In this article we will discuss about:- 1. Meaning of Drilling Machine 2. How to Specify a Drilling Machine 3. Operations Performed by Drilling Machine 4. Cutting Speeds and Feeds of Drilling Machine 5. Machining Time in Drilling 6. Power for Drilling 7. Drills and Drilling 8. Cutting Process in Drilling Operation 9. Drill Wear and Sharpening a Drill.
Meaning of Drilling Machine:
Drilling machine is one of the simplest, moderate and accurate machine tool used in production shop and tool room. It consists of a spindle which imparts rotary motion to the drilling tool, a mechanism for feeding the tool into the work, a table on which the work rests and a frame. It is considered as a single purpose machine tool since its chief function is to make holes. However, it can and does perform operations other than drilling also.
Drilling is a process of making hole or enlarging a hole in an object by forcing a rotating tool called Drill. The same operation can be accomplished in some other machine by holding the drill stationary and rotating the work. The most general example of this class is drilling in a lathe, in which the drill is held in the tail stock and the work is held and rotated by a chuck.
Boring is the process of enlarging a hole that has already been drilled or cored. Principally, it is an operation of truing a hole that has been drilled previously, with a single point tool. To perform this operation on drilling machine, a special holder for the boring tool is necessary.
How to Specify a Drilling Machine:
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(a) Portable drilling machine is specified by the maximum diameter of drill which can be held.
(b) Sensitive and upright drilling machines are specified by the diameter of the largest work piece that can be drilled.
(c) The radial drilling machine is specified by the length of the arm and column diameter.
(d) Multiple spindle drilling machine is specified by the drilling area, the size and the number of holes a machine can drill.
Operations Performed by Drilling Machine:
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Although the drill press is mainly meant for drilling operation, it can also be used for performing the following operations:
(i) Reaming,
(ii) Boring,
(iii) Counter boring,
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(iv) Counter sinking,
(v) Spot facing,
(vi) Tapping,
(vii) Trepanning,
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(viii) Rivet spinning,
(ix) Polishing.
Cutting Speeds and Feeds of Drilling Machine:
The cutting speed is a measure of peripheral speed of the drill in metres per minute. Frequently this speed is chosen arbitrarily without regard of efficient operation. The cutting speed for high speed drills should be double than that of carbon steel drills. Depending upon the material to be drilled, the cutting speed varies from 10 to 90 metres/minute.
Mathematically it is expressed as:
Feed:
Feed of drill is the distance, which it moves into the work on every revolution of the drill and is generally expressed in millimetres per minute. The feed varies from 0.05 to 0.35 mm. per revolution.
The amount of metal removed is a function of cutting speed and feed. If the feed is held constant, the tool life increases as the cutting speed is decreased. The best tool life for a given rate of metal removal is obtained by using the highest possible feed.
Apparent cutting edge engagement b = D/2, Actual cutting edge engagement ba = b/ cos θs
where θs = cutting edge angle = 90° – Point angle of drill/2 Apparent or nominal uncut chip thickness (f) = drill feed in mm/rev. (F)/2
Uncut chip thickness fa = f cos θS
Area of uncut chip = f x b,
Metal removal rate = π D2N x drill feed (F)
Machining Time in Drilling:
The machining time in drilling can be calculated as below:
T = L/(N x f) minutes
Where N = r.p.m. of drill,
f = feed per revolution of drill,
L = length or depth of hole in mm.
T = drilling time in minutes.
But in drilling operation, for calculating the length or depth of drilling hole, one has to take the length of approach into account. The length of approach (X) is taken as 0.29 D (where D = diameter of drill). In an open hole, the total length L = (H + X + X) = (H + 0.29 D x 2)
In a blind hole, the total length L = (H + X) = (H + 0.29 D).
Example 1:
Find the time required for drilling a 18 mm hole in a workpiece having thickness 50 mm. Assume cutting speed 12 metres / minute and feed 0.2 mm / revolution. Neglect the length of approach.
