Ch. 26 – Abrasive Machining and Finishing Operations Brenton Elisberg, Jacob Hunner, Michael Snider, Michael Anderson

Abrasive Machining and Finishing Operations There are many situations where the processes of manufacturing we’ve learned about cannot produce the required dimensional accuracy and/or surface finish. Fine finishes on ball/roller bearings, pistons, valves, gears, cams, etc. The best methods for producing such accuracy and finishes involve abrasive machining.

Abrasives and Bonded Abrasives An abrasive is a small, hard particle having sharp edges and an irregular shape. Abrasives are capable of removing small amounts of material through a cutting process that produces tiny chips.

Abrasives and Bonded Abrasives Commonly used abrasives in abrasive machining are: Conventional Abrasives Aluminum Oxide Silicon Carbide Superabrasives Cubic boron nitride Diamond

Friability Characteristic of abrasives. Defined as the ability of abrasive grains to fracture into smaller pieces, essential to maintaining sharpness of abrasive during use. High friable abrasive grains fragment more under grinding forces, low friable abrasive grains fragment less.

Abrasive Types Abrasives commonly found in nature include: Emery Corundum Quartz Garnet Diamond

Abrasive Types Synthetically created abrasives include: Aluminum oxide (1893) Seeded gel (1987) Silicon carbide (1891) Cubic-boron nitride (1970’s) Synthetic diamond (1955)

Abrasive Grain Size Abrasives are usually much smaller than the cutting tools in manufacturing processes. Size of abrasive grain measured by grit number. Smaller grain size, the larger the grit number. Ex: with sandpaper 10 is very coarse, 100 is fine, and 500 is very fine grain.

Grinding Wheels Large amounts can be removed when many grains act together. This is done by using bonded abrasives. This is typically in the form of a grinding wheel. The abrasive grains in a grinding wheel are held together by a bonding material.

Bonding Abrasives Bonding materials act as supporting posts or braces between grains. Bonding abrasives are marked with letters and numbers indicating: Type of abrasive Grain size Grade Structure Bond type

Bond Types Vitrified: a glass bond, most commonly used bonding material. However, it is a brittle bond. Resinoid: bond consiting of thermosetting resins, bond is an organic compound. More flexible bond than vitrified, also more resistant to higher temps.

Bond Types Reinforced Wheels: bond consisting of one or more layers of fiberglass. Prevents breakage rather than improving strength. Rubber: flexible bond type, inexpensive. Metal: different metals can be used for strength, ductility, etc. Most inexpensive bond type.

The Grinding Process Grinding is a chip removal process that uses an individual abrasive grain as the cutting tool. The differences between grinding and a single point cutting tool is: The abrasive grains have irregular shapes and are spaced randomly along the periphery of the wheel. The average rake angle of the grain is typically -60 degrees. Consequently, grinding chips undergo much larger plastic deformation than they do in other machining processes. Not all grains are active on the wheel. Surface speeds involving grinding are very fast.

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Grinding Forces A knowledge of grinding forces is essential for: Estimating power requirements. Designing grinding machines and work-holding fixtures and devices. Determining the deflections that the work-piece as well as the grinding machine may undergo. Deflections adversely affect dimensioning.

Grinding Forces Forces in grinding are usually smaller than those in machining operations because of the smaller dimensions involved. Low grinding forces are recommended for dimensional accuracy.

Problems with Grinding Wear Flat After some use, grains along the periphery of the wheel develop a wear flat. Wear flats rub along the ground surface, creating friction, and making grinding very inefficient.

Problems with Grinding Sparks Sparks produced from grinding are actually glowing hot chips. Tempering Excessive heat, often times from friction, can soften the work-piece. Burning Excessive heat may burn the surface being ground. Characterized as a bluish color on ground steel surfaces.

Problems with Grinding Heat Checking High temps in grinding may cause cracks in the work-piece, usually perpendicular to the grinding surface.

Grain Fracture Abrasive grains are brittle, and their fracture characteristics are important. Wear flat creates unwanted high temps. Ideally, the grain should fracture at a moderate rate so as to create new sharp cutting edges continuously.

Bond Fracture The strength of the abrasive bond is very important! If the bond is too strong, dull grains cannot dislodge to make way for new sharp grains. Hard grade bonds are meant for soft materials. If too weak, grains dislodge too easily and the wear of the wheel increases greatly. Soft grade bonds are meant for hard materials.

