Milling Operation
Pageproduced. The operation is continuous and only restricted in speed by time and heat needs. This approach is appropriate for items such as transformer terminals and container lips. The technology is extremely useful in the assembly of printed-circuit boards.
Because the temperatures involved are lower, a hand-type heated copper or iron-plated copper bit (soldering iron) is commonly utilised. Before applying the iron to the job, the tip is tinned. The most popular type of iron is electric, and it is utilised in numerous industries such as radio and electronics. To solder, the iron is heated to 250 - 300°C, the tip is soaked in flux, tinned with solder, and then applied to the soldered pieces to heat them at the junction. Simultaneously, solder is applied to the junction, where it is melted by the iron as it travels along the joint and enters the clearing. The solder cools and produces the seam here.
Introduction to milling
Milling is a machining operation that uses a revolving cutting tool to remove superfluous material from a work item. The revolving cutting tool known as a "Milling cutter" is a multiple-point tool with the shape of a solid of revolution with cutting teeth organised (equally spaced) on the periphery, end face, or both. The work could be held in a vice, a three-jaw chuck, an index head, a rotary table, between centres, in a customised fixture, or attached to the machine table. Milling is a process that includes simultaneous rotating cutter and linear (occasionally rotary) work motion, with the work fed against the cutter. The milling process is used to create flat, curved, or helical surfaces, threads, toothed gears, and helical grooves. In general, all milling processes can be classified into two types:
1. Peripheral milling or horizontal milling Here, the finished surface is parallel to the axis of the cutter and is machined by cutter teeth located on the periphery of the cutter as
2. Face milling or vertical milling The final surface in face milling is at right angles to the cutter axis and is obtained by the teeth on the cutter's periphery and the flat end as shown in Fig. 3.28 (b). Methods for Creating Surfaces Depending on the relative direction of feed of the worktable and the rotation of the cutter, there are two ways for milling flat surfaces with plain milling cutters (these cutters have teeth just on the cutter's periphery).
3.Conventional (up) milling Here, the direction of feed of the worktable is opposite to the direction of rotation of the cutter
3. Climb (down) milling The rotation of the cutter and the direction of the worktable feed are the same in this case, as shown in Fig. 3.28 (b).
In either instance, the individual milling chips have a varied thickness and a cross section resembling a comma. The thickness progressively grows during the cut in traditional milling; in climb milling, the cutter tooth takes a chip of maximum thickness at the start of the cut and zero thickness at the finish. The advantages of conventional milling are that the pressure on each cutter tooth is gradually increased and the teeth begin cutting beneath the workpiece's metal skin. The initial tooth contact is normally in clean metal and concludes with the rough surface scale being lifted or peeled off. As a result, this kind of milling is suited for machining sand castings, forgings, and metals with a rough or hard abrasive surface scale. The downsides are that the cutter tends to lift the workpiece from the worktable, and because the teeth tend to dull, each tooth glides across the workpiece surface a minute distance before beginning to cut, resulting in a distinctive wavy surface. There is a propensity to lift the work table as well as the job off the table or fixture (since the cutting forces are directed upwards). This Manufacturing Engineering 112 / Page increases the amount of space between the table and the bed or saddle ways. When making large cuts, such clearances cause vibrations that degrade the job's surface polish. Chips build again ahead of the cutting zone, where they can be picked up by the teeth and moved around, causing the finish to deteriorate. Furthermore, where clamping is insufficient, it is difficult to process thin sheets and workpieces. Because the table must be fed against the cutting force, the feed motor and main motor power consumption are higher. The work must be rigidly held to offset the tendency of the cutting forces to raise or drag the piece out of the vise or fixture, which is advantageous for conventional milling. The advantages of climb milling are as follows: the task is driven against the table, and the table is forced against the ways. Excess clearance in jointing surfaces and the resulting vibrations are eliminated. Furthermore, the chips are deposited behind the cutter, out of the way. These characteristics result in a superior surface polish than traditional milling. Again, less power is required in climb milling because the feed action consumes less power. Cutting speeds and feed rates can be increased. Thin sheets can also be easily machined. The following criteria are favourable for climb milling: the workpiece has no hard skin, the milling machine is in good working order, and there is no excessive backlash in the table screw and nut, as any looseness will allow the cutter to draw the workpiece ahead and take too large bites. The technique is typically utilised for milling operations on thin and complicated objects.
Types of milling machines
Milling machines were basically develop-ed to machine flat surfaces. But, the present milling machines can machine flat, contoured and helical surfaces, cut gears and do various other jobs. Due to all this, a milling machine is one of the most useful and necessary machine tools found in the shop and it ranks next to the lathe in importance. Milling machines are designed to hold and rotate milling cutter or cutters, hold the workpiece and feed the workpiece to the milling cutter in one of several directions. Milling machines can be classified in different ways:
(a) According to the axis of the spindle of the machine, we have: Horizontal milling machines with horizontal spindle.
(b) Vertical milling machines with vertical spindle. According to their purpose, the milling machines may be classified as:
(i) General purpose milling machines (ii) Production milling machines and (iii) Special purpose milling machines.
1 General purpose milling machines The most common kinds of milling machines in this category are the column and knee type models which are all single spindle machines. The various models under this category are:
(a) Plain (Horizontal spindle) (b) Universal (Horizontal spindle with swivel table) (c) Universal (Horizontal spindle with swivel table and swivel knee). (d) Vertical spindle.
(a) Universal milling machine. The universal horizontal milling machine varies from the plain horizontal milling machine in that its table may be swivelled through a 45-degree angle in the horizontal plane to mill helical grooves (e.g. the helical flutes of twist drills or the teeth of helical gears). The saddle is divided into two pieces so that the table can be rotated horizontally.
(b) Omniversal milling machine. When compared to a universal milling machine, an omniversal milling machine features an additional movement. On an axis perpendicular to the column face, the knee can be rotated. This allows for the creation of tapered spiral grooves in reamers, teeth on bevel gears, and angular holes, among other things.
(c) Vertical milling machines. The vertical milling machine is very similar to the plain horizontal milling machine except that the spindle is held in a vertical position instead of horizontal. Vertical milling machines are of two types:
Fixed bed type, and column - and - knee type, (Fig. 3.31). The column-and-knee type vertical machine has a vertical head and spindle, but the same feeds and adjustments as the plain horizontal machine. The overarm provides a strong support to the spindle. On some models, the head can be moved up and down to perform operations like grooving, slotting, die forming, facing, drilling and boring of accurate holes, with their exact locations. The machine can be with a fixed head and of swiveling head type with the head swiveling around a horizontal axis.
