A Model To Increase Productivity In Gear Generation By Using a Radial Drilling Machine

ABSTRACT

 In general, Gears are manufactured on Shaper, Planer, Hobbing and Milling machines which are very expensive, quite large in size and cannot be used in small workplaces. In this thesis, we introduce a model to overcome the difficulties in which gear generation can be accomplished using a Radial Drilling Machine. The model acts as an add-on attachment in the Radial Drilling Machine to manufacture Spur and Helical gear. The model incorporates a simple modified indexing mechanism for easy operation.

We expect this new approach to dramatically reduce the cost of gear generation with simple operations and without any loss of quality, hence making mass production viable. A great reduction in space is achieved.

CHAPTER-1

INTRODUCTION

A gear   is a rotating machine part having cut teeth, which mesh with another toothed part to transmit torque, in most cases with teeth on the one gear being of identical shape, and often also with that shape on the other gear.  Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, torque, and direction of a power source. A gear can also mesh with a non-rotating toothed part, called a rack, thereby producing translation instead of rotation.

The gears in a transmission are analogous to the wheels in a crossed belt pulley system. An advantage of gears is that the teeth of a gear prevent slippage. When two gears mesh, and one gear is bigger than the other, a mechanical advantage is produced, with the rotational speeds and the torques of the two gears differing in an inverse relationship.

SPUR GEAR 

Spur gears or straight-cut gears are the simplest type of gear. They consist of a cylinder or disk with the teeth projecting radially, and although they are not straight-sided in form the edge of each tooth is straight and aligned parallel to the axis of rotation. These gears can be meshed together correctly only if they are fitted to parallel shafts.

TERMINOLOGY OF SPUR GEAR

• Pitch surface: The surface of the imaginary rolling cylinder (cone, etc.) that the toothed gear may be considered to replace.
• Pitch circle: A right section of the pitch surface.
• Addendum circle: A circle bounding the ends of the teeth, in a right section of the gear.
• Root (or dedendum) circle: The circle bounding the spaces between the teeth, in a right section of the gear.
• Addendum: The radial distance between the pitch circle and the addendum circle.
• Dedendum: The radial distance between the pitch circle and the root circle.
• Clearance: The difference between the dedendum of one gear and the addendum of the mating gear.
• Working depth: Depth of engagement of two gears, that is, the sum of their operating addendums.

• Face of a tooth: That part of the tooth surface lying outside the pitch surface.
• Flank of a tooth: The part of the tooth surface lying inside the pitch surface.
• Circular thickness(also called the tooth thickness): The thickness of the tooth measured on the pitch circle. It is the length of an arc and not the length of a straight line.
• Tooth space: The distance between adjacent teeth measured on the pitch circle.
• Backlash: The difference between the circle thickness of one gear and the tooth space of the mating gear.

Backlash =Space width – Tooth thickness

• Circular pitchp: The width of a tooth and a space, measured on the pitch circle.
• Diametral pitchP: The number of teeth of a gear per inch of its pitch diameter. A toothed gear must have an integral number of teeth. The circular pitch, therefore, equals the pitch circumference divided by the number of teeth. The diametrial pitch is, by definition, the number of teeth divided by the pitch diameter.

   Diametral pitch (p) =N / d

Where ‘N’ is the number of teeth‘d’ is the pitch circle diameter

• Modulem: Pitch diameter divided by number of teeth. The pitch diameter is usually specified in inches or millimeters; in the former case the module is the inverse of diametrial pitch.

   Module (m) = d/n

• Fillet: The small radius that connects the profile of a tooth to the root circle.
• Pinion: The smallest of any pair of mating gears. The largest of the pair is called simply the gear.
• Velocity ratio: The ratio of the number of revolutions of the driving (or input) gear to the number of revolutions of the driven (or output) gear, in a unit of time.
• Pitch point: The point of tangency of the pitch circles of a pair of mating gears.
• Common tangent: The line tangent to the pitch circle at the pitch point.
• Base circle: An imaginary circle used in involute gearing to generate the involutes that form the tooth profiles.
• Pressure angle (): The complement of the angle between the direction that the teeth exert force on each other, and the line joining the centers of the two gears. For involute gears, the teeth always exert force along the line of action, the pressure angle is constant.

Three spur gear tooth forms are generally used with pressure angles of 14½, 20, and 25 degrees. The 14½ degree tooth form is being replaced and made obsolete by the 20 and 25 degrees.

HELICAL GEAR

Helical gears are similar to spur gears except that their teeth are cut at an angle to the axis rather than straight and parallel to the axis like the teeth of a spur gear. Helical gears are used to connect non-intersecting shafts. Helical gears are manufactured as both right and left-hand gears. The teeth of a left-hand helical gear lean to the left when the gear is placed on a flat surface. The teeth of a right-hand helical gear lean to the right when placed on a flat surface. Opposite hand helical gears run on parallel shafts. Gears of the same hand operate with shafts of 90º.

