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Classification of machining processes
Introduction to Machining
IntroductionAll machining processes remove material to form shapes. As metals are still the most widely used materials in manufacturing, machining processes are usually used for metals. However, machining can also be used to shape plastics and other materials which are becoming more widespread.
Basically all the different forms of machining involve removing material from a component using a rotating cutter. The differences between the various types arise from the relative motion between cutting tool and workpiece and the type of cutting tool used.
Typically machining will be done using a machine tool. This tool holds the workpiece and the rotating cutting tool and allows relative movement between the two. Usually machine tools are dedicated to one type of machining operation, although some more flexible tools allow more than one type of machining to be performed. The machine tool can either be under manual or automatic (Computer Numeric Control - CNC) control. Automatic control is more expensive because of the need to invest in the necessary control mechanisms however it becomes more desirable as the number of components produced increases and labour costs can be reduced.
The speed at which a machine tool can process individual components is a function of the cutting speed of the tool and the downtime involved in changing the workpiece and maintaining the tool (this will usually involve changing the cutting edges of the tool). Some very flexible tools allow automatic changing of components and cutting tools, however they greatly add to initial purchase price of the machine tool.
The cutting speed of the tool is usually dictated by the type of material being machined, in general the harder the material, the slower the machining time. Machining speed can be increased by increasing the rotational speed of the cutter, however this will be at the expense of the tool life. Hence for machining processes there is an optimum cutting speed that balances tooling costs with cutting speed.
In order to dissipate the heat generated between the workpiece and the cutting tool, cutting fluids are sprayed onto the tool. The cutting fluid also acts to remove cut material away from the cutting region and lubricates the tool - workpiece interface but may require that the component is cleaned afterwards.
- Machining processes allow high precision components to be rapidly produced.
- Machining processes are not suitable for removing large amounts of mateial.
- There can be a large amount of wastage.
Types of machining operationThere are a number of different types of machining operations available to for removal of material. These include,
Processes which remove metal from a workpiece can be broadly classified into three main groups i.e Chip Removal, Chipless removal and metal removed by heat.
Chip Removal Processes..
| || |
| Automation /CNC control || Modern machine tools are generally controlled using CNC /DNC.. |
| Drilling || Drilling is the production of holes.. |
| Turning || The workpiece is rotated around its axis and a cutting tool is fed parallel to the axis to create a cylinder or at right angles to the axis to create a face |
| Milling || Milling involves feeding the workpiece past a rotating cutter with cutting edges on its side or end or both |
| Grinding || Used to produce a good accurate surface finish...|
| Shaping || Used to produce flat surfaces by reciprocating tool motion...|
| Planing || Used to produce flat surfaces by reciprocating Workpiece motion...|
| Saws || Saws are used to cut short lengths of long sections.. |
| Drilling Process|
The drilling machine (drill press) is a single purpose machine for the production of holes. Drilling is generally the best method of producing holes. The drill is a cylinderical bar with helical flutes and radial cutting edges at one end. The drilling operation simply consist of rotating the drill and feeding it into the workpiece being drilled. The process is simple and reasonably accurate and the drill is easily controlled both in cutting speed and feed rate. The drill is probably one of the original machining processes and is the most widely used.
Drilling machine -important features/dimensions
Normal pillar drilling machines (Drill Press) are specified basically by the size of hole the machine can drill in Mild Steel i.e a 16mm machine can drill holes upto and including 16mm dia in mild steel. The speed range of a drilling machine is related to the size e.g. machines for small holes down to 1mm can have speed ranges up to 8000rpm. Larger drilling machines more suited for drilling holes. up to 25mm will have a more limited range. A machine which is used to drill larger holes ( >15mm) is not generally suitable for drilling small diameter holes (< 1 mm). Smaller machines are provided with permanent chucks whilst larger machines gnerally include morse tapers for fixing the drills.
Most pillar drills are manually fed using a rotating lever driving the vertical motion of the spindle. Larger machines can have power drives feeds.
A belt driven spindle is often a convenient low cost option but there is a tendency in modern times to use geared /inverter drives.
