There is a wide variety of types and sizes of drills. Fig. 18.17 shows the various types of conventional and special purpose drills. Most twist drills are made of high speed steel with or without a carbon-steel shank joined to the body. Some drills are made of cobalt alloys and others have inserts of cemented carbides.
Type # 1. Taper-Shank Drills:
These are general purpose drills and have a morse taper shank. The size varies with drill body diameter. Cobalt H.S.S. with heavy duty taper shank and two- flute type twist drills for drilling high-manganese steel are also available.
ADVERTISEMENTS:
Type # 2. Straight-Shank Drills, Taper Length:
These drills have straight shanks for the same diameters as the bodies and the same over-all lengths and flute length as taper-shank drills. Flute lengths are approximately 60% of the overall length.
Type # 3. Straight-Shank Drills, Jobber’s Length:
These have straight shanks of the same diameters as the drill bodies. These are general purpose drills for most materials and are available in four series viz., fractional, wire gauge, letter and metric.
ADVERTISEMENTS:
Type # 4. Heavy Duty Drills:
These are used to drill tough material. For deep-hole drilling, extra-long lengths are provided. These have larger helix angles and heavier webs than conventional drills. The webs are parallel and tapered at the point.
Type # 5. Cotter Pin Drills:
These are used for drilling cross holes in axles and shafts without the use of guide bushings. They are made of H.S.S. and have straight shanks, heavier webs, and slightly higher helix angle.
ADVERTISEMENTS:
Type # 6. Straight Fluted Drill:
It is best suited for drilling holes in soft metals like brass, copper etc., because a twist drill (drill with rake) has a tendency to “dig” or “grab” if used for drilling soft metals. It is also advisable to use a drill without rake when drilling very thin stock because of the tendency of the drill to “hook” into the work when it is breaking through.
Type # 7. Flat Drill:
ADVERTISEMENTS:
It is preferred for drilling a soft material such as brass, as it will not feed itself into the material more quickly than is desired. Another reason for its use is that, whereas hard spots in steel will cause an ordinary drill to slide off centre, flat drills are not affected in this manner. Also flat drills make fine chips instead of long coils.
Type # 8. Half-Round Drills:
In this case length equal to half of the diameter at the drilling end is ground away. The flat thus provides space for chip removal and offers a zero-degree cutting rake face. A slight back taper is incorporated behind the tip. This drill has an offset conical point to provide radial relief for the single cutting lip.
ADVERTISEMENTS:
The conical point near the cutting edge ensures an accurate start and eliminates drift. The diameter tolerance and internal finish of hole produced by this drill is better than that with twist drill. Depth to diameter ratio can be 10: 1 in horizontal position and 2: 1 in vertical position.
Type # 9. Three-Fluted Drill:
It resembles a twist drill. It is actually three-tooth spiral reamer; its function is enlarging cored, punched, or drilled holes. It will not drill the initial hole, but being very sturdy and having wide cutting edges, it is an efficient tool when a hole must be considerably enlarged.
Type # 10. Multi-Cut Drill:
These are used for drilling two or more diameters of hole or for combining operations such as drilling, counter boring, chamfering, counter sinking or reaming.
Type # 11. Oil Hole Drill:
It has holes through the body of the drill from the shank to the point by means of which lubricant flows down to cool the point of the drill. This type of drill is generally used for deep-hole drilling.
Type # 12. Step Drills:
These have two or more steps (diameters) of square or angular cutting edge produced by grinding various steps on the diameter of conventional drills. These are used for drilling multiple diameter holes and operations like counter boring, countersinking and chamfering.
Type # 13. Core Drills:
These are used for enlarging and correcting the location of previously made holes as their cutting edges do not extend upto the centre. These are provided with three or four flutes and thus due to more number of cutting edges, these give better finish.
Type # 14. Shank and Shell Type Core Drill:
It looks like twist drill but it has three flutes and three lips. Thus productivity is increased. Chamfer does bulk of cutting. Section 2 serves to size the hole and guide the tool. Shank 5 enables clamping in a holder. Chamfer angle ɸ varies from 45 to 60° for drills made of HSS and from 60 to 75° for drills made of carbide tips.
