OPTIMIZATION OF HIGH-PRECISION CYLINDER MANUFACTURING

Original scientific paper This paper deals with the reaming process optimization. The aim of optimization is that with controlled process suitable geometrical specifications could be achieved without final honing. To attain systematic improvements in manufacturing technology, transparent approach with the use of quality tools has been selected. SPC of bore cylindricity and surface roughness (Ra) of the original reaming process shows poor process capability and demonstrates shortcomings of the clamping device. Feed rate and spindle speed were identified as key factors in achieving required bore tolerances. Plan of experiments and process modelling was performed by DOE methodology. Regression analysis gives models for both responses, and they are adequate at 73 % and 96 % confidence level respectively. Optimization model was implemented in real production.


Introduction
Small batch production is due to the competitiveness of the market mainly orientated in achieving the required specifications with minimal manufacturing processes using standard cutting tools [1].Costume made hydraulic valves are one of unique cases of small batch production in combination with high tolerances requirements [2].Honing represents the final manufacturing process, which enables achieving geometric requirements for valve's cylinder bore and requires pre-processing on CNC machining centre with successive cutting processesdrilling, milling and reaming.Pre-processing on CNC machining centre is required because work piece should be machined in one clamping only in order to reach technological specifications.Specifications of the cylinder bore of the hydraulic valve housing prescribe cylindricity /○/ ≤ 12 µm and surface roughness Ra ≤ 1,6 µm after reaming and cylindricity /○/≤ 3 µm and surface roughness Ra ≤ 0,4µm after honing.
This paper deals with the optimization of the reaming process on CNC centre.Process monitoring and optimization was performed by Statistical Process Control (SPC) and Design of Experiments (DOE) methodology [3].Measuring method for bore cylindricity was defined as suggested in studies of De Chiffre et al. [4], Goeldel et al. [5], Stepien [6], and Wen et al. [7] and surface topology with mean roughness parameter Ra as in Brito et al. study [8].
Factors that have been identified as key factors to achieve specifications are bold highlighted in cause-andeffect diagram (Fig. 1), and have been studied within the experimental work.Authors as De Chiffre et al. [4], Korkut et al. [9], Müller et al. [10] and Xiao et al. [11] investigated spindle speed Sandfeed rate f as numeric control factors within the reaming process studies.In this study feed rate and spindle speed of the reaming process, were selected as key control factors as well.Region of operability was defined, for both control factors, during pre-experiments.
Burlikowska [12] and Hassan et al. [13] are focused on pattern recognition in SPC charts in their studies.Jose et al. [14], Goldarag et al. [15] and Tadic et al. [16] present weaknesses and strengths of clamping devices on the CNC machining centres in achieving the desired results of experimental work and further production.
With complementariness of conclusions from mentioned studies and pre-experiments results, lack of clamping device rigidity was defined and minimized with focusing on only one (of three) clamping nests.
Impacts of the drill bit and the milling cutter tool life, the material of the cutting tool, the work piece material, cutting tool holder, the geometry of the work piece, and the characteristics of lubricant were neglected during research.
Figure 1 Cause-and-effect diagram for reaming process They were neglected because the experiments were carried out on the same type of product, i.e. material and batch were the same and the same cutting tool was used.Drilling and milling parameters settings were not changed during experimental work.

Resulted
multi-criteria optimization enables achieving of required specifications of cylinder bore by CNC machining and improves reaming process capability, which results in drastic honing time reduction.

Approach and method
In order to achieve improvements in manufacturing technology a systematic approach with the use of SPC as in Burlikowska [12] and DOE as in Montgomery [17] has been selected.The original production technology provides a reference level to assess the improvements that have been achieved with a DOE strategy with the objective of fulfilling specifications in a cost-effective manner.First monitoring of the original reaming process was performed with measuring random samples from regular production.SPC x-control charts are given in Fig. 2 and Fig. 3, and show poor capability of the process.Specifications of original reaming process were for cylindricity /○/ ≤ 12 µm and for roughness Ra ≤ 1,6 µm.In order to improve poor process capability quality actions for machining technology must be taken.In order to reach those improvements systematic problem solving approach is needed

