Wednesday, 23 October 2013

Metal Casting Terms

Alloy — A substance containing two or more chemical elements, at least one of which is metal. The substance usually has metallic properties and possesses qualities different from those of the individual components.

AOD — A refining treatment used to control the carbon content of steel.

Binders — Binders are added to mold materials in order to create a mold of sufficient hardness. Binders can be either organic or inorganic materials.

CAD — Computer Aided Design.

CAM — Computer Aided Manufacturing.

Captive Foundry — A foundry operation that is wholly incorporated into a larger manufacturing operation. A captive foundry only produces castings for the operation that it is a part of.

Casting — A process used to form solid metal shapes out of molten metal. The molten metal is poured into a cavity or a mold.

Charge — Metal and alloy materials that make up the composition of a melt.

Cope — The top half of a horizontally parted mold.

Core — The part of a mold used in the casting process that forms the internal shapes of a casting.

Crucible Furnace — A furnace that melts metals in a refractory crucible. The furnace is typically fueled with coke, oil, gas or electricity.

Cupola Furnace — The traditional furnace used for melting metal. The furnace is typically fueled with coke.

Direct-Arc Furnace — An electric arc furnace in which the metal being melted is one of the poles.

Draft — Taper on the vertical sides of a pattern or corebox that permits the core or pattern to be removed without distorting or tearing of the sand.

Drag — The bottom half of a horizontally parted mold.

Ferrous Metal — An alloy that has iron as the predominant metal.

Flash — A thin section of metal formed at the mold, core, die joint or parting in a casting. Flash usually forms when the cope and drag do not match completely or when the core and the coreprint do not match.

Gas Porosity — A condition in a casting that occurs when gas is trapped in molten metal or as a result of mold gasses that evolved when the casting was poured.

Gating Systems — The channel(s) that allow the molten metal to enter the mold cavity.

Green Sand — Moist sand that is bonded by a mixture that contains silica, bentonite clay, carbonaceous material, and water.

Gross Weight of Casting — The weight of the casting which includes the actual product plus the metal in the gating system as poured.

Hot Isostatic Pressing (HIP) — The process of using high hydrostatic pressure and high temperature to compress fine particles into coherent parts.

Impurity — An element that is allowed into a metal or alloy. Impurities slightly change the properties of the material in some circumstances and can completely damage it in others.

Inclusion — Nonmetallic materials that become part of a metal matrix usually through reoxidation, refractories, slag, and deoxidization products.

Indirect-Arc Furnace — An alternating current electric-arc furnace. The metal being melted is not one of the poles.

Induction Furnace — An alternating current melting furnace that uses the heat of electrical induction.

Investment Casting — A casting process that uses a wax or thermoplastic pattern and is surrounded in a refractory slurry to form a mold. Once the refractory slurry is dry, the wax or plastic is melted out and the mold is formed. Molten metal is then poured in to fill the area that was previously filled with wax.

Jobbing Foundry — A foundry that creates a wide variety of castings, in small quantities for a range of customers.

Ladle — The name for a variety of receptacles used to move and pour molten metal during the casting process.

Lining — The refractory layer of firebrick, clay, sand or other materials that coat the inside of a furnace or ladle.

Lost Foam Process — A casting process that uses foam to form the pattern. The foam is eventually melted out of the mold when the molten metal is poured in.

Metal Yield — The difference between the weight of a finished casting and the total weight of the metal poured.

Mold — The cavity that the molten metal is poured in to form the final shape. A mold usually consists of a top and bottom piece made of sand or ceramic material.

Net Weight of Casting — The final weight of a casting that is determined once all of the excess metal from the gating system has been removed.

Non-ferrous Metal — An alloy that doesn’t have iron as the predominant metal.

Oxidation Losses — The loss of metal or alloy through the process of oxidation.

Oxidizing Atmosphere — Furnace atmosphere which gives off oxygen under certain conditions or where there is an excess of oxygen in the product of combustion, or the products of combustion are oxidizing to the metal being heated.

Pattern — The wood, metal, foam or plastic replica of the final product to be made. Patterns usually include gating systems.

Pattern Draft — The taper allowed on the vertical faces of a pattern to enable removal of the mold or die.

