Analysis of Positioning Adjustment Approaches for Cutting Inserts of Thread Cutters

The inclination angle of cutting inserts of thread cutters should be adjustable to provide the required geometry when machining threads. In serial production, this adjustment is realized by changing the carbide shims. In the present paper, the analysis of relationships between geometric parameters and positioning accuracy errors for thread cutters with inserts is presented.


Introduction
In Russian standard GOST 24705-81 "Metric thread. Main dimensions" the application of the threads with different pitches is described and regulated. This allows to take into account the purpose and requirements of the thread when designing new products, but increases the range of threading tools due to a change in the thread helix angle. Using the universal cutters with the cutting edge inclination angle λ = 0° with a variety of the thread helix angles τ ≠ 0 does not guarantee optimal cutting conditions and significantly degrades the quality of production.

Methods and Results
To reduce the product range and to create optimal cutting conditions for machining threads using lathe turning machines, various designs of cutters were created with the ability to tilt the cutting element at the thread helix angle related to the workpiece axis along the pitch diameter in order to optimize the geometric parameters of the cutter [1-3]. This is confirmed by the diagram shown in Fig. 1, which shows two options for the position of the cutting insert. In Fig. 1a the thread helix angle is τ>0 and the cutting-edge inclination angle λ=0°, i.e. the cutting angles on the right and left edges of the cutting part are different. In Fig. 1b, due to the basing shim with the angle τ=λ, the cutting conditions for the cutting edges become symmetric.
The values of the clearance angles at the side cutting edges (Fig. 1a) at the angle of inclination of the cutting element λ = 0 ° are determined by the formulas [4][5]: γ1= +τ; (1) γ2= -τ; (2) α1=αstτ; α2=αst + τ; tgτ=p/(πdс), where γ 1 ,γ 2 are the rake angles on the left and right cutting edge, respectively; α 1 ,α 2 are the clearance angles on the left and right cutting edge; τ is the thread helix angle on the pitch diameter; α st is the clearance angle of cutting edges in static coordinate plane; P is the thread pitch; dc is the pitch diameter, mm; For this case, the basing shim is manufactured with the inclination angle λ= 0.
When fitting the rake face of the cutter by dc in the plane normal to the turns of thread (Figure 1, b) at λ insert =λ=τ, the angles on the left and right cutting edges are determined by the formulas: γ1=γ2=0; (6) α1=α2=αst (7) The cutting conditions become equal for the right and left sides of the cutting edge . Selection of the dimensions range for basing shims using the values of the angle λ is performed by various analytic approaches. For a large dimension range, the number of shims decreases, but it becomes more difficult to choose a suitable angle of the shim. When the dimension range is small, the available quantity of shims increases. Thus, on average, the cutting tools manufacturers have from 7 to 8 shims providing a range of angles from -2.0 to 4.5 degrees.
There are technical solutions for the design of thread cutters with the adjustable angle λ which allows to shorten a dimension range for shims.
The thread cutting tool with a rotary holder having the adjustable cutting-edge inclination angle (with λ > 0) is shown in Figure 2(Patent 60414).  These designs are not widely used due to the complexity of their manufacture and large overall dimensions. And, as a consequence, the base shims remain the main option for adjusting the angle λ. The selection diagram for the shims is shown in Figure 4 (for products by Vargus) and Figure 5 (for products by Sandvik Coromant). According to the selection diagram and depending on the diameter of the workpiece and the thread pitch, one can determine the required basing shim. Table 1 summarizes both solutions in a form convenient for analysis.     After combining the graph of the choice of λ and the sorted diagram of the thread helix angles τ, it can be concluded that the basing shims by Vargus are closer to the calculated values of the thread helix angles the thread on the d с (Figure 9).

Conclusions
The basing shims by Vargus are more preferable for machining threads according to GOST 24705-81 compared to Sandvik Coromant shims as they provide optimal cutting angles for the right and left cutting edges.
Increasing the range of basing shims reduces the installation errors of cutting inserts calculated using the values of the thread helix angle τ.