Issues in Fixture Design for Machining Shafts on Multi-Cutter Machines

V. D. Varlyakov

(Presented by Prof. A. N. Eremin, Doctor of Technical Sciences)

Under the conditions of modern machine-building production, one can observe a clearly expressed tendency toward the ever wider introduction into machining practice of standard technological processes. Using standard technology under conditions of series production, subject-closed sections for machining parts are created. Proceeding further along the path of improving the organization and technology of production, variable-flow lines are created; these are characterized by the machining, on the same machines, of a number of parts united by a common machining route. Subject sections and variable-flow lines make it possible to introduce considerable order into the movement of parts through the shop, to increase the responsibility of foremen for the quality of products, to create a straight-through flow of production, and to reduce significantly the duration of the production cycle.

In variable-flow production, at each workplace not one but several products are machined and, consequently, changeover and readjustment of equipment from one product to another is required. During such readjustments, tools and fixtures are changed; this means that the equipment stands idle and interruptions occur in the machining of parts, leading to a decrease in the productivity of the equipment and the workers.

In striving to increase productivity, technologists reduce the operating time, that is, the time spent directly on machining, while completely forgetting or paying little attention to the considerable reserve of time hidden in the possibility of reducing other elements of the time standard and, in particular, the setup and completion time.

A reduction of this part of the time standard, as analysis shows, can be achieved through the standardization of fixtures and a more attentive approach to their design. The fixture used must make it possible to set it up away from the machine and to use it for machining several products without readjustment, or with minimal readjustments.

It is precisely the question of rational fixture design for multi-cutter machining of shafts that we shall address in our article. The fixture-design method we propose for multi-cutter machining makes it possible to reduce significantly the range of tool holders and cutters, and to simplify the content of machine changeovers from one type of shaft to another.

Under conditions of machine building, and especially electrical machine building, we encounter the need to organize the machining of a number of shafts that are very similar in their construction and differ from one another only in individual elements. Under such conditions, the correct approach to pro-

…the design of the fixture (tool holders) can yield significant results.

Examination of the shafts in Fig. 1 shows that they differ: shaft “A” from shaft “B” and shaft “V” from shaft “G” by the presence of an extra neck at the right end; and shafts “A”—“V” and “B”—“G” by the length of the middle neck.

When designing tool holders for machining these four types of shafts, one may proceed in the following ways:

1) by dividing the entire length of the shaft between the tool holders in half;

2) by adopting the same stroke length for machining the shafts in pairs when machining one end of the shaft;

3) by adopting the same stroke length for machining all four shaft modifications by means of a single tool holder from one end of the shaft.

The first design approach will inevitably make it necessary to have 8 different tool holders, that is, two for each shaft.

The second approach requires the use of 4–6 tool holders with six settings.

And the third approach can make it possible to machine all four shaft modifications by means of 3, or even 2, tool holders, using three of their settings.

The possibility of machining all four shaft modifications with the use of three, or even two, tool holders can be confirmed by the following general calculations.

On the basis of the drawing (Fig. 1), the values of the shaft lengths, expressed through the length of shaft “A,” may be written as:

\[ L_2 = L_1 - l_5,\qquad L_3 = L_1 - a, \]

\[ L_4 = L_1 - (a + l_5). \]

Taking the turning length in machining the first end of the shaft as equal to half the length of the larger shaft, we obtain the machining length of the second end of the shafts:

\[ L_1^{1} = \frac{L_1}{2}\qquad L_3^{1} = \frac{L_1}{2} - a, \]

\[ L_2^{1} = \frac{L_1}{2} - l_5 \qquad L_4^{1} = \frac{L_1}{2} - (a + l_5). \]

Comparing the values obtained, we see that this length differs from the machining length of the longest shaft by \(l_5\), \(a\), and \((a + l_5)\)—by the length of the additional step in shafts of types “B” and “G,” and also by the difference in the length of the middle step. From what has been said, it may be concluded that:

1) the equalization of the turning length should be carried out from the end of the shaft having the same construction, since in this case shafts of all modifications can be machined by means of one tool holder (Fig. 2);

2) machining of the second end can be carried out using one or several tool holders. Several tool holders (in our case two) are required when it is structurally impossible to arrange the cutters so that compensation for the greater length of the middle step is obtained either by setting one of the cutters in two positions (Fig. 4), or by setting an additional cutter which, during mach—

Fig. 1. Shaft designs

Fig. 2. Cutter holder for rough machining of the first end of shafts of all modifications

Fig. 3. Cutter holder for rough machining of the second end of shafts of all modifications

Note: Install the following cutters in the cutter holder:

Inserts

Fig. 4. Cutter holder for rough machining of the second end of shafts of all modifications (with inserts)

at the point of shafts with a shorter length of the middle step, is removed from the tool holder (Fig. 3). Using such a tool holder, machining is carried out by using one of the following settings (Table 1).

Table 1

Cutter “1” in settings Б and Г need not be removed, since, being in the tool holder, it will not take part in the work.

