The influence of hole finishing strategies on quality

There are many strategies that the operator can use for hole finishing, depending on the equipment available to him. The issue at hand is choosing the optimal strategy when equipment is available for more than one possible manufacturing method. The present work analyses four types of finishing strategies for holes and determines the advantages and disadvantages of each, from a quality perspective.


Introduction
In our days, there are many technologies for manufacturing a part.Each technology has its advantages and disadvantages.Depending on the requirements that the part must satisfy, the most appropriate technology is chosen.If theoretically this is easily done, in practice it is limited by existing possibilities.The usage of optimal technologies is not always possible and, thus, it is necessary to use the optimal existing technologies [1].
In the case of hole finishing there are multiple ways of obtaining the same hole through different strategies or by using different tools.The same diameter hole can be obtained by using a mill, reamer, boring bar, turning tool or abrasive body [2].Choosing the optimal strategy depends on the existing equipment, the machine tools used, the shape of the part being machined and many other factors that influence the machining strategy.A precise hole is an interior revolution surface that is different from a hole through its superior accuracy, low shape errors (which are standardized) and high surface quality [2][3][4].
The aim of this article is to study the way in which the quality of a precision hole is influenced by the chosen manufacturing strategy.The quality of a precision hole considers four aspects deemed relevant: dimensional, shape and actual position accuracy, as well as the resulting surface.All these aspects are important in precision holes with a functional role (sliding or ball bearings, guide bushings, centering elements etc.).Thus, the surface is important for precision holes that have shafts performing rotational or reciprocating movement because of the need of adequate lubrication.In such situations, the surface can have an influence on the maintenance of the oil film [5].The actual position accuracy is important in the case when the precision hole is used for guiding an element passing through it.The form accuracy is also important when the precision hole needs to ensure a certain load capacity for its corresponding element.The lack of an appropriate form Corresponding author: nicolae.panc@tcm.utcluj.roaccuracy can lead to an uneven distribution of the load on the hole surface leading to uneven wear and, as a result, a low reliability [6,7].
Next, we will compare the quality of the precision holes obtained though the most common manufacturing techniques and identify the optimal technology but also determine the influence of certain factors on hole quality.

Research methodology
To reach the proposed aim, a certain methodology was used, which is presented briefly in figure 1.In total four different techniques were used on three different material.After the rough boring, the four methods were applied for the finishing part.The characteristics were measured and the data was analyzed.The obtained results were analyzed quantitatively.The two parts that were machined from each material are presented in Figure 2 and 3. Figure 2 shows the part drawing while Figure 3 shows the actual parts that were machined.The holes were first drilled to Ø19.80 mm and then, the Ø20H7 diameter was obtained through a single pass of the tool.

Equipment and measurements
The following equipment was used for machining and measuring the precision holes: The measurement was done on a Zeiss Prismo Navigator Coordinate Measuring Machine (CMM) equipped with a VAST sensor and 5 mm ruby tactile probe.According to the manufacturer the CMM has a length measurement error of 0.5 + L/500 μm.

Results
The quality of the precision holes was determined through the measuring of the dimensional and surface quality characteristics.In the case of the dimensional measurements the dimensional accuracy, error of shape and position error were determined.
After the measurement on the CMM, the data was stored in an Excel worksheet.When processing the data, two main variables were taken into account: the working method used and the material of the part.The averages were calculated for the diameter, the circularity and the cylindricity.

Dimensional accuracy
If we compare the methods numerically, we get the data in Table 2. From the table, it can be seen that although head boring and reaming give on average the least deviation from the nominal value, their spread is much greater than that of helical and profile milling.

Conclusions
The present paper analyzed the influence of machining strategy on hole quality.During the research both machining strategy and material were taken into consideration.
After interpreting the results of this study, the following conclusions were reached: • The highest quality can be obtained by helical milling, it being the most stable strategy, with a high repeatability rate resulting from the low standard deviation (<0.01 mm); • Reaming is the most unstable strategy, being highly influenceable by tool wear;

Figure 4 .Fig. 4 .Fig. 5 . 3 . 2 Fig. 6 .Fig. 7 .Fig. 8 .
Fig. 4. Influence of the working method on the diameter, a) absolute values, b) relative to the nominal value.If we consider the influence of the material (Figure5) we notice that plastic has the least variation across working methods in comparison with metals.We also see that, in the case of metals, reaming skews the distribution of values quite a lot.

• Vertical machining center (VMC); • TRM50/50 Boring head Testarossa D'Andrea; • HSS Reamer for Ø20H7; • Drill Ø19.80 mm; • Solid carbide end mill, Ø16, z=4 mm; The machining parameters that were used are presented in Table 1. The same work conditions were used for all material types in order to do a comparative quantitative study.Table 1 .
Parameters of cutting bores.