Solution:
Speed and Feed Range:
It has already been indicated in chapter 9, that the extreme speed limits, i.e. the highest and lowest speed in the range are generally related to the extremes of size for which the machine is designed. The intermediate speeds are chosen generally in G.P. series. The various ranges of feeds are also generally chosen in G.P. series.
Recommended values of speed for drilling are 20-23 m/min. for mild steels, 18-22 m/min for alloy steels, 12-15 m/ min for stainless steels, 20-23 m/mt. for grey C.I., 35-55 m/ min for aluminium alloys, 30-45 m/min for copper alloys and 60-105 m/min for magnesium alloys.
The feed per revolution should be increased with increasing drill size in order to maintain reasonable chip thickness.
The typical feed in mm/rev for various drill diameters for hard (carbon steel and alloy steel) and soft (cast iron, brass, bronze, aluminium alloys) materials respectively are ; 3 mm : 0.05, 0.07 ; 6 mm : 0.07, 0.10 ; 9 mm : 0.1, 0.15 ; 12 mm : 0.12, 0.20 ; 25 mm : 0.22, 0.35.
Example 2:
Calculate a suitable range of six speeds for a drilling machine if the machine is to take drills from 6.25 to 25 mm size and a cutting speed of 18 m/min is to be used.
Solution:
Cutting speed (m/min)
1st speed = 229 R.P.M.,
2nd speed = 229 x 1.32 = 300 R.P.M.
3rd speed = 300 x 1.32 = 396 R.P.M.,
4th speed = 396 x 1.32 = 525 R.P.M.
5th speed = 525 x 1.32 = 690 R.P.M.,
6th speed = 690 x 1.32 = 915 R.P.M.
For these speed ranges, nearest available drill sizes
1st speed, 229 R.P.M. suitable for 25 mm drill
2nd speed, 300 R.P.M. suitable for 25 x 229/300 =̃ 20 mm
3rd speed, 396 R.P.M. suitable for 25 x 229/396 =̃ 15 mm drill
4th speed, 525 R.P.M. suitable for 25 x 229/525 =̃ 10 mm drill
5th speed, 690 R.P.M. suitable for 25 x 229/690 =̃ 7.5 mm drill
6th speed, 915 R.P.M. suitable for 6.25 mm drill.
Power for Drilling:
When a drill is cutting it has to overcome the resistance offered by the metal and a twisting effort is necessary to turn it. The torque required to operate the drill is dependent upon various factors, but for most of the practical purposes, the relation between the torque, the diameter of drill and the feed (T = Cf0.75 d1.8) has been found to be most satisfactory, based on experiment results.
Drills and Drilling:
It is possible to do the following operations on a drilling machine:
(i) Drilling:
It is the operation of producing a circular hole, using a drill, by removing solid metal.
(ii) Reaming:
It is the operation of sizing and finishing a hole by means of a reamer having several cutting edges.
(iii) Boring:
It is the operation of enlarging a hole by means of an adjustable cutting tool with only one cutting edge.
(iv) Counter Boring:
It is the operation of enlarging the end of a hole cylindrical, in order to accommodate the screw head so that it does not protrude outside.
(v) Counter Sinking:
It is the operation of a cone- shaped enlargement of the end of a hole.
(vi) Spot Facing:
It is the operation of smoothing and squaring the surface around a hole, as for the seat for a nut or the head to a cap screw.
(vii) Tapping:
It is the operation of forming internal threads by means of a tool called a tap.
(viii) Trepanning:
It is a hole making operation in which an annular groove is produced leaving a solid cylindrical case in the centre. A cutter consisting of one or more cutting edges placed along the circumference of a circle is used to produce the annular groove. It is used for holes more than 50 mm in diameter.
Tool Holding Device:
The cutting tools used for any of the drill-press operations are generally made with straight shanks for sizes under 12.5 mm diameter as they can be conveniently and firmly held in a chuck, and those of bigger size are made with taper shanks (Morse standard).
The cutting tools may be either held directly in the spindle hole of the machine, or be held in a taper socket, or drill chunk or other device, the shank of which fits the tapered hole in the spindle.
The drilling-machine spindle is provided with a morse standard taper hole of a size in proportion to the size of the machine. The bigger sized drills have shanks which will fit the spindle.
