Production Technology

What to remember

• Production technology covers shaping metals by casting, welding, forming and machining. The choice of process depends on the shape, material, number of parts and required accuracy.
• Cutting speed V = π D N ÷ 1000 (m/min), where D is the work diameter in mm and N is the speed in rpm.
• Limits and fits control interchangeability. Tolerance is the permitted variation of a size. Allowance is the intended difference between the mating parts.

Casting

In casting, molten metal is poured into a mould cavity and allowed to solidify. The main steps are pattern making, mould and core making, melting, pouring, solidification, cleaning and inspection.

Pattern is a replica of the part, made larger to allow for several allowances:

Pattern materials are wood (cheap and easy to shape, used for small numbers), metal (long life, for large numbers), plastic and wax. Pattern types are the single piece, split, loose piece, match plate and sweep patterns. A shrinkage scale (shrink rule) is longer than a standard scale. Patterns are colour-coded: red for surfaces to be machined, black for unmachined surfaces and yellow for core prints.

Moulding sand is silica sand mixed with a binder (clay, bentonite) and water. Properties needed: permeability (letting gases escape), cohesiveness, refractoriness (resisting high temperature), collapsibility and flowability. Cores form internal cavities, held in position by core prints and chaplets.

Gating system: pouring basin, sprue (vertical channel), runner, gates (ingates) and riser. The riser is a reservoir of liquid metal that feeds the casting when it shrinks. It should solidify last. Chills are metal pieces used to speed up the solidification of thick parts.

Casting processes:

• Sand casting: cheapest, suits all sizes and metals.
• Die casting: molten metal is forced into a metal die. It gives a good finish and accurate size, but is used for low melting alloys such as zinc and aluminium.
• Centrifugal casting: the mould is rotated to throw the metal outwards. It is used for pipes and cylinder liners.
• Investment (lost wax) casting: a wax pattern is coated with a ceramic slurry and melted out. It gives very fine detail.
• Shell moulding: a thin sand-resin shell is formed over a heated pattern.

Common defects: blow holes (trapped gas), shrinkage cavities, cold shut (two streams of metal do not fuse), misrun (the cavity is not filled), hot tears (cracks during cooling) and sand inclusion.

Welding, forming and sheet metal work

Welding joins metals by heat, with or without pressure, often with a filler.

Flame types in gas welding: neutral (equal gases, used for steel), carburising (excess acetylene, used for hard facing) and oxidising (excess oxygen, used for brass). In DC arc welding, straight polarity (electrode negative, DCSP) puts more heat on the work, giving deeper penetration. Reverse polarity (electrode positive, DCRP) puts more heat on the electrode, giving shallow penetration, and is used for thin sheets. Flux cleans the surface and protects the molten metal from oxidation. Welding defects: porosity, slag inclusion, undercut, incomplete penetration and cracks.

Hot working takes place above the recrystallisation temperature. Large deformations are easy, grains are refined and there is no work hardening. Cold working takes place below it. It gives a better finish and accuracy, and increases the strength through strain hardening but reduces ductility.

Forming operations:

• Forging: shaping by compressive hammering or pressing, usually hot. Types are open die and closed die (drop forging).
• Rolling: passing metal between rolls to reduce the thickness.
• Extrusion: forcing metal through a die to get a long uniform cross-section.
• Drawing: pulling a wire or tube through a die.
• Spinning, bending and deep drawing shape sheet metal.

Sheet metal operations: blanking (the cut piece is the product), piercing (the cut piece is waste, so a hole is made), shearing, bending and coining. Cutting force = perimeter of the cut × thickness × shear strength of material.

Worked example: A circular blank with a diameter of 50 mm and a thickness of 2 mm, with a shear strength of 300 N/mm², needs a force of π × 50 × 2 × 300 = 94,248 N, which is about 94.2 kN.

Machining and machine tools

The lathe is the main machine tool. Operations are turning, facing, taper turning, threading, drilling, boring, knurling and parting. The tool moves along the axis for turning. Parts: bed, headstock, tailstock, carriage and lead screw.

