Machining

Most common tool coatings

1. Titanium nitride (TiN) 2. Titanium carbide 3. Titanium carbonitride 4. Aluminum oxide

Sintering

A process where heat (not enough to melt the metal) and pressure are applied to a powdered metal, curing it in some shape.

Friction Stir Welding

A small rotating tool is plunged into the joint, which heats up and stirs the material. Relatively low heat input is required (450 to 500 degrees F), producing minimal distortion. No fumes or spatter produced. Heavy machinery is required to produce the forces required to drive the tool into the joint, and maintain tool depth along the length of the weld

Taylor tool life equation

An equation used to determine the expected lifespan of a tool. V*t^n = C C and n are workpiece and tool material dependent variables that are empirically derived.

Stages of the chemical reaction for gas welding

Two stages. First stage is the inner cone of the flame. The second stage is the outer envelope. The inner cone is much hotter relative to the outer envelope, but offers less heat (one third of total heat) because of how concentrated it is. - Flame looks different based on oxygen to acetylene ratio

Cutting Speed

velocity of cutting tool relative to workpiece

Built-Up-Edge Chips

(BUE) consists of thin layers of metal from the workpiece that deposit on the rake face of the tool. The BUE grows larger with time, until it breaks up, and part of it remains stuck to the tool face, the other part gets deposited on the surface of the workpiece (adversely affecting the surface finish). While this built-up-edge has the advantage of forming a protective layer on the tool, this protective layer effectively dulls the tool at the same time

Adaptive Control

(Machine intelligence) - sensors monitor force, torque, vibration, temperature, tool wear, chipping, etc. The machine then makes appropriate changes to optimize efficiency and desired outcome

Ceramic coatings

(commonly aluminum oxide): high temperature performance, chemical inertness, low thermal conductivity, resistance to flank and crater wear. HOWEVER, they bond weakly to the substrate, and tend to peel off the tool if not used in a multiphase coating

Titanium Nitride (TiN) coatings

(gold color): low coefficient of friction, high hardness, retention of properties at high temperatures, good adhesion to substrate. They DO NOT perform as well as the uncoated tool AT LOW SPEEDS, because the coating is susceptible to chip adhesion. Cutting fluids discourage chip adhesion. Flank wear is reduced

Titanium Carbide coatings

(silver grey): high resistance to flank wear

Titanium Carbonitride

(violet color): harder and tougher than TiN, effective at cutting stainless steels

Explain Shielded Metal Arc Welding (SMAW) ("stick welding")

- 1/2 of all industrial maintenance welding is performed by this process Arc is generated by touching the tip of the coated electrode to the workpiece, then withdrawing it quickly to generate an arc, and keeping it at a distance to maintain the arc. The electrodes are thin, long sticks (thus stick welding). The arc melts the electrode, as well as the workpiece, and the weld forms as the molten metals solidify. The electrode coating forms a deoxidizing shielding gas as it melts, meaning it reacts with the surrounding oxygen, preventing the weld itself from oxidizing

Setup for Shielded Metal Arc Welding (SMAW) ("stick welding")

- Electrode clamped to one terminal of the power source - Part being welded is attached to the other terminal. You can select the polarity (direction of current flow) by switching which terminal the workpiece and electrode are attached to. If the workpiece is (+) and the electrode is (-), i.e. electrons are flowing from the electrode to the workpiece, shallow penetration results, so it is preferred for sheet metals. If the polarity is reversed, i.e. the workpiece is negative and the electrode is positive, deeper heat penetration results

Thermite (or Exothermic) Welding

- Mix metal powder with a metal oxide, and use a high temperature ignition source to cause an oxidation-reduction reaction. - Aluminum powder + Iron oxide (rust) powder + magnesium fuse = chemical reaction that forms aluminum oxide and iron. - The iron reaches up to 2500 C, which melts the iron, which then flows into a pouring basin, and into a mold where the parts to be joined are inserted. Therefore, in casting terminology, the parts to be joined together act as "cores." *There is also a place in the mold where an oxyacetylene torch is inserted (prior to filling the mold with molten metal). The purpose of the torch is to heat the parts to be joined. This pre-heating helps resist thermal shock of the materials to be welded

