Planing, Shaping and Slotting

 

Planing and shaping machines are used to machine plane surfaces. Shaping and planning machines remove metal in series of straight cuts, in contrast to the rotary cutting action of other machines. The process is accomplished either by reciprocating a cutting tool while the work is fed, as in case of shapers, or by reciprocating the work while the tool is fed, as in planning (Fig. 6.13a, b).

Shaping machines (shapers) (Fig. 6.14) are also used for cutting of recess or grooves in workpieces, for manufacturing of dies, for tool making and similar purposes. Shapers are distinguished by the fact that a single-point cutting tool is reciprocated perpendicular to the longitudinal axis of the workpiece, which is clamped on the table 6. Working motion of the ram 9 is up to 1000 mm.

Slotting and relief-slotting machines have the same working principle as shaping machines but working and free running of tool is vertical. Examples of jobs that can be done by the slotting machines are shown in Fig. 6.15.

Fig. 6.13. Shaping (a), planing (b) and slotting (c)

Fig. 6.14. Shaping machine: 1 – foundation plate; 2 - body; 3 – traverse for transference S_h

of the table 6 horizontally; 4 – column; 5 – screw for transference S_v of the table 6 vertically; 7 – tool;

8 – support; 9 - ram, doing horizontal motions: V_w – working stroke and V_r – return stroke; 10 – drive

Fig. 6.15. Examples of jobs that can be done by the slotter

 

The planer is the largest of the reciprocating machine tools (Fig. 6.16). Since it is larger than the shaper and the miller, the planer can take work, which neither of those machines can handle. It is also capable of taking mach heavier cuts than either of those machines. The shaper moves a tool against the workpiece, but the planer moves the work against the tool. The work is mounted on a table, which is supported throughout its entire movement, so a maximum support is obtained. Like the shaper, the planer is intended to produce vertical, horizontal or inclined planes. It can also produce certain warped surfaces if these surfaces are composed of straight-line elements and the work can be indexed or turned. Modern planers have tables of up to 25...30 m in length.

Fig. 6.16. Block diagram of a planer: 1 – body; 2 - table; 3, 8 – colomns; 4, 6, 9,11 - tool-heads;

5, 10 – traverses; 12 – drive

 

Milling

 

Milling is the metal cutting operation, in which chip is detached by series of cutting edges arranged on the circumference of rotary milling cutters. Due to the high number of cutting edges, a large volume of chip can be removed in the single operation. Frequently, the workpieces may be finished in the single operation.

Milling tool is a cutter, which rotates around either horizontal or vertical axis. In accordance with this there are two types of milling (horizontal and vertical one) and three types of milling machines: horizontal, vertical (Fig. 6.17) and universal. The latter machine has the spindle, which can incline. Two methods of horizontal milling are distinguished: cut-up (conventional) and down-cut (climb) milling
(Fig. 6.18).

 

Fig. 6.17. Horizontal (a) and vertical (b) milling machines: 1-base; 2-column;

3-electric motor with belt transmission; 4-spindle speed gear box; 5-horizontal or vertical spindle;

6-overarm; 7-outer arbor support; 8-table; 9-slide; 10-knee; 11-feed gear box

Fig. 6.18. Conventional (up) (a) and climb (down) (b) milling: thick-ness of removed metal;

d_max – maximum chip thickness; S_z – feed motion on one tooth of cutter

 

The milling cutters used in horizontal machines are called hobs. The longitudinal axis of the cutter runs in parallel to the surface of the workpiece to be milled. Teeth of the cutter penetrate into the material when cutter is rotated. In cut-up milling process (Fig. 6.18a) the cutter rotates (cutting motion V) against the direction of movement of the work (feed motion S_L). The workpiece runs against the milling cutter. Each tooth or cutting edge of the cutter starts with zero chip thickness (point A) and increases it to maximum d_max as tooth leaves the work (point B). So, the load on the tooth increases gradually. But cutting forces aspire to lift the workpiece causing its vibration and deterioration of the machined surface. Nevertheless, cut-up milling method is the ordinary or conventional method used in horizontal milling machine work.

In the down-cut milling method (Fig.6.18b) the cutter revolves in the same direction as the work moves. This results in maximum chip thickness d_max at the beginning of the cut tapering off to zero thickness at the end of the cut and in high shocking loads on the teeth. Therefore, this method can be used only with particularly steady milling machines. The workpiece in this method is pressed to a table of the machine that results in high quality of surface finish.

Both in conventional and down-cut milling the cutting tool and machine are loaded not uniformly. This non-uniform stress may be balanced to a considerable extent by a special arrangement of the cutting edges of the cutter, e.g. in spiral teeth milling cutter (Fig. 6.19a).

