Special Purpose Machines Unit 7 Special Purpose Machines

Part (A) Broaching

Broaching Before understanding the process of broaching and the related machining tools it is required to know the clear difference between single point and multipoint cutting tool. A single point cutting tool normally consists of a single cutting point or an edge used for cutting. The example of single point cutting tool is turning tool used for turning tool is generally has more than one cutting points or cutting edges in action at a time. Example of multipoint cutting tool are milling cutter or a broaching tool.

Principal of Broaching Broaching is one of the metal machining operations done by a multipoint cutting tool called broaching tool or broach. The tool is made reciprocating linearly relative to the workpiece in the direction of tool axis. The relative movement, necessary fixtures for workpiece and the broach are provided by a machine tool called broaching machine. The broaching operation is depicted in Figure. The broaching is a high productivity method as so many cutting edges work to machine the workpiece at a time. The tool may be pulled or pushed through the surfaces to be finished. Surfaces finished by broaching either internal or external. External broaching is performed on the outside surface of the workpiece to create a pre-decided shape with dimensional accuracy and high degree of surface finish. Internal broaching is done on the internal surfaces of the workpiece. This way internal surfaces are brought to exact size with the required surface finish.

Its Construction Both pull and push type broaches are made in the form of slender rods or bars of varying section having along its length one or more rows of cutting teeth with increasing height (and width occasionally). Push type broaches are subjected to compressive load and hence are made shorter in length to avoid buckling. The general configuration of pull type broaches, which are widely used for enlarging and finishing preformed holes, is schematically shown in Fig.

The essential elements of the broach are : 1. Pull end for engaging the broach in the machine, pull end is made to attach the broach to the broaching machine through the puller head. 2.Neck of shorter diameter and length, where the broach is allowed to fail, if at all, under overloading 3.Front pilot for initial locating the broach in the hole, this centers the broach in the hole to be finished just before start of processing 4.Roughing teeth, these are the cutting edges which remove larger amount of stocks during cutting. Larger amount removal generates poor quality of surface finish but makes the operation faster. 5.Finishing and burnishing teeth, these are cutting edges removing smaller stocks of material. These are used for final finishing of the surfaces and their accurate sizing. 6.Rear pilot and follower rest or retriever, this is a supporting device to the broach when it is likely to complete its operation of broaching.

Broaching Methods On the basis of method of broaching the operation of broaching is classified as discussed below. 1.Pull Broaching Workpiece is clamped to the broaching machine in stationary position and the broach is pulled through the work. Broaches are usually long and are held in a special head. Pull broaching is mostly used for internal broaching. 2.Push Broaching Workpiece is held in the broaching machine in stationary position and broach is pushed through the portion of workpiece to the machined. Normally push broaching is done by hand and arbor presses (hydraulic press). This method is also recommended for internal broaching like for sizing and finishing the holes, cavities, and key ways.

3.Surface Broaching Any one of two, either workpiece or the broach (tool) is kept moving and other is kept stationary. The method is widely used as surface finishing operation. In case of surface broaching, the broaching tool is specifically designed for the shape to be finished. 4.Continuous Broaching In continuous broaching the broach is held stationary in the broaching machine and workpiece is moved continuously. The teeth of movement of the workpiece may be either straight, horizontal or circular. This is generally used for broaching a large number of similar workpiece at a time.

Material of Broach Common broach material is 18-4-1 stainless steel. As its name indicates, it has 4% chromium, 1% vanadium and 18% tungsten. This is corrosion and wear resistant steel. Carbide is also recommended for broach making, these broaches are used for broaching brittle material like cast iron in automobile industry. Inserted bit type and cemented carbide type broaches are also preferred to reduce the cost of broaches.

Advantages & Limitations of Broaching Major Advantages are • Very high production rate (much higher than milling, planing, boring etc.) • High dimensional and form accuracy and surface finish of the product • Roughing and finishing in single stroke of the same cutter • Needs only one motion (cutting), so design, construction, operation and control are simpler • Extremely suitable and economic for mass production Limitations are • Only through holes and surfaces can be machined • Usable only for light cuts, i.e. low chip load and less hard materials • Cutting speed cannot be high • Defects or damages in the broach (cutting edges) severely affect product quality • Design, manufacture and restoration of the broaches are difficult and expensive • Economic only when the production volume is large.

