Non-Traditional Manufacturing & Abrasive Jet Machining

• Module 9 focuses on Non-traditional Manufacturing, specifically introducing the subject and exploring abrasive jet machining (AJM).

Instructional Objectives

• Identify characteristics of conventional machining processes.
• Learn general characteristics of non-traditional machining processes.
• Differentiate between conventional and non-traditional machining.
• Understand the classification of non-traditional machining processes.
• Explain the need for non-traditional machining despite the existence of conventional methods.
• Understand the basic mechanism of material removal in AJM.
• Identify major components of AJM equipment and their working principles.
• Learn to draw an AJM equipment diagram.
• Understand AJM process parameters and machining characteristics.
• Analyze the effect of process parameters on material removal rate (MRR).
• Develop mathematical models relating MRR to AJM parameters.
• Understand application areas and limitations of AJM.

Classification of Manufacturing Processes

• Primary manufacturing imparts basic shape and size to a material.
  • Can be done in liquid, solid, or powder state.
  • Liquid state processes include casting (die casting, green mold casting).
  • Solid state processes include hot/cold working and metalworking (forging, rolling, extrusion).
  • Powder state processes include powder metallurgy (used for cutting tool inserts).
• Secondary manufacturing imparts final shape and size with tight dimensional control.
  • Primarily material removal processes.

Classification of Secondary Manufacturing Processes

• Conventional manufacturing/machining processes include turning, milling, drilling, grinding, and shaping.
• Non-traditional machining processes include:
  • Abrasive jet machining (AJM).
  • Ultrasonic machining.
  • Water jet and abrasive water jet machining.
  • Electrochemical machining.
  • Electro-discharge machining.
  • Laser beam machining.
• Other non-traditional processes exist (plasma machining, ion beam machining, electron beam machining) but will not be covered in detail.

Characteristics of Conventional Machining Processes

• Macroscopic chip formation occurs.
• Chip formation occurs through shear deformation.
• Requires a cutting tool harder than the workpiece.
• Material removal occurs due to the application of cutting force.
• The energy domain is mechanical.

Characteristics of Non-Traditional Machining Processes

• Material removal may or may not involve chip formation; chips can be microscopic or absent.
• A physical tool may or may not be required.
• Tool hardness is not always a necessity; softer tools can be used.
• The energy domain can be mechanical or other forms (electrochemical, thermal).

Classification of Non-Traditional Machining Processes (Based on Energy Domain)

• Mechanical processes: Material removal via mechanical energy.
  • Abrasive jet machining (AJM), Ultrasonic Machining (USM), Water Jet Machining (WJM), Abrasive Water Jet Machining.
• Electrochemical processes: Material removal via electrochemical action.
  • Electrochemical Machining (ECM), Electrochemical Grinding, Electro Jet Drilling.
• Electro-thermal processes:
  • Electro-discharge Machining (EDM), Electron Beam Machining, Laser Beam Machining.
• Chemical processes:
  • Chemical Milling, Photochemical Milling (used in electronics).

Examples of Non-Traditional Machining Processes

• Ultrasonic Machining:
  • A tool vibrates at ultrasonic frequencies (20-25 kHz) with small amplitudes (10-25 microns).
  • Abrasive slurry is used between the tool and workpiece.
  • Brittle materials are often machined this way.
  • Material removal occurs through controlled fracture induced by abrasive particles.
• Electro-Discharge Machining (EDM):
  • An "electro-thermal" process.
  • Tool and workpiece are submerged in a dielectric medium.
  • A potential difference creates sparking, melting material for removal.
• Electrochemical Machining:
  • Material removal via electrochemical dissolution.
  • Tool and workpiece are immersed in an electrolyte medium.
  • Potential difference applied, causing material removal without sparking.

Need for Non-Traditional Machining

• Conventional machining limitations with new exotic materials
• Inability to efficiently machine certain materials due to hardness or other properties.
• Requirements for innovative designs, tighter tolerances, and micromachining.
• Difficulties in machining certain geometries (e.g., square blind holes).
• Issues with conventional machining processes, such as inducing tensile residual stress during grinding.
• The need for low-stress grinding.