Pigging — Pouring molten metal back into lined containers so that it can be returned to the furnace.

Rapid Prototyping — The computerized equipment that builds a three-dimensional model of a casting from a CAD drawing.

Reducing atmosphere — Furnace atmosphere which absorbs oxygen under suitable conditions or in which there is insufficient air to completely burn the fuel, or the product of combustion is reducing to the metal being heated.

Sand Casting — Producing metal castings out of sand molds.

Sand Reclamation — Processing used sand grains into usable forms so that they can be used in the casting process as a replacement for new sand.

Scrap — All non-product metal produced during the casting process.

Shell Molding — Bringing a resin-bonded sand mixture into contact with a pre-heated metal pattern to form a mold.

Shell Process — The process in which clay-free silica sand coated with a thermostatic resin or mixed with resin is placed on a heated metal pattern for a short period of time to form a partially hardened shell. The bulk of the sand mixture inside the resulting shell is removed for further use. The pattern and shell are heated further to harden or polymerize the resin-sand mix, and the shell is removed from the pattern. Frequently, shell cores are made using this process.

Shotblasting — A process for cleaning castings that involves using a metal abrasive that is propelled by centrifugal or air force.

Shrinkage — The reduction in the volume of metal that occurs as it solidifies.

Shrink Hole — A cavity that forms in a metal part when there was not enough source metal fed into the mold during the casting process.

Slag — A film that forms on top of molten metal as a result of impurities. Slag is composed of non-metal elements.

Slag Inclusions — Imperfections of the surface of metal caused by slag (impurities in the molten mix).

Slurry — The watery mixture such as the gypsum mixture for plaster molding, the molding medium used for investment casting, core dips, and mold washes.

Sprue — The opening in the mold where the metal is first poured.

V-process — A molding process, developed in Japan, in which the mold is formed by stretching a sheet of mylar plastic over a heated metal pattern so that it conforms to the shape of the pattern. A box of loose sand is placed over the pattern, and a vacuum is applied to the sand, which then conforms to the shape of the mylar film. Thus supported, the sand-backed film is removed from the pattern and is used as one part of mold. When the metal is poured, the vacuum is released, and the loose sand falls away from the casting.

Vacuum Casting — The process of casting that uses a vacuum to draw molten metal into a mold that is placed into it.





Other Topics

METAL CASTING PROCESSES

Tuesday, 22 October 2013

Spark Ignition Engines

Internal combusiton engines are divided into spark ignition engines and com-
pression ignition engines. Almost all automobiles today use spark ignition
engines while trailers and some big trucks use compression ignition engines.
The main di erence between the two is the way in which the air to fuel mix-
ture is ignited, and the design of the chamber which leads to certain power
and e ciency characteristics.
Spark ignition engines use an air to fuel mixture that is compressed at high
pressures. At this high pressure the mixture has to be near stoichiometric
to be chemically inert and able to ignite. Stoichiometric means that there
is a one to one ratio between the air and fuel mixture. So the mixture in
order to ignite needs not to be either with too much fuel or too much air
but rather have an overall even amount. There are several components to
the spark ignition engine. Chamber design, mixture and the injection system
are some of the most important aspects of the spark ignition engine. The
importance of the chamber design will be discussed. The four basic designs
for combustion chambers are as follow:
--the distance travelled by the
ame front should be minimised
--the exhaust valve and spark plug should be close together
--there should be su cient turbulence 17
--the end gas should be in a cool part of the combustion chamber.




Other Topics






INTERNAL COMBUSTION (IC) ENGINES

An IC engine is one in which the heat transfer to the working
fluid occurs within the engine itself, usually by the combustion
of fuel with the oxygen of air.
In external combustion engines heat is transferred to the
working fluid from the combustion gases via a heat
exchanger. e.g. steam engines; Stirling engines.
IC engines include spark ignition (SI) engines using
petrol as a fuel, and compression ignition (CI) engines
(usually referred to as Diesel engines) using fuel oil,
DERV, etc as a fuel.
In these engines there is a sequence of processes:
· compression
· combustion
· expansion
· exhaust / induction

There are two basic mechanical designs to achieve these four processes in either:-
The basic difference between the petrol engine and the diesel
engine is in the method of ignition and the combustion process,

four strokes of the piston - hence the 4-stroke engine, or
two strokes of the piston - hence 2-stroke engines.





