Study of the Machinability of Cast Heat-Resistant Steels Kh25SNZD and EI 316 in Face Milling
Abstract Generated abstract
This study investigates the face milling machinability of cast heat-resistant steels Kh25SN3D and EI 316, focusing on tool material, cutter geometry, cutting speeds, feeds, and tool-life criteria. Experiments on a horizontal milling machine compared several carbide grades and examined milling with and without scale, using flank wear of about 0.35 to 0.4 mm as the dulling criterion. VK8 carbide inserts gave the best tool life for both steels, and recommended cutter geometries and cutting regimes were established. For Kh25SN3D, advisable cutting speeds were 30 to 47 m/min without scale and 15 to 30 m/min with scale, while EI 316 without scale required 17 to 20 m/min, with corresponding feed ranges specified for practical tool lives.
Full Text
STUDY OF THE MACHINABILITY OF CAST HEAT-RESISTANT STEELS Kh25SN3D AND EI 316 IN FACE MILLING
A. M. ROZENBERG, A. K. BAIKALOV, A. F. ANISIMOV,
V. A. GOVORUKHIN, A. V. POZOLOTIN
(Presented by Prof. Dr. A. N. Eremin)
In the metal-cutting laboratory of the Tomsk Polytechnic Institute, since January 1962 work has been carried out on the study of the machinability of heat-resistant steels in face milling. The work is being conducted under a contract with the Krasnoyarsk Sovnarkhoz.
The purpose of this work is to find the most optimal material for the cutting tool, the grinding geometry of the milling-cutter knives, and to determine permissible feed values and a range of cutting speeds that make it possible to obtain a practically expedient value of tool life. The hardness of steel EI 316 is \(HB = 180—210\ \text{kg/mm}^2\); Kh25SN3D, \(HB = 300\ \text{kg/mm}^2\).
The study was carried out on a horizontal milling machine. The diameter of the milling cutter with which machining was performed was \(200\ \text{mm}\). To obtain the required feed-per-tooth values, a special reduction gear with a gear ratio of \(1/64\) was designed and manufactured.
Wear of the cutter on the flank face was determined with the aid of a Brinell magnifier with tenfold magnification. Tool-life comparative tests were conducted with milling cutters having hard-alloy grades VK8, VK6M, VK4, TT7K12, T5K10 under the following milling conditions:
\[ B = 150\ \text{mm},\quad V = 47\ \frac{\text{m}}{\text{min}},\quad s_z = 0.154\ \frac{\text{mm}}{\text{tooth}},\quad t = 3\ \text{mm},\quad D = 200\ \text{mm}, \]
where \(B\) is the milling width in \(\text{mm}\),
\(t\) is the milling depth in \(\text{mm}\),
\(D\) is the milling-cutter diameter in \(\text{mm}\).
The greatest tool life was shown by cutters equipped with inserts of hard alloy grade VK8.
The optimal cutter geometry when milling steel Kh25SN3D without scale is characterized by the following angles:
\[ \alpha_1 = 10^\circ;\quad \alpha = 10^\circ—12^\circ;\quad \gamma = 0^\circ;\quad \lambda = 0^\circ \div 5^\circ;\quad \gamma_{\phi} = -5^\circ; \]
\[ f = (1.4—1.6)s_z;\quad \varphi = 60^\circ;\quad \varphi_1 = 10^\circ. \]
As the blunting criterion, flank wear \(h_z = 0.4\ \text{mm}\) was adopted, since during subsequent operation of the cutter catastrophic destruction of the cutting edge occurs because of the gradual emergence of the crater on the rake face onto the cutting edge, which causes a sharp increase in temperature and cutting forces.
In carrying out tool-life tests to determine the comparative life of milling cutters with different cutting-part materials, it was established that, with a cutter life of 30 min for a cutter with VK8 inserts (for flank wear \(h_3 = 0.4\) mm), the life of cutters with VK6M inserts was 10 min, with VK4 inserts—5 min, with TT7K12 inserts—1 min, and with T5K10 inserts—1.5 min.
Further investigations were carried out with cutters equipped with VK8 hard-alloy inserts, which is the optimum cutting material for milling steels of the given grades.
