In this article we will discuss about:- 1. Features of Boring Machines 2. Types of Boring Machines 3. Machining Time in Boring 4. Tools.
Features of Boring Machines:
There are several types of machines developed for enlarging the holes and for closely allied operations such as facing and tapping.
The important features of the boring machines are described in the following heads:
This is most important unit of the machine. The entire machine is built around it. It supports, drives and feeds the tool. It may contain one or two spindles. One spindle is heavy and slow moving for the heavier operations of boring, or drilling.
The other spindle is lighter and faster for drilling and tapping small holes and end mill work. Spindle rotation is reversible in either case for backing out tools and for right and left hand cutters.
The column provides support for the headstock and guides it up and down accurately by means of ways. It is heavily constructed and is hollow to house the counterweights which balance the headstock and make it easier to move. Columns are keyed, dowelled, and bolted to bases. Some columns are stationary whereas others move with their bases.
iii. The Column Base:
The base supports and secures the column. It houses the various gears and driving mechanisms. On floor type machines the column base is mounted so that the spindle is at right angle to the ways of the run way. On planer type machines, the spindle is parallel to the ways of the run way.
iv. End Support Column:
For operations involving the use of long boring bars and heavy tools, an out board bearing is utilised to support the end of the bar. There is an open and a closed type of end support. On table and planer type machines, the bearing block travels in synchronism with the headstock of the machine. When such an end support is used with floor type machines, it is adjusted separately from the headstock and aligned by means of an accurate scale and vernier.
v. Run Ways:
These are used on floor or planer type machines to carry the main column, end-support column, and in some cases, a rotary table. When the column base, column and headstock are traversed as a complete unit, the member upon which it travels is called a run way and not a bed.
vi. Table and Saddle:
The chief function of the table is to provide a support for holding the workpiece. It also provides a means of locking and clamping the work. It is equipped with suitable ranges of feeds as well as rapid traverse.
The table usually traverses at right angles to the axis of the spindles unless provided with saddle. The function of the saddle is to provide a compound movement of the table, so that it can move axially as well as transversely to the spindle.
It may be cast in one or several pieces. It serves to support the column and headstock, the end supports, the table, the saddle and the various feed and control shafts. It contains all the necessary feeding mechanisms for the table as well as a coolant tank.
viii. Floor Plate:
Floor-plates are made up of several sections keyed and dowelled together to form a continuous surface for fastening work. The floor plates are mounted on a foundation and are not adjustable after levelling into position. T-slots are provided for work-clamping.
ix. Cross Rail:
It is used only on the multiple-head machine.
The size of horizontal boring, mill is given by the size of the largest bar the machine can handle. An average machine may be expected to take a 75 mm bar, although machines upto 150 mm in size may be purchased.
The spindle speed and horse power are of order of 22 to 1600 r.p.m. and 10 to 80 h.p. respectively. While specifying capacity of the machine, one should mention the following points also in addition to the above points; column height, table size, floor area and weight of the job.
Types of Boring Machines:
A. Horizontal Boring Machine:
The horizontal boring mill is also known as horizontal boring, drilling and milling machine, and is intended to perform operations on relatively large pieces which cannot be rotated easily, are irregular and unsymmetrical, and require operations on many surfaces.
Fig. 18.40 shows the main features of a horizontal boring machine. It also indicates the relative movements of its sliding and rotating elements. It may be noted that the main spindle can be rotated in either direction. It is possible to feed the main spindle axially. The work table can be traversed along and across the machine bed.
It is possible to machine a flat surface square with bored hole in one setting of workpiece by mounting facing head over the spindle.
