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Machining

Leaving cert Engineering HL Q7

QuestionAnswer
What is Gang milling A number of milling cutters mounted side by side on an arbor on the horizontal milling machine
What is Straddle milling 2 milling cutters mounted on an arbor with a spacer in between on the horizontal milling machine
What is Loading Small pieces of debris from the work piece get caught in between the abrasive grains on the grinding wheel. The cutting edges on the abrasive grains are covered up and the work piece will get burned.
What is Glazing The abrasive grains on the grinding wheel go blunt but are not released. This happens when the bond is too strong. The smooth edges will not cut the work piece, it will only burn it.
What is an End Mill This is a milling cutter that has 4 cutting edges. It can mill slots but cannot drill holes as the cutting edges do not meet in the middle.
What is a Slot drill This is a milling cutter that has 2 cutting edges. It can mill slots and drill holes. The 2 cutting edges meet in the middle like a drill bit.
What is Up cut milling Horizontal milling process. The cut starts thin and finishes thick. The work piece is fed against the direction of the cutter. Leaves a slightly rougher finish
What is Down cut milling Horizontal milling process. The cut starts thick & finishes thin. The work piece is fed in the same direction as the cutter. This leads to vibration in the machine. A backlash eliminator is needed to stop this vibration. Leaves a very good surface finish.
What does the term Self Dressing mean in relation to a grinding wheel This is where the bond in the grinding wheel releases worm grains on the outside of the grinding wheel to uncover fresh sharp grains so that the wheel can keep grinding correctly
What is Continuous chip Long continuous spirals of the material cut from the work piece when machining. This happens to soft, ductile materials such as Aluminium
What is Discontinuous chip Very small chips of material (like filings) cut from the work piece when machining. This happens to more brittle materials such as brass
What is Built up edge This is when some chip gets welded to the tip of the cutting tool. As the built-up edge enlarges, the cutting edge will not cut effectively as the tool cutting angles are compromised. The surface finish will deteriorate and machining will not be smooth.
Compare Safety Hazard vs Safety precaution Hazard - Dangerous things that can happen to you when using machines or tools Precautions - Things you can do to protect yourself from hazzards
What is Surface grinding A metal cutting process in which flat and extremely smooth surfaces are produced. The grinding wheel rotates, the wpis held on a magnetic chuck and is fed to and fro continuously. The grinding wheel can be lowered to take a new cut.
What is Countersinking This is the enlarging of the mouths of holes to allow countersunk head screws or rivets sit flush with the surface of the piece.
What is Counterboring This is increasing the diameter of the hole to a certain depth to allow a cheese head screw sit flush with the surface of the piece
What is a Plug gauge The function Plug gauge is to accurately determine if a selected hole is within a specific range of limits.
What is a Gap gauge The function of a Gap gauge is to accurately check if an external diameter is within a specific range of limits.
Name 2 lubricating materials used in machines Oil and grease
Outline the functions of the chuck guard on the lathe: - Protect user from cutting chips and swarf - Interlock guards will prevent the machine operating when not in place - Allows a viewing window to safely monitor machining - Protects the user from rotating chuck
Outline the important properties of cutting tool materials. They must be harder than the material being machined. They must maintain a sharp edge at high temperatures. They need to have the ability to be sharpened or replaced easily. They need to be able to withstand the shock loads of machining processes.
Outline the safety features integrated into a milling machine. • Moving parts must be guarded (usually by interlocking guards). • Stopping controls are to be prominent and accessible. • A braking system that will stop the cutter quickly. • Minimal vibration due to robust construction.
Why do you need to dress a grinding wheel with a dressing tool Wheel dressing will renew a sharp cutting face and correct irregularities such as wheel concentricity, loaded and glazed wheels. The process can remove any undulations from the wheel.
