Structure and types of lathe tools
Knowledge of the lathe tools, their forms and application is the precondition of efficient working.
Structure of the lathe tool
Every lathe tool consists of the shank and the tool point. At the tool point, there are the top face and the flank. Shank and tool point may be made of the same material forming a whole.
Figure 10. Structure of the lathe tool
1 flank, 2 cutting face, 3 tool point, 4 shank
In order to save valuable cutting material (high-speed steel) or because it is required by the qualities of other cutting materials such as hard metal, ceramic cutting materials, diamonds, the shank is often made from mild steel.
Figure 11. Lathe tool shank and cutting edge from solid steel or different materials
lathe tool from solid steel
1 shank, 2 cutting head
welded on (butt welded)
1 hardness threshold, 2 welding point, 3 high-speed steel, 4 mild steel
welded on
1 high-speed steel, 2 mild steel
welded on
1 soldering seam
seized
The cutting tip is welded or soldered or clamped in place in the form of a plate. The flank of the lathe tool is the surface of the tool point which is directed against the area of cut at the workpiece. The top face is the surface of the tool point over which the chip is removed.
Top face and flank must always be well and smoothly ground, so that no additional heat is created by friction during the turning process and a long service life of the tool is achieved. |
Cutting edges at the lathe tool
The lathe tool has a primary cutting edge and a secondary cutting edge. The primary cutting edge faces the feed direction. The secondary cutting edge is adjacent to the primary cutting edge.
Figure 12. Cutting edges at the lathe tool
1 tool point, 2 shank, 3 main cutting edge 4 secondary cutting edge
Describe the structure of the lathe tools.
_________________________________
_________________________________
Angles at the lathe tool
Only if the cutting edges are ground correctly, the lathe tools can work economically. Therefore, one must know the correct angles at the cutting edge. The form of the cutting edge of the lathe tool is determined by the following angles:
a = angle of clearance
b = cutting-wedge angle
g = rake angle
d = cutting angle (a + b)
Figure 13. Angles at the lathe tool
1 angle of clearance a, 2 rake angle g, 3 cutting-wedge angle b, 4 cutting angle d
- The cutting-wedge angle b is situated between flank and cutting face. It is measured in the normal (vertical) to the cutting edge. Its size is determined by the strength of the material to be worked.
Hard materials require a large cutting-wedge angle, for instance steel: b = 60 - 75°; soft materials require a small cutting-wedge angle, for instance aluminium: b = 40°.
Figure 14. Lathe tool with small or large cutting-wedge angle
1 soft material - small cutting-wedge angle, 2 hard material - large cutting-wedge angle, 3 cutting-wedge angle b
- The angle of clearance a is determined by the flank and the tangent runing through the points of contact of the cutting edge with the circumferential surface of the workpiece.
Figure 15. Position of the angle of clearance
1 angle of clearance a, 2 flank, 3 cutting-wedge angle b, 4 vertical, 5 top face, 6 point of contact, 7 tangent
The angle of clearance must always be chosen only that large so that there is not too much friction between tool and work-piece.
- The rake angle g is formed by the cutting face and the vertical drawn on the tangent in the point of contact. In principle, the rake angle should be kept large in order to enable an easy removal of the chips. However, the size of the rake angle is limited by the size of the cutting-wedge angle which depends on the material.
Figure 16. Position of the rake angle
1 top face, 2 rake angle g, 3 vertical
- The cutting angle d plays a secondary part only.
It is formed by the angle for clearance and the cutting-wedge angle (d = a + b). It is situated between the cutting face and the vertical plane to the cutting edge.
Table 1 gives a general survey of the sizes of the angles at the lathe tool related to the cutting material “high-speed steel”.
Why is it necessary to know the correct angles at the cutting edge of the tool?
_________________________________
_________________________________
_________________________________
Table 1
Material to be worked | Angle of clearance | Cutting-wedge angle | Rake angle1) |
| (indication of angles in °) |
|
Steel | 8 | 62 - 68 | 14 - 20 |
Alloyed steel | 8 | 68 - 74 | 8 - 14 |
Tool steel | 8 | 72 | 10 |
Grey cast iron | 8 | 80 | 2 |
Copper | 10 | 55 | 25 |
Brass | 8 | 74 | 8 |
Bronze | 8 | 74 | 8 |
Aluminium | 10 | 60 | 20 |
1) The angles apply to rigid cutting conditions. In semi-rigid conditions, only the rake angle has to be increased up to 15 %, with unstable conditions up to 25 % and with soft, smearing materials up to 30 % in order to achieve a good non-torn finish-machined surface.
Types of lathe tools:
The type of lathe tool to be used in each respective case is determined by the shape of the workpiece which has to be worked.
For longitudinal turning, roughing and finishing lathe tools are required, for turning internal surfaces such as corners side cutting turning tools, for plunging and cutting-off parting-off tools etc.
If much material has to be removed, the roughing tool has to be used first. If high demands are made on the surface quality of the workpiece, the finishing lathe tool has to be used. Each operation requires the corresponding lathe tool. It would be a waste of time and expensive material to permanently adapt one lathe tool - for instance a side cutting turning tool for all sorts of turning.
Figure 17. Tools for turning external diameters
1 straight left roughing lathe tool, 2 bent right roughing lathe tool, 3 straight finishing tool, 4 broad finishing tool, 5 straight right-end-cut turning tool, 6 offset side cutting turning tool, 7 vee thread cutting tool, 8 form turning tool
Figure 18. Internal turning tools
single-point boring tool; internal side cutting turning tool; thread groove plunging tool; right undercutting tool; internal screw-cutting tool
The most important lathe tools are standardizes as to their shapes and dimensions. As far as the designations of the angles and surfaces as well as of the various types of lathe tools are concerned, there are generally valid international arrangements, too.
Lathe tools for turning internal and external surfaces are generally distinguished as shown in the pictures. What does the use of the respective types of lathe tools depend on?
