Surface Finish Symbols PDF

Surface Finish Symbols Notes: Refer to SUNLearn Simmons and Maguire: 4th Edition Chapter 26: Surface Texture General  The role that correct surface finishes on components play in modern technology and machines to ensure the correct performance and...

Surface Finish Symbols Notes: Refer to SUNLearn Simmons and Maguire: 4th Edition Chapter 26: Surface Texture General  The role that correct surface finishes on components play in modern technology and machines to ensure the correct performance and longevity cannot be underestimated. Just a few components that rely heavily on certain surface finishes, are:  Piston rings and cylinder liners  Solid and rolling contact bearings  Sealing surfaces  Valves and valve seats  Gears  Plastic injection moulds  Any parts that require transition or interference fits  A hydraulic cylinder is a good example of the necessity of certain surface finishes on certain parts.  Without a honed finish on the inside surface of the cylinder and the outer surface of the piston rod, the seals would not be able to work.  Common steel pipe is relatively cheap, but a piece of honed pipe is much more expensive and not so readily available.  a honing tool used to hone the inside surfaces of pipes has a mandrel with two or more honing stones that do the work as the honing tool is rotated.  Just as with geometric tolerances, finer surface finishes are not always necessary on a component and must be specified sparingly to avoid unnecessary production costs. Instrumentation and process  Even if it appears smooth by eye, if a surface is viewed under the microscope, one can clearly see it is full of irregularities such as grooves and undulations.  The machine that is used for the measurement of surface finish irregularities is the surface roughness gauge or tester.  It consists of a tracer head connected to an amplifier and a decoder that provides a digital readout.  The tracer head has a diamond or sapphire stylus with a maximum radius of 0,013mm (± a hundredth of a millimetre – the dimension is equivalent to 0.0005 of an inch).  The stylus is moved over the surface and its small movements are changed into electronic fluctuations that give a reading on the meter.  The reading is measured in microns.  The average peak-to-valley height of irregularities is calculated over a certain measurement sample length.  One of two methods can be used to obtain the reading; either the arithmetic mean deviation (Ra) method, or the root mean square (RMS) method.  In the equations shown below, ‘y’ is the height above or below the ‘zero line’, which is the average peak-to-valley height of irregularities over a certain measurement sample length.  ‘a’, ‘b’, ‘c’, etc. are the irregularities measured over that sample length, and ‘n’ is the total number of irregularities measured: 𝑦𝑎 + 𝑦𝑏 + 𝑦𝑐 + ⋯ 𝑅𝑎 = 𝑛 𝑦𝑎2 + 𝑦𝑏2 + 𝑦𝑐2 + ⋯ 𝑅𝑀𝑆 = √ 𝑛  The difference in height from the zero line can be either positive (above the zero line), or negative (below the zero line), but the negative sign is not taken into account with the Ra method.  The RMS method gives a result approximately 11% higher than the Ra method. Application of the drawing symbol  The surface finish symbol that is put on detail drawings shows three possibilities; (obligatory, optional, or prohibited).  With the old format, there are selected micron values that are indicated by the capital letter ‘N’ together with the numbers one to twelve.  The smaller the micron value, (i.e., the finer the surface finish), the smaller the ‘N’ number. ‘N’ values can be specified as upper and lower limits.  With the new format, the ‘N’ is replaced with a ‘Ra’ value in micrometres; it is put on a new position on the symbol. Just like with the old method, the Ra value can also be specified with upper and lower limits.  There is also additional information that is added on certain places around the symbol; if necessary, the production method, treatment, or coating is specified above the symbol’s ‘tail’. Other information includes machining allowance, direction of lay and sampling length.  The placement of the symbol is the same as for dimensions; preferably horizontal, but if vertical, it must be right reading. It is never placed upside down.  The direction of the lay is indicated with symbols:  Parallel  Perpendicular  Crossed  Multidirectional   Circular  Radial  Particulate or non-directional  Different manufacturing processes, machining methods and surface finishes give a range of ‘N’ or ‘Ra’ values.  Radial lip seals require Ra 0,2 – Ra 0,4 (N4 – N5) on the shaft. Hydraulic cylinder piston rods require Ra 0,1 – Ra 0,3 (N3 – N4/N5). Bearing seats require Ra 1,6 (N7).  