Methodology for Creating Series of Standard Turret Setups
Abstract Generated abstract
This paper proposes a systematic methodology for creating series of standard setups for turret lathes and, more cautiously, automatic turret lathes in small-batch instrument and machine-building production. It argues that setup time is reduced most effectively by unifying auxiliary tooling and arranging setups in graded series, where more complex setups add tools while retaining the ability to machine parts assigned to simpler setups. The method combines classification of parts by technological similarity, calculation of machine load by planned period, and assignment of standard setup codes to determine the required equipment and process sheets. The paper concludes that such standardization can expand the economical use of turret lathes for small batches, while automatic-machine applications require case-by-case economic analysis because productivity losses may be larger.
Full Text
Methodology for Creating Series of Standard Turret Setups
G. B. Kats
In modern machine building, especially in instrument making, turret lathes and automatic lathes have become widespread. A characteristic feature of these types of machines is the possibility of carrying out on them a relatively high degree of sequential and partially combined concentration of operations.
As is known, the greatest effectiveness of sequential concentration of operations is achieved when auxiliary time has a significant share in the overall piece-time norm; this is precisely the case in instrument making when machining small and medium-sized parts of the shaft and bushing classes.
The small dimensions of the surfaces being machined necessitate frequent transitions from machining one surface to another. This, in turn, requires from the operator intense attention and a high intensity of labor, which increase as the dimensions of the parts being machined decrease.
The concentration of operations inevitably creates the need to use multi-tool setups, the complexity of which depends primarily on the combination of the main and especially the auxiliary machining surfaces.
In turn, the economy of multi-tool setups depends on the ratio between setup-and-completion time and operating time, as well as on the size of the batches of parts being machined. The lower the cost and, correspondingly, the duration of the setup, the smaller will be the minimum economically justified batch of parts. Consequently, reducing setup duration expands the field of application of turret lathes and automatic lathes in small-batch production.
Let us consider methods for reducing the setup duration of turret lathes. Everyday experience and observations of the process of setting up turret lathes show that the greatest amount of time during setup is spent on obtaining and installing auxiliary tooling: collets, holders, reducing bushings, and so on. At the same time, it must be taken into account that in a number of cases the shops do not have in their storerooms, in sufficient quantity, the entire necessary and highly varied assortment of auxiliary tooling. The process of selecting auxiliary tooling is made still more complicated when a shop has various types and makes of turret lathes, since the dimensions of the tool seats are insufficiently standardized. Therefore, for each size type of machine it is necessary to have its own set of auxiliary tooling. The duration of setting the cutting tool to size under these conditions is relatively small, especially if standard parts are used for the setup, by means of which one can very quickly and accurately set the measuring tool and stops.
From this it may be concluded that the most effective means of reducing setup time is the use of unified, so-called standard setups, which make it possible, when equipping a machine with a definite set of auxiliary and partly cutting tools, to machine entire series of different, technologically similar parts. In this case it is necessary only to replace, for each part, the measuring cutting tool: drills, counterbores, reamers, taps, dies, etc., as well as collets. The very idea of creating standard setups is not new. It arose long ago in connection with the great variety in the nomenclature of turning operations and their high labor content, which lead to long production cycles and increased cost of products in serial and especially small-lot production. Even now turret lathes are still not used to the proper extent because of small batches of parts, while engine lathes are, as a rule, overloaded. The need for better use of turret lathes, in particular by means of a normal set of tools and permanent settings for an entire group of parts, is pointed out by Professor A. P. Sokolovskii in the textbook Course in Mechanical-Engineering Technology.
The extension of the limits of economical machining on a turret lathe toward small-lot production by the method of so-called standard settings is discussed in the book by Professor A. B. Yakhin, Matalin, and others, Technology of Precision Instrument Making. At present, many plants use such standard setups; however, they have an almost accidental character, when a number of parts is selected for a definite set of tools, which can be machined on the corresponding setup consisting of that given set of tools.
A scientific generalization of the experience of using standard setups and the creation of a system for constructing them have not yet received proper treatment in the existing literature.
Let us consider the process of creating any setups, including standard ones, on turret lathes. In doing so, it is necessary to consider separately machines with a vertical and with a horizontal axis of rotation of the turret head.
Let us consider the first case: machining parts of the shaft class, without holes, from bar stock on turret lathes with a vertical axis of rotation of the turret heads, equipped, as a rule, with a cross slide.
