Technologiczne Przygotowanie Produkcji
czas główny - określanie

Determining the machining time – main time

The main time (tg – main machine time or main manual time) is a component of the standard of technological times. Its definition is one of the key components of the production process schedule. The more efficiently determine the main time, the more accurate estimates of production costs. Scheduling is the basis of any process, including the production process, and depends on many factors. The main time, whether machine or manual, is only part of the technological standard of the times.

Determination of the main time depends on what kind of machine tool is machined (conventional and CNC machine tools – figures 1 and 2). In the case of numerically controlled machine tools, the main time can be specified in the CAM software. The photo shows the Mazatrol control system from Yamazaki Mazak. This system is equipped with Smooth technology. Thanks to this technology and full control over the process of creating their own control systems, it is possible to reduce the unit time. This shows that determining the main time solely on the basis of the CAM software is not sufficiently reliable.

main time - conventional machine tool

Fig.1. Conventional horizontal milling machine equipped with a ruler and position reading.

main time - CNC machine tool

Fig.2. CNC Turning and Milling Center Nakamura-Tome – Abplanalp stand – International Fair STOM 2019.

In both cases, the estimation of the technological times takes place experimentally during machining. Multiple experimental determination of particular times makes it possible to determine individual components of times (eg auxiliary and supplementary) characteristic for a given industrial plant. The use of tabular data collected and written over 50 years ago is not completely meaningless. However, as the most accurate estimation of costs requires the most accurate estimation of the technological standard times. An individual approach seems to be the most effective here.

In the following, the determination of the main time for the processing operation of the machine on a conventional machine tool is presented.

Time standardization card

The time standard card for the technological operation includes auxiliary times tp (tabular data), supplementary time here (12% of execution time is taken tw = tg + tp). Unit time is the sum of execution times and supplementary time. The time standardization card is mainly used for the technological operation carried out on a conventional machine tool. The exhaustive development of the technological process in mass production can use time-norm cards for each technological operation. Thanks to this, all times are determined within the standard of technological times.

The time norm sheet, which is available in this article and in the download section, is an example card. Depending on the production plant, the documentation differs from one another. First of all, this card is tailored to the individual needs of a given industrial plant. In the era of universal computerization and the use of various types of information systems, the handling of technological documentation is also diversified. Regardless of the form of documentation and how to manage it, the data does not change.

Below are two examples of determining machine main times. They constitute only a fraction of the tabular data, sketches and drawings contained in the norms [1, 5].

Main time – straight passage turning

Figure 3 shows an overview of the straight passage turning.

main time - straight passage turning

Fig.3. An illustrative technological sketch showing external longitudinal turning when it approaches: ld – access path, l – machined length, lw – exit path, ap – cutting depth κr – tool cutting edge angle.

Machining parameters (cuts) are selected separately. From the machining parameters, we need the work feedrate f [mm / rev]. In the case of longitudinal external turning, the main time tg [min] is determined on the basis of formula 1:

main time - formula 1

Formula 1.

The value L [mm], the length with which the tool travels with the machining parameters in the present case includes the length at which the material is removed from the workpiece and the path of approach and exit. The length of the approach ld is recommended to be determined on the basis of the formula 2, where ap – depth of cut [mm], and κr – the tool cutting edge angle.

main time - formula 2

Formula 2.

The length of the exit path lw of the tool is arbitrarily accepted between 1 and 5 mm. In the case of other types of machining, such as external and internal longitudinal turning to the shaft shoulder, there is no exit path.

The main time – drilling the through hole
main time - drilling the through hole.

Fig.4. Drilling the through hole.

Figure 4 shows an overview of the drilling of the through hole. As in the previous example, here too, we have the paths access and exit. The value of the main time tg [min] is also determined on the basis of formula 1. The recommended values of the route of access ld (pattern 3) and the exit of lw are different. The exit path is taken within 1 to 3 mm.

Formula 3 takes into account the angle of the drill bit, which is different for different materials of workpieces. In the metalworking most often the angle φ is 120 °. In the case of numerically controlled CNC machine tools, among others tool probes that allow measuring and its interpretation (determining the tool length and verifying the degree of wear of the blade). Measuring the tool when drilling holes gives us information about the above tool length. Whether it’s CAM software or independently on this length, we determine the path, without taking into account the angle φ of the drill cone. It boils down to a similar discretionary determination of the length of the approach, as in determining the length of the exit.

If, however, we decide to base on norms [1, 5] then formula 3 (d – drill diameter, φ – drill cone angle) is as follows:

main time - formula 3

Formula 3.

In [5] 51 different types of machining methods have been accurately described in terms of calculating the main machine time. Above, only two selected from among the 51 ones are presented. Determining the main machine time, or more manual, is not a simple task and requires, within a given industrial plant, an unambiguous and, above all, consistent application of its own internal standards.

I invite you to read the article: Norm time in the technological process.

Sources
  1. Wołk R., Normowanie czasu pracy na obrabiarkach do obróbki skrawaniem, WNT 1972
  2. Karpiuk M., Szacowanie kosztu wytwarzania metodą obróbki skrawaniem na podstawie modelu 3D CAD, Politechnika Krakowska, 2014
  3. Feld M., Podstawy projektowania procesów technologicznych typowych części maszyn, WNT 2000
  4. Durlik I., Inżynieria zarządzania. Strategia i projektowanie systemów produkcyjnych. Strategie organizacji i zarządzania produkcją, Placet 1993
  5. Górski E. (koordynator), Poradnik inżyniera. Obróbka Skrawaniem, tom II, WNT 1993
  6. Author’s own notes.

About author

morek

Born 1973. In 1993, I graduated from Technical Secondary School No. 1. In 1998, the Faculty of Mechanical Engineering and Automation (now Faculty of Production Engineering) - Warsaw University of Technology. 1997-2000 cutting tools manufacturer at VIS Precise Products Factory S.A. 2004. Unfortunately, this company no longer exists. PhD in gear technology. Production technologies and technological processes are my passion.

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