Milling cutters for horizontal milling machines. Cylindrical milling cutters are primarily used for milling the plane surfaces (Fig.6.19a).

If both plane surfaces and end faces are to be milled at the same time shell-end cutters (Fig. 6.19c) are used (sometimes they are called corner cutters).

If the workpieces are to be slotted circular saw blades of the required width are used
(Fig. 6.19q).

Angular cutters (Fig. 6.19i) are used to produce straight-sided shapes. To manufacture contoured surfaces or profiles formed milling cutters (Fig. 6.19k) of the appropriate shapes are used.

V

Fig. 6.19. Basic milling operation and tools

 

If surfaces of different dimensions or certain contours are to be produced on a workpiece, it is possible to use a set of milling cutters (gang cutters) (Fig. 6.19h) with a view to machining the workpiece in one single operation.

Basic operations and tools of vertical milling machines. The cutter, mounted in the work spindle of vertical milling machines, normally operates in a position perpendicular to the surface of the workpiece to be milled. The teeth of the cutter remove a uniformly thick chip. This results in more uniform stress on the milling machine. In vertical milling the cutter performs the cutting action with the teeth on the end face. Therefore, this cutting operation is also known as face milling (Fig. 6.19b). To mill grooves end (Fig. 6.19o) or side milling cutters are used; their width should be equal to that of the groove to be milled. In many cases milling by means of end milling cutters has the advantage over horizontal milling. Vertical milling machines are used to produce flat surfaces and are also employed to cut recesses of various type in work pieces.

 

 

Gear - Cutting Methods

 

The two principal methods are employed in gears manufacturing: form cutting and generating. In form cutting process cutting tool with cutting edges formed to the shape of the tooth space to be cut is used. Gear-tooth milling cutters of the disk or end-mill type are examples of such tools (Fig. 6.20).

If a single tool is employed, the cutting alternates with indexing, i. e. turning the blank to the next tooth space or 1/z revolution, where z is the number of teeth of the gear. The production capacity of this method is low since each tooth space is machined separately, and time is lost in returning the tool to its initial position and indexing the gear blank.

Fig. 6.20. Form gear-cutting: a, b – scheme; a₁, b₁ – drawing; 1 – cutting tool; 2 – billet;

3 – device for periodical turn of the billet on 1/z

 

In mass production the form cutting principle is applied in the multiple tool slotting cutter head used to cut all the tooth spaces of the gear at the same time (Fig. 6.21). This cutter head has as many radially arranged form tools as the number of teeth spaces (teeth) in the gear to be cut. The profile of the tools is exactly of the same shape as the gear-tooth spaces. During each full stroke V (cutting and return) of gear blank each tool is fed radially towards the blank V_r, by the amount equal to the infeed prior to each cutting stroke. All the tools by motion V_v of keeper 3 down are simultaneously retracted from the work in the return stroke to avoid rubbing of tool clearance surface against the machined surfaces. All the tooth spaces are cut simultaneously, and the gear is finished when the tools reach their full depth of cut.

Fig. 6.21. Gear cutting by multiple tool slotting head: 1-gear; 2-radially arranged tools

 

The production capacity of this gear-cutting method is very high because accuracy of the cut gears depends only upon the accuracy of the cutter head, which may be sufficiently high.

Comparatively complex manufacture of the cutter heads and the necessity to have a separate head for each gear are among the drawbacks of this method.

The generating process is based upon the meshing of the cutter with gear being produced to develop the tooth by the relative rolling motion of the cutter and the work. For this purpose the cutter is shaped like a gear, gear rack or worm, i.e. a part which could mesh with the gear being cut; or the tool is made so that its cutting edges describe in space the surfaces of the tooth profiles of a certain imaginary gear or rack, known as the generating gear or rack.

In this general type of gear-cutting machines the cutting tool resembles either a gear and is called a rotary gear cutter (shaper), or a gear rack, in which case it is called a rack-type cutter.

A gear slotter (operating with the rotary type cutter 1) has the following principal motions (Fig. 6.22):

- straight line primary cutting motion V accomplished by travel of the cutter only in one direction (cutting stroke) and return of the cutter to the initial position (return stroke);

- continuous rotation of the cutter V_c and the gear blank V_b to obtain the circular feed (indexing motion);

- feeding motion (radial infeed) of the cutter S_c is obtained by travel of the cutter 1 axis in a direction towards the blank 2 axis with reciprocation of the blank V_b to avoid friction while return stroke.