Grinding Ratio G = (Volume of material removed)/ Volume of wheel wear) The higher the ratio, the longer the wheel will last. During grinding, the wheel may act “soft” or hard” regardless of wheel grade. Ex: pencil acting hard on soft paper and soft on rough paper.

Dressing, Truing, Shaping “Dressing” a wheel is the process of: Conditioning worn grains by producing sharp new edges. Truing, which is producing a true circle on the wheel that has become out of round. Grinding wheels can also be shaped to the form of the piece you are grinding. These are important because they affect the grinding forces and surface finish.

Grinding Operations and Machines Surface Grinding Cylindrical Grinding Internal Grinding Centerless Grinding Creep-feed Grinding Heavy Stock Removal by Grinding Grinding fluids

Grinding Operations and Machines Surface Grinding - grinding of flat surfaces Cylindrical Grinding – axially ground

Grinding Operations and Machines Internal Grinding - grinding the inside diameter of a part Creep-feed Grinding – large rates of grinding for a close to finished piece

Grinding Operations and Machines Centerless Grinding – continuously ground cylindrical surfaces

Grinding Operations and Machines Heavy Stock Removal - economical process to remove large amount of material Grinding Fluids Prevent workpiece temperature rise Improves surface finish and dimensional accuracy Reduces wheel wear, loading, and power consumption

Design Consideration for Grinding Part design should include secure mounting into workholding devices. Holes and keyways may cause vibration and chatter, reducing dimensional accuracy. Cylindrically ground pieces should be balanced. Fillets and radii made as large as possible, or relieved by prior machining.

Design Considerations for Grinding Long pieces are given better support in centerless grinding, and only the largest diameter may be ground in through-feed grinding. Avoid frequent wheel dressing by keeping the piece simple. A relief should be include in small and blind holes needing internal grinding.

Finishing Operations Coated abrasives Belt Grinding Wire Brushing Honing Superfinishing Lapping Chemical-Mechanical Polishing Electroplating

Finishing Operations Coated Abrasives – have a more pointed and open structure than grinding wheels Belt Grinding – high rate of material removal with good surface finish

Finishing Operations Wire Brushing - produces a fine or controlled texture Honing – improves surface after boring, drilling, or internal grinding

Finishing Operations Superfinishing – very light pressure in a different path to the piece Lapping – abrasive or slurry wears the piece’s ridges down softly

Finishing Operations Chemical-mechanical Polishing – slurry of abrasive particles and a controlled chemical corrosive Electropolishing – an unidirectional pattern by removing metal from the surface

Deburring Operations Manual Deburring Mechanical Deburring Vibratory and Barrel Finishing Shot Blasting Abrasive-Flow Machining Thermal Energy Deburring Robotic Deburring

Deburring Operations Vibratory and Barrel Finishing – abrasive pellets are tumbled or vibrated to deburr Abrasive-flow Machining – a putty of abrasive grains is forced through a piece

Deburring Operations Thermal Energy Deburring – natural gas and oxygen are ignited to melt the burr Robotic Deburring – uses a force-feedback program to control the rate and path of deburring

Economics of Abrasive Machining and Finishing Operations Creep-feed grinding is an economical alternative to other machining operations. The use of abrasives and finishing operations achieve a higher dimensional accuracy than the solitary machining process. Automation has reduced labor cost and production times. The greater the surface-finish, the more operations involved, increases the product cost. Abrasive processes and finishing processes are important to include in the design analysis for pieces requiring a surface finish and dimensional accuracy.

Chapter 27 – Advanced Machining Processes

Chapter 27 – Advanced Mechanical Processes Advanced Machining Processes can be used when mechanical methods are not satisfactory, economical or possible due to: High strength or hardness Too brittle or too flexible Complex shapes Special finish and dimensional tolerance requirements Temperature rise and residual stresses

Advanced Mechanical Processes These advanced methods began to be introduced in the 1940's. Removes material by chemical dissolution, etching, melting, evaporation, and hydrodynamic action. These requirements led to chemical, electrical, laser, and high-energy beams as energy sources for removing material from metallic or non-metallic workpieces.

Chemical Machining Chemical machining Uses chemical dissolution to dissolve material from the workpiece. Can be used on stones, most metals and some ceramics. Oldest of the advanced machining processes.

Chemical Machining Chemical milling - shallow cavities are produced on plates, sheets, forgings, and extrusions, generally for the overall reduction of weight. Can be used with depths of metal removal as large as 12 mm. Masking is used to protect areas that are not meant to be attacked by the chemical.