The formula for calculating cutting speeds when milling steel Kh25SN3D without scale has the form
\[ V=\frac{193.5}{T^{0.527}\cdot t^{0.368}\cdot s_z^{0.635}}\ \frac{m}{min} \left(\frac{B}{D}=0.75\right). \]
The most rational ranges of cutting conditions are
\[ V=30 \div 47\ \frac{m}{min};\qquad s_z=0.078 \div 0.2\ \frac{mm}{tooth}, \]
which corresponds to 360–180 min of cutter life. When the feed per tooth is increased above \(0.2\ \frac{mm}{tooth}\), the cutter life drops sharply. Suffice it to say that, when operating at a cutting speed of \(V=30\ m/min\), changing the feed per tooth from \(0.2\ mm/tooth\) to \(0.254\ mm/tooth\) causes the life to decrease by approximately 5–6 times.
Such a sharp decrease in life can probably be explained by insufficient mechanical strength of the cutting edge of the cutter insert at a certain maximum feed value (“breaking” feed).
When working through the scale on steel Kh25SN3D, the most expedient cutting regime may be characterized by the following values: \(v=15 \div 30\ m/min\); \(S_z=0.078 \div 0.2\ mm/tooth\), while the formula for calculating the cutting speed has the following form: (for \(\frac{B}{D}=\frac{150}{200}=0.75\)):
\[ V=\frac{180}{T^{1.5}\cdot t^{0.35}\cdot s_z^{0.41}}\ \frac{m}{min}. \]
The considerably lower level of cutting speeds when working through the scale is explained by the increasing role of abrasive wear due to the hard particles of the scale.
However, even in this case the tool geometry differs little from the geometry of the tool used for machining steels without scale.
The angle values for this case are as follows:
\[ \alpha=\alpha_1=\varphi_1=10^\circ;\quad \varphi=60^\circ;\quad i=0^\circ;\quad \lambda=+10^\circ;\quad \gamma_\varphi=-5^\circ; \]
\[ f=(1.4-1.6)s_z. \]
Along with the development of cutting regimes, several comparative tests were carried out to determine the influence of the amount of displacement of the cutter axis relative to the axis of the workpiece—
\[ \frac{k}{D}, \]
where \(k\) is the amount by which the cutter tooth projects beyond the side surface of the workpiece;
\(D\) is the cutter diameter in mm.
When operating without scale, with an offset of \(\frac{k}{D} = 0.05\), a twofold increase in cutter life was obtained in comparison with the symmetrical arrangement of the axes of the cutter and the workpiece.
In studying the offset when machining over scale, practically no increase in life was observed.
Similar work to determine machinability was also carried out on steel EI 316.
It was established that the optimum values of the cutter angles when machining this steel without scale are as follows:
\[ \varphi_1 = 10^\circ;\quad \varphi = 60^\circ;\quad \alpha = 15^\circ;\quad \gamma = 0^\circ;\quad \gamma_{\mathrm{f}} = -5^\circ;\quad f = (1.4 - 1.6)s_z; \]
\[ \lambda = +20^\circ. \]
The optimum grade of hard alloy is VK8.
The level of cutting speeds and feeds corresponding to a tool life of 180–260 minutes is:
\[ V = 17 \div 20\ \text{m/min};\quad s_z = 0.08 \div 0.155\ \text{mm/tooth}. \]
Conclusions
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In face milling of heat-resistant steels Kh25SN3D and EI 316, it is advisable to use carbide cutters equipped with inserts of VK8 hard alloy.
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The range of the most advisable cutting speeds and feeds when machining steel Kh25SN3D without scale is \(V = 30 \div 47\ \text{m/min};\ s_z = 0.078 \div 0.2\ \text{mm/tooth}\); with scale, \(V = 15 \div 30\ \text{m/min};\ s_z = 0.078 \div 0.2\ \text{mm/tooth}\).
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The range of cutting speeds and feeds when machining steel EI 316 without scale is as follows:
\[ V = 17 \div 20\ \text{m/min};\quad s_z = 0.08 \div 0.155\ \text{mm/tooth}. \]
- The criterion for cutter dulling is wear on the flank face up to a value of \(0.35\text{–}0.4\ \text{mm}\).