Basic formulae:

• Cutting speed V = π D N ÷ 1000 m/min.
• Machining time for turning: T = L ÷ (f × N), where L is the length of cut, f is the feed per revolution and N is the rpm.
• Material removal rate (turning) ≈ V × f × d, with V in mm/min, f the feed and d the depth of cut.

Worked example: For D = 60 mm and N = 400 rpm, V = π × 60 × 400 ÷ 1000 = 75.4 m/min. For L = 120 mm, f = 0.2 mm/rev and N = 300 rpm, T = 120 ÷ (0.2 × 300) = 2 minutes.

Other machines:

• Drilling machine: makes round holes with a twist drill. Reaming and tapping finish the hole and cut internal threads.
• Shaper: the tool reciprocates and the work moves slowly across. It cuts only on the forward stroke. A planer has the work reciprocating, used for large work.
• Milling machine: a rotating multi-tooth cutter. In up milling (conventional) the cutter rotates against the feed. In down milling (climb) it rotates along with the feed, and this gives a better surface finish.
• Grinding: an abrasive wheel removes small amounts of metal to give a fine finish and close tolerance.

Cutting tool materials: high carbon steel, high speed steel (HSS), cemented carbides, ceramics and diamond. Their hot hardness increases roughly in this order. Important tool angles on a single-point tool are the back rake angle, side rake angle, end relief angle, side relief angle and nose radius. Cutting fluids cool and lubricate the cutting zone.

Jigs and fixtures hold and locate workpieces. A jig guides the cutting tool (as in drilling). A fixture only holds the work and does not guide the tool (as in milling). The 3-2-1 principle locates a body using 3 pins on the base, 2 on the side and 1 at the end, to restrict 6 degrees of freedom.

Non-traditional machining: EDM (spark erosion), ECM (electrochemical), USM (ultrasonic), LBM (laser) and abrasive jet. They are used for very hard materials and complex shapes.

Powder metallurgy: metal powders are blended, compacted and sintered. It is used for self-lubricating bearings and carbide tools.

Heat treatment, limits, fits and measurement

Heat treatment changes properties through heating and cooling.

Limits, fits and tolerances (hole basis and shaft basis systems):

• Basic size is the nominal size. Tolerance = upper limit − lower limit. Allowance = the smallest hole size − the largest shaft size (when positive, it gives a clearance).
• Clearance fit: the shaft is always smaller than the hole. Interference (force or press) fit: the shaft is always larger than the hole. Transition fit: it may be either.
• In the hole basis system, the hole's lower deviation is zero and the shaft size is varied. This is more common because holes are harder to finish accurately.

Measuring instruments: Vernier caliper least count = 1 main scale division − 1 vernier scale division, usually 0.02 mm. Micrometer least count = pitch ÷ number of divisions on the thimble, usually 0.01 mm (pitch 0.5 mm with 50 divisions). Slip gauges give precise reference lengths. Go and no-go gauges check limits quickly.

Exam traps

• Blanking: the cut piece is the product. Piercing: the cut piece is waste.
• Shrinkage allowance makes the pattern larger, but shake allowance makes it slightly smaller.
• A jig guides the tool; a fixture does not.
• Hot working is above recrystallisation temperature and cold working is below it.
• DCSP gives deeper penetration. DCRP gives shallow penetration.
• Neutral flame is used for steel and carburising for hard facing.
• Up milling: the cutter rotates against the feed. Down milling: along with the feed.
• Allowance is the intended difference between mating parts. Tolerance is the permitted variation in one part.

One-liners

• 1. The riser is a reservoir that feeds the casting during shrinkage.
• 2. Draft allowance eases withdrawal of the pattern.
• 3. Spot welding is a resistance welding process.
• 4. Cutting speed V = π D N ÷ 1000.
• 5. A shaper cuts on the forward stroke only.
• 6. Annealing softens the metal.
• 7. Hardening is followed by tempering.
• 8. The vernier caliper least count is typically 0.02 mm.
• 9. The micrometer least count is typically 0.01 mm.
• 10. A clearance fit always leaves a gap between the shaft and the hole.
• 11. Die casting suits low melting point alloys.
• 12. A fixture holds the work but does not guide the tool.

