Keywords

1 Introduction

In processing materials by machining, cutting fluids (CFs) are commonly used to improve product quality and enhance productivity. CFs extensive use imposes increased risk to health and environment [1]. Therefore, eliminating or minimizing their application has been a subject for many researchers. Despite their limited requirement in time and space, superfluous volumes are being supplied, e.g. in flood strategy.

Dry machining can be the best way to sustain health and environment. However, it cannot be realized in many machining applications due to poor product quality outcomes, increased tool wear and reduced productivity. Therefore, rational consumption and optimization of CFs have been researched. For instance, minimum quantity lubrication (MQL) has been successful in many machining applications where very low consumption of CFs can be realized [2].

While it is important to observe the effectiveness of CFs for enhancing machinability in terms of machined surface quality and improved material removal rate, it is quite essential to understand and fundamentally explore the physical and chemical limitations that bound their effectiveness. In particular, the theory of varying contact conditions within tool-chip interface (i.e., sticking and sliding zones) [3,4,5] can shed the light into better understanding where CFs can provide lubrication and/or cooling. High thermal and mechanical loads, which exist between tool and workpiece, work against proper CFs access to the contact areas where it is most needed. For instance, high normal and shear stresses, located close to the cutting edge, squeeze the lubricant film out of the contact area leaving it to as minimum as the average height of the asperities [6]. Moreover, high temperatures that may reach up to the melting point of the workpiece leave no chance for CFs to provide almost any lubrication action.

It has been observed that interrupted machining can enhance the machinability of low thermal conductivity materials such as stainless steels, titanium or nickel-base alloys (e.g. Inconel 718) [7, 8]. These difficult-to-cut materials require relatively low cutting speeds to avoid high temperatures caused by poor heat dissipation out of the tool-chip interface that is rather carried away with the removed chips. In continuous machining operations such as turning or drilling, the cutting tool is intimately engaged with the workpieces for long period of time. As a result, CFs’ actions are restricted or even absent after a short distance of cutting. Lubricant effect supplied in pockets along the cutting path was investigated by Saelzer et al. [9]. Significant reduction in mechanical loads lasted a finite period of cutting time after exposure to the lubricant. Itoigawa et al. [6] investigated the effect of MQL in interrupted machining of an aluminum alloy where transient reduction of feed forces was observed at the beginning of cutting intervals. For efficient application of CFs, it is still necessary to fundamentally investigate the boundaries of interrupted machining characteristics in relation to machining setup and environment.

In this study, initial period of chip formation (IPCF), where low tool-chip contact may occur within a finite cutting length, is closely investigated in an orthogonal machining setup. Mechanical loads were observed and the existence of IPCF is further investigated under interrupted cutting process at different intervals. Moreover, 2D numerical chip formation model is proposed to better understand IPCF mechanisms.

2 Methodology

In order to investigate the contact condition at the start of the cut, stepped workpieces allowing for variable cutting lengths were prepared, as shown in Fig. 1. The evolution of contact as appears on the tool is examined. The total tool-chip contact length lc on the rake face of the tool was measured until the end of adhesion marks taking into account the development of sticking and sliding zones that appear progressively on the rake face. Moreover, slotted workpieces were prepared to investigate the influence of interrupted cutting on sustaining IPCF. IPCF effect was modelled using 2D chip formation simulations with modified friction model.

2.1 Tools and Materials

The machine tool used was a custom-built CNC machine that allows fundamental analysis of chip formation in orthogonal cutting, see Fig. 2. A three-axis piezoelectric dynamometer type 9263 (Kistler AG) was used to monitor process forces during machining. Synthetic MQL oil Vascomill MMS HD 1 (Blaser Swisslube AG) was applied on the rake face using air brush prior to the cutting occurrence during continuous cut and supplied as MQL during interrupted cutting trials. Optical microscope VHX5000 (Keyence GmbH) was used for taking rake images and quantifying tool-chip contact zones.