Part (B) Gear Cutting

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 : (a) Gear cutting by gear shaper. (b) Rack planning process. (c) Hobbing process. Out of above three methods, Gear Shaping & Gear Hobbing is to be covered in this chapter according to syllabus.

Gears cutting by 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 predefined amplitude.

The cutter and the gear blank both are set to rotate at very low rpm about their respective axis. 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.

Advantages & Limitations Main advantages of gear shaping process are described below : (a) Shorter product cycle time and suitable for making medium and large sized gears in mass production. (b) Different types of gears can be made except worm and worm wheels. (c) Close tolerance in gear cutting can be maintained. (d) Accuracy and repeatability of gear tooth profile can be maintained comfortably. Limitations Main limitations of gear shaping process are described below : (a) It cannot be used to make worm and work wheel which is a particular type of gear. (b) There is no cutting in the return stroke of the gear cutter, so there is a need to make return stroke faster than the cutting stroke. (c) In case of cutting of helical gears, a specially designed guide containing a particular helix and helix angle, corresponding to the teeth to be made, is always needed on urgent basis.

Gear Hobbing Operation & Principle: 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 and gear sprockets, etc.

Gear Hob Process of Gear Hobbing

Setup of Gear Hobbing Machine

Advantages & Limitation (a) Gear hobbing is a fast and continuous process so it is realized as economical process as compared to other gear generation processes. (b) Lower production cycle time, i.e. faster production rate. (c) The process has a larger variability’s in the following sense as compared to other gear machining processes. (i) Capable to make wide variety of gears like spur gear, helical gears, worms, splines, sprockets, etc. (ii) Process of required indexing (named so) is quite simplified and capable to make any number of teeth with consistent accuracy of module. (iii) A special type of gear named herringbone gear can be generated by gear hobbing exclusively. (iv) Wide variety of batch size (small to large volume) can be accommodated by this process. (e) Hob is multipoint cutting tool having multi cutting teeth or edges at a time few number of cutting edges work so lots of time is available to dissipate the generated heat. There is no over heating and cutting tool.

Limitation : In addition to above mentioned large number of advantages there is one glaring limitation of the process of gear hobbing. That is manufacturing of internal gears is not possible.

Gears Finishing Operations Surface of gear teeth produced by any of the generating process is not accurate and of good quality (smooth). Dimensional inaccuracies and rough surface generated so become the source of lot of noise, excessive wear and play between the pair of gears in mesh. These all result in loss of power to be transmitted and incorrect velocity ratios. This can be summarized as inefficient power transmission. In order to over come these problems some finishing operations are recommended for the produced gears. Commonly used gear finishing operations are described below : Gear Shaving Roll Finishing of Gear tooth Gear Burnishing Gear Honing Gear Lapping Out of the above 5 processes, shaving , honing & lapping is to be covered in this chapter

Gear Shaving Gear shaving is a process of finishing of gear tooth by running it at very high rpm in mesh with a gear shaving tool. A gear shaving tool is of a type of rack or pinion having hardened teeth provided with serrations. These serrations serve as cutting edges which do a scrapping operation on the mating faces of gear to be finished. Both are gears in mesh are pressed to make proper mating contact. A shaving tool with serrated teeth is explained by illustration in figure

Gear Lapping Those gears which are finish machined or shaved and then heat treated(hardened) are ultimately lapped to remove scale, improve surface finish on the teeth and rectify small errors due to distortion during the hardening process. In this process, gear to be lapped is run under load in mesh with one or more cast iron toothed laps. Either an abrasive paste is introduced between the teeth or fine abrasive powder, mixed in oil, is made to flow through the teeth during the operation. A very negligible amount or almost no metal removal takes place during the operation. As such, this process cannot be used to correct the tooth profiles or dimensional accuracies. It only helps in improving the surface finish

Gear Honing It is rather a newer process used for finishing gear teeth after shaving and heat treatment. It enables removal of scale, burrs and nicks from the surface of hardened teeth. 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 last 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.

Thank You……