A surface roughness comparator gives a good visual and tactile indication of various surface finishes.  N.B.: ‘Figure 26.13’ on page 225 of Simmons and Maguire is totally incorrect; a geometric tolerance of position has nothing to do with surfaces – only axes. Lecture 7 Maximum Material Condition Simmons and Maguire: 4th Edition Chapter 24: Maximum Material and Least Material Principles (Note: We do not cover Least Material Principles.)  When a shaft or a pin is at maximum material condition, it is on its upper limit, (its largest diameter).  When a hole is at maximum material condition, it is on its lower limit, (its smallest diameter).  Maximum material condition is known as a ‘modifier’, because it alters the meaning of the geometric tolerance.  The maximum material condition is indicated by means of the capital letter ‘M’ inside a circle, depicted inside the tolerance frame; it can sit next to the tolerance digit, or next to the datum letter or letters, or both.  It is only applicable on straightness, the three attitude tolerances, and the three location tolerances.  Its application is restricted to features whose size is specified by toleranced dimensions incorporating an axis or a median plane; it can never be applied to a plane, surface, or line on a surface.  Maximum material condition should also not be applied to moving mechanical linkages and precision components such as bearings and gears, features such as threaded holes, or holes intended for an interference fit.  Any dimensional deviation away from maximum material condition serves as additional clearance on the given geometric tolerance.  It is possible to find a maximum material tolerance of zero on a drawing; it simply means that the upper limit indicates the largest permissible effective assembly dimension. Lecture 8 Keys, Keyways and Circlips Notes: refer to SUNLearn Simmons and Maguire: 4th Edition Chapter 19: Keys and Keyways (Note: there is no information on circlips in Simmons and Maguire) Keys and Keyways Keys transfer the load from a shaft, to the hole in the hub or body of a component on the shaft, such as a pulley or a gear. The sunken key is the most common type used, where one half of the key fits in the shaft, while the other half fits in the hub or body of the component. Common sunken key profiles:  Square key.  Rectangular key.  Round key Square and rectangular keys are called parallel keys. Parallel keys with rounded ends are called feather keys. Even though we only do keyway calculations with parallel keys, you must know the various other types of keys listed in Chapter 19 of Simmons and Maguire (4th Edition). Keys are usually manufactured out of EN8 key steel. A keyway in the hole of the hub or body of a component, is manufactured by a process called broaching. Keyways on shafts are milled with end or face milling cutters. How is a keyway dimensioned on a drawing? A typical question reads: ‘determine the dimensions of the keyway cut into the 35 diameter shaft or hole; the keyway has a tolerance of H8/f7.’ The width of a keyway requires a certain fit from the Hole Basis Table. We use the keyway table to merely determine the depth of the keyway. Follow the steps below: 1. According to the diameter of the shaft or hole, identify which row of the table must be used. 2. Look at the ‘Depth of Keyway’ columns. 3. If the keyway is cut into a shaft, subtract the value of ‘t1’ from the shaft’s diameter; this gives the dimension from the opposite side of the shaft to the bottom of the  keyway groove; (it is not dimensioned from the centre line, or one of the edges of the groove.) 4. If the keyway is cut into a hole, add the value of ‘t2’ from the ‘Feather Key’ column to the hole’s diameter; this gives the dimension from the opposite side of the hole to the bottom of the keyway groove; do not use the ‘Taper Key’ column. 5. Both ‘t1’ and ‘t2’ give the required depth of the keyway, plus additional clearance above the key – in other words, neither of the two dimensions require additional tolerance. 6. Look at the width ‘b’ of the keyway; follow the Hole Basis Table to give it the right tolerance according to the fit that is asked for. 7. On a 35 diameter shaft or hole, the width of the keyway is 10mm. 8. Ignore the ‘f7’ (shaft) part of the fit: a fit of H8 on a 10mm wide groove has a tolerance 10,02 of +22 microns (10 H8( )). 