Pre-experiments
Fig. 2 and Fig. 3 show results of monitoring process for the initial cylinder bore manufacturing process.Measurements included a half-year period.Sample from each batch is accounted for 6 consecutive products manufactured within the same series.Process capability index for cylindricity C pk(Cyl) = 0,18 and surface roughness C pk(Ra) = 0,07 represents very poor manufacturing process control.The large dispersion of the measurements within the same series and uniqueness of results from batch to batch shows low measurement reproducibility.
Honing gives best results when reaming process specifications are reached and if there is small deviation in cylindricity between consecutive work pieces.Even if mentioned requirements are not respected all derogations could be compensated during honing process.In this case additional setting of honing parameters for each work peace individually is needed.This way honing process becomes production bottle neck and manufacturing costs are increased.The existing clamping device allows mounting of three pieces, as shown in Fig. 4. According to the findings monitoring of the process within one batch was performed.Cylindricity /○/ control chart in Fig. 5 demonstrates the shortcoming of the clamping device.Shortcomings are evident as trend deviations of measurements of three consecutive samples.The results show that only 1 st nest ensures an adequate rigidity, allows the desired results and ensure reproducibility of the measurement (nest No. 1 in Fig. 4).Nests No. 2 and 3, due to poor rigidity, do not allow adequate reproducibility.Region of operability for reaming was determined with pre-experiments using regular procedure [18].Reamer with in-operation time (tool life) t = 250 min was utilised.Experiments were started from the existing production parameters settings (bold point in Fig. 6; spindle speed S = 350 rev/min, feed rate f = 100 mm/min) and carried out in a diagonal direction of the S/f diagram.
Extreme values of technological window (rectangle in Fig. 6) were determined with a point where machining conditions were still acceptable.The criterion for determining the technological window was set on; cylindricity /○/ ≤ 4 µm and surface roughness Ra ≤ 1,1 µm.Goal of using such criteria was determination of more robust region of operability that would allow experiments conducting in regular production, without additional honing parameters setting.Resulted technological window limits shown in Fig. 6 are listed in Tab. 1, as first (−1) and second (1) level of the reaming parameters.

Experimental setup
The experimental part was carried out in regular production and thus includes real production noise factors, which are in laboratory testing difficult to simulate.Control factors and their levels were chosen on a base of pre-experiments and review of the reaming process studies [4, 9÷11].Experiments were carried out using L 12 orthogonal array and analysed with ANOVA.
In the experimental work the effects of reamer's tool life, cutting parameters and the part position in the clamping device on the cylindricity and surface roughness of the cylinder bore have been studied.Experiments were carried out at ambient temperature T = 23.°C with standard reamer DIN 8094B with diameter of 11,75 mm.Control factors -parameters and their values (see Tab. 1) were determined by pre-experiments and constitute an adequate range of parameters, which allow achieving the required specifications.
Experiments were carried out on the MoriSeikiNH5000 machine tool equipped with the option of tool life monitoring and it allows measurements of tool in-operation time in minutes.The workpiece material was C45E steel with dimensions of 70 × 100 × 100 mm.The maximum reamers tool life, which was determined by the experience of the preceding series, is 180 min.In the present experimental work reamer with a 40 % greater operation time was used.Reamer tool operation time was t = 250 min.The goal of using such tool was to demonstrate that it is possible to achieve the required specifications with longer tool life and thus create additional savings.
To characterize surface roughness, surface topology parameter Ra as suggested by Brito et al. [8] and Raykar et al. [19], was used.Measurements were carried out using roughness tester MarSurf GD25.For each specimen four profiles at four different cylinder bore positions were recorded.Results for individual experiment are given in Tab. 1.

Results and discussion
Bore cylindricity and average surface roughness Ra resulting during machining of cylinder bore were measured after the experiments.The experimental results are given in Tab. 1.The correlation between the control factors and results of cylinder bore machining were obtained with multiple regressions.A multiple analysis of variance (ANOVA) was used for identifying the factors significantly affecting the preference measures, cylindricity and surface roughness, after machining.The experimental results and optimization procedure were analysed with Design Expert software [20].

Regression model for cylindricity
The results of cylindricity formed in reaming cylinder bore are evaluated in Tab. 2. For determination of the optimal set of parameters 'cylindricity' linear regression graphs were used (Fig. 8).Both control factors namely spindle speed S and feed rate f were recognized as significant (Tab.2) and give the best result at lower level setting (−1 level).Regression model for cylindricity is defined with regression Eq. ( 1): Cylindricity regression model shows that cylindricity increases with the increase of feed rate f and spindle speed S. The feed rate has the most dominant effect on machining cylinder bore cylindricity, while spindle speed influence is much lower.
The Model F-value of 12,25 implies the model is significant.There is only a 0,27 % chance that a 'Model F-Value' this large could occur due to noise.The signal to noise ratio (S/N) greater than 4 is desirable.Cylindricity model S/N ratio of 7,528 indicates an adequate signal.This model can be used to navigate the design space for cylindricity.