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Wednesday, 9 October 2013

Gear Terms & Definitions










Addendum: Height of tooth above pitch circle or the radial distance between the pitch circle and the top of the tooth.

Approach Ratio: The ratio of the arc of approach to the arc of action.

Arc of Action: Arc of the pitch circle through which a tooth travels from the first point of contact with the mating tooth to the point where contact ceases. 

Arc of Approach: Arc of the pitch circle through which a tooth travels from the first point of contact with the mating tooth to the pitch point.

Arc of Recess: Arc of the pitch circle through which a tooth travels from its contact with the mating tooth at the pitch point to the point where its contact ceases. 

Axial Plane:  In a pair of gears it is the plane that contains the two axes.  In a single gear, it may be any plane containing the axis and a given point. 

Backlash: The amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles.  As actually indicated by measuring devices, backlash may be determined variously in the transverse, normal, or axial planes, and wither in the direction of the pitch circles or on the line of action.  Such measurements should be converted to corresponding values on transverse pitch circles for general comparison. 

Base Circle: The circle from which an involute tooth curve is generated or developed. 

Base Helix Angle: The angle, at the base cylinder of an involute gear, that the tooth makes with the gear axis. 

Base Pitch: In an involute gear it is the pitch on the base circle or along the line of action.  Corresponding sides of involute teeth are parallel curves, and the base pitch is the constant and fundamental distance between them along a common normal in a plane of rotation.  The Normal Base Pitch is the base pitch in the normal plane, and the Axial Base Pitch is the base pitch in the axial plane. 

Center Distance: The distance between the parallel axes of spur gears and parallel helical gears, or between the crossed axes of crossed helical gears and worm gears.  Also, it is the distance between the centers of the pitch circles. 

Central Plane: In a worm gear this is the plane perpendicular to the gear axis and contains the common perpendicular of the gear and worm axes.  In the usual case with the axes at right angles, it contains the worm axis

Chordal Addendum: The height from the top of the tooth to the chord subtending the circular-thickness arc. 

Chordal Thickness: Length of the chord subtended by the circular thickness arc (the dimension obtained when a gear-tooth caliper is used to measure the thickness at the pitch circle).

Circular Pitch: Length of the arc of the pitch circle between the centers or other corresponding points of adjacent teeth. Normal Circular Pitch is the circular pitch in the normal plane. 

Circular Thickness: The length of arc between the two sides of a gear tooth, on the pitch circle unless otherwise specified.  Normal Circular Thickness is the circular thickness in the normal plane. 

Clearance: The amount by which the dedendum in a given gear exceeds the addendum of its mating gear.  It is also the radial distance between the top of a tooth and the bottom o the mating tooth space. 

Contact Diameter: The smallest diameter on a gear tooth with which the mating gear makes contact. 

Contact Ratio: The ratio of the arc of action to the circular pitch.  It is sometimes thought of as the average number of teeth in contact.  For involute gears, the contact ration is obtained most directly as the ration of the length of action to the base pitch. 

Contact Stress: The maximum compressive stress within the contact area between mating gear tooth profiles.  It is also called Hertz stress. 





Cycloid: The curve formed by the path of a point on a circle as it rolls along a straight line.  When this circle rolls along the outer side of another circle, the curve is called an Epicycloid; when it rolls along the inner side of another circle it is called a Hypocycloid.  These curves are used in defining the former American Standard composite tooth form. 

Dedendum: The depth of tooth space below the pitch circle or the radial dimension between the pitch circle and the bottom of the tooth space.

Diametral Pitch: The ratio of the number of teeth to the number of inches of pitch diameter-equals number of gear teeth to each inch of pitch diameter.  Normal Diametral Pitch is the diametral pitch as calculated in the normal plane and is equal to the diametral pitch divided by the cosine of the helix angle. 

Effective Face Width: That portion of the face width that actually comes into contact with mating teeth, as occasionally one member of a pair of gears may have a greater face width than the other. 

Efficiency: The actual torque ratio of a gear set divided by its gear ratio. 