10,00 Circlips (Retaining Rings) External circlips hold components such as pulleys, gears or bearings in place on the shaft. Internal circlips perform the same function, but inside a hole on a component. Circlips are made from carbon spring steel. Special circlip pliers are required to locate or remove them. How is a circlip groove dimensioned on a drawing? We use the circlip table to determine the width, depth and position of the groove. Circlips are made for specific nominal diameters (‘d1’). Sizes ‘t’ and ‘b’ refer to the circlips and may be ignored. Diameter ‘d2’, width ‘m’, and minimum distance ‘n’ from the edge of the component to the edge of the circlip groove, are the critical dimensions that must be applied. Lecture 9 Seals Notes: refer to SUNLearn Simmons and Maguire: 4th Edition There is no information on seals in Simmons and Maguire  We look at three basic seal types that you get:  Static seals: The components remain static relative to one another; (e.g. the head gasket between the cylinder head and the engine block); the sealing occurs mechanically. (With a series of bolts)  Seals for reciprocating linear motion: When there is linear movement between two components relative to one another, e.g. an engine’s piston rings, or the cup seals and wiper seals on the shaft of a hydraulic cylinder.  Seals for rotational motion: There is rotation of one component relative to another, e.g. a pump or boat propeller shaft’s stuffing box, or a radial lip seal.  A radial lip seal relies on three factors to work correctly:  Point contact; even though the friction is very little, over time the rotation of the shaft wears a flat surface onto the lip of the seal –that is when the sealing fails and the seal must be replaced.  Lubrication; the seal’s point keeps a thin layer of oil or lubricating fluid in position.  The correct surface finish on the shaft; it doesn’t matter how good the seal is, if the surface is not smooth enough, it will not be able to seal.  There are two types of sealing forces; all contact seals work with one, or the other, or a combination of both:  Mechanical force: (Bolt and nut, spring / garter spring etc. A ‘garter spring’ is like a sock garter).  Automatic or self-actuating force: The pressure force against the seal causes the sealing.  A radial lip seal relies on both forces.  In the case of a piston ring in a motorcar engine, the combustion gas pushes the piston ring against the wall of the cylinder, which improves the sealing.  A piston ring also works as a large spring that pushes against the wall of the cylinder.  Piston rings also rely on both forces.  The automatic force is the concept behind the seal inside the lid of a pressure cooker. As long as the contents are cold, the seal leaks, but as they heat up, it causes steam inside the pot that pushes the seal against the pot and the lid.   The same happens with rubber O-rings; the width of an O-ring’s groove is critically important to allow the pressure to push the O-ring flat against the one side of the groove.  If the groove is too narrow, with enough pressure the gas or liquid will leak over the top of the O-ring and the sealing will fail.  The general pressure limit for a conventional O-ring is 10MPa (100 bar). If the pressure is greater, anti-extrusion rings must be used with the O-ring.  O-rings are manufactured out of three types of rubber:  Viton (The best material)  Nitrile (The most common type)  Silicon rubber (For certain applications)  There are no contact seals that last indefinitely; all seals have to be replaced sooner or later.  You also get seals that work without contact, on the principle of enlargement of volume and constant pressure drop, like a labyrinth seal; there is leakage out of a labyrinth seal, but very little; they are used in gas turbines.  The design and materials of seals are critical, especially in environments with high temperature, pressure, friction, or a vacuum.  Seals are made from a wide range of materials; paper, rope, asbestos, cotton, cork, rubber, plastic, copper, aluminium, carbon, ceramic, cast steel and even stainless steel.  Many of the materials are impregnated with oil, grease, graphite, wax or silicon to improve the sealing.  This is the concept that makes a stuffing box work so well; the packing rings inside the housing are impregnated with wax or a mixture of grease and graphite; these are compressed by the lid and forced against the shaft, which causes the sealing.  Bearings that are filled with grease must be sealed on the bearing itself, (which the manufacture provides), or in the assembly where the bearing is mounted.  A bearing’s seals are only there to keep dust out; in the case of a gearbox, to prevent the oil in the box from leaking out between the gaps in the bearing’s rollers, a radial lip seal is essential.  