The first auxiliary tool, in order, mounted on the turret head is, as is known, an adjustable stop. The second most universal tool is a cut-off tool, which it is advisable to mount in inverted position on the rear side of the cross slide. With such a simplest setup it is possible to machine smooth small shafts, without chamfers, from calibrated bar stock, which do not require external machining. Facing is performed by a facing tool secured in the front cross slide. The outer surface of the smooth shaft is machined by a tool secured in the simplest tool holder.
Stepped small shafts should expediently be machined with the aid of tools secured in a multiple-tool holder, on which either a fixed or a roller steady rest may be mounted for machining long parts. To form a chamfer, a chamfering tool mounted on the turret head is used. Thread cutting is performed either by a die head or by a die, also mounted on the turret head. Grooves on not very long parts may be formed by using a cantilever multiple-tool holder with manual transverse feed. Short contoured surfaces are machined by forming tools mounted in the front cross slide.
By installing the tools listed above on a turret lathe, it is possible to machine almost all complex parts of the shaft class without holes that are encountered in instrument-making practice.
The machining of the most complex parts can be carried out on turret lathes with a horizontal axis of rotation of the turret heads, which have a significantly larger number of stations for tools.
Naturally, on a machine equipped with such a complex universal setup, parts of less complex configuration can be machined, omitting during machining those positions whose tools are not needed for the given, simpler part. Thus, for example, to machine a multi-step shaft but without a thread, one can use the specified complex universal setup, omitting during machining the corresponding position of the turret head on which the thread-cutting head is installed.
On this principle, series of standard setups can be created; moreover, on each subsequent setup it is possible to machine both all the parts for the preceding setups and more complex parts having certain additional surfaces characteristic of the given setup.
Consequently, in such a series each subsequent, “higher,” setup is formed by adding one of the tools to the preceding, “lower,” setup. An example of the formation of such series—standard turret setups for machining shaft-class parts—is shown in Fig. 1.
The question arises: how is the nomenclature of setups to be established in each specific case?
For this purpose, for the simplest three-tool setup (Fig. 1, setup A) we select an assortment of parts that can and must be machined on this setup during the nearest planned period (month, quarter). Knowing the production program and the standard piece time for machining each part, one can determine the load, in hours, of the corresponding turret lathe.
In the same way we determine the duration of the machine load for machining more complex parts requiring the next setup in degree of complexity (Fig. 1, setup B).
By distributing all parts of the given group and type according to the degrees of complexity of the setups, we determine in an analogous way the time fund of the machine required for machining parts at each setup level. Then, knowing the amount of the available time fund and taking into account that at each subsequent setup level all less complex parts assigned to the preceding setup levels can be machined, it is possible without difficulty to determine the assortment of setups with which it is necessary to equip those turret lathes on which the corresponding set of parts is to be machined during the given planned period, best of all a month. Let us explain the foregoing with a schematic example.
Suppose that for machining all parts requiring the simplest setup A, 55 hours of piece time are needed; for parts requiring setup B—293 hours, etc. (see Table 1).
Table 1
| No. of part group | Setups | Machine load for the given planned period (in hours) |
|---|---|---|
| 1 | A | 55 |
| 2 | B | 293 |
| 3 | V | 184 |
| 4 | G | 342 |
| 5 | D | 74 |
| 6 | E | 75 |
If the nominal time fund is \(208.2—416\) hours, then, with a calculated load factor \(K = 0.85\), the available time fund is 350 hours. Then, on setup B, the first
| Setup code | Setup | Sketches of parts | Characteristics of parts |
|---|---|---|---|
| A | [schematic of a turret setup] | [sketches of smooth shafts with external machining, with chamfers or points] | Smooth shafts for external machining with chamfers or points |
| Б′ | [schematic of a turret setup] | [sketches of smooth and stepped shafts] | Smooth and stepped shafts for machining for chamfers and for cutting internal chamfers of sections of the external surface |
| В′ | [schematic of a turret setup] | — ″ — ″ — | The same, with machining for [[unclear: symbol]] |
| Б″ | [schematic of a turret setup] | [sketches of smooth and stepped shafts with holes/undercuts] | Smooth and stepped shafts with machining for chamfers and undercutting of ends and with chamfering of sections of the external surface |
| В″ | [schematic of a turret setup] | — ″ — ″ — | The same, with machining for [[unclear: symbol]] |
| Г | [schematic of a turret setup] | [sketches of stepped shafts] | The same as in item B′, with chamfers, undercutting of ends, and with chamfering of sections of the external surface |
| Д | [schematic of a turret setup] | [sketches of stepped shafts] | The same, with grooves |
| Е | [schematic of a turret setup] | [sketches of stepped shafts] | The same, with thread |
Fig. 1. Series of standard turret setups for machining parts of the shaft class.
two groups of parts (\(55 + 293 = 348\) hours) will be machined; on setup Г, group of parts No. 4 (342 hours); and on setup Е, groups of parts Nos. 3, 5, and 6 (\(184 + 74 + 75 = 333\) hours).