Cutting tools with taper shanks that are too small to fit the taper hole in the spindle of the machine are held in a smaller taper hole in a socket, the shank of which fits the spindle hole. If a suitable socket or sleeve is not at hand, a combination of socket and sleeve or two sockets or two sleeves may be used.
The taper shank from the taper hole must be removed by a tapered key or drift as shown in Fig. 18.12 (a). It may be noted that the taper shanks of drills, reamers, counter borers, etc. and also of the sockets and sleeves, have their end flattened to form a tang which fits in a suitable slot at the end of the taper hole in which the shank is held.
The tang helps in driving the drill, since the hold of the taper alone is not sufficient. As the tang alone is also not sufficient to drive the drill or other cutting tool, the taper shank and hole must be properly fitted, cleaned and made dry. A taper shank may be mounted into the spindle by thrusting the taper shank into the socket and the socket into the spindle.
To secure the drill and drill socket, put a block of wood on the table and with the feed handle, bring the drill down sharply against the block to drive the drill and socket tight.
A drill chuck is a gripping device with two or more adjustable jaws set radially to hold straight-shank drills or other cutting tools and is provided with a taper shank which fits the taper hole in the spindle. These are made in various sizes; and a series of three to four chucks will hold drills from the smallest size upto 25 mm in diameter.
Another holding device commonly used is the floating holder which is used to compensate for out-of-alignment of drill spindles or work holder for reamers, counter borers, tapes etc. and permits self-aligning of the tool.
Drills with taper shanks are placed directly into the taper bore of the tail-stock spindle, or, when the tapers’ sizes differ, into taper sleeves (Fig. 18.13 (a)).
Drills with straight shanks up to 16 mm in diameter are held by drill chucks (Fig. 18.14) which are placed into the tailstock spindle. A drill is gripped by jaws 6 which can expand and contract as they move in the slots of body 2.
The jaws have teeth which are in mesh with threads made on the inside surface of ring 4. Key 5, whose pinion is in mesh with the teeth of sleeve 3, thus forming a bevel gearing, turns the sleeve together with ring 4, which drives jaws 6 up and down and makes them expand or contract. Taper shank 1 serves to mount the chuck into the tailstock spindle.
Before drilling, set the tailstock on the slideways at such a distance from the workpiece that it is possible to drill a full-depth hole with the minimum extension of the tailstock spindle; Start rotation of the work. Advance the drill to the work by turning the tailstock hand-wheel slowly to avoid impact of the drill against the work, and start drilling the hole, drilling a small depth.
Then retract the drill, stop the workpiece, and check the alignment of the hole. To prevent the drill from running off centre, start drilling the hole with a short large-diameter drill or a special centre drill with a 90° point angle.
Owing to the centre hole thus obtained, the chisel edge of the drill does not act at first, and that reduces the tendency of the drill to wander off centre. To replace the drill, turn the tailstock hand wheel until the spindle comes to the extreme right-hand position and the screw pushes the drill from the spindle. Then place another drill into the spindle. When drilling a hole whose depth is greater than its diameter, retract the drill periodically to clean the hole and the drill flutes from the chips.
Friction of the drill against the hole is reduced by drilling with a cutting fluid which is particularly advantageous with steel and aluminium work parts. Cast iron and bronze can be drilled without any coolant. The use of cutting fluids makes it possible to increase the cutting speed 1.4 to 1.5 times.
Cutting fluids include soluble oil emulsions for break structural steels), oil compounds (for alloy steels), oil emulsions and kerosene (for cast iron and aluminium alloys). If there is no coolant supply on the machine, use can be made of a mixture of machine oil with kerosene.
The use of cutting fluids allows the axial and tangential cutting forces to be reduced by 10 to 35 per cent in drilling steels, by 10 to 18 per cent in drilling cast irons and non-ferrous alloys, and by 30 to 40 per cent in drilling aluminium alloys.
When drilling through, it is necessary to sharply reduce the feed rate as the drill comes out of the work; otherwise the drill may break down. The drill will serve longer if it is used at the maximum permissible cutting speeds and the minimum permissible feed rates.
