AISI 304 stainless steel milling process state diagnosing

. The article presents the results of works on the analysis of using cutting forces and torque to determine AISI 304 stainless steel milling process state. The research and analyzes were carried out for the determine tool state and the process state itself. The tests were performed using Sandvik milling head, equipped with 345R-1305M-PM 4230 inserts . This study presents and discusses the obtained results determining the resultant value of the minimum and maximum cutting forces and the tool life. The results of observations of the tool's condition were presented.


Introduction
Milling of high-alloy steels with high chromium or nickel content is common in the industry associated with the production of various types of processing tools.Due to the need to achieve high productivity during machining, machining parameters that result in a significant load on the cutting tool should be used.Carrying out machining with high volumetric machining efficiency is associated with a relatively short tool life, which in many cases is defined as few minutes.
The available literature includes many publications on determining machining forces, determining and modeling the state of the cutting process of high-alloy steels in terms of selecting process conditions, determining its state or analyzing load, tool wear [8,13,14,25] or tool damage [5].Machining of alloy steels, due to its alloy components that are difficult to cut, is generally problematic and requires the use of wear-resistant tools; attempts are also made to carry out machining in conditions different from standard ones, for example dry or using MQL [7,12,17,18,20,23].At the same time, work is being carried out on modifying the cutting edge geometry [1,11] or process modeling [21,24].
Due to the significant machining costs, also resulting from tool costs, it seems reasonable to diagnose [15,16] or monitor the cutting edge condition, enabling tool optimal use and preventing tool damage and/or the workpiece surface [2,3,6,10 ,19] Many works describe the use of force measurement or acoustic emission to determine the state of the process, mainly due to the cutting edges condition [4,9,13,22].

Characteristics and diagnosing AISI 304 stainless steel milling processes state
An attempt was made to determine the suitability of the cutting force generated in the toolholder-workpiece system during milling AISI 304 stainless steel process as state determine signal.Original results were compared to cutting edge state.

General set-up, methodology and technique of experiment
The article presents the results of original studies using the Kistler 9257B dynamometer with the 5017 signal amplifier.Measuring stand; its setup and settings are presented in detail in [16].AISI 304 stainless steel milling conditions were: • dimensions : 40x40x140mm, 18mm removed (18 passes of 1mm each), • tool: Sandvik milling head, 345R-1305M-PM 4230 inserts; sintered carbides with a TiCN+Al2O3+TiN coating applied using the CVD technique, • cutting speed vc = 176 m/min, feed rate vf = 460 mm/min, cutting depth ap = 1 mm, cutting width ae = 20 mm, • dry machining, • down-cut and up-cut milling (milling in both directions).
The tests were carried out on a DMG DMU50 5-axis machining center, the workpiece was clamped in a vice attached to a force gauge.Orientation axis force gauge was consistent with the orientation axis machine tools.
Optical analysis of the drills was performed using a Keyence VHX-7000 digital stereoscopic microscope.The photos were captured at x100 magnification.Dimensions markers were placed in the drawings

Measurements results
Cutting force measurement results; Fx, Fy, Fz and torque Mz were depicted in the form of selected graphs of forces and torque values changes during dry AISI 304 stainless steel milling.A hardware low-pass filter of the amplifier 10 Hz and sample rate 100 Hz was used and post process filtration was used with low-pass filtration; edge frequencies -lower (-3dB) 1Hz and upper (-3dB) 100Hz.The results of the changes in the values of forces and torque were also presented on two summary charts (Fig. 1,2).The machining conditions were selected on the basis of the analysis of the available literature and previous own work so as to obtain high process efficiency and standard tool wear.Fig. 1 shows force Fx, Fy, Fz and Fig. 2 -torque Mz values changes during whole milling process [15,16].The values of the total cutting force were determined from the relationship: When determining the value of the total cutting force, the time drift of the force gauge was taken into account and eliminated.The obtained results of the total cutting force determined on the basis of measurements and transformations are presented graphically in Fig. 3. Due to the decrease in the cutting force value during a single pass, the minimum and maximum force values were displayed.The direction of cuts of the cutting tool (down-cut and up-cut) was also taken into account, and thus the following output variables were determined: • down-cut minimum resulting force, • down-cut maximum resulting force, • up-cut minimum resulting force, • up-cut maximum resulting.The analysis of the obtained results clearly indicated the occurrence of three characteristic zones, which can be attributed to different states of the cutting tool.The first zone, marked in Fig. 3 as I -initial wear zone , the second zone, marked as N is normal operation zone and the third zone, marked E to excessive wear zone.The obtained course of changes in the value of the total cutting force is consistent with the results obtained by the authors of other works [6,24,25].To determine the durability period of the shortening edge, the formula (2) was used: where: Tc -blade durability, min, l -cutting path in mm, determined as the product of the sample length and the number of passes to the end of zone N (fig.3), v f -feed speed, mm/min.
A visual verification of the main cutting edge state, visible from the side of the rake plane and the flank surface of the firs insert (fig.4) and second insert (fig.5) cutting edge was carried out after making whole milling process.The places of the greatest tool wear (cutting edge chipping), resulting in a change in its geometry, are marked -indicated by red arrows and tool coating abrasive wear -indicated by yellow lines.

Discussion and conclusions
The tests of AISI 304 stainless steel milling resulted in tool wear and changes in the values of the measured quantities, i.e. cutting forces and torque, which were diagnostic signals of the state of the process.It was shown, similarly to previous works [15,16], that measurements of changes in the values of forces or torque can be a good diagnostic signal of the state of the cutting process.The experiment was stopped when increased force values were obtained in order to avoid catastrophic wear of the tool.The edge durability period obtained during the tests, approximately 9 minutes, was lower than expected, however, probably if the force values had not been measured, the processing would not have been interrupted; the increase in power demand observed at the same time (on the machine controller screen) was small and it is unlikely that in production conditions the operator would interrupt the processing due to such a small increase in power demand.Very intensive wear of the anti-wear multicoat TiCN+Al2O3+TiN coating of inserts was observed.This indicates the abrasive nature of wear of the coating itself and the cutting edge.This form of wear was undoubtedly influenced by the processing conditions used; significant cutting speed, feed and dry machining.Cutting forces and torque online measurements enabled the critical condition in the form of accelerated wear of the cutting edge detection and prevented damage.
Based on the literature review and the results of own work, the following conclusions were formulated: • online measurements of forces during the milling process of AISI 304 steel enabled supervision of the process and prevented damage, • the durability period of the cutting edge under the conditions of the experiment was approximately 9 minutes and was shorter than the assumed durability period of the blade, • force measurement systems require intervention in the processing zone and are costintensive, which translates into limited applicability of such solutions in industrial conditions.

Fig. 4 .
Fig. 4. Main cutting edges and surfaces view of first insert, description in text.

Fig. 5 .
Fig. 5. Main cutting edges and surfaces view of second insert, description in text.