Basics and Principles of Broaching Operation - Research Paper Example

INTRODUCTION: Broaching is an attractive method of material processing technique existing since the Industrial Revolution, and finding increasingly novel applications day-by-day in automated mass production of both metallic and plastic components with high degree of dimensional accuracy and surface finish required for high-tech industry. Underlying Principle of Broaching: Fg-1 Schematic Drawing showing Underlying Principles of Broaching If a slot has to be cut in a rectangular work-piece, as shown in Fig-1, through the use of shaping process, it would have to be cut layer by layer in thin layers, all the while cutting small depths at a time. Therefore, the total depth is completed in a number of passes. In-feed is given down layer after layer so as to completely remove the total depth of material to be removed. The in-feed is not done in a single pass because, it is not possible to remove the complete material in one single pass since, by adopting such a course of action, enormous force would be developed on the cutting tool causing it to break. So the total layer is subjected to a number of passes, each pass removing a thin layer at a time, so that the tool is not subjected to very large forces. So the tool is allowed to be cut layer by layer, to allow the cutting tool to withstand the load, till the total depth is completed. In such a shaping process as above, the productivity of the method will be very low because the tool starts from zero velocity, goes to maximum velocity, again reaches zero velocity, then comes back in idle stroke to return to the original position, attains zero velocity and starts the process of cutting in the next pass repeating the above sequence. Time and energy get wasted due to repeated movement of cutting tool as a number of passes have to be given manually or semi-automatically causing wastage of time energy (Chattopadhyay). Now suppose, we join the different positions of the cutting tool in Fig-1(a) to convert it into one large tool with a number of teeth in series, with each teeth gradually protruding more from the axis of the tool than the previous one as shown in Fig-1(b). The in-feed is now called tooth-rise or rise per tooth (RPT). The resultant large single cutting tool can be used, to remove the entire material in one single stroke, as the feed of the cutting edges is built into the cutting tool. The total force required for the tool is large, but the force per cutting tool will be less .The entire material to be machined can be removed in one stroke. This is the underlying principle of broaching. The cutting tool designed for the operation is similar to the bigger single tool with a series of cutting teeth with gradually increasing tooth-rise. The above machining process is known as broaching shown at Fig-3 and the cutting tool with series of teeth is known as a broach shown at Fig-4.The work piece subjected to broaching is shown at Fig-5. The number of teeth in a broach can be up to 40 or more. Therefore the force per tooth is now decreased but the total force and power requirement will be increased in this second method, called the broaching process, in which finish and quality are better than those attained by shaping process. The most common application of broaching is in the enlarging and finishing operation that has to done on holes drilled in castings or forging. The broaching tool used, will have a number of rings with cutting edges, the diameter of the rings gradually increasing in one direction. The diameter of the finished hole will be decided by the last of the series of cutting teeth in the broach. The broach used for this purpose can be a push-type broach or a pull-type broach. According to Zohdi M.E. et al (n.d,40-41) The maximum value of force that can be withstood by a pull broach is given by the equation P=Ay Fy /s in lbs where Ay = minimum tool selection, in sq.inches Fy = tensile yield strength of material of the tool, in pounds per sq.inches and s = Safety factor. The maximum allowable pushing force is given by the equation P=Ay Fy /s in lbs where Fy = minimum compressive yield strength P= 5.6x 107D4/sL2 in lbs where D= minimum tool dia and L= length of the broach. In actual shop-floor practice, the applicable metal removal rate(MRT) and motor power to be applied are obtained from standard alignment charts, which are available for F.P.S as well as metric units of dimensions (Zohdi M.E. et al 40-41). The parts produced by a broach can be externally broached (Fig-6) or internally broached parts (Fig-7). Fig-8 shows the parts of a typical broach with nomenclature. Geometry The angle between the rake surface or the reference plane perpendicular to the direction of motion of the cutting tool and the cutting edge of the cutting tool is the Rake Angle which is a very vital parameter. Rake Angle has a positive value or is zero for carbides while it very high values for HSS. Clearance Angle is provided on the flank surface so that the cutting tool does not rub with the finished surface. There are two Clearance Angles, namely Primary Clearance angle of 30-50 and Secondary Clearance Angle of 60 -250. Pitch is the gap between the consecutive teeth with values of around 1.25√ Lc to 1.5√ L c where L c is the length of the work piece to be cut. The number of teeth in a broach is the value of Total length/pitch(Chattopadhyay) Broaching operation Criteria for Selection of Broaching Machines,Type of the job, size and material Geometry and volume considerations require that broaches have to be longer if more volume of material is to be removed. The machine which suits the work at hand has to be chosen since different types of machines are available. For high Process capability, rigid machines are required.