Influence of Coolant Supply Method and Pressure on Cutting Temperature During Drilling of Steels with High-Speed Steel Drills
Abstract Generated abstract
This paper examines how coolant delivery method and pressure affect cutting temperature during drilling of several steels with high-speed steel drills. Experiments used 23 mm drills with internal coolant holes on a modified vertical drilling machine, measuring temperature by the natural thermocouple method while comparing flood cooling with pressure-fed coolant supplied directly to the cutting zone. The results show that internal coolant delivery substantially lowers cutting temperature, with diminishing additional benefit beyond a material-dependent critical pressure that is higher for steels of lower thermal conductivity. The study also reports that, for selected stainless and heat-resistant steels, replacing flood cooling with internal cooling at 25 kg/cm² increased drill life by about 20 to 25 times.
Full Text
EFFECT OF THE METHOD OF SUPPLY AND PRESSURE OF THE COOLANT ON CUTTING TEMPERATURE DURING DRILLING OF STEELS WITH HIGH-SPEED STEEL DRILLS
D. V. KOZHEVNIKOV, Yu. V. SHCHЕPETILNIKOV
(Presented by the scientific seminar of the Department of Machine Tools and Metal Cutting)
The drilling process is accompanied by intense heat generation, the source of which is the plastic deformation of the metal being machined as it is transformed into chips, and friction on the contact surfaces of the tool and the workpiece.
The cutting temperature is closely connected with all the physical phenomena accompanying the process of metal cutting and has a decisive influence on the wear and life of the cutting tool. The specific conditions of the drilling process—the variability of cutting speed and geometry along the length of the cutting edges, and the hindered removal of chips—considerably complicate the pattern of heat generation and temperature distribution. In addition, the enclosed nature of the cutting process during drilling leads, as a result of prolonged contact of the chip, which carries the main share of the generated heat, with the drill body, to its additional heating.
In order to reduce the cutting temperature, drilling of steels is carried out with the supply of a cutting fluid. However, the cooling method usually used in drilling—free flooding of the drill—is of little effectiveness, since the coolant first encounters the heated chip and the rotating drill body, which hinder its penetration into the cutting zone. Tool life is affected mainly by the cutting heat concentrated in the surface layers of the cutting tool, in the zone of its contact with the chip and the material being machined. It is precisely from here that heat must first of all be removed intensively. Thermophysical analysis has shown that the greatest reduction in cutting temperature is achieved when the cooling medium first encounters the tool on its path, and only then the chip and the workpiece [1]. In drilling, this condition is fully satisfied only when coolant is supplied directly into the cutting zone through internal holes in the drill.
Investigations carried out in the metal-cutting laboratory of TPI with high-speed steel R9 drills of 23 mm diameter with rolled holes (VNII design), on steels differing in their physical properties, convincingly confirm the above.
The experiments were conducted on a modernized vertical drilling machine 2A150. Measurement of cutting temperature was carried out by the me—
by the method of a natural thermocouple when drilling steels 45G17YuZ, EI—316, Kh25SN3D, 1Kh18N9G, St. 45, and St. 20. The layout of the installation for measuring the cutting temperature is shown in Fig. 1. The cooling liquid was supplied to the cutting zone under a pressure of up to 45 kg/cm² from a special pumping station through chuck 1, mounted on the machine spindle, and through holes in the drills. To prevent the occurrence of parasitic thermocouples, the drilling zone was insulated from the surrounding space by a protective casing 2, from which the cooling liquid was discharged through openings into a tank not connected with the cup. The thermoelectric emf arising during cutting between the drill and workpiece 6 was taken off from copper ring 3, fastened on the machine spindle, by copper-graphite
Fig. 1
brushes 4 and was recorded by a low-inertia millivoltmeter 5 of type M-136/A with a scale division of 0.05 mv.
Measurements were carried out on sharpened drills; the angle at the tip was \(2\varphi = 120^\circ\), the clearance angle \(\alpha = 12^\circ\). The coolant was a 5% emulsol solution in water.
The results of measurements of cutting temperature at a speed of \(21.2\ \text{m/min}\), as a function of coolant pressure for all steels, are presented in Fig. 2. From an analysis of the dependences shown, it follows that the greatest reduction in cutting temperature is observed when flood cooling is replaced by delivery of coolant directly into the cutting zone. As the coolant pressure is increased, the cutting temperature at first continues to decrease and, upon reaching some “critical” value for the given material and cutting conditions, subsequently remains practically unchanged. Moreover, this “critical value” of pressure is greater for steels with lower thermal conductivity. Thus, for steel 45G17Yu3 it is about \(25\ \text{kg/cm}^2\), while for steel 45 the cutting temperature remains practically unchanged already beginning at a pressure of \(5\ \text{kg/cm}^2\). Therefore, under identical drilling conditions for steels with different thermal conductivity, the difference in cutting temperature decreases with increasing pressure (Table 1).
Fig. 2
Table 1
| Type of cooling | Flooding | \(1\ \text{kg/cm}^2\) | \(5\ \text{kg/cm}^2\) | \(15\ \text{kg/cm}^2\) | \(25\ \text{kg/cm}^2\) | \(35\ \text{kg/cm}^2\) | \(45\ \text{kg/cm}^2\) |
|---|---|---|---|---|---|---|---|
| Temperature difference when drilling steel 45G17Yu3 and steel 45 | \(446^\circ\) | \(356^\circ\) | \(268^\circ\) | \(252^\circ\) | \(245^\circ\) | \(240^\circ\) | \(235^\circ\) |
Consequently, drilling heat-resistant and stainless steels, which have poorer thermal conductivity than structural materials, should be carried out at a higher pressure of the coolant in the supply system than when drilling structural steels.
With an increase in cutting speed, the cutting temperature for all steels increases more intensively in the case of drilling without coolant supply and with flood cooling. With internal cooling, as the coolant pressure increases, a decrease in temperature is observed at all cutting speeds; moreover, the intensity of this decrease diminishes as the coolant pressure increases (Fig. 3).
A significant decrease in cutting temperature when replacing cooling by internal pressure delivery provides a substantial increase in drill life. As subsequent life tests showed, the life of high-speed-steel drills when drilling steels 1Kh18N9T and 45G17Yu3, in the case of replacing flood cooling with internal cooling at a coolant pressure of 25 kg/cm², increases by 20–25 times.
Fig. 3
Conclusions
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Supplying coolant under pressure to the cutting zone through internal holes in the drill ensures a significant reduction in cutting temperature, especially when drilling steels with low thermal conductivity.
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The value of the coolant pressure that provides the greatest reduction in temperature compared with its supply by flooding is the higher, the lower the thermal conductivity of the material being machined.
LITERATURE
- A. N. Reznikov. Design and Practice of Introducing High-Durability Cutting Tools. “Mashinostroitel,” NTO Mashprom, No. 6, Moscow, 1966.