Machine Tool Characteristics and Operation - Assignment Example

INЕ ТООL СНАRАСТЕRISТISТIСS АND ОРЕRАТIОN Table of Contents Summary 2 Introduction 2 Discussion: - Task Analyses. 3 Task One: 3 a)Machine Tools Axis Conventions 3 b)How the Drives are controlled 5 Task Two: Tool and Work-holding Devices 6 Center Lathe 6 Vertical Milling Machine 6 Surface Grinder 7 Task Three: Degrees of Freedom of a Rigid Body 7 Task Four: 9 a)Suitability of Selected Machines/Tools 9 b)Operational Plan 10 Task Five: Engineering Drawings 13 a)Cutting Operations 13 b)Description of Operations 15 Conclusion 17 References 18 Summary An analysis of the axis conventions of various machine tools was undertaken and an explanation of how the conventions are controlled and driven during operation. The report also looked into the various tool and work holding devices while explaining the suitability of each device to the specific operation being undertaken. The concept of degrees of freedom and principles of location was also analyzed with the use of illustrations to visualize the six degrees of freedom. The suitability of workshop machinery relative to the work undertaken in the workshop was analyzed and an operational plan drawn. The report also looked into the various cutting methods and identified various parts and the methods of cutting applied on them. A description of broaching and bending of the work-piece is also presented at the end of the report. Introduction In industry, manufacturing firms use a range of machine tools to produce a variety of components. When a product requires manufacturing a firm will decide what machine to use and develop a plan to supply the customer with the demand. It is therefore vital to understand the characteristics of machine tools and their operational feasibility. It is this subject that has been of interest to machine tool designers and users alike. It is the basis for which tool life and general metal cutting economics are based. This report is aimed at describing the characteristics and operational features of some provided machines. The report is written from the perspective of a Senior Manufacturing Technician who has received a job from a customer and have been asked to plan a sequence of operations for the production of specific articles. The report investigates and analyses the typical manual machining operations that relate to the individual machine’s ability, characteristic, capability, suitability. Discussion: - Task Analyses. Task One: a) Machine Tools Axis Conventions It is important to employ a machine tool system of axes that is based on the ISO standards. Such basis facilitates the description of the machine tool motion. The system describes possible motions of a machine tool and is based on a right-hand coordinate system as shown below[Kni06]. Figure 1: Machine tools coordinate system for motions. (a) Possible tool motion; (b) work-piece motion The axis conventions of the following machines are identified and explained as follows; i) Centre Lathe It refers to a single point tool whose main purpose is to produce concentric work. The tool is ground to point that accurately positions the work piece on an axis. Centre Lathe operates through cylindrical turning where the work-piece rotates in a -C’ motion and the carriage moves in the -Z axis direction along the lathe bed as shown in figure 2 below[Kni06]. Figure 2: Axis convention for Center Lathe machine ii) Vertical Mill The vertical miller is a multipoint tool whose spindle is vertically oriented such that the milling cutters are held and rotate on its axis. A turret mill is an example of vertical mill and the cutting occurs in the vertical Z direction. The primary motion occurs in the –C axis direction while the continuous feed motion is in the –X’ direction as show in figure 3[Kni06]. Figure 3: Axis convention, Knee-type vertical mill iii) Surface Grinder The surface grinder provides a primary motion in the C axis direction due to the fact that it has a horizontal spindle. The principal feed motion is a result of hydraulically operated worktable movement in the X’ axis direction. Further infeed may be provide in the –Y motion axis or cross feed in -Z’ axis direction as shown below. Figure 4: Axis convention on horizontal-spindle surface grinder b) How the Drives are controlled For the Centre Lathe machine, the operation is controlled through automated feed action whereby the primary motion (motion C’) is generated by a the movement of a series of gears automatically driven by motors. The motion Z is automatically driven a lead screw and feed rod operated by gears that are mounted in the headstock[Kni06]. In the case of the vertical mill, vertical Z-motion is achieved by manually moving the knee up or down. The primary –C motion is automatically driven by a series of gears that are run by electric motors mounted on the headstock. The continuous feed motion (-X’ motion) can either be manual or automatically driven depending on the heaviness of the work-piece and the accuracy desired. For the surface grinder, the principal feed motion is controlled by the traverse movement of the worktable on which the piece is mounted and is hydraulically operated. Infeed (-Y motion), is manually controlled through moving the wheel head down the column. Cross-feed (Z’ motion), on the other hand is controlled by moving the wheel head parallel to the spindle. Task Two: Tool and Work-holding Devices Center Lathe The most prevalent work holding method for the center lathe is the chuck that can have either three or four jaws depending on the type of work. Three-jawed