The design of the tool holder shown in Fig. 4 is used for machining shafts with a small variation in the length of the middle step and, chiefly, when the travel length of the tool holder is considerable and its magnitude is determined on the basis of the distance between the cutters machining the middle step, which is a multiple of the length of this step for the longest shaft.

The proposed considerations are valid for both rough and finish turning of shafts. However, the indicated problem in designing tool holders for finish turning is solved somewhat more complicatedly, because in this case we often encounter the need to machine the middle step with one nominal diameter but with different limit deviations on different sections. In finish machining of the first end of the shaft, the length of the turning is not determined arbitrarily, as was the case in rough turning, where we selected it and, in our example, took it equal to \(1/2\,L_1\), but rather by the lengths of the shaft sections on the middle step that have different limit deviations. Nevertheless, in finish machining as well, unification of tool holders can be carried out. Thus, machining of the first end of the shaft can be performed with the aid of one tool holder in its two settings. Here, compensation for the different lengths of the sections of the middle step is achieved by placing cutter 4 in two different positions (Fig. 5) with the use of an insert.

Fig. 5. Tool holder for finish machining of the first end of shafts of all modifications

Turning of shafts in finish machining as well should be started from the end of identical design, using one tool holder for this purpose. Ob-

The machining of the second end, however, can rarely be accomplished with the aid of a single tool holder. Usually, in this case, as many tool holders are required as there are different lengths of the middle step (in our case, two—Fig. 6). This is explained by the fact that here it is usually necessary to change the position not of one cutter, but of a group of cutters (cutters 4, 5, and 6). The number of cutters machining each section of the shaft is determined by dividing the length of that section by the travel length, rounding the value obtained up to the next larger integer. The travel length itself is determined by the greatest distance between cutters plus a certain overtravel (overlap of travel).

In our example (Fig. 6), the travel length is equal to 70 and 57 mm for the greatest distances between cutters of 70 and 55 mm.

Fig. 6. Tool holders for finishing the second end of the shaft in all modifications

The distances between cutters machining one step are usually determined by dividing the length of the given step by the number of cutters. Thus they have been determined in our—

example (Fig. 6). For case “b” this is correct, since here the length of travel itself was determined by the distance between the indicated cutters. In case “a,” however, the mechanical division of section “k” in half is incorrect, since in this case cutter “6” will overlap the work of cutter “5” by 15 mm. In the overlap section, cutter “5” will pass over an already machined surface, which will lead to a reduction in the load on the cutters and, consequently, to a reduction in the deflection of the shaft under the action of the cutting forces. The reduction in shaft deformation will lead to the removal of an additional chip in the overlap section. When the cutters are set to a diameter close to the smallest limiting shaft size, this may lead to an undersize of the diameter in this section and, consequently, to scrap. To eliminate the indicated circumstance, it is necessary to set the cutters so that the distances between cutters machining one and the same step are less than the length of travel by the maximum permissible overlap that does not cause an undersize of the shaft and its rejection. It is best to take the overlap value as equal to 1–2 mm. Then the arrangement of the cutters for case “a” (Fig. 6) will be as shown in Fig. 7.

Fig. 7. Cutter holder for finish turning the second end of shafts of types A and B

Using the principles considered, we were able to reduce the number of cutter holders from sixteen to eight, that is, by a factor of two (the cutter holders of the cross slides are also included here). Reducing the number of cutter holders simplifies production preparation, but at the same time is accompanied by a very substantial simplification and reduction in the amount of machine adjustment and, consequently, a reduction in the time spent on resetting the machines when changing from one shaft modification to another.

Thus, in operation I—rough turning of the first end—when changing from one shaft modification to another, it is only necessary to change the position of the stops limiting the slide travel.

In operation II—rough turning of the second end—it is necessary to install the required cutters in the cutter holder (Table 1) and change the stops when changing from long shafts to short ones and vice versa. Shafts with the same length of the middle portion can be machined without changing the position of the stops.

In operation III—finish turning of the first end—the stops and cutter 4 are reset.

In operation IV—finish turning of the second end—the cutter holders are changed and the stops are reset when changing from the long shaft modification to the short one.

If the principles we have set forth were not used, it would be necessary to carry out a complete readjustment of the machines in all cases of changing from one modification to another—to change the cutter holders and stops and, consequently, to spend many times more time.

Reducing the time spent on setup naturally leads to higher productivity and to an increase in output per unit of equipment and floor space. This circumstance is especially important in the operation of transfer and automatic lines, since in this case a delay in setting up one machine causes the entire line to stop and, consequently, reduces the output of the line as a whole. By using this principle, it is possible to raise substantially the utilization factor of such lines, to make possible a more rapid transition from one shaft modification to another, and to make possible a significant reduction in the volume of work in progress by decreasing the size of launch batches. This provision is especially important when it is absolutely necessary to fulfill plans not only in terms of gross output, but also in terms of the product mix.