Discontinuous chips are caused by:

1) Brittleness of the workpiece material (material can't undergo the high strains associated with machining) 2) Hard inclusions and impurities in the workpiece. These serve as nucleation points for cracks. 3) The cutting speed being too low or too high 4) The depth of cut being too large 5) The rake angle being too low 6) Low stiffness and low damping of the machine tool 7) Lack of cutting fluid

Important characteristics of tool materials

1) Hardness, and particularly "hot hardness": hardness maintained at higher temperatures - Hardness is resistance to deforming and flattening 2) Toughness, so that impact forces do not chip the tool - Resistance to breakage and chipping 3) Wear resistance, to extend tool life - Resistance to abrasion and erosion 4) Chemical stability or inertness, so that reactions contributing to tool wear are avoided - Resistance to corrosion

The BUE decreases with?

1) Increased cutting speed, V 2) Decreased depth of cut, d 3) Increased rake angle, 4) Decreased tip radius of tool 5) Use of cutting fluid

Four main metrics of machinability

1) Surface finish and integrity of machined part (good surface finish) 2) Tool life (long tool life) 3) Force and power requirements (low force and power requirements) 4) Chip control (produced chip is easily collected and doesn't interfere with machining)

The two main types of carbides

1) Tungsten Carbide: Tungsten-carbide particles in a cobalt matrix. These tools are manufactured by powder metallurgy 2) Titanium Carbide (TiC): higher wear resistance than Tungsten Carbide, but less toughness. Suitable for machining hard materials, like steels and cast irons, and for machining at speeds higher than those for tungsten carbide

Characteristics of tool coatings

1. High hot hardness 2. Chemical stability and inertness to workpiece material 3. Low thermal conductivity 4. Good bonding to the substrate (the material we are coating) 5. Little or no porosity

Carbide tools have:

1. High hot hardness 2. High elastic modulus 3. High thermal conductivity 4. Low thermal expansion

Why do we need Joining and Fastening?

1. Product is impossible or uneconomical to manufacture as a single part 2. Product is easier or less costly to manufacture in individual parts which are then assembled 3. Product may have to be taken apart for repair or maintenance 4. Different parts of the product require different properties 5. Transportation of the product in individual components and subsequent assembly may be more economical

What are you going to get on this exam?

A 100% :D

Chips

A chip is produced ahead of the tool by shearing material along the shear plane. One side of the chip is shiny, due to friction with the rake face of the tool. The other side is rough, due to shearing. Due to strain hardening from shear deformation, chips are harder, stronger, and less ductile than the workpiece they are cut from

What is the frictional component of the total power and what does it equal?

A component of the total energy that is required to overcome friction at the interface of the tool rake face and the chip. Pf = Ff*Vc, Where Vc is chip velocity

Inserts

After tools become dull, they need to be sharpened. To increase efficiency, we now have tool inserts. Carbide tools are most commonly used as inserts. These inserts are of different shapes, and have a bunch of cutting edges on them. Inserts often have chip breakers. Inserts are clamped on the tool shank.

Arc

An electrical breakdown of gas (in this case, air) that causes an ongoing electrical discharge. - Lightning is a naturally occurring electrical arc. - Characterized by high current density and high temperature (3000-6000 degrees Fahrenheit) - The science of electric arc started formally being developed in the 1800's, but it wasn't until the 1890's that this form of electrical energy started being applied to welding. "Stick welding" was the first implementation of arc welding

Oxyfuel Gas Welding (OFW)

Any welding process that uses fuel gas combined with oxygen to produce a flame to melt the metals at a joint. The most common gas used is acetylene, and the process is called oxyacetylene gas welding (OAW)

Filler metals

Apply additional metal to the welding zone. Can be coated with flux, which helps minimize oxidation of the surfaces being welded. The molten metal heated up during welding is susceptible to oxidation, so the flux is necessary to prevent this

Explain temperature distribution on a Merchant Model with respect to cutting speed

As cutting speed increases, a greater percentage of the heat is diverted to the chip! However, higher cutting speed implies higher total energy, so--this doesn't mean necessarily that by increasing your cutting speed you reduce the temperature of the workpiece!