(Chattopadhyay) MACHINE SELECTION: A TYPICAL MACHINE WITH BRAND, MODEL NO.AND PRINCIPAL FEATURES The elmass P10-200-NC broaching machine shown at Fig-2, is Fig-2. The elmass P10-200-NC broaching machine. a Vertical Broaching Machine marketed by elmass production of Belgium and is designed exclusively for broaching of key-ways and splines in through holes and blind bores . It combines the latest control technology with a dynamic positioning system and live guided broaching tools to enable mass manufacture of accurate splines and key-ways with excellent surface finish and standard tolerances. It can be used to broach either for mass production or small batch production of up to 200 mm long internal key-ways and splines using 2mm to 10mm wide cutters. Principal features: The elmass P10-200-NC broaching machine has a Stroke of 200mm, key width of 2-10mm,Parts diameter of 300mm,Maximum pressure of 10kN fixed setting, cutting speeds of 12m/min to 20m/min, dimensions L x W x H of 600x750x2250mm and weighs 600Kg.It offers an interactive 15” touch-screen panel programming facility in English, French, Dutch and German and can be adapted to either metric or imperial F.P.S system of dimensions. It operates on an electric supply of input voltage 3x400VAC, frequency 50/60hz,and amperage 5kW. The workspace of the machine is completely shut which facilitates cleaning and prevents oil and chips from ejecting out. The machine is of rugged construction and can be erected on uneven floors using adjustable feet or lifted up using built-in supports to optimize work flow processes by stackers. The machine’s functions are all controlled by an embedded system which displays interactive user-friendly icons facilitating input of necessary parameters such as bore diameter, key-way width, cutting depth and material in a step-by-step process, which can be converted to a manual mode of operation also, if required. Mobile network based internet-enabled services are available for maintenance and trouble-shooting of the machine functions round-the-clock through the customer care services of the manufacturer which can access all machine functions and diagnostic software and download latest updates of required software. The operation of the machine is carried out by a highly dynamic electrical motor which positions the cutting tool automatically to pre-determined values of work piece material-specific parameters such as broaching speed, retracting speed, cutting pressure, cutter speed, and retraction. The broaching tools used are available to both metric and imperial sizes and are interchangeable. The work pieces can be positioned by means of an integrated cross-table. since the machine is designed to occupy minimal space, productivity in the shop-floor can be maximized by full utilization of space. This is a versatile machine which can be pre-set for manufacture iin small batch quantities or for mass production, in both F.P.S and metric dimensions and can be used by most of the European Union and English speaking nations. The design of the machine has been improved over the years based on feedback from customers and clients over the years and the present model is intended for long use even under rough operating conditions. The effects of torsion are minimized by the solid and rigid frame construction while the adjustable feet of the machine enable leveling of the machine even on uneven floor. The standard tolerances of this machine are H9/P9/JS9. (http://s3.amazonaws.com/machinetools_production/uploads/848015/P10-200-NC-BROACHING-MACHINE.pdf) MACHINE REQUIREMENTS: An Overview of Resources Required to Operate and Maintain the Broaching Machine: Selecting the Correct Broach and Configuration The pull-type broach is constructed in the shape of a rod with a series of rings each having cutting edges. The pull-end is connected to an adaptor, which is connected to a slide which in turn is connected to the broach. The slide will pull the broach to make it travel through the work piece, by a hydraulic force. The neck is the region of the broach, having the smallest diameter design so as to break in case of more load, so that the no damage will occur to the other parts. The configuration of the cutting teeth with increasing tooth rise consists of a series of roughing teeth, followed by semi-finishing teeth with lesser tooth rise , finishing teeth with still lesser tooth-rise. This is finally followed by the burnishing teeth. If solid ,sectional and module or segmented type broaches are used, the single pieces or modules can be removed and replaced, enabling design, manufacture and repair of broaches to be easier and also decrease cost of tool.(Chattopadhyay) Material for Construction of Broaches High Speed steel (HSS) and Cemented Carbide are the most commonly used materials. HSS has low values of hardness, wear resistance and heat resistance. If more speed and productivity is the requirement, cemented carbide tools should be used which can be used in the form of blades which are brazed on to the cutting tool or as ready-to-use inserts available commercially. The HSS tools can be coated with cemented carbides. Tungsten carbide and cobalt carbide are also used. Titanium Nitride is also used as coating material for cutting tool of broaches made out of HSS. The material of construction of the broach has to be selected based on the type, shape and material of the work piece, geometrical complexity and amount of material to be removed,length of stroke and broach and the type of broaching machine selected for the operation(Chattopadhyay). Mounting and Clamping accessories Mounting of Broaches The tool holding device of the broaching machine performs mounting, clamping and movement action of the broaches. The mounting methods are different for pulling type and pushing type broaches. The broach is fixed to the machine by an adaptor and a pin. In pull-head and socket type arrangement, used for pull type