Fig. 6.22. Rotary type gear cutter: a – drawing; b – cutting angles of the cutter;

c – work part of gear-cutting machine; d – gear-cutting machine: 1 – cutter; 2 – blank;

3 – driving mechanism; 4 – cutting head; 5 – traverse; 6 – table

 

Grinding

 

Grinding is a metal cutting process carried out with abrasive grains, which are bonded by special materials. Usually grinding tool has a shape of a disk; sometimes it has a shape of a cone, a bar and so on.

The abrasive cutting has peculiarities:

- grains have negative top rake angle g (Fig. 6.23);

- the cutting speed is very high (v>30 m/sec); the rotary speed of wheels is limited by strength of wheel bond;

- the grinding wheel is similar in action to multi-tooth milling cutter; it presents about 300 million cutting edges to a workpiece in 1 minute; grains break off periodically to present new cutting edges; bond wears away  faster than grains, it allows to expose new grain for cutting; grinding wheel produces a chip just like any other cutting tool, but it is dust-fine chip, being visible only under a microscope;

- these chips are given in the form of sparks because of their high temperature.

 Corundum A1₂O₃, silica carbide SiC, boron carbide B₄C and diamond C are used as abrasive materials. According to their sizes the distinction is made between:

- grains, which have sizes from 2.00 to 0.16 mm;

- powders, which have sizes from 0.12 to 0.03 mm;

- micropowders, which have sizes from 0.04 to 0.005 mm.

 

Fig. 6.23. Cutting by grinding tool: 1 – abrasive grain; 2 – bond (matrix); 3 – pore; 4 – blank

 

Fireclay, water glass, quartzite (non-organic substances) and bakelite, rubber, phenol-formaldehyde resins (organic substances) and other substances are used as a bond.

A distinguishing feature of grinding machines is a rotative abrasive tool. This group of machines is employed chiefly in finish machining operations accomplished by removing layers of metal from the work surface with an accuracy that may reach tenths of a micron and producing a very high class of surface finish.

Grinding machines handle workpieces that have been previously machined, in the most cases, in other types of machine tools, leaving a small grinding allowance of magnitude depending upon the required class of accuracy, size of the work and the proceeding machining operations to which it has been subjected.

Operations efficiently performed by grinding machine include:

-roughing and cutting off blanks;

- precise machining of flat surfaces, surfaces of revolution, profiles of gear teeth, thread and other helical surfaces, contoured surfaces, etc.;

- sharpening of all types of cutting tools.

Cylindrical grinding machines (Fig. 6.24 a, b, c) are intended for grinding external cylindrical and tapered surfaces. The primary cutting motion is a grinding wheel rotation V_d. The auxiliary cutting motion is a workpiece rotation V_d. The longitudinal feed motion S_L is a work motion and infeed traverse motion S_t is the periodic crosswise tool motion D is diameter of a workpiece, t is thickness of removed metal.

Internal grinding machines are intended for grinding cylindrical (Fig.6.24d) and tapered holes. The end faces of the workpiece are usually ground in these machines as well.

In addition to the primary cutting motion of the grinding wheel rotation V_w internal grinders of the chucking type have the following working motions:

- work revolution V_p;

- longitudinal motion (the reciprocating motion of the work or grinding wheel) S_L;

- set-up - infeed motion (the periodic crosswise motion of the wheel (wheel head) S_t.

In the planetary-type internal grinders (Fig. 6.24e), designed for finishing holes in workpieces of irregular shape, or large heavy workpieces, work rotation is replaced by rotation of the axis of grinding wheel in a circle about the axis of the hole being ground V_w.p.. The longitudinal motion S_L is obtained by reciprocation of either the grinding wheel or work table, on which the work is clamped.

Fig. 6.24. Basic grinding operations: a, b, c, i – grinding of external cylindrical and tapered surfaces;

d, e – grinding of internal surfaces; f, g, h, j – grinding of flat surfaces; 1, 2 – abrasive disks;

3 – support (blade)

 

Today the most widely used types of surface grinders at the present time are the following: horizontal-spindle reciprocating-table grinders (Fig. 6.24f); vertical-spindle reciprocating-table grinders (Fig. 6.24g); horizontal-spindle rotary-table grinders (Fig. 6.24h); vertical-spindle rotary-table grinders (Fig. 6.24j).

In respect to the kind of surface they can grind, centreless grinders are classified as external (Fig. 6.24i) and internal centreless grinders. Workpiece is supported in the external centreless grinder on work-rest blade 3 and between grinding wheel 1 and regulating wheel 2. The grinding wheel rotates at a peripheral speed of 30 to 40 m per second and removes the grinding allowance from the work-piece. The regulating wheel rotates at a peripheral speed of 10 to 50 m per minute. It imparts both rotation and axial motion to the workpiece.

In the internal centreless grinder a work-piece is rotated between support roll, pressure roll and regulating roll, and is grounded by grinding wheel.