Chemical Machining Chemical Blanking – similar to the blanking of sheet metals with the exception that the material is removed by chemical dissolution rather than by shearing. Printed circuit boards. Decorative panels. Thin sheet-metal stampings. Complex or small shapes.

Chemical Machining Surface Roughness and Tolerance table

Chemical Machining Photochemical blanking/machining Applications Modification of chemical milling. Can be used on metals as thin as .0025 mm. Applications Fine screens. Printed circuit boards. Electric-motor laminations. Flat springs. Masks for color televisions.

Chemical Machining Chemical machining design considerations No sharp corners, deep or narrow cavities, severe tapers, folded seam, or porous workpiece materials. Undercuts may develop. The bulk of the workpiece should be shaped by other processes prior to chemical machining.

Electrochemical Machining Electrochemical machining (ECM) An electrolyte acts as a current carrier which washes metal ions away from the workpiece (anode) before they have a chance to plate on the tool (cathode). The shaped tool is either solid or tubular. Generally made of brass, copper, bronze or stainless steel. The electrolyte is a highly conductive inorganic fluid.

Electrochemical Machining Electrochemical machining cont. The cavity produced is the female mating image of the tool shape. Process capabilities Generally used to machine complex cavities and shapes in high strength materials. Design considerations Not suited for producing sharp square corners or flat bottoms. No irregular cavities.

Electrochemical Machining

Electrochemical Machining Pulsed electrochemical machining (PECM) Refinement of ECM. The current is pulsed instead of a direct current. Lower electrolyte flow rate. Improves fatigue life. Tolerance obtained 20 to 100 micro-meters.

Electrochemical Grinding Electrochemical grinding (ECG) Combines ECM with conventional grinding. Similar to a conventional grinder, except that the wheel is a rotating cathode with abrasive particles. The abrasive particles serve as insulators and they remove electrolytic products from the working area. Less then 5% of the metal is removed by the abrasive action of the wheel.

Electrochemical Grinding Electrochemical honing Combines the fine abrasive action of honing with electrochemical action. Costs more than conventional honing. 5 times faster than conventional honing. The tool lasts up to 10 times longer. Design considerations for EGC Avoid sharp inside radii.

Electrical Discharge Machining (EDM) Principle of operation Based on the erosion of metal by spark discharge Components of operation Shaped tool Electrode Workpiece Connected to a DC power supply Dielectric Nonconductive fluid

Electrical Discharge Machining (EDM) When the potential difference is sufficiently high, the dielectric breaks down and a transient spark discharges through the fluid, removing a very small amount of material from the workpiece Capacitor discharge 200-500 kHz This process can be used on any electrically conductive material

Electrical Discharge Machining (EDM) Volume of material removed per discharge 10^-10 to 10^-8 in^3 Material removal can be predicted MRR = 4*10^4 I*Tw^-1.23 MRR is mm^3/min I is current in amperes Tw is melting point (C) Mechanical energy is not a factor The hardness, strength, and toughness do not necessarily influence the removal rate

Electrical Discharge Machining (EDM) Movement in the X&Y axis is controlled by CNC systems Overcut (in the Z axis) is the gap between the electrode and the workpiece Controlled by servomechanisms Critical to maintain a constant gap

Electrical Discharge Machining (EDM)                     Electrical Discharge Machining (EDM) Dielectric fluids Act as a dielectric Provide a cooling medium Provide a flushing medium Common fluids Mineral oils Distilled/Deionized water Kerosene Other clear low viscosity fluids are available which are easier to clean but more expensive

Electrical Discharge Machining (EDM) Electrodes Graphite Brass Copper-tungsten alloys Formed by casting, powder metallurgy, or CNC machining On right, human hair with a 0.0012 inch hole drilled through

Electrical Discharge Machining (EDM) Electrode wear Important factor in maintaining the gap between the electrode and the workpiece Wear ratio is defined as the amount of material removed to the volume of electrode wear 3:1 to 100:1 is typical No-wear EDM is defined as the EDM process with reversed polarity using copper electrodes

Electrical Discharge Machining (EDM) Process capabilities Used in the forming of dies for forging, extrusion, die casting, and injection molding Typically intricate shapes                 

Electrical Discharge Machining (EDM) Material removal rates affect finish quality High removal rates produce very rough surface finish with poor surface integrity Finishing cuts are often made at low removal rates so surface finish can be improved Design considerations Design so that electrodes can be simple/economical to produce Deep slots and narrow openings should be avoided Conventional techniques should be used to remove the bulk of material