Where the drill axis coincides with the lathe spindle axis, and the drill is properly ground and securely clamped, the drilled hole has insignificant errors. In a properly ground drill, both lips are in action and the chips emerge in both flutes.
An oversize hole may be caused by the following factors: the drill lips are unequal in length although ground at equal angles; the lips are equal in length but ground at different angles; the lips are unequal in length and ground at different angles.
Drills that have improper geometry and are not sharp enough produce off-centre holes with rough surfaces. In addition, drills with dull cutting edges produce burrs at the rear side of through holes.
A drill with unsymmetrically ground cutting edges of different length, with an eccentrically disposed chisel edge, and with margins of different Width may wedge in the hole as a result, of mounting friction as the drill goes deeper down, and finally break down.
Holes with a depth-to-diameter ratio of 5 and more are classed as deep. In drilling deep holes use is made of long twist drills with regular geometrical elements. The drill is periodically retracted from the hole for cooling and cleaning the flutes from the chips.
To increase productivity, use is made of special drills which have ducts for delivery of liquid or air under pressure to the cutting zone to eject chips from the hole.
With greater depth of drilling the working conditions deteriorate, the removal of heat becomes more difficult, the friction of chips against the walls of the drill flutes increases, and less coolant reaches the cutting edges. Hence, the cutting speed should be lowered if the depth of the hole to be drilled exceeds three times its diameter.
Twist drills operate at cutting speeds that range from 25 to 35 m/min for high speed steel drills, from 12 to 18 m/ min for drills of carbon tool steels, and from 50 to 70 m/min for carbide-tipped drills, the greater values being used for larger-diameter drills and lower feed rates. It is difficult to feed a drill by hand at a uniform rate; therefore various methods are used to feed the tool by power.
Twist Drill:
A drill consists of a cylindrical piece of steel with special grooves. One end of the cylinder is pointed and the other end is shaped so that it may be attached to the drilling machine.
The grooves, usually called flutes, may be cut into the steel cylinder, or flutes may be formed by twisting a flat piece of steel into a cylindrical shape. Drills of this kind are sometimes referred to as twist drills.
Twist drills are of two types viz.,
i. High-Helix (Fast spiral) Drills and
ii. Low Helix (Slow spiral) Drills.
High helix drills are provided with helix angles of 34 – 40° which provide more bearing area per linear length making them suitable for deep hole drilling in materials of low tensile strength such as aluminium, magnesium, copper, die casting materials, wood, plastics etc.
The web is slightly heavier and the grooves are approximately 30% wider than in conventional drills and are highly polished. These drills have wider flutes which assist in clearing chips. Low helix drills are provided with low helix angles which makes them more rigid capable of taking more torque.
These are capable of clearing large volumes of chips. These are used for drilling plastics, fibre asbestos, hard rubber and bakelite. They are also used for shallow drilling on aluminium and magnesium alloys. Because of rigidity of these drills, these can take heavier feeds.
Twist drill suffers from problems like chip elimination, heating, variable cutting speeds, limitation of depth of cut. Further uneven grinding of the point angle or wear of the cutting edges causes instability in the drilling process. In case of drilling of long holes, the instability of the force system leads to waviness and considerable run out.
These are most commonly used in the machine shop and are made in number sizes No. 1 (0.228 in. diameter) to No. 80 (0.0135 in. diameter), and in letter sizes-A (0.234 in. diameter) to Z (0.413 in. diameter). They are also made in sizes in ranging from 1/64 in. to 4 inch diameter or larger, and in metric sizes as well.
The smaller sizes of drills are not usually marked and the size is found by the use of a drill gauge (which is a sheet of metal having a large number of holes with their size marked). The smaller size drill is passed through these holes and the drill size is corresponding to one which just fits in by force or corresponding to one such that the drill passes easily in the next higher one and does not pass in the next smaller one.
Cutting Process in Drilling Operation:
Drilling differs from turning, because the twist drill is a multi- edge tool which cuts with five cutting edges (two lips, two leading edges, and the chisel edge). Cutting forces that act on the drill during cutting are shown in Fig. 18.26. Force F at each point A of the lips can be resolved into component forces Fx, Fy and Fz acting on axes X, Y and Z.