When drilling holes in a material a number of factors should be considered including
The machines below are the classical designs. The modern trend is towards machining centres which are CNC controlled machines with tool changing facilities and ability to perform multiple machining operations including drilling.
Notes on Selection of Drilling Machines
The most common form of drilling machine is the bench drill. As the name implies this machine is normally bolted down to a bench. The workpiece can be clamped onto the worktable or onto the base. Tee slots are normally provided for this function. The worktable can be moved up and down the vertical column. The worktable can be clamped at the selected height. The drill is normally located in a three jaw chuck which is rotated by the drive system. The figure below shows a belt drive. Modern bench drills are driven by more sophisticated arrangements. The chuck is moved up and down by a feed handle which drives rotating spindle via a rack and pinion mechanism.
The pillar drill has the same features as the bench drill. This drill is however free standing and is of a far heavier construction able to take larger drills. The larger drills normally have taper shanks which are located within a taper bore in the spindle end. These tapers are standardised as morse tapers.
The radial drill is a free standing and the workpiece is clamped in position on the base. The drill head is positioned using motorised drives.
There are two common types of twist drills, high-speed steel drills, and carbide-tipped drills. The most common type used for normal workshop practice is the high-speed steel twist drill because of its low cost. Carbide-tipped metal drills are used in production work where the drill must remain sharp for extended periods, such as in a numerically controlled drilling machine. Other types of drills available include solid carbide drills, TiN coated drills, diamond drills etc. etc.
Twist drills shanks are either straight shank or tapered shank (Morse taper). Straight shank twist drills are usually 12mm or smaller and are gripped in the drill chucks. Tapered shank drills are usually for the larger drills that need more strength which is provided by the taper socket chucks.
Common twist drill sizes range from 0.3mm to 90mm in diameter. Larger holes are cut by special drills that are not considered as twist drills.
Types of Drills Bits
Typical Drilling Processes
The notes below ralate to HSS drills. For drills manufactured with more exotic material combinations much higher feed and speed rates are viable
Drilling feeds range from 0,03m to 0,5mm rev the feed rate being higher as the drill size increases from say 1mm to 60mm.
Table of drilling speeds
Turning/ Lathes etc
| Introduction The lathe is available in many forms as listed below. All lathes are based upon the centre lathe as shown in the figure below; The basic operations that can be carried out on lathes include:-Turning, Facing, Boring, Drilling, Reaming, Counterboring, Countersinking, Threading, Knurling and Parting.|
Types of Lathes
All of the important features of the lathe are shown on the above figure.
The headstock spindle which is hollow to allow bar stock to be fed through is generally provided with a special male thread to allow chucks etc. to be easily fitted and removed. Some of the different work holding units which can be fitted onto the headstock spindle are shown below.
Work Holding Devices
Cutting Tool Terms
The cutting speed is the speed at which the work surface passes the cutting tool. This is mainly dependent on the material being turned and the cutting tool material. The table below indicates some approximate cutting speed for turning on a lathe.
For information on cutting materials refer Cutting Materials. For information on cutting fluids refer Cutting Fluids
A milling machine is a machine tool that cuts metal with a multiple-tooth cutting tool called a milling cutter. The workpiece is fastened to the milling machine table and is fed against the revolving milling cutter. The milling cutters can have cutting teeth on the periphery or sidesor both. The cutting teeth can be straight or spiral.
Milling machines can be classified under three main headings:..
Cutting Tool Materials... Cutting Tool Materials..
Cutting Fluids... Cutting Fluids..
There are a wide range of milling cutters as illustrated in BS 122-1. these include
Tee Slot Milling Cutter
Shell End Mill
High Helix Cylinderical Cutter
BS 122-1:1953..Milling cutters. Milling cutters
BS 122-4:1980..Milling cutters. Screwed shank end mills and slot drills
BS 122-5:1986..Milling cutters. Specification for mounting dimensions and accessories
BS 122-6:1995, ISO 240:1994..Milling cutters. Specification for dimensions for interchangeability of cutter arbors and cutter mandrels
Column and Knee Machine -Vertical Milling MachineVertical milling machines have their cutting tool spindles vertical and are characterised by by their heavy overarms which are integral with their columns. This provides rigid support for the spindle housing which is set at a fixed distance from from the column. The milling head can be adjusted vertically
Column and Knee Machine - Horizontal Milling MachineThis is the basic milling machine configuration. The workpiece can be fed in all three axes and is suitable for short production runs but it is not as rigid as a fixed bed machine and should not be used for heavy duty work. Because of the large number of controls it is relatively slow to operate.