There is a more modern mechanical seal for rotational movement that can replace a traditional stuffing box, namely the ceramic gland seal; it consists of three main components:  The primary ring.  The mating ring.  A set of springs. Between the primary ring and the mating ring is a layer of oil or fluid film; it must be clean and viscous, and the temperature and pressure thereof must be controlled. Lecture 10 Surface Finish Symbols Notes: Refer to SUNLearn Simmons and Maguire: 4th Edition Chapter 26: Surface Texture General  The role that correct surface finishes on components play in modern technology and machines to ensure the correct performance and longevity cannot be underestimated. Just a few components that rely heavily on certain surface finishes, are:  Piston rings and cylinder liners  Solid and rolling contact bearings  Sealing surfaces  Valves and valve seats  Gears  Plastic injection moulds  Any parts that require transition or interference fits  A hydraulic cylinder is a good example of the necessity of certain surface finishes on certain parts.  Without a honed finish on the inside surface of the cylinder and the outer surface of the piston rod, the seals would not be able to work.  Common steel pipe is relatively cheap, but a piece of honed pipe is much more expensive and not so readily available.  a honing tool used to hone the inside surfaces of pipes has a mandrel with two or more honing stones that do the work as the honing tool is rotated.  Just as with geometric tolerances, finer surface finishes are not always necessary on a component and must be specified sparingly to avoid unnecessary production costs. Instrumentation and process  Even if it appears smooth by eye, if a surface is viewed under the microscope, one can clearly see it is full of irregularities such as grooves and undulations.  The machine that is used for the measurement of surface finish irregularities is the surface roughness gauge or tester.  It consists of a tracer head connected to an amplifier and a decoder that provides a digital readout.  The tracer head has a diamond or sapphire stylus with a maximum radius of 0,013mm (± a hundredth of a millimetre – the dimension is equivalent to 0.0005 of an inch).  The stylus is moved over the surface and its small movements are changed into electronic fluctuations that give a reading on the meter.  The reading is measured in microns.  The average peak-to-valley height of irregularities is calculated over a certain measurement sample length.  One of two methods can be used to obtain the reading; either the arithmetic mean deviation (Ra) method, or the root mean square (RMS) method.  In the equations shown below, ‘y’ is the height above or below the ‘zero line’, which is the average peak-to-valley height of irregularities over a certain measurement sample length.  ‘a’, ‘b’, ‘c’, etc. are the irregularities measured over that sample length, and ‘n’ is the total number of irregularities measured: 𝑦𝑎 + 𝑦𝑏 + 𝑦𝑐 + ⋯ 𝑅𝑎 = 𝑛 𝑦𝑎2 + 𝑦𝑏2 + 𝑦𝑐2 + ⋯ 𝑅𝑀𝑆 = √ 𝑛  The difference in height from the zero line can be either positive (above the zero line), or negative (below the zero line), but the negative sign is not taken into account with the Ra method.  The RMS method gives a result approximately 11% higher than the Ra method. Application of the drawing symbol  The surface finish symbol that is put on detail drawings shows three possibilities; (obligatory, optional, or prohibited).  With the old format, there are selected micron values that are indicated by the capital letter ‘N’ together with the numbers one to twelve.  The smaller the micron value, (i.e., the finer the surface finish), the smaller the ‘N’ number. ‘N’ values can be specified as upper and lower limits.  With the new format, the ‘N’ is replaced with a ‘Ra’ value in micrometres; it is put on a new position on the symbol. Just like with the old method, the Ra value can also be specified with upper and lower limits.  There is also additional information that is added on certain places around the symbol; if necessary, the production method, treatment, or coating is specified above the symbol’s ‘tail’. Other information includes machining allowance, direction of lay and sampling length.  The placement of the symbol is the same as for dimensions; preferably horizontal, but if vertical, it must be right reading. It is never placed upside down.  The direction of the lay is indicated with symbols:  Parallel  Perpendicular  Crossed  Multidirectional   Circular  Radial  Particulate or non-directional  Different manufacturing processes, machining methods and surface finishes give a range of ‘N’ or ‘Ra’ values.  