The teeth on helical gears are cut at an angle to the face of the gear. When two teeth on a helical gear system engage, the contact starts at one end of the tooth and gradually spreads as the gears rotate, until the two teeth are in full engagement. This gradual engagement makes helical gears operate much more smoothly and quietly than spur gears. For this reason, helical gears are used in almost all car transmissions. Because of the angle of the teeth on helical gears, they create a thrust load on the gear when they mesh. Devices that use helical gears have bearings that can support this thrust load. One interesting thing about helical gears is that if the angles of the gear teeth are correct, they can be mounted on perpendicular shafts, adjusting the rotation angle by 90 degrees. Helical gears are used to connect non-intersecting shafts. Boston standard helical gears with 45-degree helix angles (a term that will be discussed below) are used to connect parallel shafts or shafts at right (90º) angles. Helical gears are manufactured as both right and left-hand gears. The teeth of a left-hand helical gear lean to the left when the gear is placed on a flat surface. The teeth of a right-hand helical gear lean to the right when placed on a flat surface

The tooth profile of a helical gear is an involute curve from an axial view, or in the plane perpendicular to the axis. The helical gear has two kinds of tooth profiles – one is based on a normal system, the other is based on a transverse system. Pitch measured perpendicular to teeth is called normal pitch, pn. And pn divided by π is then a normal module, mn = p/π. The tooth profile of a helical gear with applied normal module, mn, and normal pressure angle αn belongs to a normal system. In the axial view, the pitch on the reference is called the transverse pitch, pt . And pt divided by π is the transverse module, mt = p/π. These transverse module mt and transverse pressure angle αt at are the basic configuration of transverse system helical gear. In the normal system, helical gears can be cut by the same gear πpn.

Hob if module mn and pressure angle at are constant, no matter what the value of helix angle β. It is not that simple in the transverse system. The gear hob design must be altered in accordance with the changing of helix angle β, even when the module mt and the pressure angle at are the same. Obviously, the manufacturing of helical gears is easier with the normal system than with the transverse system in the plane perpendicular to the axis. When meshing helical gears, they must have the same helix angle but with opposite hands.

METHODS OF GEAR MANUFACTURING

In broader sense the gears can be manufactured by the following three methods:

 (a) Casting

 For casting of gears sand moulds or permanent moulds are prepared, and then molten metal is poured into the mold cavity to get the required gear. Cast iron gears are made by this method comfortably. These gears (casted gears) cannot be very fine, these are rough, low strength, and with some inaccuracies in operation. There cost of production is very low. This method is recommended for manufacturing of large sized gears where cost and power transmission are important than operating accuracy and noise level.

(b) Plastic Moulding

Plastic mould is also one of the ways of gear manufacturing. In plastic moulding gears of plastic material can be manufactured by using injection moulding or compression moulding. These are the very light duties gears used for transmission of very low amount of power but maintains velocity ratio accurately. Plastic moulding is also used for making gears of metal. In this process the metallic workpiece is heated first to bring it to a plastic state and then it is moulded to the required shape with the help of mechanical tools, die, and application of pressure. This process is used to make light duty smaller gears with accuracy. Non-ferrous metals can also be used as raw material for gear making by plastic moulding methods.

(c) Machining

 This is most widely used gear manufacturing method. Gear blank of accurate size and shape is first prepared by cutting it from metal stock. The gear blank can also be the metal casting. This method lies under the category of chip forming process. Gear is prepared by cutting one by one tooth in the gear blank of desired shape and size along it periphery. Different gear cutting methods are used in this category. These methods are described in details

(d) Gears Shaping

 Gear shaping is one of the gear generating methods. In this process gear tooth are accurately sized and shaped by cutting them by a multipoint cutting tool. Various gear shaping processes are listed and then described below:

(i). Gear cutting by gear shaper.

(ii). Rack planning process.

(iii). Hobbing process.

 (i). Gear Cutting by Gear Shaper

This process uses a pinion shaped cutter carrying clearance on the tooth face and sides and a hole at its centre for mounting it on a stub arbor or spindle of the machine. The cutter is mounted by keeping its axis in vertical position. It is also made reciprocating along the vertical axis up and down with adjustable and predecide amplitude. The relative rpm of both (cutter and blank) can be fixed to any of the available value with the help of a gear train. This way all the cutting teeth of cutter come is action one-by-one giving sufficient time for their cooling and incorporating a longer tool life. The specific advantages of the process over other processes, its product cycle time is very low and negligible dimensional variability from one unit to other in case of mass production. The principle of gear cutting by this process as explained above is depicted in the Figure 1.4. The main parameters to be controlled in the process are described below.