The Fy forces act on the drill lips in the opposite directions. They are equal in magnitude if the lips are ground symmetrically. Hence, the resultant force acting on the drill along they-axis equals zero. The axial force Fax acting along the drill is Fax = 2Fx + Fe + Ff, where Fe is the force acting on the chisel edge, and Ff is the force of friction of the margin against the wall of the hole.
In drilling, the main work of cutting is effected by the lips of the drill, whereas the chisel edge, whose cutting angle is 90°, crushes the metal with the force Fe‘ 0.5 Fax. The total moment of the cutting forces is Mt = Mz + Me + Mm, where Mz = (0.8-0.9) Mt is the moment of the force Fz ; Mc is the moment of the force Fe, and Mm is the moment of the force Ff.
As the drill wears on the flanks, the axial force and its moment rise; for instance, with the flank wear reaching 1 mm, the axial force and its moment increase by 60 to 80 per cent.
The efficiency of twist drills is improved by web thinning, margin thinning, and double-point grinding; by changing the point angle; by making holes of a smaller diameter before drilling to size; and so forth. Standard twist drills come with a point angle of 118°, but it is advisable to use drills with a point angle of 135° for harder materials (and deeper holes). Methods of drill point grinding are shown in Fig. 18.27.
Drill Wear and Sharpening a Drill:
As soon as drill is worn out, it should be sharpened. Worn out drill experiences high cutting force and produces high temperature. It results in poor surface finish, over size holes, noise etc.
In a drill, wear occurs mainly on the flank surface. It is predominant at the outer corner and the chisel edge. The allowable wear is dependent on the drill diameter, being about 0.2 mm for 5 mm drill, 0.53 mm for 10 mm diameter drill, 0.5 mm for 20 mm, 0.85 mm for 40 mm, 1.0 mm for 50 mm and 1.3 mm for 80 mm diameter drills. As drill diameter increases tool life also increases.
Usually drill grinding machine is employed for sharpening a drill on which the drill of any length or diameter can be quickly adjusted and supported. The machine is so designed that it is a very simple matter to grind the drill properly, i.e. with the lips of equal length, at the correct angle with the axis, and with the correct clearance.
Drills can be made of:
(i) Carbon steel,
(ii) High speed steel,
(iii) Carbide tipped.
In grinding carbon steel drill, extra care must be taken not to let it get hot enough to loose the temper (which is indicated by the cutting edge showing blue colour).
Plenty of Water should be used:
A drill of high-speed steel should be ground on a dry grinding wheel of medium grain and soft grade. It must not be immersed in water after grinding as this will cause the point and lips of the drill to crack.
In grinding the drill, its cutting angles, viz joint angle, chisel edge angle and the various clearance angles along with concentricity of lips should be given due care and maintained at the desired values to obtain best cutting results.
Point Angle:
Usually point angle of 118° is provided and flute profile is designed to produce straight cutting edges. Point angle much less than 118° would result in convex lips and seriously affect the cutting efficiency. High point angle results in concave lips which weaken the cutting corners and again impair the efficiency of drill.
It is essential that point angle be symmetrical with drill axis; otherwise only one lip would do most of the cutting, thereby deflecting the drill to opposite side producing oversize hole. Further both the lips should be of same size, otherwise the chisel edge will be offset and oversized holes will be produced. The condition of unsymmetrical point angle and/or lips of unequal size is indicated by greater volume of chips from one flute from the other.
Lip Clearance:
Lip clearance is the relief given to the cutting edges in order to allow them to enter the metal without interference. General purpose drills are given a clearance of 8 to 12 degrees. If there is little or no clearance, then the surface of the drill can only rub on the stock with which it comes into contact. The heel, which is on the same plane as the lip, prevents the lip from cutting the work.
In case lip clearance is zero, and when pressure is applied, the drill will not cut, sometimes resulting in a cracked drill. If the clearance angle is too large, the corners of the cutting edges may break away for lack of support.
