Lecture slides on drilling and milling By: Dr H N Dhakal Lecturer in Mechanical and Marine Engineering, School of Engineering, University of Plymouth
Drilling Hole making is among the most important operations in manufacturing, and drilling is a major and common hole-making process. In drilling, the surface of interest is the side wall of the hole, which is burnished by the rubbing action of the twisted flutes of the drill and the material that escapes outward.
Drilling machine configuration Drilling is the most common (multi-point) cutting technique targeted for the production of small-diameter holes: Yet, the complexity of its tool geometry makes it one of the most complex operations to model mathematically: Fixed head Adjustable head Spindle Tool
Column
Worktable
Workpiece
Base
Drilling machines The types of drilling machine range from simple bench type drills used to drill small diameter holes to large radial drills which can accommodate large workpieces. Also three axis CNC drilling machines has turret that holds as many as eight different tools, such as drills, taps and reamers There are various types of drill machines. The most common machine is the drill press. The workpiece is placed on the adjustable table, either by clamping or directly into the slots and holes on the table or by using a vice which is in turn clamped to the table.
Common types of drills
Figure: Two common types of drills: (a) Chisel-point drill (b) crankshaft drill Source: S. Kalpakjian, S.R. Schmid: Manufacturing Engineering & Technology, 5th edition, Prentice-Hall International, 2006; pp. 705.
Various types of drills
Reamer, adjustable
drill and counterbore
Various types of drills The most common drill is the conventional standard-point twist drill. The geometry of the drill point is such that the normal rake angle and the velocity of the cutting edge vary with the distance from the centre of the drill. The main features of the twist drills are: a. point angle 118135o; b. lip-relief angle 7-15o, c. chisel edge angle 125- 135o; d. helix angle 15-30o
Various types of drills and operations
Source: S. Kalpakjian, S.R. Schmid: Manufacturing Engineering & Technology, 5th edition, Prentice-Hall International, 2006; pp. 707.
Various types of drills and operations A step drill produces holes with two or more different diameters. A core drill is used to make an existing hole larger. Counterboring and countershinking drills produce depressions on the surface to accommodate the heads of screws and bolts below the workpiece A centre drill is short and is used to produce a hole at the end of a piece of stock, so that it may be mounted between centres of the head stock and tailstock of a lathe . A spot drill is used to spot (to start) a hole at the desired location on a surface.
Various types of drills and operations Solid carbide and carbide tipped drills are available for drilling hard materials such as cast irons, concrete and bricks and composite materials with abrasive fibre reinforcements. Gun drilling is used for drilling deep holes and requires a special drill.
Drilling cutting velocity Drilling:
π N dt V= 1000
N: spindle speed (rev/min); dt: (drill) tool diameter (mm);
(mm/min)
MRR in drilling The material removal rate (MRR) in drilling is the volume of material removed by per unit time. For a drill with a diameter D, the cross-sectional area is : π 4D The velocity of the drill perpendicular to the workpiece is the product of the feed f (the distance of the drill penetrates per unit revolution), and the rotational speed, N, where N = V/πD. Thus, 2
MRR =
π D2 4
fN
N: spindle speed (rev/min); D: (drill) tool diameter (mm); f: feed distance per revolution (mm/rev, or mm).
MRR in drilling-example A hole is being drilled in a block of magnesium alloy with a 12 mm drill bit, at a feed of 0.3 mm/rev, and the spindle running at N= 700 rpm. Calculate the material-removal rate and the torque on the drill. Solution: The MMR is calculated using equation:
MRR = MRR =
(3.14)(12) 2 4
π D2 4
fN 3
(
3
(0.3)(700) = 23738.4 mm / min 395.65mm / s
Taking average unit power of 0.5 W.s/mm3 for magnesium alloy. The power required then; Power= (396) (0.5)= 198 W Power is the product of the torque on the drill and the rotational speed, which in this case is (700) (2π)/60 = 73.26 radians per second. (J = N.m) T = 198/73.26=2.70 N.m
Design consideration for drilling Design should allow holes to be drilled on flat surfaces and perpendicular to the drill motion. Exit surfaces for drill also should be flat. Interrupted hole surfaces should be avoided or minimized for improved dimensional accuracy, drill life and to avoid vibrations. Through holes are preferred over blind holes. If holes with large diameters are required, the w/p should have a pre-existing hole, preferably made during fabrication of the part Blind holes must be drilled deeper than subsequent reaming or tapping operations that may be performed.
