The drilling or boring of long, relatively small diameter holes presents special difficulties which prevent the employment of ordinary drilling methods. Spade drills were originally used for this work, but lack of accuracy, poor quality of finish, and short tool life, coupled with the slowness of the operation due to need for frequent withdrawal of the drill in order to clear the swarf, made them far from satisfactory. Also, inspite of the provision of internal passages to conduct coolant to the tip, over heating still occurred.
One solution of this problem lies in the use of trepanning, an operation employed long before for producing large diameter holes in the plate, but it is being only recently adopted for machining bores in the solid metal. As a guide to the success of the operation it may be mentioned that, under ordinary production conditions, an accuracy of 0.025 mm per 1000 mm length for taper and ovality is readily maintained.
Cutting is done with a hollow head shown in Fig. 18.43 carrying one or more tungsten carbide tools, the head being secured to the end of a long hollow bar. The tools produce bore of slightly large diameter than that of the head end bar, and thus there is an annular space between them.
In some machines this is used for feeding high pressure cutting oil to cool the tips and wash the chips away, the oil and chips return to settling tanks and filters via the hollow cutter head bar; on others the reverse applies, the oil entering along the hollow bar, and the oil and chips returning along the annular clearance.
To a large extent trepan boring is only possible because of the availability of the tungsten carbides, because even the best tool steel has a very short life under the extremely arduous conditions encountered in machining deep holes.
Another solution of this problem lies in the use of pierce boring. In most of the respects, this operation is similar to trepan boring, except that the tool is designed to cut the waste metal completely into swarf, i.e. no core is produced.
To do this, two cutters shown in Fig. 18.43 are arranged to cover the entire radius of the hole, the width of the inner cutter being such that it slightly overlaps the centre of the hole and also the inner edge of the other cutter. Also one cutter is slightly in advance axially of the other. Each cutter has 5° positive top rake and front clearance of 6°.
A 0.5 mm high chip breaking step is provided approximately 1.5 mm behind the cutting edge. To support and centralise the head, three equally spaced tungsten carbide pads are provided. The piercing boring process can bore and machine a hole of size 60 mm diameter and 6.6 metres deep.
Deep Hole Tools:
These are substantially longer than their diameter. These must be designed to machine in a straight line without the aid of outside support and provide circulatory channels for coolant. The cylindrical peripheral elements of the tool body behind the cutting tip revolve on a finished surface and confine the direction of travel.
The various tools are:
A. Single Lipped Rifle Drill:
Its cutting edge is ground to lead in the hole and machine in clockwise rotation. It opens up holes from the solids and also sizes accurately in one operation. It is used for drilling holes upto 25 mm.
B. Two Lip Drills:
It drills holes of size 25 mm to 75 mm in the solid material. Coolant circulation to drill is permitted by provision of clearance between shank and machine hole. The drill body between flutes is relieved to bring the coolant to the cutting tip where it flushes the chips back through the flute and into the centre hole of the drill and shank.
C. Two-Lipped Deep Hole with Replaceable Tip:
Tool body can be used for long since the tip is replaceable.
D. Two-Blade Pack Bit (Back Reamer):
It is capable of boring long accurate hole with exceptionally small runout even when the pre-existing hole is rough or somewhat out. It consists of a central flat steel body equipped with two replaceable blades. Two pack strips are mounted on the body and these are turned slightly larger than the hole to be bored. The packs thus fit tight in the bore and guide the cutter. The coolant is carried through milled grooves in the flat body and ahead of the cut.
E. Four-Lipped Hollow Core Drill:
These are used for core drilling and enlarging deep holes where concentricity or axial straightness is not of major importance. The coolant circuit for blind hole is provided by the relief between the flutes and the return channel and through the centre of the tool.
F. Multi-Fluted Reamer:
It is similar to ordinary reamer. Coolant is carried around the tool shank and chips are washed out ahead of the cut or returned through the hollow of the tool in blind holes.