Instead of this, the parts can be combined differently: on setup Г—group No. 3 (184 hours) and about half (by time fund) of the parts of the fourth group
\[ \left(\frac{342}{2}=171 \text{ hours}\right), \]
and the other half of the parts of the fourth group can be machined together with the parts of groups 5 and 6 on setup Е
\[ \left(\frac{342}{2}+74+75=320 \text{ hours}\right). \]
Thus, optimal utilization of standard setups is achieved by a proper combination of production preparation with operational planning.
When calculating the piece time for a batch of parts, it is necessary to take into account only the duration of readjustment. The form of the calculation sheet for determining the types of setups is given in Table 2. On the basis of Table 2, prepared once for the forthcoming month or quarter, the required number of machines equipped with a setup is established according to the indicated code (Fig. 1). The setups are installed on the corresponding machines for the duration of the entire planned period. The measuring tool required for each part is issued in the usual manner when a change of part is scheduled in accordance with the production plan.
Table 2
| No. of part group | Setup code | Part No. | Program | Operating time: per piece | Operating time: for program | No. of batches | Setup duration | Piece time (total), min. | Piece time (total), hr. |
|---|---|---|---|---|---|---|---|---|---|
| 1 | А | 41—17 | 100 | 3,0 | 300 | 1 | 15 | 315 | 5,23 |
| 1 | А | 60—24 | 200 | 4,5 | 900 | 1 | 15 | 915 | 15,25 |
| 1 | А | 60—71 | 150 | 2,0 | 300 | 1 | 10 | 310 | 5,17 |
| 1 | А | 42—109 | 400 | 3,2 | 1280 | 2 | 30 | 1310 | 21,80 |
| 1 | А | 60А—21 | 100 | 4,5 | 450 | 1 | 15 | 465 | 7,75 |
| 1 | А | Total | Total | 55,20 | |||||
| 2 | Б | 41—110 | 1000 | 4,0 | 4000 | 3 | 50 | 4050 | 67,40 |
| 2 | Б | 40—28 | 600 | 5,2 | 3120 | 2 | 45 | 3165 | 52,50 |
| 2 | Б | 61—22 | 1400 | 6,4 | 8860 | 5 | 65 | 8925 | 149,50 |
| 2 | Б | 40—79 | 400 | 3,5 | 1400 | 2 | 30 | 1430 | 23,80 |
| 2 | Б | Total | Total | 293,20 | |||||
| 4 | Е | 60—21 | 200 | 4,5 | 900 | 1 | 15 | 915 | 15,25 |
| 4 | Е | 61—13 | 400 | 3,0 | 1200 | 2 | 30 | 1230 | 20,50 |
| 4 | Е | 40—71 | 150 | 3,5 | 525 | 1 | 10 | 535 | 8,90 |
| 4 | Е | 42—19 | 350 | 5,2 | 1840 | 2 | 20 | 1860 | 31,00 |
| 4 | Е | Total | Total | 75,65 |
On the series of setups given above for shafts without holes, it is possible, as was indicated, to machine almost all types of such parts of medium and large sizes encountered in practice. For individual more complex parts, which are encountered comparatively less often in practice, it is advisable to use turret lathes with a horizontal axis of rotation
type 1336 turret heads. On the turret head of machines of this type it is possible to mount up to 16 tools (in practice, up to 12).
When machining hollow shafts and bushings, the number of surfaces encountered increases sharply. If one also takes into account that, in a number of cases, machining accurate holes requires the use of 3 or 4 tools (a drill, a countersink, one or two reamers), not counting the center drill, then the necessity of creating several series of standard turret setups becomes clear.
For machining complex parts, a considerable expansion of the technological capabilities of the machines can be achieved not only by the simultaneous machining of several surfaces (drilling plus rough machining of the external surface), but also by the use of double holders, four-cutter heads, and blocks on the cross slides. Such more complicated setups are used in separate series of standard setups intended for machining the most complex parts. This complication of the setups is justified by their universalization and by the possibility of sharply reducing the overall assortment of holders used and shortening the time required to set up the machines.
All this makes it possible to considerably expand the field of application of turret lathes. An example of forming a series of standard turret setups for parts of the bushing class is given in Fig. 2.