Regression model for surface roughness
The quality of machined surface generated in reaming cylinder bore is evaluated in terms of average surface roughness (Ra) values.The graph in Fig. 9 and ANOVA in Tab. 3 were used to determine the optimal set of parameters.From the linear regression graph and ANOVA, the control factor -spindle feed f is recognized as significant and gives the best result at -1 level.On the other hand control factor -spindle speed S was recognized as not significant.Regression model for surface roughness Ra is defined with the following Eq.( 2): Figure 9 Surface roughness one factor graph for feed rate Feed rate has main role in achieving surface roughness.The model F-value of 309,39 implies the model is significant.There is only a 0,01 % chance that value this large could occur to noise.

Optimization with conformation test for target cylindricity and surface roughness
For the reaming process optimization both regression models for bore cylindricity and surface roughness were used.Even specifications prescribe cylindricity /○/ < 3 µm, in order to get more robust reaming process, target value for bore cylindricity was set at /○/ = 2 µm (first criteria).Cylindricity /○/ low = 1,5 µm represents lower limit and cylindricity /○/ up = 0,3 µm upper limit in the optimization procedure (Tab.4).Target value for surface roughness Ra (second criteria) was set at Ra = 0,3 µm where Ra low = 0,15 µm is the lower and Ra up = 0,4 µm is the upper limit in the optimization procedure (Tab. 4).
Review of the studies on reaming process [4,10] and preceding experiences reveal that from machining point of view bore cylindricity specification is more difficult to meet than surface roughness specifications.From this reason the greatest weight was set on cylindricity target.Regression models optimisation with above target criteria settings resulted in a solution for control factors i.e. process parameters settings shown in Tab. 5.In order to use optimization results in real small batch production, robustness of the optimization model needs to be confirmed in practice as well.Optimised control factors values were implemented in production and 35 successive samples were measured.Results are presented in x-control charts for both cylindricity and Ra in Fig. 10 and Fig. 11.
Average values for cylindricity /○/ ave = 2,677 µm and roughness Ra ave = 0,203 µm are in reasonable agreement with predicted responses values.Interpretation of such results could lead to honing process elimination, but dispersion of individual measurements is rather high.
Tab. 6 shows reaming process capabilities improvements.Process capability index C pk that was calculated for original and improved reaming process respectively is shown in Tab. 6. USL represents Upper Specification Limit and σ standard deviation of the measurements.It can be seen that reaming process capability improvement is enormous.Even when honing specifications are used for improved reaming process its capability index is higher than in original state.Optimisation of reaming process results in drastic honing time reduction.Hydraulic valves housings (cylinders) with more loose tolerances (/○/ ≤ 4 µm and Ra ≤ 0,5 µm) could be manufactured without honing.

Conclusions
Reaming process for cylinder manufacturing was improved using SPC and DOE methodology.The following conclusions can be made: • SPC results indicate that original reaming process does not meet specifications.All reaming process derogations were at original manufacturing process compensated with additional honing parameters settings for each work piece individually.
• Clamping device lack of rigidity was minimized with focusing on only one (of three) nest.
• Reaming process region of operability was determined with pre-experiments using regular procedure.Criterion of determining was set in the way that would allow experiments conducting in regular production, without additional honing parameters setting.
• Regression analysis gives models for both responses under consideration, namely cylinder cylindricity and Technical Gazette 23, 6(2016), 1717-1722 surface roughness, and they are adequate at 73 % and 96 % confidence level respectively.
• Results show that feed rate influences both responses while also small effect of spindle speed on cylindricity was recognised.
• Two criteria for reaming process optimisation were applied; cylindricity /○/ ≤ 2 µm and Ra ≤ 0,3 µm.Conformation test in real production environment confirms regression models and the optimisation strategy.
With reaming process optimisation capability of the process improved enormously and results in drastic honing time reduction.Cylinders with more loose tolerances (/○/ ≤ 4 µm and Ra ≤ 0,5 µm) could be manufactured without honing.

•
In experiments reamer with 40 % longer in-operation time as in original production was used.

•
In order to reach original specification with resulting multi-criteria optimization model (with reaming only): first upgrade of original clamping device is needed and second new in-operation tool life limit must be defined.

Figure 2 Figure 3
Figure 2 Cylindricity x-control chart; monitoring of original reaming process

Figure 4
Figure 4 Clamping device with three nests

Figure 5
Figure 5 Cylindricity x -control chart: showing clamping device weakness

Figure 6
Figure 6 Determination of technological window for reaming process

Figure 7
Figure 7 Measuring points of cylinder bore

Figure 8
Figure 8 Cylindricity interaction graph for feed rate (A) and spindle speed (B)

Table 1
DOE and experimental results

Table 2
ANOVA for cylindricity factorial model

Table 3
ANOVA for roughness factorial model

Table 4
Responses criteria for optimization

Table 5
Control factors and predicted responses values

Table 6
Reaming process improvements