’s tool life equation, but we may also graph it normally to emphasize the effects.

 

• There are two basic conditions to trade off,

Low cost: exemplified by low speeds, low mrr, longer tool life

High production rates: exemplified by high speeds, short tool life, high mrr

*** There are many factors in addition to these, but these are the most commonly considered

 

• A simplified treatment of the problem is given below for optimizing cost,

 

 

• We can also look at optimizing production rates,

 

• We can now put the two optimums in perspective,

 

4.4 References

4.1 Ullman, D.G., The Mechanical Design Process, McGraw-Hill, 1997.

4.5 Problems

Problem 4.1 If a bar of SAE 1040 is to be turned with a high speed steel tool with a feed of 0.015” per revolution, and a depth of 0.050”. Previous experiments have revealed that the following cutting velocities yielded the following tool lives,

90 fpm for 30 min.

80 fpm for 90 min.

75 fpm for 150 min.

a) estimate the cutting speeds to get tool lives of 60 and 120 minutes.

b) calculate the mrr at the two speeds found in part a).

Problem 4.2 Two tools are being compared for their costs. The table below summarizes the details of each tool. Find the economic tool life and cutting speed for each tool, and determine the least expensive tool.

Answer 4.2 tool A T = 45.9 min., V = 232.6 fpm, tool B T = 11.73 min., V = 305.6 fpm, both A and B cost $0.062/min.

Problem 4.3 What happens to the cutting process as the temperature rises?

Answer 4.3 As temperatures rise both the tool and work change. Heat causes expansion, therefore the dimensions change, and accuracy decreases. Heat also causes decreased strength of the material. This causes faster wear in the tool, but also makes the work easier to cut.

Problem 4.4 We are going to estimate the effects of feedrate on tool life. Some simple calculations yield the Taylor tool life coefficients of n = 0.4 and a C = 400. Find the change in tool life (in %) when velocity drops by a) 20% and b) 40%. [based on Kalpakijian]

Answer 4.4

Problem 4.5 Some tools use coatings that reduce the coefficient of friction. How does this affect the cutting process?

Answer 4.5 Reduced friction in cutting reduces heat in the chip and tool, and this will prolong tool life. The reduced friction also decreases the wear rate and prolongs tool life.

Problem 4.6 Describe the factors that are used to decide when a tool should be reconditioned, recycled or discarded.

Answer 4.6 Two failures typically occur; wear and fracture. If a tool is worn, and the material and geometry permit, we can recondition a tool: grinding is common. If a tool is fractured or can’t be reconditioned, it can be discarded. In some cases tools contain parts that can be reclaimed, or materials that can be recycled.

Problem 4.7 As cutting temperatures rise materials expand. How does this affect the cutting process?

Problem 4.8 Consider that at a certain velocity we will get the lowest cost per piece. As the cutting velocity rises the cost per piece rises (but we will improve the production rate) what cost components rise or drop?

Problem 4.9 Describe at least two methods that generate heat during machining.

Problem 4.10 How does the heat generated during cutting affect the operation?

Problem 4.11 What are the main failure types found in tools? Where do these typically occur on the tool?

Problem 4.12 What does the parameter ‘n’ mean in Taylor’s tool life equation? How is ‘C’ different?

Problem 4.13 What properties are desired in a material for a cutting tool?

Problem 4.14 What are the main functions of cutting fluids?

Problem 4.15 We have been asked to calculate the cutting speeds that gives the maximum possible production rate and lowest cost for an existing job. The current tool will last for 4 hours if we cut at 300 fpm and 2 hours at 345 fpm. The following things are known about the job.

the tool costs $6.50 and has 2 edges that can be reground 5 times before discarding.

it takes 5 minutes to change the tool, and 10 minutes to regrind it.

the labor rates for the operators is $25.00/hr.

the tool room labor rate is $35.00/hr for regrinding tools.

Answer 4.15 Vecon = 525fpm, Vcost = 404

 

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