The use of standard setups for machining less complex parts inevitably causes some increase in unit time, mainly on account of unnecessary indexing of the turret head. This increase in unit time is compensated by the reduction in setup time or, in individual cases, by the possibility of using multi-cutter machining and combining operations, which in small-lot production with ordinary setups would be irrational to apply because of their increased complexity.
Calculations show that, in almost all cases, the use of standard setups is economically justified and gives a considerable effect.
Analyzing the features of using series of standard setups, we arrive at the conclusion that their rational application requires a preliminary classification of the parts to be machined. It goes without saying that such a classification is applied only to parts that can be machined on turret lathes. The system of this classification is not stable and depends entirely on the nature of the corresponding series of standard setups. Another specific feature of this classification is the need to determine the number of surfaces to be machined for hollow parts (shafts with a hole and bushings), as well as the accuracy class of the hole dimensions. The usual indexing system would be excessively complex. It is much simpler to designate parts by the setup index: the series number and a letter denoting the setup stage in the given series. After the parts have been classified and the nomenclature of setups established, a standard technological process is developed for each group of parts assigned to the given setup. As an example, a standard technological process developed by us is presented (Table 3).
As can be seen from this table, one technological process covers the machining of 24 parts manufactured on one setup of a turret lathe. The process sheet contains the necessary data for setting up the machine and machining each part. The sheet was compiled on the assumption that the worker has the appropriate training (grade IV–V) and skills in adjustment. In those cases where the worker is less trained, and the setup and adjustment of the machines are performed by special personnel (setters), it proved expedient to supplement the sheets with sketches of the listed parts.
Practice shows that the use of standard turret setups is profitable even when machining small batches of parts.
The advantages of standard turret setups are obvious. This is evidenced not only by the experience we have obtained, but also by the data presented
| Setup cipher | Setup | Sketches of parts to be machined | Characteristics of parts |
|---|---|---|---|
| A | [schematic of turret setup] | [sketches of bushings and stepped parts] | Smooth and stepped bushings with machining of the bore by a drill and by a boring cutter, and of the external surface by a cutter (for threading) |
| B | [schematic of turret setup] | [sketches of stepped bushings] | The same, with countersinking |
| V | [schematic of turret setup] | The same, with external machining at ∇∇ | |
| G | [schematic of turret setup] | [sketches of shaped parts and flange-like part] | The same, with a section of shaped external surface and facets |
Fig. 2. Series of standard turret setups for machining parts of the bushing class.
in individual sources, including materials published by Lonitomash in the section “Progressive Technology of Machine Building,” part 1.
Turning to consideration of the question of standard setups for automatic turret lathes, it should be noted that the field of their application
considerably narrower than that of ordinary turret lathes. The use of standard automatic-machine setups in a number of cases makes it possible to expand significantly the field of application of automatic machines in small-series production by using one and the same set of cams for machining a number of parts. In these cases, for machining on standard setups, parts of very similar configuration must be selected, differing from one another in the dimensions of the surfaces being machined: grooves, chamfers, etc. Most suitable for machining on standard automatic setups are fasteners: bolts, screws, wood screws, nuts, as well as simple rollers and bushings.
When setting up an automatic machine and making cams for machining the largest part in a given series of parts, with the greatest number of additional surfaces, it is possible, as is known, to machine on such cams parts of smaller dimensions, as well as parts that do not have the corresponding grooves, chamfers, etc. In this case, however, the machining time of the less complex and smaller parts will be equal to the machining time of the largest and most complex part in the given series, since a known portion of the idle movements of the automatic machine’s working members will be performed at reduced speeds corresponding to cutting speeds. Thus, for example, when machining a screw 30 mm long on cams intended for machining an analogous part 50 mm long, the movement of the tool for turning over a length of 20 mm will be carried out at the working feed rate (0.1–0.15 mm/rev) instead of at an accelerated idle stroke. If there is no groove on the simplified part, the tool is removed from the corresponding side slide. However, manufacturing such a simplified part requires the same amount of time as was needed to machine the more complex one.
Under these conditions, productivity losses with standard setups on automatic machines are considerably greater than on turret lathes, which causes a significant increase in machining cost. However, under the conditions of small-series production this increase in cost is offset by a reduction in the cost of the cams, since a number of parts can be machined with one set of cams, while spending only a comparatively small amount of time on a simple resetting.
The field of application of standard automatic-machine setups must be established in each individual case by economic analysis.
The methodology for determining the economic expediency of using standard automatic-machine setups in small-series production is presented by the author in Izvestiya TPI, vol. 73.