Safety in drilling Should not wear loose clothing such as tie or jewellery which could become entangled Guards should always be in place and suitable face protection should always be worn The floor space around the machine should be without obstructions and slippery substances. The machine should not be kept running when not in use. Provide holding clamps or fixtures to secure the work to the drill press bed. Use brushes, chip pullers or other tools to remove chips from table or work; do not allow removal by unprotected hands.
Milling Milling is a material-removal process for non-rotational objects: It uses a multi-point tool that rotates about a fixed axis, while the prismatic workpiece is fed into the tool according to a pre-specified travel path: • This intermittent-cutting process is commonly classified as “face” (or “end”) milling versus “peripheral” (or “plain”, or “slab”) milling: Tool Workpiece
Tool Workpiece
Metal cutting - milling The milling process: Typically uses a multi-tooth cutter Work is fed into the rotating cutter Capable of high MRR Well suited for mass production applications Cutting tools for this process are called milling cutters
Different milling machines
Schematic illustrations of (a) A horizontal-spindle column and knee type milling machine and (b) vertical spindle column and knee type milling machine Source: S. Kalpakjian, S.R. Schmid: Manufacturing Engineering & Technology, 5th edition, Prentice-Hall International, 2006; pp. 739.
Metal cutting - milling
Source: S. Kalpakjian, S.R. Schmid: Manufacturing Engineering & Technology, 5th edition, Prentice-Hall International, 2006; pp. 725.
Milling operations
Source: S. Kalpakjian, S.R. Schmid: Manufacturing Engineering & Technology, 5th edition, Prentice-Hall International, 2006; pp. 734.
MRR in milling
Milling with max MRR w. a. f m MRR= ? , [cm3/min] w : width of cut (step depth) [mm]; 1000 a: depth of cut [mm]; fm: feed rate of the table [mm/min] , fm = ft..N.n . N: spindle speed [rev/min]; dt: (milling) tool diameter [mm];
fm a
w
Feed rates/cutting velocity in milling
In milling, the thickness of material removed is the depth-ofcut (DOC). The feed rate per tooth ft (tabulated, mm/tooth) is related to the amount of material each tooth removes during a revolution. It is given as the travel rate of the tool divided by the number of teeth n. So, the feed rate of the table fm can be calculated as follows: fm = ft..N.n , mm/min The cutting velocity V is the linear velocity of the tool tip as it engages the workpiece.
π N d t , m/min V= 1000
Selecting cutting conditions
The surface finish achieved by cutting is a function of: cutting speed depth-of-cut feed rate
Sequence of cutting: • rough cuts, carried out at high speed and feed rates and large depths-of-cut and big tool size for a maximum materialremoval rate (MMR), • finish cuts, where RPM can remain the same, but at much lower values for feeds and depth-of-cut for a better surface finish and dimensional accuracy.
Cutting time calculation Calculate the cutting time for skimming 2mm of the top surface of a 200 mm long aluminium plate using 4 tooth diam 80 mm mill.
Tool Workpiece
Speed = L/T = fm= ft..N.n , mm/min hence: T=L/ft.N.n , min From the table: feed rate per tooth ft = 0.2 mm/tooth To find N:
π N dt V= 1000
, m/min
From the table: cutting speed V= 300 m/min hence: N=1000V/π dt = 1000x300/π x 80=1194~1200 rev/min Hence: T= 200/(0.2x1200x4)= 0.2 min
References 1. S. Kalpakjian, S.R. Schmid: Manufacturing Engineering & Technology, 5th edition, Prentice-Hall International, 2006. 2. E. Paul Degarmo, J. R. Black, R. A. Kohser; Materials and Processes in Manufacturing, 9th edition, John Wiley & Sons, Inc, 2003. 3. R. L. Timings, S. P. Wilkinson: Manufacturing Technology, 2nd edition, Pearson Education Limited, London, 2000.