Machinability of copper alloys

Brasses, especially those containing lead, are easy to machine. Bronzes are more difficult than brass to machine

Machinability of Tungsten

Brittle, strong, very abrasive. Machinability is low

High-Speed steel tools

Can be hardened to various depths, have good wear resistance, relatively inexpensive.

Can you label all parts of The Merchant Model?

Can you define all parts of The Merchant Model?

Can you label all forces in The Merchant's Force Circle?

Can you label all forces in The Merchant's Force Circle?

Flank wear

Caused by sliding on machined surface (abrasive wear). Abrasive wear is enhanced by temperature rise and its adverse affects on material properties of the tool

What force(s) can we measure from The Merchant's Force Circle?

Chip thickness. We know the rake angle from tool geometry. Using a force dynamometer, we can measure the cutting force and the thrust force

Why is Machining Needed?

Closer dimensional accuracy, Necessary for certain geometric features, finishing operation to correct defects from heat treating, specific surface characteristics (like for a highly reflective mirror), More economical for producing a small amount of parts.

What are coated tools for?

Coatings are often applied to high-speed steel and carbide tools. The base tool material is called the "substrate." Coated tools can be used at much higher cutting speeds. 2-3x greater wear resistance than uncoated tools, 10-100x less crater wear

Numerical Control

Control machining components using coded instructions. A system interprets the commands to do things like turn on/off spindles, change tools, move workpiece and tools along specific paths, and turn on/off cutting fluids

Crater wear

Crater-shaped wear pattern on rake face, caused by increased temperature at rake-chip interface. The crater is located at the highest temperature location. Crater wear is usually accompanied by discoloration of the tool (called "loss of temper")

Current Ranges for Shielded Metal Arc Welding (SMAW) ("stick welding")

Current ranges from 50 to 300 A, with power less than 10 kW. - If the current is too low, poor fusion of the metal is achieved. If the current is too high, it can damage the electrode coating, which reduces the effectiveness of the shielding gas. - DC current produces a steady arc, whereas AC current causes the arc to pulse rapidly.

Diamond coatings

Diamond is the hardest substance of known materials - Low tool-chip friction - High wear resistance - Maintenance of sharp edge Susceptible to chipping! For this reason, it used primarily for light, uninterrupted finishing cuts. For example, machining copper-front high-precision optical mirrors

Machinability of molybdenum

Ductile and work hardening, and therefore less machinable

Computer Numerical Control (CNC)

Each machine controlled locally, by its own computer. This is more typical nowadays.

Machinability of Aluminum

Easy to machine, although soft aluminums have built up edges and thus a poor surface finish. High cutting speeds, high rake angles, high relief angles are recommended (high rake angle + high relief angle=small included angle)

Machinability of magnesium

Easy, with good surface finish and long tool life. HOWEVER, there is danger of fire because of the high rate of oxidation

Explain temperature, heat, and their affects during machining

Energy dissipated in machining is converted to heat, and causes a temperature rise in cutting zone. High temperatures adversely affect the strength, hardness, and wear resistance of the cutting tool. Heat results in dimensional changes of workpiece, and makes it hard to control tolerances. The surface of the workpiece can suffer thermal damage. Principal sources of heat generation are the primary shear zone, the secondary shear zone (due to friction at the tool-chip interface), and finally the flank face/workpiece interface. A dull tool also generates extra heat

The total power of cutting is given by

Fc*V (the product of the cutting force and the cutting velocity)

Inertia Friction Welding

Flywheel accelerates, members are brought into contact, axial force is applied, axial force increases as flywheel slows, weld is completed when flywheel finally stops. Can only be used where angular orientation at the weld interface isn't important

In the Merchant's Force Circle, Fn is equal to:

Ft*cos(fi) + Fc*sin(fi)

Energy Balance of Arc Welding

H/l = e*V*I/nu - H is heat input - l is weld length - e is efficiency - V is voltage - I is current - nu is welding speed H can be expressed as: H = u*A*l - u is specific energy of welded material - A is cross-sectional area of weld bead - l is length of weld To get: nu = e*V*I/(u*A)