broaches, spring type clamping is used to secure he broach. Broaches for external broaching are mounted by guide bushes and frames. Mounting by using Allen screw is also in use. Mounting of Work Piece The clamps are of such a design as to enable rapid and uniform performance of activities such as locating, clamping and mounting of work pieces, since broaching is a mass production machining process. Fixtures Internal broaching requires no special fixtures other than plate, plug or horn fixtures.Very deep key ways require insertion of shims made of tool steel.Special fixtures designed to withstand enormous pressures are required for carrying out external broaching(Donaldson,8-6). Process Parameters and accessorry resources for Cutting Tool Since broaching operation generates intense heat, the chip-tool interface needs to be cooled continuously by lubricants and cutting fluids. Besides providing lubrication and cooling, the metal chips are also flushed away by cutting fluid which is made to impinge upon the tool-work piece interface with high pressure. Cutting Velocity of the tool differs from tool to tool depending upon the material of construction of the tool and of the work piece. It also differs with the capability of the broaching machine.Cutting velocity is 10-20 metrers/minute for HSS and for Cemented Carbides, it is 20-40 meters/minute. The machine tool should be properly designed to have the capacity to withstand the velocity and power of broaching operation. Broah Sharpening Broaches need to be re-sharpened very often depending upon usage and hence a less abrasion resistant material should be selected as material for tool material . NET SHAPE and CONCLUSION: A discussion of processes and part condition which precede and succeed the technique you have been asked to discuss The Broaching process is carried out in a sequential way as per the following steps(Chattopadhyay): 1) Broach and broaching machine are selected 2) Broach is clamped in the broaching machine 3) Work piece is then clamped on to the broaching machine 4) The work-tool motions and process parameters are calculated and set including rise per tooth and metal removal rate , and are programmed into the machine control system. 5) Broaching Operation is carried out As per Holden Ventures,LLC, the following methods are used to ensure adequate cooling and lubrication: 6) Cutting fluid is applied by high pressure jet or by submerging the working piece under the cutting fluid during the period of broaching process.Care is taken to ensure that the lubricant cutting fluid is delivered to the exact point of interface between work piece and cutting tool. 7) The speed of the broach is decreased to facilitate cooling of the work piece by conduction. 8) Use of water based synthetic cutting fluids tailor made for heat reduction. The condition of the work piece which succeeds the broaching is characterized by the generation of immense heat which has to be carried away by the cutting fluid. The intense heat causes softening of both the tool material and the work material. Fig 9 and fig-10 show internal broaches as used for internal gears and the cutting tools that produce them. Broaching machines are characterized by only one motion- i.e., the cutting motion. The profile of the job is totally dependent on the form of the tooth and any defect on the job will be transmitted to the job. Internal broaches are capable of imparting close tolerances to extreme levels of dimensional accuracy in through holes of different form and dimensions, such as holes of circular sections, holes of internal keyways, internal gears, and internal splines. External broaches find application in shaping, planing and milling of unobstructed flat surfaces, grooves, slots, key-ways, external splines, teeth of external spur gears and gear sectors. All the gear teeth of an external gear can be constructed in one stroke, by using a complex broach in the form of a hollow cylinder. To the inside walls of this hollow cylinder, several broaches fixed by joining. When this cylindrical broach is drawn over the cylindrical gear blank work piece as shown in Fig-11, the result of this operation is that the gear is made in just one stroke. Horizontal machines are generally of pulling type, while vertical machines occupy less space, are more rigid and are mostly used for external broaches. Automation of broaching process by suitable methods such as clamping and turret indexing, where different broaches fitted on the faces of a polygonal turret can quicken the removal of finished jobs and such an arrangement can be used turn by turn to do different types of broacing operation such as shaping, and finishing on the part, just by turning the side of the polygon. This can transform the broaching operation into a very rapid process. CONCLUSION Broaching is mass production process, suitable for surfaces which are parallel, and compatible with automated operations, and for forming tough materials. When combined with automation, this process is capable of delivering exceptionally high productivity, which is the need of the present day. References: Lesson 26, Broaching-Principles, systems and Applications, IIT Kharagpur at www.scribd.com at URL http://www.scribd.com/doc/11621699/26-Broaching-Principles-Systems-and-Applications Zohdi M.E, Biles W.E., Webster D.B. Production Processes and Equipment For Metals Online Internet 27 Nov 2010 available athttp://85.185.231.194/mekanik/Handbook%20of%20Materials%20Selection%20-%20Kutz,Myer/production%20processes.pdf Chattopadhyay A.B., http://www.youtube.com/watch?v=vsNAPRIKAVs, IIT ,Kharagpur http://s3.amazonaws.com/machinetools_production/uploads/848015/P10-200-NC-BROACHING-MACHINE.pdf Holden Ventures LLC at http://holdenventures.com/fl.aspx Donaldson, Cyril & Donaldson Mn Tool Design Edition 1976 39th Reprint 2008 Tata McGraw-Hill, New Delhi Read More