Compare Forming vs Generating >Forming is when the surface produced is a copy of the tool producing it. eg Contour work and screwcutting >Generating moves the tool in various directions until the required surface is machined. eg Facing and taper turning
Discuss two reasons why inaccuracies may occur during precision measurement. Inaccuracies may occur with the use of incorrectly set measuring tools >lack of care in reading equipment >equipment not sufficiently accurate ?surface debris and roughness.
Outline the function of cutting fluids Reducing the build up of heat. Washing chips and swarf away from the cutting edge. May have lubrication action to improve machining and surface finish. Reduces vibration in machine. Prolongs tool and machine life. Allows higher cutting speeds.
Name • a single point cutting tool • a multi-point cutting tool • an abrasive cutting tool. >Parting off tool >Drill bit >Grinding wheel
Outline the advantages of Tungsten Carbide cutting tools stay sharp at high temperatures >ease of replacement >multiple cutting points on each tool >cutting angles are ground into inserts
List the safety features of the centre lathe • chuck guard • Stopping controls are to be prominent and accessible. • A braking system that will stop the chuck quickly. • Minimal vibration due to robust construction.
Outline three methods of machining flat surfaces on metals. • Milling; • Surface grinding; • Facing on the lathe;
List the disadvantages of Tungsten carbide cutting tools expensive to replace >brittle when subjected to shock load >limited use on materials other than round bars
List the safety hazards associated with using a bench grinder - WP can heat up - Clothes getting caught in the wheel - Small pieces of debris coming out at high speed - wheel must be balanced to avoid vibration
Outline the function of a dividing head on a milling machine; The dividing head is used for accurate spacing of cuts on a workpiece, for example, holes on a pitch circle, gear teeth or slots on the periphery of a disk.
Explain the term tolerance in engineering measurement systems; A tolerance is the extent by which a dimension is allowed to deviate from the nominal or basic size. If a nominal size of an object is 20.00mm with a tolerance of 0.15 then the upper limit is 20.15mm and the lower limit is 19.85mm.
List the factors that influence metal surface finish when parallel turning on the lathe Surface finish is influenced by machine speed, tool feed, machine condition, vibration and type of metal to be cut.
State one advantage and one disadvantage of using magnetic chucks on a grinding machine. >Advantages: quick clamping with a firm grip, can accommodate large batches of workpieces, can often accommodate unusual workpiece shapes >Disadvantages: metal needs to be magnetic, the workpiece must have a flat surface
Identify two safety precautions that reduce the formation of a built-up edge while turning on a centre lathe; • Choose suitable cutting tools for each machining process. • Run the machine at the correct speed. • Ensure that the machine is in good condition and not prone to excessive vibration. • Use the correct cutting feed for the material.
Distinguish between oblique cutting and orthogonal cutting; Orthogonal cutting has two forces, the tangential and axial force acting on the cutting tool. Oblique cutting consists of a three force system. As the radial force increases, the axial force decreases.
Explain one main function of a feeler gauge; Used to gauge small gaps or spaces between components
State the function of a reamer Produce accurate holes with a good surface finish.
Describe the main features of a Morse taper sleeve; Morse taper shanks are used to fix components such as drills and centres in to machines, they come in a variety of sizes. To increase a morse taper size, a sleeve can be used.
Outline the function of a backlash eliminator The backlash eliminator removes the free play in the leadscrew during down cut milling. This reduces the vibration in the machine.Down cut milling should not be done on a machine without a backlash eliminator fitted.
State two advantages of a keyless chuck You won't loose the chuck key. The teeth on a chuck key can wear away and make it difficult to tighten the chuck, this doesn't happen with a keyless chuck
Explain the term rancidity in relation to cutting fluids |Caused by bacteria and other microscopic organisms, growing causing bad odors to form. bactericides can control growth of bacteria and make fluids more resistant to rancidity. Prevented by continuous filtering of the cutting fluid and changing it
Identify two types of cutting fluids Soluble oils, pastes, synthetic fluids, oils, etc.