Radial lip seals require Ra 0,2 – Ra 0,4 (N4 – N5) on the shaft. Hydraulic cylinder piston rods require Ra 0,1 – Ra 0,3 (N3 – N4/N5). Bearing seats require Ra 1,6 (N7).  A surface roughness comparator gives a good visual and tactile indication of various surface finishes.  N.B.: ‘Figure 26.13’ on page 225 of Simmons and Maguire is totally incorrect; a geometric tolerance of position has nothing to do with surfaces – only axes. Lecture 11 Welding Symbols Notes: Refer to SUNLearn Simmons and Maguire: 4th Edition Chapter 33: Welding and Welding Symbols Basic Principle of Welding The welding process joins two pieces of metal together by melting them, with or without the use of a welding rod as additional filler material. It involves the use of very high temperature focused on a small area. (Steel melts at ±1500°C.) If a welding rod is used, there is also penetration of the molten rod in both pieces of metal, which strengthens the joint. It is different from soldering and brazing; these processes join two pieces of metal by using solder or brazing wire as the binding agent without melting the pieces of metal themselves. Solder and brazing wire also melt at a much lower temperature. (±180°C – 450°C.) Common Types of Welding Arc welding: a strong, continuous electric current forms an arc between a consumable welding rod coated with solid flux, and the workpieces, which causes the workpieces and the rod to melt together. As the flux melts with the rod, it works as a lubricant which helps the molten metal to flow, and it forms a layer of protective gas around the pool of molten metal which reduces contact with the atmosphere to prevent corrosion. TIG welding (‘Tungsten Inert Gas’): this was the old way to weld aluminium. The electric arc is created by a non-consumable tungsten electrode inside a tube which also delivers an inert gas such as argon to the weld area. The filler rod is fed apart by hand. The process is slow and requires much skill. MIG welding (‘Metal Inert Gas’): It’s become the most popular method to weld almost all metals. It’s similar to TIG welding, except that the electric arc is created by a consumable filler wire fed automatically by the machine. The wire is fed through a tube which also delivers a mixture of argon and CO2 gases to the weld area. It is quicker and much easier than TIG welding to master. Spot welding: the pieces of metal are melted together by a strong electric current, without a filler material. Two electrodes that hold the workpiece together act as the conductors. The microscopic layer of air between the two workpieces offers a high resistance to the current, which causes the heat. (That is how motorcar panels are welded together by welding robots)  Gas welding (Oxy-Acetylene): Oxygen and acetylene gases are burned by means of a torch to melt the workpieces and hand-fed filler rod together. There are various types of joints that are created by welding one or more interfaces between two pieces of metal: Butt joint Corner joint Lap joint T-Joint Edge joint There are numerous types of welds that are possible, and each is denoted by a specific welding symbol placed on an engineering drawing. Refer to the samples of the most common ones shown in the lecture slides: Surfacing (Two pieces are not joined) Bead weld Square butt weld V-Groove weld Double V-Groove weld Bevel weld Fillet weld Plug weld Spot weld U-Groove weld J-groove weld Edge flange weld Corner flange weld Superimposed welds (Welds layered on top of each other) The Welding Symbol There are numerous welding standards applied around the world, but there is no general consensus as to which standard should be applied as the overriding one. The situation is no different in South Africa, where the commonly favoured and applied welding standard is SANS 10044-2, which is largely based on the old BS 499-1 standard. In the lecture slides on SUNLearn, I present the so-called ‘old’ welding symbol arrow from this legacy standard. Chapter 33 in Simmons, Maguire and Phelps is based on the hybrid BS EN ISO 2553: 2012 standard, which is itself a mix of the ISO 2553 and American AWS A2.4 standards. In the lecture slides on SUNLearn, I present the so-called ‘new’ welding symbol arrow from this standard. I have stuck to so-called ‘old’ welding symbol arrow standard when presenting all the examples of welding symbols and their interpretation. FOR THE BENEFIT OF IMD 244 STUDENTS, ALL MAJOR ASSESSMENT QUESTIONS ON WELDING SYMBOLS WILL LIKEWISE BE BASED ON THE OLD STANDARD, NOT THE NEW BS EN ISO 2553: 2012 STANDARD PRESENTED IN THE TEXTBOOK.