Practice questions

1. The cutting speed for a 60 mm diameter job rotating at 400 rpm is about

   1. 150.8 m/min
   2. 24.0 m/min
   3. 7.54 m/min
   4. 75.4 m/min
   Answer

   D. 75.4 m/min

   V = π D N ÷ 1000 = π × 60 × 400 ÷ 1000 = 75.4 m/min.

2. To obtain a cutting speed of about 62.8 m/min on a 40 mm diameter bar, the spindle speed should be about

   1. 1000 rpm
   2. 250 rpm
   3. 1570 rpm
   4. 500 rpm
   Answer

   D. 500 rpm

   N = 1000 V ÷ (π D) = 62,800 ÷ (π × 40) = 500 rpm.

3. The machining time to turn a length of 120 mm at a feed of 0.2 mm/rev and 300 rpm is

   1. 0.5 min
   2. 2 min
   3. 1 min
   4. 4 min
   Answer

   B. 2 min

   T = L ÷ (f N) = 120 ÷ (0.2 × 300) = 2 min.

4. The force needed to blank a circular disc of diameter 50 mm from 2 mm thick sheet with a shear strength of 300 N/mm² is about

   1. 188.5 kN
   2. 47.1 kN
   3. 30.0 kN
   4. 94.2 kN
   Answer

   D. 94.2 kN

   F = π d t τ = π × 50 × 2 × 300 = 94,248 N.

5. A micrometer screw has a pitch of 0.5 mm and 50 divisions on the thimble. Its least count is

   1. 0.1 mm
   2. 0.02 mm
   3. 0.01 mm
   4. 0.05 mm
   Answer

   C. 0.01 mm

   Least count = pitch ÷ divisions = 0.5 ÷ 50 = 0.01 mm.

6. On a vernier caliper, 50 vernier divisions equal 49 main scale divisions of 1 mm each. The least count is

   1. 0.1 mm
   2. 0.02 mm
   3. 0.05 mm
   4. 0.01 mm
   Answer

   B. 0.02 mm

   LC = 1 − 49 ÷ 50 = 0.02 mm.

7. A shaft has upper and lower limits of 50.05 mm and 49.95 mm. Its tolerance is

   1. 100 mm
   2. 0.10 mm
   3. 0.01 mm
   4. 0.05 mm
   Answer

   B. 0.10 mm

   Tolerance = upper limit − lower limit = 50.05 − 49.95 = 0.10 mm.

8. The smallest hole is 50.00 mm and the largest shaft is 49.95 mm. The allowance is

   1. 0.15 mm
   2. 0.01 mm
   3. 0.10 mm
   4. 0.05 mm
   Answer

   D. 0.05 mm

   Allowance = smallest hole − largest shaft = 0.05 mm.

9. A casting is to be 500 mm long, with a shrinkage allowance of 1%. The pattern length should be

   1. 500 mm
   2. 510 mm
   3. 505 mm
   4. 495 mm
   Answer

   C. 505 mm

   Pattern is larger by 1%: 500 × 1.01 = 505 mm.

10. The force to pierce a 20 mm hole in 3 mm plate with a shear strength of 250 N/mm² is about

    1. 47.1 kN
    2. 15.0 kN
    3. 94.2 kN
    4. 23.6 kN
    Answer

    A. 47.1 kN

    F = π d t τ = π × 20 × 3 × 250 = 47,124 N.

11. A turning cut has V = 50 m/min, feed 0.2 mm/rev and depth of cut 2 mm. The metal removal rate is about

    1. 10,000 mm³/min
    2. 2,000 mm³/min
    3. 200 mm³/min
    4. 20,000 mm³/min
    Answer

    D. 20,000 mm³/min

    MRR = V × f × d = 50,000 × 0.2 × 2 = 20,000 mm³/min.

