Design of Tool Shank:

• The tool shanks normally are of square cross section but rectangular or circular sections are also used. Square section is preferred because when a seat is cut for an insert reduction in strength of the shank is less compared to that with other section.

• Square shank tools are used for boring turret lathe and screw machines and where the distance between the base of the tool and the line of centers of the machine tool is not enough to accommodate rectangular shank. Round shanks are used for thread cutting tools. These tools can be rotated for adjustment.

• The cross section of the shank is decided from consideration of its bending strength against the cutting force as shown in Figure 1.

• If $F_p$ is the cutting force, (Newtons) and l is the overhang of the tip of the tool, (mm) the bending moment acting on the tool is

$M = F_p \times l \ Nmm$

For a rectangular tool shank

$F_p \times l = \frac{1}{6} \ bh^2 \ \sigma_b$

As for a round tool

$F_p \times l =\frac{\pi}{32} \ d^3 \times \ \sigma_b$

Where b and h are the width and height of the rectangular shank and d is the diameter of a round shank.

• Normally, $\frac{h}{b}$ varies from 1.25 for roughing operations to 1.6 for finishing operations while l is usually kept from 1 to 1.5 h.

• The deflection of the tool point can be checked using the relation:

$\delta = \frac{4 \ F_p \ l^3}{Ebh^3}$

• The analysis given above is approximate because only the cutting force component of tool force is considered. The stress system actually existing will be more complex when the other two components of the tool force are also considered. The stresses are also influenced by the approach angle of the tool with respect to the work piece.

• Because of these considerations the safe bending stress for shank material for unhardened shank for different values of approach angle $\lambda$ is assumed to be as given in below table.

• For hardened shank material, the values given in the table can be doubled.

• The value of the Young’s modulus E for shank material is taken as 200 GPa.

• The standard cross sections of rectangular tool shanks are $10 \times 16, 12 \times 16, 12 \times 20, 16 \times 20, 16 \times 25, 20 \times 25, 20 \times 32, 25 \times 32, 25 \times 40, 32 \times 40, 32 \times 50 \ and \ 40 \times 50 \ mm $

• The length of the tool varies from 100 to 500 mm depending upon the size of the tool post, dimensions of the tool holder, tool overhang, number of clamping screws, etc. (at least two)