External Gear: A gear with teeth on the outer cylindrical surface.

Face of Tooth: That surface of the tooth which is between the pitch circle and the top of the tooth. 

Fillet Curve: The concave portion of the tooth profile where it joins the bottom of the tooth space.  The approximate radius of this curve is called the Fillet Radius.

Fillet Stress: The maximum tensile stress in the gear tooth fillet.

Flank of Tooth: That surface which is between the pitch circle and the bottom land.  The flank includes the fillet. 

Helical Overlap: The effective face width of a helical gear divided by the gear axial pitch; also called the Face Overlap.

Helix Angle: The angle that a helical gear tooth makes with the gear axis at the pitch circle unless otherwise specified. 

Highest Point of Single Tooth Contact: The largest diameter on a spur gear at which a single tooth is in contact with the mating gear.  Often referred to as HPSTC.

Internal Diameter: The diameter of a circle coinciding with the topes of the teeth of an internal gear. 

Internal Gear: A gear with teeth on the inner cylindrical surface. 

Involute: The curve formed by the path of a point on a straight line, called the generatrix, as it rolls along a convex base curve.  (The base curve is usually a circle.)  This curve is generally used as the profile of gear teeth. 

Land: The Top Land is the top surface of a tooth, and the Bottom Land is the surface of the gear between the fillets of adjacent teeth.

Lead: The distance a helical gear or worm would thread along its axis in one revolution if it were free to move axially.





Length of Action: The distance on an involute line of action through which the point of contact moves during the action of the tooth profile.

Line of Action: The path of contact in involute gears.  It is the straight line passing through the pitch point and tangent to the base circles.

Lowest Point of Single Tooth Contact: The smallest diameter on a spur gear at which a single tooth of one gear is in contact with its mating gear.  Often referred to as LPSTC.  Gear set contact stress is determined with a load placed at this point on the pinion. 

Module: Ration of the pitch diameter to the number of teeth.  Ordinarily, module is understood to mean ration of pitch diameter in millimeters to the number of teeth.  The English Module is a ratio of the pitch diameter in inches to the number of teeth.

Normal Plane: A plane normal to the tooth surfaces at a point of contact, and perpendicular to the pitch plane.

Pitch: The distance between similar, equally-spaced tooth surfaces, in a given direction and along a given curve or line.  The single word “pitch” without qualification has been used to designate circular pitch, axial pitch, and diametral pitch, but such confusing usage should be avoided. 

Pitch Circle: A circle the radius of which is equal to the distance from the gear axis to the pitch point.

Pitch Diameter: The diameter of the pitch circle.  In parallel shaft gears the pitch diameters can be determined directly from the center distance and the numbers of teeth by proportionality.  Operating Pitch Diameter is the pitch diameter at which the gears operate.  Generating Pitch Diameter is the pitch diameter at which the gear is generated.  In a bevel gear the pitch diameter is understood to be at the outer ends of the teeth unless otherwise specified.  (See also reference to standard pitch diameter under Pressure Angle.)

Pitch Plane: In a pair of gears it is the plane perpendicular to the axial plane and tangent to the pitch surfaces.  In a single gear it may be any plane tangent to its pitch surface.

Pitch Point: This is the point of tangency of two pitch circles (or of a pitch circle and a pitch line) and is on the line of centers.  The pitch point of a tooth profile is at its intersection with the pitch circle.

Plane of Rotation: Any plane perpendicular to a gear axis.

Pressure Angle: The angle between a tooth profile and a radial line at its pitch point.  In involute teeth, pressure angle is often described as the angle between the line of action and the line tangent to the pitch circle.  Standard Pressure Angles are established in connection with standard gear-tooth proportions.  A given pair of involute profiles will transmit smooth motion at the same velocity ratio even when the center distance is changed.  Changes in center distance, however, in gear design and gear manufacturing operations, are accompanied by changes in pitch diameter, pitch, and pressure angle.  Different values of pitch diameter and pressure angle therefore may occur in the same gear under different conditions.  Usually in a gear design, and unless otherwise specified, the pressure angle is the standard pressure angle at the standard pitch diameter, and is standard for the hob or cutter used to generate teeth.  The Operating Pressure Angle is determined by the center distance at which a pair of gears operates.  The Generating Pressure Angle is the angle at the pitch diameter in effect when the gear is generated.  Other pressure angles may be considered in gear calculations.  In gear cutting tools and cutters, the pressure angle indicates the direction of the cutting edge as referred to some principal direction.  In oblique teeth, that is helical, spiral, etc., the pressure angle may be specified in the transverse, normal, or axial plane.  For a spur gear or a straight bevel gear, in which only one direction of cross-section needs to be considered, the general term pressure angle may be used without qualification to indicate transverse pressure angel.  In spiral bevel gears, unless otherwise specified, pressure angle means normal pressure angle at the mean cone distance. 