12. A 100 mm bar turned at 318 rpm has a cutting speed of about

    1. 318 m/min
    2. 100 m/min
    3. 31.8 m/min
    4. 10 m/min
    Answer

    B. 100 m/min

    V = π × 100 × 318 ÷ 1000 = 99.9 m/min.

13. A 60 mm deep hole is drilled at a feed of 0.3 mm/rev and 200 rpm. The drilling time is about

    1. 0.2 min
    2. 0.5 min
    3. 1 min
    4. 2 min
    Answer

    C. 1 min

    T = L ÷ (f N) = 60 ÷ (0.3 × 200) = 1 min.

14. A hole has limits 50.00 to 50.05 mm and a shaft has limits 49.90 to 49.95 mm. This fit is a

    1. transition fit
    2. force fit
    3. clearance fit
    4. interference fit
    Answer

    C. clearance fit

    The largest shaft (49.95) is smaller than the smallest hole (50.00), so there is always a gap.

15. The allowance given to compensate for the contraction of metal on cooling is called the

    1. machining allowance
    2. shrinkage allowance
    3. draft allowance
    4. shake allowance
    Answer

    B. shrinkage allowance

    The pattern is made larger because the metal contracts on cooling.

16. The shake (rapping) allowance on a pattern is

    1. a negative allowance
    2. a positive allowance
    3. zero
    4. made only for cores
    Answer

    A. a negative allowance

    Rapping enlarges the cavity slightly, so the pattern is made a little smaller.

17. The purpose of a riser in casting is to

    1. cool the casting faster
    2. let the air in
    3. hold the core
    4. feed molten metal to the casting as it shrinks
    Answer

    D. feed molten metal to the casting as it shrinks

    The riser is a reservoir that solidifies last.

18. The ability of moulding sand to let gases escape is called

    1. refractoriness
    2. permeability
    3. collapsibility
    4. cohesiveness
    Answer

    B. permeability

    Permeability prevents blow holes.

19. Which casting process is mainly used for making pipes?

    1. Investment casting
    2. Shell moulding
    3. Pressure die casting
    4. Centrifugal casting
    Answer

    D. Centrifugal casting

    Rotation of the mould throws metal outwards, forming the pipe.

20. Lost wax casting is another name for

    1. die casting
    2. centrifugal casting
    3. investment casting
    4. shell moulding
    Answer

    C. investment casting

    A wax pattern is coated with ceramic and melted out.

21. A cold shut defect is caused by

    1. two streams of metal failing to fuse
    2. trapped gases
    3. metal not filling the mould
    4. contraction cracks
    Answer

    A. two streams of metal failing to fuse

    Cold shuts occur when metal streams meet but do not join.

22. For welding steel with oxy-acetylene, the flame used is

    1. carburising
    2. oxidising
    3. neutral
    4. reducing with excess oxygen
    Answer

    C. neutral

    A neutral flame has equal oxygen and acetylene.

23. Spot welding is an example of

    1. gas welding
    2. brazing
    3. arc welding
    4. resistance welding
    Answer

    D. resistance welding

    Heat is produced by I²R and pressure joins the parts without filler.

24. In TIG welding, the electrode is

    1. non-consumable tungsten
    2. consumable wire
    3. carbon steel rod
    4. copper wire
    Answer

    A. non-consumable tungsten

    Inert gas shields the weld and a filler rod is added separately.

25. Hot working is carried out

    1. below the recrystallisation temperature
    2. above the recrystallisation temperature
    3. at room temperature only
    4. at absolute zero
    Answer

    B. above the recrystallisation temperature

    Strain hardening is removed as the grains recrystallise.

26. In sheet metal work, the operation in which the cut-out piece is the required product is

    1. bending
    2. blanking
    3. piercing
    4. coining
    Answer

    B. blanking

    In piercing the cut-out piece is scrap and the sheet with a hole is the product.

27. In up milling (conventional milling), the cutter rotates

    1. along the direction of feed
    2. at right angles to the feed
    3. only at slow speed
    4. against the direction of feed
    Answer

    D. against the direction of feed

    In down milling the cutter rotates in the feed direction.

