Sawing Machines: Meaning and Classification | Machine Tools | Engineering

In this article we will discuss about the meaning and classification of sewing machines.

Meaning of Sawing Machine:

The most common used materials like steel, aluminium, brass, plastics etc. are available in standard sizes and shapes. But before the metal is presented to a machine tool it is usually cut to some specified length and operation of cutting is generally performed by sawing. Therefore, metal sawing is chiefly concerned with cutting bar-stock to convenient length for machining.

It is very important that the designer should know and consider the standard sizes before completing his dimensions of the part to be designed. In many instances of design, a part size can be altered slightly in dimension in order to use a standard stock size and save money. Knowing the resulting variations in size of work resulting from the various sawing machines may be an important part of good designing.

Full utilisation of raw material will depend upon the design of the part, the skill and knowledge of the operators involved and the limitations and waste of the material during sawing operation used to prepare the work. Good planning may save money in many ways by reducing the cost of direct material, machine time and labour time. Sawing process has the advantages of doing job faster, saving material, using less power, and minimum material lost as chips.

Classification of Sawing Machines:

The sawing machines are broadly classified as:

(i) Band saw,

(ii) Circular saw

(iii) Reciprocating saw.

(i) Band Saw:

The band saw can rightfully be classified as a machine tool since it produces chips and assists in shaping work to our needs. Originally band saw was used for severing workpieces and doing rough contour sawing. But the modern band saw of today is a precision machine capable of performing a variety of operations quite accurately.

These machines are more expensive that hack-saws but are capable of faster cutting because of continuous action. The larger number of teeth, only a few in contact with the work at a given time, allows high surface speeds since the teeth cool off during their travel. Further, wear is distributed over many cutting teeth and tool life is prolonged. Average thickness of the cut in one travel of the saw is 1.5 mm.

A big advantage of band sawing machines is contouring ability and thus intricate curved shapes can be cut by a combination of hand and power feeds. Sawing has the advantage of uncovering the workpiece shape in a minimum number of cuts and only a small amount of waste material is reduced to chips.

There is no limit to the length, angle and direction of the cut. Some band saw machines are equipped with a hydraulic tracing attachment. Workpieces larger than the machine tool can be cut.

(a) Friction Sawing:

It is used to cut hard as well as tough materials which would be very difficult to cut by other means. In friction sawing, the workpiece is heated by friction created between it and cutting blade. The blade melts its way through the work.

Cutting teeth are not needed for friction sawing. Various kinds of indentations, or projections, of uniform height, along the saw edge, may be used. Speed upto about 6000 m/mt. is needed for circular sawing machines and 900 to 4500 m/mt. for band-sawing machines.

(b) Band Filing:

The band file consists of file segments attached to a spring steel band. As the band file passes through the work, an interlock device closes and keeps the file segment tight. The interlock then releases, permitting the file segment to roll around the band wheel. The band file has a locking slot so that the ends can be joined to form a continuous loop. Band files can be used in both internal and external filing applications.

(c) Band Polishing:

In band polishing, a continuous abrasive strip is used.

(ii) Circular Saw:

It is called circular saw as it has a circular blade with inserted teeth for cutting small or large bars to desired length. It has many of the characteristics of an inserted tooth milling cutter, although it is of much larger diameter. It runs at relative slow speed and is very powerful. Coolants may or may not be used with this type of saw. Average thickness of the cut is 6.25 mm.

(iii) Reciprocating Saw:

Reciprocating saw provides a relatively slow and expensive method of cutting.

Several bars may be cut at the same time and continuous attention is not needed since the saw will shut off automatically when the cut is finished. A lubricant is often used for easier cutting action and longer blade life.

A crank mechanism is provided to raise the blade on each return stroke so that teeth are not damaged by being dragged backward over the work. Average thickness of the cut is 3 mm. Fig. 17.7 shows the cutting action of reciprocating saw.

It is important to note the provision of dash pot mechanism by a cylinder and piston filled with viscous oil and connected between the base and reciprocating arm carrying the blade. It avoids the rapid wear of blade teeth, thereby increasing the blade life.

Automatic Cut-Off Saw:

Automatic cut-off saw can be programmed to cut a variety of workpiece sizes. Such a high speed, volume production vertical band saw has a mechanical bar feed that can be quickly and easily programmed for automatic operation. Programming cut-off lengths and number of cuts to be made is done on a keypad built into the operator’s console.

Several programs can be stored in the controlled memory, and such programs can be called up by touching appropriate keys on the pad. Programmable control adds a new dimension of efficiency, accuracy and economy to band sawing.

Selection of Blade for Band Saw Blade Material:

Band saw blade could be made of:

(i) Carbon steel (fully hardened saw teeth and back of blade is soft), available in wide variety of combination of width, set, pitch and thickness.

(ii) Carbon alloy steel (both teeth and back being hard­ened). Higher tension is permitted because of increased ten­sile strength and thus the cutting accuracy is greatly im­proved. It is well suited to contour sawing.

(iii) High speed steel and bimetallic high speed steel- used for high production and severe sawing applications. Wear is very less and it can withstand much more heat. These blades can cut faster, longer and more accurately.

Blade Pitch:

Pitch should be such that at least two teeth are in contact with the workpiece. Thus thin materials call for fine pitch blade and thick materials require coarse pitch blade. Latter helps in easy clearance of chips from the kerf.