Identify three safety features integrated into a lathe. • Emergency stop button. • Chuck interlocking guard. • Leadscrew guard. • Emergency foot brake pedal.
Explain how measurement errors may occur when using digital vernier calipers. • The callipers maybe “zero” when the jaws are open. • The battery may be weak. • Part of the jaws maybe damaged or chipped. • The locknut may not be tightened and jaw can move.
Outline one use of a vee-block and clamp. Vee block and clamp are precision metalworking jigs typically used to hold round metal rods or pipes for performing drilling or milling operations.
Distinguish between a pilot hole and a tapping size hole. Pilot Hole is a hole which is drilled prior to a larger hole being drilled. Tapping Size Hole is a hole drilled prior to threading. The hole is always smaller than the tap to be used.
Explain the difference between a rake angle and a clearance angle in cutting tools. The rake angle facilitates the lifting of the chip during cutting. A rake angle can be positive, negative or zero. Clearance angle is formed to allow one point of the cutting contact the workpiece. It prevents rubbing off the work piece.
Outline two reasons why a sealed lubrication system is used in engineering machines. Lubrication systems reduce friction between moving parts of machines. Machine parts will last longer. Heat will be dissipated. Sealed systems ensure that lubrication materials do not escape.
Identify any two lubricating materials commonly used in engineering machines. Oil, grease, graphite, PTFE, etc.
Describe briefly how tungsten carbide inserts are manufactured. cobalt and tungsten powder are mixed with cemented carbide and pressed into shape. There are several stages of cooking and heating (too long to explain here)
Explain the term tolerance used in metrology A tolerance is the extent by which a dimension is allowed to deviate from the nominal or basic size. If a nominal size of an object is 20.00mm with a tolerance of ±0.15 then the upper limit is 20.15mm and the lower limit is 19.85mm.
Identify two types of cutting fluids Soluble oils, pastes, synthetic fluids, oils, etc
Describe the impact of cutting fluids on: • metal cutting machines and • metal cutting efficiency. Metal cutting machines: prolong machine life, reduces vibration, maintains tool sharpness, etc. Metal cutting efficiency: makes metal cutting more effective, reduces heat generated, improves finish, etc.
Explain, with the aid of a diagram(s), the basic operation of a magnetic chuck. Consists of 2 plates with magnetic inserts. One permanent and one sliding. When out of line, the magnetic fields are contained within the chuck. When in line, the magnetic fields extend outside the chuck and grip the work piece. Used on steel only.
State one advantage and one disadvantage of magnetic chucks • They provide quick clamping with a firm grip. • They can accommodate large batches of workpieces. • They can often accommodate unusual workpiece shapes. • The metals need to be magnetic. • The WP must have a flat surface
Identify two materials which may produce a continuous chip formation when machined Aluminium • Copper • Steel • Nylon • Acetal
Explain the impact of continuous chip formation on: • Surface finish • Automated machinery Continuous chips are more likely to produce a stable cutting action which results in a good surface finish. For automated machinery, the chip can wrap around the workpiece and get in the way of the cutting process.
Explain the difference between a dovetail cutter and a slot drill in milling. Dovetail cutter will machine an angled profile suitable for dovetail joints. Slot drill will machine a slot in the middle of a piece to a certain depth or it can also machine an edge profile.
Outline one reason for balancing a grinding wheel If a grinding wheel is out of balance the spindle will vibrate and a poor surface finish will result. This has consequences for machine safety. A wheel is placed on a balancing stand and weights are moved around the flange to balance the wheel.
Explain why a three-jaw chuck is not suitable for holding a square bar. A three-jaw chuck has its jaws at 120° whereas a square bar has it sides at 90° therefore the square bar will not be held on center in a three-jaw chuck
Describe the use of a fixed steady when operating a lathe. The fixed steady provides support for lengthy work which needs an intermediate support to prevent whip or wobble. This is useful where long lengths of bar are being machined at either end and need to be securely supported.
Created by: todd.ciaran