Arc Welding

Heat required for welding is obtained from electrical energy. Using a consumable or non-consumable electrode, an arc is produced between the tip of the electrode and the parts to be welded

Cast-Cobalt Alloys (Stellite tools)

High hardness and hot hardness, good wear resistance. 38 to 53% cobalt, 30 to 33% chromium, 10 to 20% tungsten. These are often used as hip and knee replacement materials, because of wear resistance. Not as tough as high speed steels, and sensitive to impact forces. Therefore they are less suitable for interrupted cutting operations. These tools are being faded out because of their cost of production compared to other, better performing tools.

What is usually done to high-speed steel tools?

High speed steel tools are either cast, wrought, or sintered. They can also be coated for improved performance

What contributes to tool wear?

High stresses, elevated temperatures, and sliding over the machined surface

When and why did Taylor leave Bethlehem Steel?

In 1901 Taylor had to leave Bethlehem steel because he didn't get along with the other managers

How were high-speed steel tools made?

Introduced by F.W. Taylor and Maunsel White in 1900, who were working at Bethlehem Steel in Pennsylvania. They took "Mushet" steel, a steel alloy including carbon, chromium, and tungsten; and they heat treated it to levels unprecedented at the time. The result was a steel with high toughness and resistance to chipping, higher hot hardness than tool steels, resulting in a 4-5x decrease in machining time!

Carbide tools

Introduced in the 1930s, Carbides (also known as cemented or sintered carbides) have increased strength and hardness relative to the previous tools. They are produced by sintering. They became popular in WWII due to the 4-5x machining speed increase over high-speed steels.

Machinability of wrought (metal formed) copper

Is difficult to machine, due to B.U.E

Disadvantages of Machining

Lots of material waste, More labor required, Removing material takes more time than other processes

Machinability of titanium

Lowest thermal conductivity of all metals, which causes significant localized temperature rise and the formation of built up edges, and also serrated chips. Therefore, achieving a good surface finish is difficult

Machinbility of Gray cast irons

Machinable, but abrasive

Machinability of leaded steel

Made by adding lead to molten steel while casting. Lead particles are dispersed finely in solidified product. Lead has low shear strength, and while machining, the lead particles act as a solid lubricant. Lead reduces stress in primary shear zone, and reduces cutting forces and power consumption. Toxicity concerns! Therefore, other soft metals like bismuth and tin are preferable. When these alternative to lead are used, they are called "free-machining" steels

Direct Numerical Control (DNC)

Many machines controlled by a central computer. Becoming less common as computers become less and less expensive, and more and more powerful. Also, this framework falls prey to the classic SCI-FI villain movie blunder: what if the central computer fails?

Multiphase coatings

Mixture of the different coatings. a) First layer must bond well to the substrate b) Outer layer must resist wear and have low thermal conductivity c) Intermediate layer should be compatible with the first layer and outer layer

Gas Tungsten Arc Welding (GTAW) (TIG (tungsten inert gas) welding)

Non-consumable electrode made of Tungsten. Because the Tungsten is not consumed, this helps maintain a constant and stable arc gap at a constant current. For example, think of stick welding: because the electrode is being consumed and constantly getting shorter, the welder has to compensate for this. Such is not the case with TIG welding. However—because the electrode is not consumed, you run the risk of contaminating it by touching it to the molten metal of the weld *Filler metal does not contain flux - rather, a shielding gas, typically of argon and helium, to protect against oxidation. - Typically creates a very high quality weld, however it is more expensive than stick welding due to the inert gas expensive than stick welding due to the inert gas

Do we always want to generate continuous chips?