Angle of Clearance at Centre of Drill:
The angle of clearance at the centre of drill should be greater than the angle at the circumference of the drill. The reason for this is that when 0.05 mm of stock is removed as the drill turns one-quarter of a revolution, it is distributed around a much larger sector on the circumference of the drill than at the centre.
The angle of clearance at the centre must be proportional to angle at the outside. The clearance on a drill is about 11° at the cutting edge. If correctly sharpened, the edge of the angle across the web of the drill (the dead centre of the drill) will be about 45° with the line of the cutting edges. The appearance of the dead centre is therefore an index to the clearance.
Lip clearance, as already indicated, is very important as it takes considerable pressure to feed the drill into the work under the best possible conditions, owing to the nature of the point and if the lips are not properly backed off, the drill will break under feeding pressure simply because it can’t cut.
The excessive clearance on the other hand, leaves insufficient thickness of tip to carry off the generated heat. It also leaves insufficient stock behind the cutting edge usually required for its support, thereby weakening the cutting edge. Thus a drill not having proper clearance will not be able to cut properly but it will wander, making oversized holes.
From the appearance of the ‘dead centres’ of the drill, lines l, l’, l’’ it is possible to know if the lip clearance is correct or not. (Refer Fig. 18.31).
Angle and Length of Lip:
In grinding a drill, it is very important that the angle of the point be correct, and the angles and lengths of the lips be equal.
If the drill is ground with its tip on centre, but with the cutting edges at different angles, then the drill will bind on one side of the hole (as shown in Fig. 18.32). Only one lip or cutting edge will do the work, resulting in rapid wear on the edge, and the hole will be larger than the drill.
If the angles of the cutting edges of a drill are equal but lips be of unequal length, then the result will be that the point and the lip will be off center (Refer Fig. 18.33). This will cause the hole to be larger than the drill. The effects of this condition are the same as the effects that would be obtained from a wheel with its axis placed at any point other than the exact centre of the wheel.
It will also place a strain on the drilling machine, the spindle will tend to weave and wobble, the drill will wear away rapidly and if continued, the machine will eventually break-down because of the strains of the spindle bearings and other parts.
Drill Point Geometries:
The actual cutting of a hole takes place at the drill point and not at the edges. The geometries or shapes of drill points differ according to the requirements of the drilling operation and the kind of material being drilled. In a conventional drill point the chisel edge acts like a blunt negative cutter, pushing the material instead of cutting it.
The length of the chisel edge depends on web thickness at the drill point. Wide-web tools being rigid can drill deeper, straighter and more accurately. It may be noted that in conventional drill point, the chisel edge is straight and thus tends to wander on the workpiece.
A centre punch is thus necessary to enable centering of drill. Precision cuts generally require secondary reaming operations. Sharp corners of the point break down more rapidly. It tends to produce burr when breaking through. In helical drill point, S-shaped chisel and crown enable the drill to be self-centering and thus cuts close to drill diameter.
Racon drill point geometry has a radiused conventional point which has 8—10 times the tool life of conventional point drill. The curved lips produce less heat, no burr on break through.
Split point geometry (also called crank shaft drill) is used in deep drilling, self-centering applications. Cutting edge acts as chip breaker to facilitate coolant flow. Brickford point (combines attributes of helical and Racon points) results in self-centering burr free break through, high feed rate, long tool life.
Web Thinning:
The design of twist drills is such that web thickness gradually increases from the drill point to the flute run-off in order to provide desired strength and rigidity. After grinding about one third of useful length of the drill, web needs to be thinned to reduce the chisel edge length. If web is not thinned, then centering action is lost and oversized holes would be produced.
In thinning the web, equal amounts of materials must be ground on either side, with thinning nicely blending into the flutes. Uneven thinning would result in unbalanced cutting forces resulting in drill deflection, oversized hole and drill failure. The chisel edge should not be reduced excessively. Excessive web thinning weakens the drill point and it should be done to restore approximately the original web thickness of a new drill.
Drill Maintenance:
With use, drill loses its efficiency of cutting and its proper maintenance is required from time to time.