chucks are commonly used for cylindrical work pieces that require the machined surface to be concentric to the work surfaces. The jaws of the chuck have a series of teeth that mesh with grooves to ensure grip of the work piece. The jaws are equidistant from the chuck therefore the pieces are automatically centered. On the other hand, chucks with four jaws are used when the work piece is non-cylindrical[Kni06]. Pieces that have complicated shapes can be held with circular faceplates that has radial slots. The slots provide a means of holding the work piece through bolting it to the faceplate. Collets are used for smaller lathes that are usually used on materials in form of bars. Collets are composed of sleeves that are split effectively and fit perfectly on the work. They generally have a tapper on their other surface. Collets are available in a range of standard forms based on the shape of the bars to be gripped. For instance, square and hexagonal collets for bars that are not in cylindrical form[Kni06]. A simple tool holder in the form of a typical tool-post can be used for holding this single-point tool. It is composed of a curved block that effectively rests on a concave spherical surface such that the tool can easily be inclined to the required height during operation. The tool-post is mounted on a compound rest. A slightly different form of holding devices is the four-way tool post that can hold up to for cutting tools such that specific tools can be moved into operational position by unlocking tool-post lever, rotating and re-clamping with the level. Vertical Milling Machine The vertical milling machine can have cutters in form of either straight shank or bore. Tool holding in such a case depends on the type of the cutter with bore cutters (shell end mills) being secured to an arbor. The arbor can be held in a socket in the spindle with a draw bar. Straight shank cutters on the other hand are held in position with chucks or screw bearing mounted on a flat surface in mounted unto the shank[Kni06]. Work-holding can be achieved through typical vices or T slots in contained in the machine table. It really depends on the type and size of the work-piece and the accuracy desired. Surface Grinder Tool holding can be achieved by securing the grinding wheel to the end of the spindle and preferable between two flanges to effective prevent unwanted displacements during operation. Work-holding on the other hand can be accomplished through the use of a magnetic vise that is mounted on the machine table. The magnetic vise can be controlled by an ‘ON’ or ‘OFF’ lever/button that alternates between magnetizing and demagnetizing the work-piece. This type of holding is however limited to ferromagnetic materials and for operations that do not require much grip such as finishing. For other materials, mechanical vises and other means of clamping can be used[Kni06]. It is important to perform wheel dressing that returns the wheel into its original round shape before estimating machining time. The wheel is fed in a direction normal to the work surface i.e. infeed. This is also called sparking out and results in the removal of a layer of material that is equivalent to a single stroke grinding. Task Three: Degrees of Freedom of a Rigid Body For efficient operational performance, a rigid body being operated upon ought to be able to move along the three axes as shown in fig. 1. It must also be able to rotate about the three axes and therefore has six degrees of freedom[Kni06]. A rigid body therefore has three degrees of translation and three degrees of rotation as shown below; Figure 5: The six degrees of freedom However, during operation, the body must be firmly secured in a given position to allow for work to be performed on the desired surfaces. This can be achieved by restraining the freedom of movement of the surfaces such that those that do not need machining are immobile. To eliminate the six degrees of freedom, the principle of location is applied as shown in fig. 6 below. Figure 6: The principle of location The first plane contains three restraint points along the Z- axis and about the X and Y planes (1, 2 and 3 respectively). The second plane is restrained along the Y- axis (4) and about the Z-plane (5). The third plane has one restrained that limits movement along the X – axis. The arrangement in fig 6 employs the effective clamping and therefore eliminates all the degrees of freedom. The number of restraints should be as minimum as possible when designing work-holding devices. Positive solid restraints are preferable compared to frictional restraints to achieve positional location. Task Four: a) Suitability of Selected Machines/Tools To develop the plump-bob body, a diameter of 6.8mm was drilled at the center of the 20mm bright drawn mild steel material. The material drilled is a cylindrical rod of length 82mm. A Center Lathe was therefore suitable in that case since the operation required a single point drilling tool. Given that the work piece had a length of 83mm, an extensive machine table was required to ensure the material was effectively held parallel to the spindle. It is important not to take little material at a time to ensure the material didn’t make noise. Swarfs would also become entangle around the piece and could dangerously fly off the machine. As a precaution, protective equipment including ear muffs and goggles were worn during the process[Ben14]. The most suitable work-piece holding device in that case was a three-jawed chuck. Given the work-piece was cylindrical, an even radius could only be achieved using the