Set Patterns for Saw Blade Teeth:

In order to produce saw cuts wider than the thickness of saw blade, the tips of the saw teeth are bent sideways slightly as shown in Fig. 17.8. This is called “tooth set” and its purpose is to allow a saw to move freely in the cut.

The teeth could be either racker set or wave set or straight tooth pattern. In racker set, one tooth is unset, and then two tooth after it are offset to the right and left respectively.

This pattern of 3 teeth is repeated for the whole length of the blade. It is used for heavy work on bar stock forgings, die blocks, etc. The straight set is most suitable for nonferrous metals and non-metals.

In wave set, teeth are offset in groups, alternately from right to left to form a wave pattern.

Fig. 17.9 shows the general features of saw tooth form. Pitch is the tooth spacing. Selection of pitch is dependent on the length of saw in contact with the workpiece. Longer saw cuts produce longer chips which require larger spaces for chips between the teeth. Thus longer saw cuts and softer materials call for coarse-pitched teeth.

Precision tooth form (standard form) is most widely used. It produces accurate cuts and fine finished surface. The tooth has a 0° rake angle, 30° back clearance angle, deep gullet (to provide enough chip capacity) and a radius at bottom.

Buttress (skip) form has rake angle of 0° and back clearance of 30°. Teeth are spaced wider apart to provide greater chip clearance. Recommended for thick work sections, deep cuts, and soft material.

Claw (hook) tooth has a positive rake angle. It cuts at a faster rate and at reduced feed pressures. It has long tool life. Tungsten carbide form has a positive rake angle and small clearance angle. Blades with wave set teeth are used for cutting variable cross sectional area material, like structural forms and pipes.

The force exerted by a workpiece against the saw blade in the direction of feed should be as high as possible without damaging the blade.

Cutting Action in Band Machine:

The action of the saw teeth cutting produces a huge quantity of heat.

The heat is generated at (Refer Fig. 17.11)

(1) The point of contact and along the cutting face (around 10% of total).

(2) The cutting edge (around 30% of total).

(3) The shear plane in the forming of chip (around 60% of total).

The tremendous amount of heat generated must be removed by cutting fluid, otherwise it will cause the teeth to soften and dull.

Cutting Fluids:

The cutting fluid is hacksawing is essential to:

a. Dissipate heat generated over small area of each saw tooth.

b. Remove heat to prevent softening the cutting edges of the saw teeth.

c. Reduce friction between chips and saw teeth.

d. Prevent the depositing of meta at or near the edges of the teeth.

e. Clear chips away from the workpiece.

f. Increase productivity and tool wear life (as the saw teeth dull and wear, the cutting efficiency is reduced drastically).

Cutting fluids may be:

i. Straight Oils:

These are mineral oils. It has high lubricity than heat dissipation. Used for very tough materials cut at slow speeds. Usually sulphur or other additives are added.

ii. Soluble Oils:

These mineral oils mix readily with water and the mixture has both lubricity and heat dissipating and cooling qualities. Because of evaporation of water, the mixture should be tested regularly and proper ratio maintained.

iii. Synthetic (Chemical-Water) Cutting Fluids:

No mineral oils. Used at high cutting speed to remove high heat. Chemicals result in wetter water (Ref. Fig. 17.12). Due to wetter action or better spreading, the coolant covers and makes contact with the entire area of cut to remove maximum heat.

Heavy Duty Production Hacksaws:

Power mechanisms of hacksaws are designed to automatically load, position, clamp, and cut off sections, and then stop.

The hydraulic system of a power hacksaw:

i. Moves the work table (carriage) or the saw frame.

ii. Controls down feed during cutting.

iii. Raises the frame and blade on the return stroke.

iv. Maintains a constant flow of cutting fluid.

v. Automatically raises the frame to clear the work at the end of the cut.

vi. After the movable carriage is loaded and cut off length set, the entire cutting off process is auto­matic and continuous one.

vii. When the workpiece is cut, all machine motions are stopped automatically.

Selection of Hacksaw Blades:

The following principles apply:

a. At least two teeth must be in contact with the work surface at all times.

b. The pitch is chosen proportional to cross-sectional area of cut, i.e. coarse pitch for greater cross- sectional area.

c. Coarse pitch is also used for easily machined and soft materials.

d. Finer pitches are used for hard materials and small cross-sectional areas.

Blade lengths generally used are 300 mm, 350 mm, 400 mm, 450 mm, and 600 mm. The pitch ranges from 2 mm to 6 mm.

Safe Practices in Power Hacksawing:

i. The cutting teeth and the blade should be posi­tioned to cut on the draw stroke.

ii. The blade should be so tightened that tension is adequate to hold the blade taut during the cutting operation.

iii. The blade pins should be checked regularly to en­sure that they are not being sheared.

iv. The workpiece should be tightened securely.

v. The ends of long pieces projecting from power hack­saw must be supported using a roller stand.

vi. The cut off section be cooled before handling to avoid burns and cuts from burred pieces.

vii. The cutting fluid must be directed towards the cutting area and cutting saw teeth.

viii. Before starting the power hacksaw the blade must be moved away from the work.

ix. The cutting fluid and reservoir must be kept clean. Regular testing for ratio of water and oil and cor­recting of fluid ensures that the evaporation of liq­uid does not change the efficiency of the cutting fluid.