Principle Reference Planes: These are a pitch plane, axial plane, and transverse plane, all intersecting at a point and mutually perpendicular. 

Rack: A gear with teeth spaced along a straight line, and suitable for straightline motion.  A Basic Rack is one that is adopted as the basis of a system of interchangeable gears.  Standard gear-tooth proportions are often illustrated on an outline of the basic rack.  A Generating Rack is a rack outline used to indicate tooth details and dimensions for the design of a required generating tool, such as a hob or gear-shaper cutter. 

Ratio of Gearing: Ratio of the numbers of teeth on mating gear.  Ordinarily the ratio is found by dividing the number of teeth on the larger gear by the number of teeth on the smaller gear or pinion.  For example, if the ratio is 2 or “2 to 1,” this usually means that the smaller gear or pinion makes two revolutions to one revolution of the larger mating gear. 

Roll Angle: The angle subtended at the center of a base circle from the origin of an involute to the point of tangency of the generatrix from any point on the same involute.  The radian measure of this angle is the tangent of the pressure angle of the point on the invlolute. 

Root Circle: A circle coinciding with or tangent to the bottoms of the tooth spaces.

Root Diameter: Diameter of the root circle.

Tangent Plane: A plane tangent to the tooth surfaces at a point or line of contact.

Tip Relief: An arbitrary modification of a tooth profile whereby a small amount of material is removed near the tip of the gear tooth.

Total Face Width: The actual width dimension of a gear blank.  It may exceed the effective face width, as in the case of double-helical gears where the total face width includes any distance separating the right-hand and left-hand helical teeth. 

Transverse Plane: A plane perpendicular to the axial plane and to the pitch plane.  In gears with parallel axes, the transverse plane and the plane of rotation coincide. 

Trochoid: The curve formed by the path of a point on the extension of a radius of a circle as it rolls along a curve or line.  It is also the curve formed by the path of a point on a perpendicular to a straight line as the straight line rolls along the convex side of a base curve.  By the first definition the trochoid is derived from the cycloid; by the second definition it is derived from the involute.

True Involute Form Diameter: The smallest diameter on the tooth at which the involute exists.  Usually this is the point of tangency of the involute tooth profile and the fillet curve.  This is usually referred to as the TIF diameter. 

Undercut: A condition in generated gear teeth when any part of the fillet curve lies inside of a line drawn tangent to the working profile at its lowest point.  Undercut may be deliberately introduced to facilitate finishing operations, as in preshaving. 

Whole Depth: The total depth of a tooth space, equal to addendum plus dedendum, also equal to working depth plus clearance. 

Working Depth: The depth of engagement of two gears, that is, the sum of their addendums.  The standard working distance is the depth to which a tooth extends into the tooth space of a mating gear when the center distance is standard. 




Convert Between Different Gear Pitch Types



The following are the basic formulas for convert from diametral pitch, ciurcular pitch and module pitches.

D = Diametral Pitch         C = Circular Pitch            M = Module
You Have
You Need
Rule
Formula
Diametral Pitch
Circular Pitch
Divide Pi (3.1416) by the diametral pitch
C = 3.1416 / D

Module
Divide 25.4 by the diametral pitch
M = 25.4 / D




Circular Pitch
Diametral Pitch
Divide Pi (3.1416) by the circular pitch
D = 3.1416 / C

Module
Multiply the circular pitch by 8.08505
M = C x 8.08505




Module
Diametral Pitch
Module divided by 25.4
D =  25.40 / M

Circular Pitch
Module divided by 8.08505
C = M / 8.08505





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