As workpiece material, austenitic stainless-steel alloy X8CrNiS18–9 (1.4305) was used. The specimens were separated from a round workpiece, using waterjet machining. The workpieces had a width of b = 2 mm. Stepped workpiece (WP-A) with cutting lengths Lc = 2 … 108 mm were prepared. Additionally, slotted workpieces (WP-B) were prepared with cutting length Lc = 8 mm and different interruption lengths Lint = 4 … 24 mm.

Fig. 1.
figure 1

Workpieces geometry (left) and experimental Setup (right)

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Tool holder CTFPL2525M16 with uncoated tungsten carbide (WC) inserts TPGN160308H13A from manufacturer Sandvik were used. The cutting insert has a triangular shape with cutting edge radius measured at rβ = 8 µm. Uncut chip thickness was kept constant for all trials at h = 0.2 mm, as well as the rake angle of γ0 = 6° and the clearance angle of α0 = 5°. The cutting speed was varied in the range of vc = 30, 60 and 90 m/min.

2.2 2D Chip Formation Simulations

Two-dimensional (2D) numerical chip formation simulations were carried out using DEFORM 2D V12.0. Adaptive meshing was provided in both bodies to enable an efficient calculation and the detailed resolution of the effective zones. Minimum element size was set at 6 µm and the maximum element size was set at 50 µm. For the flow stress modeling, a Johnson and Cook model was used, see Fig. 5. Model parameters were obtained from Lee et al. [10] for comparable stainless-steel alloy AISI 304. For simulating serrated chips, a Cockcroft-Latham damage criterion was considered to have a critical value at 150 and the flow stress reduces to 35% of the original flow stress value. Tool thermal conductivity was a function of temperature according to Vornberger et al. [11].

Tool-workpiece friction was modeled as hybrid friction, where the friction is considered as shear friction within sticking zone and as Coulomb friction within sliding zone. Coulomb friction coefficient varies between two values depending on tool temperature. The maximum and minimum values were selected at µmin = 0.15 and µmax = 0.5 analogues to dry and lubricated conditions, respectively. The transition temperature was set at Tcrt.oil = 200 ℃ corresponding to lubricant flash point according to manufacturer literature. In the simulation, the friction occurs in element level. Hence, the increase in overall friction changes gradually to a plateau when the temperature of tool elements in-contact exceeds Tcrt.oil.

3 Results and Discussions

3.1 Initial Chip Formation and Correlated Low Tool-Chip Contact

Chip formation undergoes complex physical evolution during machining process. In particular, at the start of the cutting interval, a transient behavior appears to exhibit an interesting behavior where low mechanical loads can be observed. It is quite important to understand how such development occurs, its limiting criteria and influencing factors. Figure 2 shows mechanical loads and calculated coefficient of friction during orthogonal cutting.

Fig. 2.
figure 2

Mechanical loads during IPCF (A) using different speeds (B) Mechanical loads at Lc = 18 mm and vc = 30 m/min and calculated coefficient of friction (COF)

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The passive force Fp is affected to large extent by the friction in the secondary shear zone where the rake side of the tool contacts the forming chip. Contaminants (e.g. lubricants and oxides) and tool surface characteristics such as roughness and coatings are main factors that determine contact behavior within tool-chip interface.

When observing mechanical loads at IPCF, low forces were detected with more reduction associated with the passive force Fp. The reduction in measured forces occurs only at the beginning of the cut. The transient region appears to reach a plateau at specific cutting length irrespective of the cutting speed. This observation may indicate possible temperature-related phenomenon that promotes adhesion and severe friction.

To investigate the history of the contact during IPCF, adhesion on the rake face resulting after machining different cutting lengths was examined. An area of intimate contact that is located towards the cutting edge, where no adhesion marks, is considered as the sticking zone while the region with adhesion marks is considered the sliding zone. Figure 3 summarizes measured contact lengths.

Fig. 3.
figure 3

Tool-chip contact zones measured as appears on tool’s rake face

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The sticking zone develops gradually as cutting progress reaching a stable value after some cutting length. Qualitatively, the development of sticking area is analogues to observations related to cutting forces. The full contact might have been delayed by contaminants that exist during initial chip formation.

IPCF appears to result lower mechanical load and lower chip-tool contact. However, to this point, IPCF analysis has been applied to single cutting occasion. Since the effects of IPCF are temporary, the goal is to maintain the favorable effects through interrupting the cutting process.