Wire EDM Similar to contour cutting with a bandsaw Typically used to cut thicker material Up to 12” thick Also used to make punches, tools and dies from hard materials

Wire EDM Wire Usually made of brass, copper, or tungsten Range in diameter from 0.012 – 0.008 inches Typically used at 60% of tensile strength Used once since it is relatively inexpensive Travels at a constant velocity ranging from 6-360 in/min Cutting speed is measured in cross sectional area per unit time (varies with material) 18,000 mm^2/hour 28 in^2/hour

Wire EDM Multiaxis EDM Computer controls for controlling the cutting path of the wire and its angle with respect to the workpiece plane Multiheads for cutting multiple parts Features to prevent and correct wire breakage Programming to optimize the operation

Electrical Discharge Grinding Similar to the standard grinder Grinding wheel is made of graphite or brass and contains no abrasives Material is removed by spark discharge between the workpiece and rotating wheel Typically used to sharpen carbide tools and dies Can also be used on fragile parts such as surgical needles, thin-wall tubes, and honeycomb structures Process can be combined with electrochemical discharge grinding Material removal rate is similar to that of EDM MRR = KI where K is the workpiece material factor in mm^3/A-min

Laser Beam Machining The source of the energy is the laser Light Amplification by Stimulated Emission of Radiation The focus of optical energy on the surface of the workpiece melts and evaporates portions of the workpiece in a controlled manner Works on both metallic and non-metallic materials Important considerations include the reflectivity and thermal conductivity of the material The lower these quantities the more efficient the process

Laser Beam Machining The cutting depth can be calculated using the formula t = CP/vd where t is the depth C is a constant for the process P is the power input v is the cutting speed d is the laser spot diameter The surface produced is usually rough and has a heat affected zone (discussed in section 30.9)

Laser Beam Machining Lasers may be used in conjunction with a gas such as oxygen, nitrogen, or argon to aid in energy absorption Commonly referred to as laser beam torches The gas helps blow away molten and vaporized material Process capabilities also include welding, localized heat treating, and marking Very flexible process Fiber optic beam delivery Simple fixtures Low setup times

Laser Beam Machining Design considerations Sharp corners should be avoided Deep cuts will produce tapered walls Reflectivity is an important consideration Dull and unpolished surfaces are preferable Any adverse effects on the properties of the machined materials caused by the high local temperatures and heat affected zones should be investigated

Electron Beam Machining Energy source is high velocity electrons which strike the workpiece Voltages range from 50-200kV Electron speeds range from 50-80% the speed of light Require a vacuum

Electron Beam Machining Plasma arc cutting Ionized gas is used to rapidly cut ferrous and nonferrous sheets and plates Temperatures range from 9400-17,000 F The process is fast, the kerf width is small, and the surface finish is good Parts as thick as 6” can be cut Much faster than the EDM and LBM process Design considerations Parts must fit in vacuum chamber Parts that only require EBM machining on a small portion should be manufactured as a number of smaller components

Water Jet Machining Also known as hydrodynamic machining The water jet acts as a saw and cuts a narrow groove in the material Pressures range from 60ksi to 200ksi

Water Jet Machining Process capabilities Can be used on any material up to 1” thick Cuts can be started at any location without predrilled holes No heat produced No flex to the material being cut Suitable for flexible materials Little wetting of the workpiece Little to no burr produced Environmentally safe

Water Jet Machining Very similar to water jet machining Water contains abrasive material Silicon carbide Aluminum oxide Higher cutting speed than that of conventional water jet machining Up to 25 ft/min for reinforce plastics Minimum hole diameter thus far is approximately 0.12 inches Maximum hole depth is approximately 1 inch

Abrasive Jet Machining Uses high velocity dry air, nitrogen, or carbon dioxide containing abrasive particles Supply pressure is on the order of 125psi The abrasive jet velocity can be as high as 100 ft/sec Abrasive size is approximately 400-2000 micro-inches

Economics of Advanced Machining Processes Advanced machining processes each have unique applications The economic production run for a particular process depends on the costs of tooling, equipment, operating costs, material removal rate required, level of operator skill required, and necessary secondary and finishing operations Chemical machining has the added cost of reagents, maskants, and disposal Table 27.1 lists material removal rates for all advanced machining processes

References http://www.electricaldischargemachining.com/ http://www.belmont4edm.com/ http://www.texasairsonics.com/Cabinet%20Style.html