Tools are generally classified as single-point and multi-point cutting tools. Single- point cutting tools could be classified as solid, butt-welded/tipped tools and throwaway tool tips. Solid tools are produced from a solid piece of shank, as in the case of some of the turning and boring tools. They are made from forged high-carbon low-alloy steel, subsequently ground and heat-treated to the required hardness to withstand wear-resistance and to have hot-hardness properties.
Butt-welded and tipped tools follow the same philosophy of either butt-welding an alloy tool material to the shank of the tool body or brazing the special cutting material to the point end of the tool. Throwaway tools use inserts screwed to the shank. These inserts are made of cemented carbides and have the advantage such as elimination of tool grinding time. A simple turning tool has a number of variables in its geometry such as:
(i) Back Rake Angle (ii) Side Rake Angle (iii) End Relief Angle (iv) Side Relief Angle (v) End Cutting Edge Angle (vi) Side Cutting Edge Angle (vii) Nose Radius
The above information has a bearing on tool life, quality and rate of production.
The information is also called ‘tool signature’. Barring rake angles, which could
A ppendix A
Metal Cutting Tools
Design of Jigs, Fixtures and Press Tools, First Edition. K. Venkataraman.
© K. Venkataraman 2015. Published by Athena Academic Ltd and John Wiley & Sons Ltd.
be either positive or negative, the other angles are measured positive and are generally less than 10° for carbide single-point tools. In the case of alloy cast steel tools, rake angles can go even up to 20° for machining materials such as stainless steel. The geometry of lathe tool is indicated in Fig. A.1.
Fig. A.1 Nomenclature of Single-point Cutting Tool
Fig. A.2 Forces Acting in Orthogonal Cutting
Cutting operation in a single-point cutting tool and the forces involved are modelled as shown in Fig. A.2. Figure A.2 represents an orthogonal cutting system (representing two-dimensional cutting operation) in which the cutting edge is perpendicular to the direction of motion relative to the work piece and is wider than the chip. FC and FN are cutting and normal forces, respectively.
These forces result in a resultant force R which could be measured by a dynamometer or through a transducer. The other forces which can be obtained from the resultant force are indicated in the figure along with the angles such as rake angle, shear angle and the friction angle.
The above geometrical analysis may be useful for carrying out the study of various forces in relation to the tool geometry, material to be turned, cutting tool material, and the cutting parameters such as speed and feed. However, in reality, three components of resultant force will be involved, namely, tangential force FT, radial force FR, and the feed force FF. The same are shown in Fig. A.3.
Fig. A.3 Resultant Force in Three-dimensional Cutting Operation
The tangential force FT (in Newtons) can be calculated for a cutting tool material of high-speed steel by a rough formula :
FT = C f n1 dn2
where C is the constant of proportionality, ranging between 1000 and 2000 f is the feed rate (in mm per revolution)
d is the depth of cut (in mm)
n1 and n2 are slopes of the plot between force versus feed and force versus depth of cut.
Here, n1 varies between 0.43 and 0.84 and n2 varies between 0.77 and 1.21.
(Variations in constant of proportionality and in the values of n1 and n2 depend on the type of material and its hardness, like hardened, cold finished, annealed, etc.)
Thus, for a feed rate of 0.5 mm per revolution and a depth of cut of 0.5 mm, a force of 700 N may be encountered in a tangential direction at the point of cutting.
In addition, moments due to such forces may also be considered depending on the component dimensions. This will provide an idea to the designer of a turning or a boring fixture about the amount of clamping forces that may be needed to counter such forces. In addition, the fixture should be so designed as to dampen
any machine tool chatter that may arise due to the sudden increase in the depth of cut, non-homogeneous material, tool wear, etc.
Figure A.4 shows a piloted boring bar fitted with single-point cutting tools.
As explained earlier, the boring tool could be from solid shank, particularly in the case of short objects.
Fig. A.4 Single-Point Cutting Tool mounted on Piloted Boring Bar