Fixed Bed Milling MachinesFixed bed or production type machines are designed for rapid metal removal requiring the minimum of operator involvement. On this type of machine the table is fixed permitting machine table movement in one horizontal direction only. the workpiece has to be fixed in one location on the table and all vertical movements for feeds and lateral positioning are by moving the spindle in the spindle carrier mounted on the headstock. A plain machine has just one spindle and a duplex machine has two spindles. this type of machine can have one column or two columns providing more rigidity.
Cutting speed HSS cutting tools , RoughingThe table below provides some very approximate cutting speed for different materials using HSS cutters on roughing cuts. For finishing cuts the speeds would increase 20% to 50%. For cermet (cemented carbide)tipped teeth significantly higher cutting speeds are possible see table below.
Cermet Surface Speed /Feed RatesThe table below provides some indicative values for the surface speeds and feeds (mm/tooth) that can be achieved using cermet type e.g cemented carbide tipped milling cutters.
Grinding /Abrasive Machining
Abrasive MachiningAbrasive machining uses hard non-metallic particles to machine the work surface. Relevant processes include grinding, honing, superfinishing / abrasive belt machining and honing. The first three processes uses abrasive particles (grit), rigidly held in a wheel, stone, or belt. Lapping is based on the abrasive particles being retained in a fluid.
GrindingThe grinding process is used to produce a high surface finish with a close tolerance and for machining hard materials. The process is a variation of polishing using abrasive materials held together by an adhesive generally in the form of a solid wheel. The wheel is rotated at high speeds and the circumferential surface of the rotating wheel is brought into contact with the material being machined.
Different Grinding Processes
Surface GrinderThe surface grinder is used in the toolroom for the production of accurate flat surfaces. This machine has a similar layout to the horizontal milling machine but only removes small thicknesses of material on the grinding passes.
The Cylindrical Grinding MachineThis machine is use to generate cylindrical surfaces and is similar to a centre lathe in appearance. The main difference is that the tool is replaced by a rotating grinding wheel.
The Vertical Grinding MachineInformation to be added...
The Centreless Grinding machineThis type of grinding machine is for pure cylindrical prism shapes which do not required mounting. The workpieces are fed through two parallel rotating wheels: a conventional grinding wheel and a rubberised regulating wheel. The regulating wheel is inclined at a slight angle to facilitate axial movement (like a screw thread).
Shaping Machine Tools
| The shaping process was used in the early days of machine tools. Shaping machines are not widely used now. Shaping produces flat surfaces by moving a single point cutting tool in a recprocating motion. The forward stroke is the cutting stroke the backward movement is completed at a faster velocity and includes the sideways feed motion. |
The shaping machine is a versitile machine able to produce flat surfaces , grooves, T-Slots, dovetails, and may be used to produce curved surfaces.
The size of components that may be machined is normally limited by the length of the stroke of the shaping machine which can vary up to a maximum of about 1500mm. (750mm is normally the maximum stroke. The shaping machine is not generally used as a production tool because of its slow cutting speed and the unproductive return stroke.
Planing Machine Tools
| Planing is used for the production of flat surfaces. The workpiece is clamped onto the worktable and the worktable is reciprocated while the tool is held stationary. The tool is only moved to provide a feed when the workpiece is moving on the return stroke.|
The worktable moves on hardened ways and is designed for large size work. As the tool post and the bedplate are designed to be very rigid the planer can take very heavy cuts and can machine very accurately. (0,5mm to 0,075mm).
The largest length of workpiece is limited by the table stroke and the largest section is limited by the size of the toolhead. The width of worktable can be up to 2,5m and the length of strokecan be up to 7m.
Planer MillersThe planer millers have all the features of planing machines but include a milling cutting head in place to the single point cutting tools
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