specific chuck with jaws set at equidistance from the spindle. Even though drilling was only done to the depth of 19mm, the material had to be held with 20mm protruding, thus further making the case for the three-jawed chuck. The clamp was made at an angle that ensured that the piece was symmetrical and the hole was at the centre[Ben14]. A paintbrush was used to remove the swarf produced as opposed to removing by hand. This was because cutting the material resulted in it becoming very hot to touch. The swarf material are also sharp and for safety reasons, it was better to use the paintbrush. The knurling process required the formation of a diamond shoulderless knurl that would form the handle of the plump bob. For the knurling process, a Dorian double wheel shoulder-forming knurling head tool was used. Figure 7: Knurl head tool The tool would also automatically form the chamfer required at the end of the knurl and therefore more suitable for the process. The paintbrush was also used to clean the machine and the work area since it was safer compared to using bare hands. In performing the threading process for the bob screw, the lathe machine was suitable since accuracy and precision was vital in threading[Kni06]. b) Operational Plan The operational plan for the making of the plump bob was divided into two; i) Plump Bob Body List of Operations         Tools and Materials  Obtain material 20mm×82mm(BDMS) Bright drawn mild steel  Measuring tapes, gauges  Obtain tools (for drill, knurl, chamfer and )    Setup lathe (centre height ,speed, feed)    Place material in chuck (10-20mm protruding)  Three- jawed chuck A face off was created that acted as the datum of the operation  Lathe Saw  Using Centre Drill, drill6.8mm×19mm  Centre Lathe  The material was then turned 19.5mm×50mm  Turning Lever  Knurl material was then undertaken  Double wheel knurl head  turn16mm×10mm  Turning lever  Chamfer1mm×45  Knurl head  Turnaround 180  Lever  Face off to length 80mm  Lathe Saw  Turn chamfer  Turning Lever  Turn taper40  Taper  Clean machine + work area  Paint brush   ii) Plump Bob Screw List of Operations         Tools and Materials  1-Obtain material 12mm×50mm(FMB/CZ121) Bright drawn mild steel  Measuring gauges  2-Obtain tools    3-Setup lathe (centre height ,speed, feed)  Center Lathe  4-Place material in chuck (10mm protruding)  Three-jawed chuck  5-Face off (datum)  Lathe saw  6-Centre drill  Center Lathe  7-Drill3.5mm×16mm  Drill bit  8-Turn11.5mm30mm  Turning Lever 9- Knurl material  Double wheel knurl head  10-Turn8mm×12mm  Turning lever  11-Chamfer 1mm45o  Knurl head  12-Undercut7mm×1mm  Lathe saw  13-Thread m8×1.25mm  Threading bit  14-Undercut to half diameter  Lathe saw  15-Turn chamfer 1mm×45o  Knurl head  16-Part off to length 16mm  Lathe saw  17-Clean machine + work area  Paint brush     Task Five: Engineering Drawings a) Cutting Operations The figure below is an illustration of the processes performed on the material. The process was either formed or generated. The different characteristic geometry of these machining operations depends on both the relative motion of the tool and work-piece and the geometry of the cutting tool. Generated Surfaces Figure 8: Generated surfaces The surfaces herein are generated depending on the feed trajectory of the cutting tool. The method of cutting involves various turning operations and milling operations as illustrated below; Figure 9: cutting operations for generated surfaces Forming Method Figure 10: Formed surfaces The operations in figure above are a result of forming. The cutting method is dependent on the geometry of the tool. In this case, the surface of the part and the cutting edge of the tool has reverse shapes. These cutting operation may include form turning, drilling, broaching etc. as illustrated below. Figure 11: Form operations b) Description of Operations Broaching The broaching process entails a series of cutting teeth that remove a portion of stock on the work-piece as the broach moves through the work piece. For precise tolerance, both roughing and finishing were combined in one operation during the creation of the keyways shown in figure below[Tod09]. Figure 12: Keyway broaching The machine provided the primary motion that is hydraulically powered in the X motion direction. The feed on the other hand was provided by staggering the teeth on the broach such that each tooth removed a small layer of the material. The illustration below shows how the broach operation works[Kni06]. Figure 13: Broaching Bending of Keep-plate When bending care ought to be taken so as not to break the material. Bend radius depends on the thickness of the material and the keep-plate’s maximum bend radius had to researched before the operation. This was based on the materials chemical composition, surface and edge conditions as well as the thickness of the plate. The bending process involved plastic deformation and the stresses associated with the deformation might have resulted in strain-hardening of the materials. The resistance to fracture and ductility of the material were therefore considered Figure 14: Bending set-up Conclusion In conclusion, it is evident that understanding machine operations is vital in producing high quality work. A balance has to be struck between the prevailing conditions and the precision required for the material being machined. The operator ought to have a detailed operational plan since it ensures no process is omitted or done before it time. References Kni06: , (Knight & Boothroyd, 2006), Ben14: , (Benson, 2014), Tod09: , (Today's Machining World, 2009), Read More