HSS drills have angles of γ = 8 – 15° for machining steel, γ = 6-8° for machining cast iron, and γ = 25-30° for machining non-ferrous metals and alloys. Carbide-tipped core drills have rake angles of γ = 5° for cast iron and γ = 0 to 5° for steel.
The relief angle is α = 8 to 10°, and the flute helix angle is ω = 10 to 25°. The blades of core drills have a margin 1.2 to 2.8 mm wide for better guidance of the tool in the hole. Shell-type core drills (Fig. 18.21) are used to enlarge holes up to 100 mm in diameter. They have four helical flutes (and hence four cutting edges), no shank, and they are arbor- mounted.
Type # 15. Deep Hole Drills:
The web thickness of such drill is very heavy (of the order of 40% of the drill diameter) and helix angle is greater. Special drills for this purpose have split point to facilitate holding of the drill and the centre without wandering parabolic flute profile to expedite flow of chip, duplex cutting edges on the lips to facilitate in breaking the chips.
Type # 16. Carbide Drills:
For drilling small size hole, solid carbide drills are used as these are very rigid; having high resistance to vibrations and these produce true and clean holes. In bigger size, these are made by brazing the carbide tip to the drill body of hardened alloy steel.
These are used for drilling cast and malleable iron, non-ferrous metals like aluminium and magnesium, copper and brass alloys and non- metallic compounds like rubber and fibre. Drills with index able carbide inserts which obviate the problem of grinding have been developed.
In one design, two square carbide inserts are used, one insert cuts at the centre and other at outer half of the hole. Inserts are held in place with a pin lock to provide maximum space for chip clearance. Two guide pads are provided for guiding the drill in the hole, and an in-built coolant hole is provided to cool the drill and dispose off chips.
Type # 17. Drills for Thin Metal Sheets:
These have a short and sturdy web design; flute length is short and chip space is less. Low web thickness permits easier centring and entry of the drill. The drill point as shown in Fig. 18.22 is used. It drills the full diameter for a greater period of the cut and thus gives a rounder hole and better surface finish.
Type # 18. Spade Drill for Hard Steels (Above 60 HRC):
These drills are specially designed so that these cut after heating the metal beneath the drill point by friction. Such drills have a round shank and a triangular fluted section. The fluted end is ground to a three-sided pyramid tip and then notched to provide chip clearance.
Type # 19. Spade Drills:
These are used for drilling large diameter and deeper holes. Spade shaped bit is held in an alloy steel body by means of screw and dowel. Since the same holder can accept bits of different sizes, it caters to a wide range of diameters.
Operations like reaming, flat bottoming, counter boring etc. can be carried out by merely choosing the proper tool bits. Web, instead of extending over the whole length of the flutes, is limited to the cutting end of the blade, resulting into higher strength, greater resistance to end thrust, minimised vibrations. They produce straight hole and can be operated as high speed.
Type # 20. Ejector Drill:
It has two edges on one side and another edge opposite to the two edges. One cutting edge starts from the periphery of the head and cuts through 0.4 of the radius. The second edge starts from the centre and cuts through 0.4 of the radius.
The third edge cuts the remainder of the area, partially overlapping the area cut by the other two edges. In this way the cutting forces are partially balanced and not much load acts on the supporting pads.
The coolant is introduced between an outer and inner tube. About one third of the coolant is drawn off through the annular nozzle straight back through the inner tube, moving with sufficient velocity to set up a partial vacuum in the chip passages.
The rest of the coolant passes through a number of holes and cools and lubricates the cutting edges and supporting pads. The coolant is then drawn by vacuum back through the inner tube and carried away with the chips.
Type # 21. Drilling with Jet Pulsing Coolant:
It has been observed that number of holes drilled by a drill can be increased considerably and the life of twist drill also increased by the supply of coolant to the drill point under pulsating pressure. The pulsating supply is obtained by an air operated jet pulser pump.
This action prevents sticking of chip segments at the drill point or along the flutes of the drill, breaks the chip into segments, keeps the drill point cool and lubricates the land. In this way drill life is high, higher feed rates can be obtained and surface finish is better. Productivity is increased because the drill need not be taken out for cooling and removing of chips.