28. A shaper cuts metal

    1. only on the forward stroke
    2. only on the return stroke
    3. on both strokes
    4. while rotating
    Answer

    A. only on the forward stroke

    The return stroke is idle.

29. A jig differs from a fixture because a jig

    1. clamps only
    2. guides the cutting tool
    3. is used only in welding
    4. is used only for inspection
    Answer

    B. guides the cutting tool

    A fixture holds the work but does not guide the tool.

30. Annealing is mainly done to

    1. coat the surface
    2. harden the metal
    3. soften the metal and relieve stresses
    4. increase brittleness
    Answer

    C. soften the metal and relieve stresses

    The metal is cooled slowly in the furnace.

31. Tempering is carried out on

    1. hardened steel to reduce brittleness
    2. cast iron to harden it
    3. soft steel to soften it
    4. aluminium to harden it
    Answer

    A. hardened steel to reduce brittleness

    Reheating below the critical temperature gives toughness.

32. A transition fit may give

    1. always interference
    2. always clearance
    3. either clearance or interference
    4. no contact at all
    Answer

    C. either clearance or interference

    The tolerance zones of hole and shaft overlap.

33. Which of the following statements is/are correct? 1. Draft allowance helps the withdrawal of the pattern from the mould. 2. Shrinkage allowance makes the pattern smaller than the casting.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Shrinkage allowance makes the pattern larger.

34. Which of the following statements is/are correct? 1. The sprue is the vertical channel through which metal enters the mould. 2. The riser is the reservoir feeding the casting.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are parts of the gating system.

35. Which of the following statements is/are correct? 1. Hot working causes strain hardening. 2. Cold working gives a better surface finish.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Hot working does not cause strain hardening.

36. Which of the following statements is/are correct? 1. In blanking the cut piece is the product. 2. In piercing the cut piece is scrap.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    These define the two operations.

37. Which of the following statements is/are correct? 1. A jig guides the cutting tool. 2. A fixture also guides the tool.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A fixture only holds and locates the work.

38. Which of the following statements is/are correct? 1. An oxidising flame is used for brass. 2. A carburising flame has an excess of oxygen.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A carburising flame has excess acetylene.

39. Which of the following statements is/are correct? 1. Straight polarity (DCSP) gives deeper penetration. 2. Reverse polarity (DCRP) gives shallow penetration.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    In DCSP more heat falls on the work.

40. Which of the following statements is/are correct? 1. In up milling the cutter rotates along the feed direction. 2. Down milling rotates the cutter against the feed.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Both are reversed.

41. Which of the following statements is/are correct? 1. In the hole basis system the lower deviation of the hole is zero. 2. In a clearance fit the shaft is always larger than the hole.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    In a clearance fit the shaft is smaller than the hole.

42. Which of the following statements is/are correct? 1. Annealing softens the metal. 2. Normalising cools the metal in still air.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Annealing uses slow furnace cooling.

43. Which of the following statements is/are correct? 1. EDM works by spark erosion. 2. ECM uses a rotating abrasive wheel.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    ECM uses electrochemical dissolution.

44. Match the casting process with its use: (a) Centrifugal casting (b) Investment casting (c) Die casting. Uses: (1) pipes (2) fine detail with a wax pattern (3) low melting alloys in a metal die

    1. a-3, b-2, c-1
    2. a-2, b-1, c-3
    3. a-1, b-3, c-2
    4. a-1, b-2, c-3
    Answer

    D. a-1, b-2, c-3

    Centrifugal for pipes, investment for fine detail, die casting for zinc and aluminium.

45. Match the heat treatment with its feature: (a) Annealing (b) Normalising (c) Tempering. Features: (1) cooling in still air (2) slow furnace cooling (3) reheating of hardened steel

    1. a-1, b-2, c-3
    2. a-2, b-3, c-1
    3. a-3, b-1, c-2
    4. a-2, b-1, c-3
    Answer

    D. a-2, b-1, c-3

    Annealing: furnace cooling; normalising: air cooling; tempering: reheating after hardening.