(a) Possible tool motion; (b) work-piece motion The axis conventions of the following machines are identified and explained as follows; i) Centre Lathe It refers to a single point tool whose main purpose is to produce concentric work. The tool is ground to point that accurately positions the work piece on an axis. Centre Lathe operates through cylindrical turning where the work-piece rotates in a -C’ motion and the carriage moves in the -Z axis direction along the lathe bed as shown in figure 2 below[Kni06].

Figure 2: Axis convention for Center Lathe machine ii) Vertical Mill The vertical miller is a multipoint tool whose spindle is vertically oriented such that the milling cutters are held and rotate on its axis. A turret mill is an example of vertical mill and the cutting occurs in the vertical Z direction. The primary motion occurs in the –C axis direction while the continuous feed motion is in the –X’ direction as show in figure 3[Kni06]. Figure 3: Axis convention, Knee-type vertical mill iii) Surface Grinder The surface grinder provides a primary motion in the C axis direction due to the fact that it has a horizontal spindle.

The principal feed motion is a result of hydraulically operated worktable movement in the X’ axis direction. Further infeed may be provide in the –Y motion axis or cross feed in -Z’ axis direction as shown below. Figure 4: Axis convention on horizontal-spindle surface grinder b) How the Drives are controlled For the Centre Lathe machine, the operation is controlled through automated feed action whereby the primary motion (motion C’) is generated by a the movement of a series of gears automatically driven by motors.

The motion Z is automatically driven a lead screw and feed rod operated by gears that are mounted in the headstock[Kni06]. In the case of the vertical mill, vertical Z-motion is achieved by manually moving the knee up or down. The primary –C motion is automatically driven by a series of gears that are run by electric motors mounted on the headstock. The continuous feed motion (-X’ motion) can either be manual or automatically driven depending on the heaviness of the work-piece and the accuracy desired.

For the surface grinder, the principal feed motion is controlled by the traverse movement of the worktable on which the piece is mounted and is hydraulically operated. Infeed (-Y motion), is manually controlled through moving the wheel head down the column. Cross-feed (Z’ motion), on the other hand is controlled by moving the wheel head parallel to the spindle. Task Two: Tool and Work-holding Devices Center Lathe The most prevalent work holding method for the center lathe is the chuck that can have either three or four jaws depending on the type of work.

Three-jawed chucks are commonly used for cylindrical work pieces that require the machined surface to be concentric to the work surfaces. The jaws of the chuck have a series of teeth that mesh with grooves to ensure grip of the work piece. The jaws are equidistant from the chuck therefore the pieces are automatically centered. On the other hand, chucks with four jaws are used when the work piece is non-cylindrical[Kni06]. Pieces that have complicated shapes can be held with circular faceplates that has radial slots.

The slots provide a means of holding the work piece through bolting it to the faceplate. Collets are used for smaller lathes that are usually used on materials in form of bars. Collets are composed of sleeves that are split effectively and fit perfectly on the work. They generally have a tapper on their other surface. Collets are available in a range of standard forms based on the shape of the bars to be gripped. For instance, square and hexagonal collets for bars that are not in cylindrical form[Kni06].

A simple tool holder in the form of a typical tool-post can be used for holding this single-point tool. It is composed of a curved block that effectively rests on a concave spherical surface such that the tool can easily be inclined to the required height during operation.

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