Not necessarily. They can become tangled around the tool and disrupt the machining process, especially with CNC machining. For this reason, chip breakers are used, which break the chip before it gets too long

Closed Loop vs. Open Loop

Open loop uses a stepper motor, and is controlled by pulses sent into it Closed loop, or "Feedback System" uses a DC motor. Directions are entered into the system, where it takes the input and subtracts the actual position (measured by a position sensor) of the workpiece to get the position error. It is called a feedback system because the sensor sends information back to the input in order to calculate the position error. For both, the user inputs: - Geometry - Cutting parameters (speed, feed, tool) - Tool changes

Flame types

Proportion of oxygen and acetylene 1. Neutral: 1:1 oxygen and acetylene ratio, no excess oxygen 2. Oxidizing: extra oxygen, can be harmful because it oxidizes the metal, especially steels! This can be desirable for copper, however. Protective slag layer formed 3. Reducing (or carburizing): lower temperature, useful for brazing, soldering, and flame hardening operations

In the Merchant's Force Circle, Fs is equal to:

R*cos(-alpha + beta + fi) OR -Ft*sin(fi) + Fc*cos(fi)

In the Merchant's Force Circle, N is equal to:

R*cos(beta) OR -Ft*sin(alpha) + Fc*cos(alpha)

In the Merchant's Force Circle, Ff is equal to:

R*sin(beta) OR Ft*cos(alpha) + Fc*sin(alpha) ALSO, Ff = mu*N and mu = tan(beta)

How does rake angle influence shear strain?

Rake angle increases shear strain

Linear Friction Welding

Reciprocating linear motion between the faces of two parts creates friction and heat. Can be used in situations where orientation is important. Can be used to join dissimilar metals, like aluminum to steel. Or even for joining plastics

Explosion Welding History

Started being commercially used a few decades after WWII, however the process was discovered in WWI, where they noticed that shrapnel would not only embed itself, but become welded to armor plating. Extreme heat (like other forms of welding) wasn't involved, so they determined that the intense explosive forces were responsible for the weld. In 1964 DuPont patented the process. In 1996, Dynamic Materials Corporation acquired Dupont's operations

Machinability of steel

Steels are not inherently the most machinable of materials, and often require some sort of treatment to make it more suitable for machining. As carbon content increases, steel becomes less and less machinable

Mchinability of resulfurized and rephosphorized steels

Sulfur forms manganese sulfide inclusions which act as stress raisers in the primary shear zone, and reduce cutting force. Phosphorous acts to harden the steel, and make the chips less continuous. These additions make the steel easier to machine, but aren't they reduce the ductility and toughness of the steel

Pressure Gas Welding

The addition of axial force to the weld interface while performing gas welding. The interfaces to be welded are heated up with a torch. Once melting begins to occur, axial force is applied until the surfaces solidify. Flash is formed (i.e., like the flash that can be formed between the cope and drag during casting. A metallic projection.)

What is the specific energy of friction?

The frictional component of the energy per unit volume of material removed. uf = Ff*Vc/(w*to*V) Vc/V can be simplified to the cutting ratio, and Ff can broken up into its components, Fc*sin(alpha) + Ft*cos(alpha)

Friction Welding

The heat of required to melt the interface of the weld is generated by friction. There are a few different techniques to generate this friction

Submerged Arc Welding

The shield from oxidation is created by a granular flux (lime, silica, manganese oxide, calcium fluoride). This sand-like mixture is fed from a hopper via gravity to the weld zone. It prevents sparks and suppresses fumes and radiation. It also acts as thermal insulator, which allows for deeper penetration of heat. - Some of the flux fuses to the metal, and becomes part of the weld. Typically a recovery tube picks up the unused flux The electrode is consumable. It is a bare wire fed through a tube called the welding gun - The process can be used on thick plates on speeds up to 17 ft/min. Due to the deep heat penetration, the process is used for heavy duty welding jobs for ships, pressure vessels, pipes

The specific energy is ______ and is equal to

The total energy per unit volume of material removed. ut = Fc*V/(w*to*V). (The velocities can cancel out to simplify the equation)

What is significant about the substrate of a coated tool?

The underlying substrate provides: hardness, toughness, high thermal conductivity. The high thermal conductivity of the underlying substrate allows for heat to be dissipated, and not concentrated, which could lead to crater wear

Explain the typical temperature distribution on a Merchant Model

The workpiece is relatively cool compared to the chip and to the tool. The highest temperature on the workpiece is along the primary shear plane, and also the contact point of the flank face, where some rubbing occurs.