Abrasive Jet Machining (AJM) - Mechanism of Material Removal

• A gas and abrasive particle mixture is fed through a nozzle (0.2-0.8 mm opening).
• The mixture is ejected at high velocity (150-300 m/s) towards the workpiece.
• Brittle materials are machined more efficiently with AJM.
• Abrasive particles impact the workpiece, converting kinetic energy into mechanical work.
• The impact creates small craters on the surface.
• Material removal occurs through brittle fracture due to multiple impacts.
• It is a pure mechanical domain process.

Abrasive Jet Machining (AJM) - Equipment

• Compressor: Provides compressed gas.
• Valve: Controls gas flow.
• Air filter: Removes oil and moisture from the compressed air.
• Pressure regulator: Regulates air pressure (e.g., reduces 12 bar to 5 bar).
• Mixing chamber: Mixes abrasive particles with the carrier gas.
• Hopper: Feeds abrasive into the mixing chamber.
• Electromechanical shaker: Vibrates the mixing chamber for proper mixing.
• Pneumatic on/off valve: Controls the flow of the abrasive/gas mixture to the nozzle.
• Nozzle: Directs the abrasive/gas mixture towards the workpiece.
• Workpiece table with lead screw: Allows movement of the workpiece below the nozzle.
• Enclosed chamber with exhaust system: Contains dust generated during machining and separates abrasive particles before releasing air.- Abrasive particles are required for abrasive jet machining.
• The material, shape, size, and mass flow rate of the abrasive are major process variables.
• A carrier gas is needed to propel the abrasive particles.
• Air, carbon dioxide, and nitrogen are commonly used as carrier gases.
• The pressure of the carrier gas is a critical parameter affecting the velocity of the abrasive particles.

Abrasive Jet Parameters

• Abrasive jet parameters include velocity mixing ratio, stand-off distance, impingement angle, and nozzle material.
• The velocity of the abrasive jet is typically between 100 to 300 meters per second.
• Mixing ratio is the ratio of the abrasive mass flow rate to the gas mass flow rate.
• Stand-off distance is the distance between the nozzle and the workpiece.
• Impingement angle is the angle at which the jet interacts with the workpiece, usually around 90 degrees.
• Nozzle material needs to be wear-resistant, with tungsten carbide or wear-resistant sapphire being common choices.
• Nozzle internal diameter typically ranges from 0.2 to 0.8 millimeters.
• Nozzle life is an important consideration in industrial applications, with an expected life of around 100 hours.
• Material removal in abrasive jet machining is assumed to occur due to brittle fracture when modeling for brittle materials.
• Each abrasive particle creates a hemispherical fracture with a diameter equal to the chordal length of the indentation.

Assumptions for Modeling

• Kinetic energy of the abrasive particles is assumed to be fully utilized in creating the crater.
• Abrasive particles are assumed to be rigid, spherical, and characterized by their mean diameter.
• The indentation radius 'r' can be expressed as the square root of the grain diameter multiplied by the indentation depth.
• The volume of material removed in a single impact is two-thirds pi r cubed, where 'r' is the indentation radius.
• Kinetic energy of an abrasive particle is half mv squared, where 'm' is the mass (volume times density).
• Work done during indentation is half the force 'F' times the indentation depth 'delta'.
• Force 'F' can be estimated as the indentation area times the hardness of the workpiece.

Equations for Modeling

• Equating total work to kinetic energy allows finding the value of delta (indentation depth).
• Indentation depth varies with grain diameter and velocity, and inversely with the hardness of the work material.
• Material removal rate is the amount of material removed in a single impact multiplied by the number of impacts.
• Number of impacts can be estimated by dividing the mass flow rate of the abrasive by the mass of a single grit.
• Increasing abrasive flow rate or velocity increases the material removal rate.
• Using a harder work material reduces the material removal rate.

Key Factors

• Increasing abrasive flow rate increases material removal rate up to a point, beyond which it decreases due to insufficient momentum of the gas to accelerate the particles.
• Increasing gas pressure increases velocity and thus material removal rate.
• Material removal rate changes linearly with abrasive flow rate if the mixing ratio is kept constant.
• Stand-off distance affects material removal rate due to the flaring of the jet as it interacts with the surrounding air.
• Material removal is highest within a certain distance from the nozzle, after which it drops due to energy loss as the jet flares out.