The various important points to be borne in mind in this connection are described below:
The drill wears starts by rounding off the corners and at the cutting edges or lips and the chisel edge. Thus a conical surface of narrow width having no relief is formed adjacent to these edges (Refer Fig. 18.34) and it tends to rub in the hole rather than cutting; requiring more power and thrust to be forced in and generates more heat and wear.
The increase in wear at corners travels back along the margins, resulting in a loss in size. This worn section would be removed (as shown in Fig. 18.34) at the earliest whenever wear is observed on tip, chisel edge, and margin. Generally maximum wear land allowed on lip is about 0.2 mm and it can be easily observed.
Removal of worn portion will shorten the drill and increase web thickness as the latter increases towards the shank of drill. The chisel edge, which does no cutting but only pushes the metal out of way, will require more force and generate more heat if it is long.
It is therefore essential to reduce the web thickness after removing the worn, chipped and burned portion of drill. It is important that web thinning cut should extend far enough up the flute so that an abrupt wedge is not formed at the extreme point.
After these two operations, it is also necessary to regrind the surfaces of the point. The two conical surfaces intersect with faces of the flutes to form the cutting lips, the back surface of which must be relieved in order to permit the edge to penetrate.
Chip Breakers:
Long continuous chips are nuisance and hazardous to operators at high speed. Drills are therefore provided with suitable chip breakers to produce smaller chips.
Various types of chip breakers are:
(i) Over curling type,
(ii) Groove type,
(iii) Crisp chip breakers,
(iv) Oxford chip breakers.
Chip breaker drills require extra energy for breaking the chips. Sometimes these affect the stability of drill and produce out-of-round holes.
Speeds and Feeds of Twist Drills:
A drill is said to be dull, if it penetrates the work very slowly or not at all, it becomes very hot, a squealing noise is made, the finished hole has a rough surface.
In drilling operation, correct speeds and feeds are determined by the judgement of the operator, who should know the following facts:
(a) The cutting edge breaks off when the feed is too heavy or the drill has been given too much clearance.
(b) The rapid dulling of the drill especially at the outer ends of the lips (corners) is evidence of too much speed and too much clearance.
(c) When the drill splits, either drill feed is too much or clearance is not sufficient.
(d) When a drill squeaks, it is usually an indication of a crooked hole or dullness caused by the margin of the drill becoming worn.
Average cutting speeds with drill are as under:
Feeds vary with size of hole being drilled; being 0.04 to 0.006 mm/rev. of 1.5 to 2.5 mm diameter holes, and 0.30 to 0.40 mm/rev. for 40 mm diameter holes.
Cutting lubricants in drilling, reaming and tapping operations are:
Torque and Thrust in Drilling:
Torque required to operate a drill depends upon various factors. Approximate results can be obtained by considering the drill diameter and the feed and the material being drilled.
It has been found experimentally that:
torque T = C. f0.75 d1.8 Newton metre.
where C = constant = 0.11 for aluminium and 0.084 for soft brass, 0.07 for cast iron and 0.36 for mild steel and 0.4 for carbon tool steel
f= drill feed (mm/rev), d = diameter of drill (mm)
If N = speed of drill in rev./min then work done/min = 2π NT Newton metre and power = 2π NT/60000 kW.
In addition to the torque, a drill requires an axial force to feed it through the work, but in power calculations this is generally neglected.
Factors Influencing Torque and Thrust in Drilling:
Torque and thrust in drilling, apart from drill diameter and feed, depend upon the following factors:
(i) Work Material:
Torque and thrust are dependent upon hardness and tensile strength of work material.
(ii) Helix Angle:
Torque and thrust decrease with increase in helix angle because chip formation becomes easier.
(iii) Point Angle:
Torque decreases with increase in point angle, but axial thrust and wear increase which may cause chatter.
(iv) Web Thickness:
Thrust force in drilling depends upon the chisel edge and the nature of flank surface. Total torque slightly increases with increase in chisel edge length. Axial thrust can be reduced by thinning the web.
(v) Drill Wear:
Torque and thrust rise sharply when drill gets dull.
Troubles in drilling and their remedial measures:
The various troubles experienced in drilling operation and their remedial measures are given below.