Explosion Welding Process

Two metals to be welded are stacked on top of each other. Then, a layer of powder explosives is placed on top of the plates. Upon detonating the explosives, a huge amount of downward force is applied to the materials, welding them together. The process creates a wavy interface between the metals, increasing the shear strength of the bond. This process can be used to weld together traditionally incompatible materials. Two typical uses of explosion welded plates are for armor plating and pressure vessels. *An angular-interface clearance gap creates higher amplitude waves than a constant-interface clearance gap

Carbon steel tools

Used since the 1880s. Inexpensive and easily shaped and sharpened, but don't have sufficient hot hardness and wear resistance for machining at high cutting speeds. Lose hardness at temperatures above 400 degrees F. Therefore, limited to low-speed machining, or woodworking

Filler Metal

Usually a wire of similar material to the metal being welded is used to provide filler metal, however this metal does not contain flux— rather, a shielding gas, typically a mixture of argon and helium, protects against oxidation. Typically creates a very high quality weld, however it is more expensive than stick welding due to the inert gas (obviously the Welding Page 13 expensive than stick welding due to the inert gas

Gas Metal Arc Welding (GMAW) (MIG (metal inert gas) welding)

Weld area shielded by an external source of gas (argon, helium, carbon dioxide, various mixtures). Deoxidizers may be present in consumable electrode, as well. The electrode is fed automatically through a nozzle - Typically creates a very high quality weld, however it is more expensive than stick welding due to the inert gas expensive than stick welding due to the inert gas

Machining

a subtractive manufacturing process, where material is removed from the workpiece

High-speed steels are suitable for

a) High positive rake angle tools (i.e. small included angle tools) b) Interrupted cuts c) Use on machine tools that are subject to vibration and chatter

Serrated (or Segmented) Chips

also known as segmented or nonhomogeneous chips, these chips are "semi-continuous," in that they behave continuously in segmented regions. That is, some zones have low shear strain, other zones have high shear strain (edge closer to workpiece has high strain, other side has low strain). The end result is that the serrated chips end up looking like saw blades, with "teeth" jutting out

Feed

axial distance tool travels in one revolution

Nose wear and chipping

catastrophic failure caused by: Mechanical shock - Occurs in regions where a small crack or defect already exists; High positive rake angles make tool more susceptible to chipping Thermal fatigue - Crater wear contributes to chipping; thermal cracks (generally perpendicular to cutting edge) contribute to chipping

Discontinuous Chips

consist of segments that are either firmly or loosely attached to one another

Machinability

describes the relative ease with which we can machine a material

Continuous Chips

formed at high cutting speeds and/or high rake angles. Deformation occurs along the primary shear layer. However, a secondary shear layer running parallel to the rake face of the tool may develop due to friction. This secondary shear layer becomes thicker and thicker with increasing friction

Depth of Cut

how much tool is plunged into surface of workpiece

The two main types of high-speed steel tools

molybdenum (M series) and tungsten (T series). M series contains up to 10% molybdenum, and T series up to 18% tungsten. They both also contain chromium, vanadium, and cobalt. M series has higher abrasion resistance, undergoes less distortion during heat treating and is less expensive

Advanced Machining

one of the three categories of machining; (non-traditional machining processes): electrical, chemical, thermal, hydrodynamic, and optical sources of energy, or a combination, to remove material

Abrasive Processes

one of the three categories of machining; grinding, honing, lapping, ultrasonic machining

Cutting

one of the three categories of machining; single-point or multipoint cutting tools such as 1) turning 2)milling 3)sawing 4)drilling 5)broaching 6)shaping (planing) and 7)grinding

In the Merchant's Force Circle, R is equal to:

sqrt(Ft^2 + Fc^2)

The cutting ratio is

to/tc (the undeformed chip thickeness (depth of cut for orthogonal cutting) over the chip thickness

When are serrated chips usually formed?

usually formed while machining materials with low thermal conductivity, and strength that reduces sharply with temperature increase. Titanium is a good example