The four basic types of machines are described below:
i. Table-Type Machine:
It is perhaps the most familiar machine and consists of headstock, column, base, table, saddle, end support and bed. The headstock can be raised and lowered, the spindle moved in and out, and the out board bearing moved up and down. The table and its saddle can be moved in two directions, so as to position the work conveniently. The end-support column can be positioned along the bed towards or away from the headstock.
ii. Floor-Type Machine:
It consists of headstock, column, column base, run way, end support runway and floor plate. The chief difference between this and the table-type machine is the spindle movement. The machine is used for heavier work. During the operation, the work is stationary and the spindle end support is traversed along the run-way past the work.
iii. Planer Type Machine:
It consists of headstock, column, column base runway, end support, end support run-way, table and bed. This type resembles with the table-type, but the work supporting mechanism is different. It has a reciprocating table very similar to the planer which makes it most suitable for big and long jobs.
iv. Multiple Head Type:
It consists of headstocks (two to four), columns (two), bed, table and cross-rails. This machine resembles the planer miller. It may have two, three or four headstocks, all of which may be swiveled for angular cuts. It is the only machine that can do vertical as well as horizontal boring of milling, e.g., boring and milling of V- type diesel engine blocks.
B. Vertical Boring Machine:
There are two types of vertical boring machines, viz.,
i . Standard vertical boring mill and
ii. Vertical turret lathe.
Standard vertical boring mill is the largest of the machine tools. It is used to machine inside and outside diameters and facing large pieces which are more or less symmetrical such as large ring-gear blanks, steam turbine castings, water turbine runners, locomotive tires, tables for machine tools, flanges for large pipes and pressure vessels. The size of such a machine is given by the diameter of the largest workpiece which can be machined.
Vertical turret lathe is basically a vertical boring mill and has turrent arrangement of holding the tools. It can do essentially the same jobs, but on a smaller scale. The machine looks like a turrent lathe, with its head stock resting on the floor and its axis vertical. The table is usually called a chuck because of its adjusting jaws for work clamping.
The distinguishing feature of the machine, however, is its five sided turrent, or tool holder mounted on the cross rail. It has five tool positions which enable successive tools to be brought into position without demounting. The typical jobs which can be machined include boring and turning rail road wheels, locomotive cross-heads, large pistons, rings and gear blanks, bowls and many other similar round and symmetrical pieces.
C. Other Boring Machines:
Two-spindle vertical way type drilling and boring machine (Fig. 18.41):
i. Double Ended Machine (Fig. 18.42):
ii. Fine Boring Machines:
These are precision boring machines designed for high accuracy and surface finish. Boring spindle is very precisely supported on ball, roller, hydrostatic or air bearings. Often, diamond tools are used. These are available in horizontal (single end or double end type) and vertical spindle fashions. Axial feed is given by cam or hydraulic cylinder. Recirculating ball screw drive is used for feeding to achieve higher position accuracy in numerically controlled boring machines.
Machining Time in Boring:
Machining time in boring operation is calculated by the same formula as for calculating the machining time in turning operation,
The length of cut in this case is the depth of hole to be bored. Cutting speeds and feeds for boring are same as for turning.
A 10 mm drilled hole in a casting of 10 mm thickness is to be brought in alignment by boring. Calculate the time taken in boring operation, assuming cutting speed 30 metres/minute and feed 0.13 mm/rev.
Boring tools are held in boring tool holders which may be either fixed or rotating type. Fixed holders are used in work-rotating machines and rotating holders in tool rotating machines. Size of boring bar should be such as to provide maximum rigidity and also permit sufficient clearance for disposal of chips. Normally boring bar diameter is taken as 0.7 x bore diameter. It should have minimum overhang.
Various types of boring tool holders are:
i. Adjustable Boring Bars:
These are built with cartridges (to hold throwaway type inserts) adjustable in axial and radial directions by set screws.
ii. Damped Boring Bars:
In it, dampers are provided at the feed end to absorb vibrations.
iii. Line Boring Bars:
These are used for long bores and can accommodate fly cutters, tool blocks, cartridges etc. These are supported at the feed end in suitable bearings.
iv. Boring Head:
It is more rigid boring tool holder and is used for roughing and finishing of large bore (100 to 500 mm). Provision to offset cutting axially and radially exists. Two cutters can be mounted radially at 180° to have balanced cutting.
v. Boring and Facing Heads:
This enables boring of large bores and facing of seating surfaces at right angles to the bore. Radial tool feeding facility also exists.