3.2 Interrupted Machining

As can be noticed form previous section, the effects associated with IPCF are transient and cannot be sustained after finite cutting distance. Therefore, interrupted cutting can offer an opportunity to sustain the favorable effects associated with IPCF. During cutting interruption, spatial and temporal windows can offer a potential to cool the tool down and restore contamination layer (e.g. lubricant film).

Cutting length was fixed, to be within IPCF range, at Lc = 8 mm. Interruption length Lint was varied within range Lint = 4 … 24 mm. Each workpiece has a constant interruption length Lint. Figure 4 (A) illustrates calculated COF = (Fp + Fc tanγ0)/(Fc − Fp tanγ0) of four consecutive cutting intervals for two outermost cases at Lint = 4 and Lint = 24 mm. The average work of single cutting intervals \(\overline{W }\) after the first interruption of different interruption lengths was calculated and summarized in Fig. 4 (B).

Fig. 4.
figure 4

Interrupted cutting under MQL with varying interruption length

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Interruption can be rather important for sustaining the effects of IPCF. At short interruption length limited chance is available to clear the chips, cool down the tool and restore contamination layer. On the other hand, as interruption duration increases by means of interruption length, subsequent cutting intervals exhibit repeatable behavior comparable to the first cutting interval. Average work per cutting interval continues to decrease as interruption length increases in a direct correlation to interruption effectiveness. It is still important to investigate which physical characteristics contribute the most into IPCF. Hence, 2D chip formation model is proposed in the next section.

3.3 2D Chip Formation Simulations

2D chip formation finite element (FE) simulation model is proposed consisting of a modified hybrid friction model that includes a transition friction coefficient as a function of tool temperature Ttool. The transition temperature was selected to be lubricant flash point at Tcrit.oil = 200 ℃. Selected simulation parameters, force, COF and maximum simulated tool temperature are shown in Fig. 5.

The modified friction model was able to reveal a transition range, characterized by passive force, representing initial chip formation that is very close to experimental results. In simulation, total tool-chip contact was 0.28 mm and 0.50 mm at 2 mm and 10 mm cutting distance, respectively, which are within comparable magnitude to experimental results shown in Fig. 2.

Fig. 5.
figure 5

IPCF 2D FE chip formation simulation with oil temperature (OT) friction criterion

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However, there were issues concerning force levels. On average, the cutting force appeared 20% higher and the passive force was 12% lower. A possible cause might be related to fundamental limitations of Johnson-Cook flow stress model at high temperatures in combination with high strain rates [12]. Also, force fluctuation range due to serrated chip formation both at low and high friction regimes appeared different than experimental results. That could be attributed to tool-workpiece dynamics that were not included in the present model. Improvements about flow stress model and system dynamics are still required to achieve better simulations results.

4 Conclusion

Initial period of chip formation (IPCF), which occurs at the start of the cutting process, was observed to exhibit reduction in mechanical load and low tool-chip contact in comparison with steady state condition. Characterization of IPCF can be beneficial for improving continuous machining processes by introducing interruptions along the cutting path. Cutting fluids (CFs) working action can be improved during IPCF leading to targeted and efficient CFs application and improved sustainability.

The present investigations indicate a finite cutting length where transient IPCF effects do exist before diminishing at steady state. In order to maintain IPCF, adequate cutting-free period is required. There seems to be a relationship between adhesion on the rake face of the tool and the end of IPCF. The sticking contact appears to develop gradually within IPCF until it reaches a final size. This delay indicates an apparent relationship between contaminants resistant to ablation and mechanical loads. Enhanced performance of MQL under interrupted cutting within IPCF was observed. The repeatability of IPCF is affected by interruption size. IPCF appears to be insensitive to cutting speed and a finite cutting length can be identified. The proposed modification in the friction model in FE simulations was capable of producing a transition period analogue to IPCF.

More investigations are required to study IPCF of different materials particularly with different thermal properties. Opportunities exist for improvements about IPCF modeling considering tool-workpiece dynamics and enhancing flow stress and friction models.