Holes deeper than 10 d are called deep . Drilling deep holes is a complex and time-consuming operation. Due to the low rigidity of long drills, under the action of cutting forces, their longitudinal bending occurs, which can lead to a curvature of the hole axis. In addition, with an increase in the length of the hole, unfavorable conditions for the formation of chips are created and it is difficult to extract it from the hole during operation.
There are two ways to obtain deep holes; solid drilling and ring drilling.
Rice. 9.5. Methods for obtaining deep holes by drilling : a – solid; b – ring
The continuous drilling method (Fig. 9.5, a) consists in obtaining a hole by turning into a chip all the metal to be removed to form a given hole size.
The ring drilling method (Fig. 9.5, b) consists in obtaining a hole by drilling an annular cavity in the workpiece with the formation of a rod in the central part of the hole, which is then broken off or cut off with a special device at the end of drilling.
This method is mainly used for drilling deep holes with a diameter of more than 100 mm with special drills on special horizontal deep drilling machines.
When drilling deep holes on vertical drilling machines, it is recommended to use the following techniques:
- first drill a hole with a short drill to a depth of approximately 4 d, and then a long one to a given depth;
- it is necessary to periodically (without stopping the rotation of the spindle) remove the drill from the hole and remove the chips formed in it;
- to facilitate the pressure of chips from a deep hole, it is advisable to use a special pneumatic device (Fig. 9.6).
Rice. 9.6. Deep Hole Chip Extractor
Tube diameter 1 for air supply in this device. they are selected so that the gap between it and the walls of the hole is at least 6-7 mm. Casing 2 serves to protect the driller from flying chips, the gap between the casing and the end of the workpiece should be 15-20 mm.
When drilling deep holes, it is necessary to supply large amounts of cutting fluid to the cutting edges of the tool, which facilitates the cutting process, ensures reliable and timely washing out of the formed chips and removal of heat from the cutting edges of the tool.
The most advanced method is to supply liquid through holes passing inside the feathers of the drill. There are a number of twist drill designs available from tool factories with coolant holes through the drill shank or through radial holes.
Such drills are made from special rolled products with screw holes, from blanks obtained by radial forging, rolling of blanks using solid fillers, rolling of tubular blanks, casting.
The use of these drills is most effective when drilling holes to a depth exceeding 3d of the tool.
Rice. 9.7. Deep hole drills :
a – spiral with holes for supplying fluid to the cutting zone;
b – gun with soldered hard-alloy plates;
c – rifle with a solid carbide working part;
g – gun with a hard-alloy plate and an intermediate high-speed plate;
e – the shape of sharpening the tops of the drills
On fig. 9.7, but shows a twist drill made of special rolled products with holes for internal fluid supply to the cutting zone.
The use of such drills makes it possible to increase the cutting speed by 1.2–1.8 times, the durability of drills by 2–2.5 times, and at the same time, it facilitates the removal of chips and eliminates the need for periodic withdrawal of the drill from the hole being machined.
Drilling deeper holes (over 10 d) is advisable to carry out special drills for deep drilling with coolant supply to the cutting zone.
These drills include gun drills , ejector drills and drills of the BTA type. Ejector and BTA type drills are still of limited use.
The body of a gun drill with a steel body and soldered cutting and two guide plates made of hard alloy of the TK or VK group (Fig. 9.7, b) is made of steels 40X, 9XC, 35XGSA and can be tubular with a chip groove formed by plastic deformation, solid or made of special steel with an eccentric coolant hole and a milled flute. Drills of this type are made with a diameter of 8-30 mm, length L = 110-: – 1700 mm.
Gun drills with a solid carbide working part 1 soldered to a steel body 2 can be made with a shank (Fig. 9.7, c).
The hard-alloy working part is made of alloys of the VK or TK group, diameter d = 2 -: – 15 mm, length l1 = 1.5-1 mm, total length L = 110 -: – 600 mm.
To supply coolant to the cutting zone, the carbide working part has holes of a round or oval (to increase the volume of fluid through) shape. The tubular body with a groove formed by plastic deformation is made of steel grades 40Kh or 35KhGSA.
The internal cavity of the housing has a sickle-shaped shape formed during deformation; it is used to supply coolant to the working part and interface with holes in the working part.
Drills of this type have not only an increased service life due to their greater length compared to the drills shown in fig. 9.7, b, but also with an increased consumption of hard alloy.
The gun drill shown in fig. 9.7, d, similar to the drill of the first type, but differs from it in the presence of an intermediate insert 3 made of high-speed steel attached to the body 2. Carbide cutting and guide plates are fixed on the rod 4.
The work of gun drills is reduced not only to cutting the allowance with cutting plates, but also to smoothing out irregularities on the surface being machined with guide plates.
The shape and geometric parameters of sharpening the drill tip are shown in fig. 9.7, e. Usually m = 0.75, K=0.6-:-1.5 mm, f=0.2-:- 0.975 mm.
Recommended modes of drilling with gun drills when machining carbon steels: v= 80 -:- 125 m/min, So=0.01-:-0.1 mm/rev; when machining cast iron with HB<=250: v=65 -:- 100 m/min, So=0.005-:- 0.2 mm/rev.
Rice. 9.8. Spade drills with indexable inserts for drilling deep holes:
1 – indexable insert, 2 – clamping screw, 3 – drill body, 4 – coolant fitting
For drilling deep holes with a diameter of 25 to 130 mm in structural and alloyed steels and cast irons on universal machines of the drilling and boring group, prefabricated spade drills with replaceable plates made of high-speed steel are also successfully used (Fig. 9.8).
Depth of processing with a horizontal method of drilling up to 10-30 diameters, vertical – up to four diameters of drills.
Replaceable high-speed cutting plate 1 is fixed in the groove of the drill body 3 with a screw 2; the holes made in the body of the drill, through the fitting 4, serve to supply coolant to the cutting zone.
Inserts are made with a wear-resistant coating that doubles tool life.
Rice. 9.9. Scheme of a device for ejector drilling of deep holes
When machining deep holes, the ejector method of supplying and removing coolant is used.
The device for fastening ejector drills (Fig. 9.9) allows you to perform high-speed drilling of deep holes with a diameter of more than 20 mm with chip removal through the hole in the inner pipe.
The outer stem 2 of the device is fixed with a nut 7 in the body 4. The body with a shank 5 is installed in the hole of the tailstock or a special stand connected to the caliper to provide machine feed to the drill 1.
A hollow drill of a special design, fixed with a threaded shank in the outer stem, has a conical nozzle 8 inside which, with the cone of the coaxial inner pipe 3, forms an ejector (jet) pump.
Lubricating fluid , which is supplied from the pumping station of the machine through the pipe 6 and the gap between the outer stem 2 and the inner pipe 3, is divided into two streams in the zone of the jet pump.
One stream, having passed the hole 9 in the drill body, is fed into the cutting zone, and the second one ensures the operation of the jet pump, which creates suction of the coolant together with the chips from the cutting zone and transports the chips through the hole of the inner pipe to the chip receiver.