 (ii). Gear Shaping by Rack Shaped Cutter

In this method, gear cutting is done by a rack shaped cutter called rack type cutter. The principle is illustrated in Fig 1.5. The working is similar to shaping process done by gear type cutter. The process involves rotation (low rpm) of the gear blank as the rack type cutter reciprocates along a vertical line. Cutting is done only in the downward stroke; the upward stroke is only a return movement. The main difference of this method with the previous one is that once the full length of the rack is utilized the gear cutting of operation is stopped to bring the gear blank to its starting position so that another pass of gear cutting can be started. So this operation is intermittent for cutting larger gears having large number of teeth over their periphery.

(iii). Rack Planning Process

This process is used for shaping of spur and helical gear teeth with the help of a rack type cutter. In this process the gear blank is mounted on a horizontal aims and rotated impertinently. At the same time the gear blank is kept in mesh with a reciprocating rack type cutter. The process is shown in Fig 1.6. The teeth cutter gradually removes material to cut the teeth and to make the required profile. The whole operation includes some important operations. These are feeding cutter into the blank, rolling the blank intermittently and keeping cutter in mesh with the rolling gear blank. After each mesh the gear blank is rolled by an amount equal to one pitch of gear tooth. After each cutting, the rack is withdrawn and re-meshed after the rotation of gear blank.

(iv). Gear Hobbing Process

In addition to the gear shaping process another process used for gear generation is gear hobbing. In this process, the gear blank is rolled with a rotating cutter called hob. Gear hobbing is done by using a multipoint cutting tool called gear hob. It looks like a worm gear having a number of straight flutes all around its periphery parallel to its axis. These flutes are so shaped by giving proper angles to them so that these work as cutting edges. In gear hobbing operation, the hob is rotated at a suitable rpm and simultaneously fed to the gear blank. The gear blank is also kept as revolving. Rpm of both, gear blank and gear hob are so synchronized that for each revolution of gear bob the gear blank rotates by a distance equal to one pitch distance of the gear to be cut. Motion of both gear blank and hob are maintained continuously and steady. The hob teeth behave like screw threads, having a definite helix angle. During operation the hob is tilted to helix angle so that its cutting edges remain square with the gear blank. Gear hobbing is used for making a wide variety of gears like spur gear, helical, hearing-bone, splines and gear sprockets, etc.

Three important parameters are to be controlled in the process of gear hobbing indexing movement, feed rate and angle between the axis of gear blank and gear hobbing tool (gear hob). The aims of hob are set at an inclination equal to the helix angle of the hob with the vertical axis of the blank. If a helical gear is to be cut, the hob axis is set at an inclination equal to the sum of the helix angle of the hob and the helix angle of the helical gear. Proper gear arrangement is used to maintain rpm ratio of gear blank and hob.

The operation of gear hobbing involves feeding the revolving hob till it reaches to the required depth of the gear tooth. Simultaneously it is fed in a direction parallel to the axis of rotation. The process of gear hobbing is classified into different types according to the directions of feeding the hob for gear cutting.

GEAR MILLING

The gear milling operation is used for gear cutting. All types of gears can be made by using gear milling. Milling cutter is selected specifically for a particular type of gear and module. The periphery of the gear blank is divided into required number of equi-spaced parts. The required number of parts should be equal to the number teeth to be made on the gear blank. The method of dividing the periphery is called indexing which is an integral part of the operation of gear milling. The details of the operation of gear milling and indexing are explained in course. Gear milling is a slower process of gear generation as compared to other gear generation process. In this process gear is generated by cutting one-by-one tooth. Gears are to be made; it is not suitable for larger batch size. The other methods required very high capital cost and setup cost as compared to gear milling so these are not economical for smaller batch size, only gear cutting by milling operation is recommended for smaller batch size.

GEAR HONNING

It is used for super finishing of the generated gear teeth. Honing machines are generally used for this operation. The hones are rubbed against the profile generated on the gear tooth. Gear lapping and gear honing are the lost finishing operations of a gear generation process. In the above gear finishing operations some operations are based on metal cutting by removing very small size of chips like gear shaving, gear grinding, lapping and honing and some other operations like gear burnishing, roll finishing and based on finishing by plastic deformation of metal.

RADIAL DRILLING MACHINE

Drilling Machine is used for making holes with removal of chips. Drilling machines are used for drilling, boring, countersinking, reaming, and tapping. Several types are used in metalworking: vertical drilling machines, horizontal drilling machines, center-drilling machines, gang drilling machines, multiple-spindle drilling machines, and special-purpose drilling machines.

This is probably one of the most popular types of drilling machine. Due to its radial orientation, the radial arm drilling machine provides a great deal of versatilit

in its operation, most especially on larger and heavier parts. The set-up of this drilling machine allows the operator to manipulate the machine over the workpieces, thus making it more convenient to operate and drill on large workpieces. Moreover, the radial arm drilling machine can be coupled with a tilting table, or a trunion table, for the drilling of intersecting and angular holes. This saves considerable time because it is much faster to reposition the drill head than it is to unclamp, move, and then re-clamp the workpiece to the table. The size of work that can be handled may be considerable, as the arm can swing out of the way of the table, allowing an overhead crane to place a bulky workpiece on the table or base.

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