(a) Rough Hole:
If rough hole is produced, feed should be reduced, point reground, coolant used and rigidity of fixture ensured.
(b) Oversize Hole:
This may occur due to loose spindle or unequal angle/length of the cutting edges.
(c) Breaking of Drill:
Drill may break if it gets dull or if the point is improperly ground, or flute is clogged by chips, or high feed or improper clamping of drill and work.
(d) Chipped Cutting-Lips:
These occur due to high feed and high clearance angle.
High Accuracy in Drilling:
Following measures will ensure high accuracy:
i. Use rigid machine tool
ii. Ensure directionally stable tool feed
iii. Properly ground drill
iv. Axis of spindle, sleeve and tool must coincide
v. Proper clamping of workpiece
vi. Use of appropriate guide bushes for guidance of drills.
Factors Influencing Finish:
Rough surface is produced in drilling due to:
i. Chip flow across the drill flute
ii. Chip particles welded to the drill land
iii. Drill feed marks
iv. Drill tapering toward the shank
Example 2:
Calculate the drilling torque, material removal rate and drilling power for drilling a 20 mm hole at a feed rate of 0.2 mm/rev. in aluminium block with specific cutting energy of2000 N/mm2.
Solution:
Chip cross sectional area:
Example 3:
It is required to drill a hole and then tap a thread in cast iron workpiece using carbide drill with 120 degree point and HSS tap of M10 x 1 mm size. If depth of hole is 40 mm, determine the machining time and material removal rate for both the operations. Take cutting speed for drilling and tapping as 100 m/min and 30 m/min respect. Take feed rate for drilling C.I. as 0.15 mm/tooth.
Solution:
Spindle rpm for drilling Nd = Vd/πD
Vd = 100 m/min = 100 x 1000 mm/min
Diameter of drill may be taken 1 mm less than diameter of tap, i.e. 10 – 1 = 9 mm
∴ Nd = 100 x 1000/3.14 x 9
= 3840 rpm spindle rpm for tap
Spindle rpm for tap = 30 x 1000/3.14 x 10 = 955 rpm
Feed rate in mm/min
= No. of teeth x feed/tooth x N
fr = 2 x 0.15 x 3840 = 1152 mm/min.
Feed rate for tapping
= pitch x N = 1 x 955 = 955 mm/min.
Cutting time for drilling
Work Holding Devices, Location of Workpiece:
The success of any job that must be clamped to the table of any machine depends almost entirely on the manner in which it is clamped. Small quantity production is usually done with the workpiece clamped to the machine table. A setter is often employed to locate and clamp the workpiece ready for the operator to carry out the drilling.
Column and radial arm drilling machines drill holes vertically and hence to drill angular holes, the workpiece must be set at required angle by utilising either tilting table or by bolting together two angle plates by one bolt through their vertical faces and upper surface of angle plate is thus inclined at required angle.
Work can be held on the drilling machine by means of clamps, vises and jigs. Equipment necessary for clamping down work is simple and inexpensive, consisting merely of clamps of various sorts, bolts and sometimes parallels.
Vices are commonly used but these do not accurately locate the work and provide no means for holding cutting tools in alignment.
Jigs are the production means of holding work for drilling; these hold the work securely. The job can be quickly loaded and unloaded. Jigs also provide means for guiding tools into work at proper relative positions and for holding holes to size.
The work table is provided with enough T-slots to enable the operator to locate conveniently, the necessary bolts for the stops and clamps and vices.
The various important points in proper clamping for the work pieces are summarised below:
(i) A clamp should be properly placed and the clamping block must be of correct height or else the work will become loosened, with probable damage to both work and machine.
(ii) The clamping bolt should be placed as close to the work as conditions permit.
(iii) The clamp should have a firm seat on both the work and the clamping block.
(iv) The clamp must not be placed over a part that will spring under pressure until a shim or packing block is placed under that part.
(v) All the chips and dirt must be cleaned off from the surfaces of vices, table, job and other clamping surfaces.
(vi) When setting up for drilling holes through the work, it should be ensured that the work is so arranged that the drill will be able to go through without drilling into the vice or the table or the parallels.