Applications

• Abrasive jet machining is typically used for machining brittle materials, drilling holes of intricate shapes and fragile objects.
• It can be used in different modes such as drilling, deburring, etching, and cutting.
• Applications include conventional and micro-machining of brittle objects.
• Advantages include machining fragile objects and heat-sensitive materials.
• Material removal rate is relatively low, around 15 mm3/min.
• Abrasive particles can get embedded in the workpiece, causing contamination.
• Tapering can occur due to flaring of the jet.
• Environmental issues include dust generation and the need for an enclosed environment.
• Abrasive jet nozzles are made of tungsten carbide due to its wear resistance.
• Material removal occurs due to mechanical impact of high energy particles with the material.

Abrasive Jet Machining (AJM) - Material Removal

• Mechanical impact is the major contributor to material removal in abrasive jet machining, not material fatigue, thermal action, or sparking.

AJM - Stand-Off Distance and Depth of Penetration

• As stand-off distance increases beyond 5mm, the depth of penetration in AJM decreases.
• The jet diameter is typically sub-millimetric (around 0.5 to 1 mm).
• The jet flares out as it interacts with the surrounding air.
• The jet remains relatively straight for a distance of 5 to 10 times its diameter.
• Beyond 5 to 10 times the diameter, the jet starts to flare out, reducing the depth of penetration.

AJM - Material Removal Rate (MRR) Estimation

• Model for MRR: MRR = (abrasive mass flow rate * abrasive velocity) / (abrasive density * flow strength). Units for flow strength should be consistent to calculate the MRR.

Calculation Example 1

• Parameters: flow strength = 4 GPa, abrasive flow rate = 2 x 10^-3 kg/min = (2 x 10^-3)/60 kg/sec, abrasive velocity = 200 m/s, abrasive density = 3000 kg/m^3.

Calculation Example 2

• Problem: Calculate material removal per impact.
• Mass of a single grit can be estimated, allowing calculation of the number of impacts per unit time.
• Dividing the material removal rate by the number of impacts gives the material removal per impact.
• Assuming a hemispherical crater shape, the indentation radius can be calculated.

Solved Problems - Key Takeaways

• Typical indentation radius: Around 10 microns.
• Typical MRR: Around 48 mm^3/minute (tens of mm^3 per minute).

Lecture Summary:

• Characteristics of conventional and non-traditional machining processes can be differentiated.
• There are different classifications of non-traditional machining processes.
• AJM is a mechanical process.
• Process parameters and equipment modules for AJM have been identified.
• Modeling of material removal rate in AJM has been demonstrated.
• Effects of process parameters on material removal rate in AJM have been studied.
• Application areas and limitations of AJM have been indicated.

Introduction to Ultrasonic Machining (USM)

• Lecture 9.2 focuses on Ultrasonic Machining (USM).
• USM falls under Non-Traditional Manufacturing processes (Module 9).
• First lecture of Module 9 covered abrasive jet machining.

Instructional Objectives of USM Lecture

• Describe the basic mechanism of material removal in USM.
• Identify process parameters of USM.
• Identify the machining characteristics of USM.
• Analyze the effect of different process parameters on material removal rate (MRR).
• Draw variations in MRR when changing different process parameters.
• Develop mathematical models relating material removal rate with different USM process parameters.
• Identify the major components of ultrasonic machining equipment.
• State the working principle of ultrasonic machining equipment modules.
• Schematically draw the ultrasonic equipment.
• List at least three applications and three limitations of USM.

Classification of Non-Traditional Manufacturing Processes

• Types: Mechanical, electrochemical, electro-thermal, and chemical processes.
• Mechanical processes include abrasive jet processes, ultrasonic machining processes, water jet, and abrasive water jet machining processes.
• Electrochemical machining includes ECM.
• Electro-thermal processes include electro-discharge machining (EDM), electron beam machining (EBM) and laser machining.

USM - Process Description

• A tool vibrates over a workpiece, causing machining.
• Tool vibration frequency is around 19 to 25 kHz.