Milling Dynamometer of Variable Stiffness

G. L. Kufarev, V. V. Ovcharenko, V. A. Govorukhin

(Presented by the joint scientific seminar of the Departments of Machine Tools and Metal Cutting and of Mechanical-Engineering Technology)

The purpose of the work in which the dynamometer described below is used is to study the instantaneous circumferential force and feed force in face milling with a single-tooth cutter. It was necessary to investigate the nonuniformity of the circumferential force during the cutting period, with the continuously varying chip thickness. In face milling with a single-tooth cutter, the feed force changes not only because of the variable chip thickness, but also because the vector of the force acting on the tooth continuously rotates together with the cutter. Therefore, when recording the forces, it was necessary to use low-inertia and at the same time very sensitive sensors in the dynamometer.

The most advanced designs of milling dynamometers, developed at Tomsk Polytechnic Institute [3] and at ENIMS [1], have a number of shortcomings. First, the inductive sensors used in them possess a certain inertia [5]; second, the elements transmitting the forces to the sensors have considerable masses, which also increase the inertia of the instruments; third, the constancy of the cutter diameter in the ENIMS dynamometer [1] limits the range of experiments.

Taking into account the above-mentioned shortcomings of existing dynamometers, an elastic-electric dynamometer for measuring the circumferential force during milling was manufactured at the Department of Machine Tools and Metal Cutting of TPI (Fig. 1).

The dynamometer is a single integral body, the upper part of which is fastened to the machine spindle 9 by means of four bolts. Centering is provided by the cylindrical recess 8 present in the body of the dynamometer. The torque is transmitted to the cutter by means of two keys bolted to the spindle and entering the corresponding slot of the dynamometer. Cutting is carried out by one cutter, secured in a special replaceable holder. The four conical holes for fastening the holder, present in the body of the dynamometer, make it possible to vary the cutter radius in the experiments.

The elastic element is the web of the cutter holder. The strain gauges glued to it have the following characteristics: base—10 mm, resistance \(R = 190\) ohm, strain-sensitivity coefficient equal to 2. The sensors are connected according to a differential electrical circuit (Fig. 2). The signal from the sensors, by means of a four-channel UT4-1 amplifier, is fed to an MPO-2 loop oscillograph and to a panel with M-24 instruments.

(microammeters) for visual monitoring. During operation (when the dynamometer is loaded), the resistance of the sensors changes. This disturbs the balance in the electrical circuit, and the instruments register the unbalance current. The greater the circumferential force on the milling cutter, the greater the unbalance current.

Fig. 2. Electrical circuit for measuring the circumferential force and the feed force during milling

The strain gauges for measuring the circumferential force during milling are located together with the dynamometer on the rotating spindle of the machine. From the rotating sensors, the emf is transmitted to the fixed measuring instruments by means of a current collector with sliding contacts.

Figure 1 shows the general view of the current collector. The emf from the strain gauges is transmitted through insulated wires passing inside the machine spindle and the rotating hollow shaft mounted on bearings in the fixed housing of the current collector. The shaft is driven in rotation from the machine spindle by a special driver 10. Six rings (two working and four spare), insulated from one another, are fastened on the shaft; the ends of the wires coming from the sensors are soldered to these rings. Two brushes are brought to the surface of each ring, and from them the emf is transmitted to the measuring instruments. The brushes, insulated from one another, and the rings of the current collector are made of silver-constantan alloy.

The feed force in face milling was measured by means of a dynamometric table (Fig. 1), made rigid, low-inertia, and at the same time sensitive.

In principle, this design is a steel frame 5, the upper part of which, with the workpiece fastened to it, under the action of ...

Fig. 1. Diagram of the attachment

Visible labels in the figure:

• Amplifier UTs-1
• Oscillograph MPO-2
• $\sim 220\ \mathrm{V}$
• $\mu\mathrm{A}\ P_{\mathrm{okr}}$
• $\mu\mathrm{A}\ P_h$
• Machine table

Fig. 3. Calibration graphs for the circumferential force and the feed force

Left graph:

• Vertical axis: \(P_{\text{circ}}\), kg
• Horizontal axis: mA
• Inset:
• Dynamometer calibration
• \(J_{\text{cal}} = 10\) mA
• \(V = 60\)
• \(\circ\) — loading
• \(\times\) — unloading

Right graph:

• Vertical axis: \(P_h\), kg
• Horizontal axis: mA
• Inset:
• Table calibration
• \(J_{\text{cal}} = 30\) mA
• \(V = 60\)
• \(\circ\) — loading
• \(\times\) — unloading

the feed force moves relative to the lower part owing to the elastic deformation of the uprights, since the base of the dynamometric table is firmly connected to the table of the milling machine by means of bolts.

In this elastic spatial system, the strain-sensitive elements are the two side uprights 13. Strain gauges 14 are glued to the outer surface of each element (base 25 mm, resistance 190 ohms, strain-sensitivity coefficient—2). A load in the direction of the feed force causes the uprights to bend. In this case, the gauges glued to one upright will be stretched, and those on the other upright will be compressed. The feed force is recorded by sixteen gauges connected in series-parallel pairs. The need to include such a large number of gauges was dictated by the increased rigidity of the elastic system of the dynamometric table in the feed direction.

The gauges are connected into bridge circuits with the intention, on the one hand, of utilizing all the possibilities of their differential connection and thereby increasing the sensitivity as much as possible, and, on the other hand, of ensuring automatic elimination (compensation) of the effect of the components of the cutting force on the feed force.

To record the magnitude of the feed force, either a microammeter or the vibrator of an MPO-2 oscillograph is connected into the diagonal of the bridge circuit.

In testing the apparatus for measuring the circumferential force and the feed force during milling, the corresponding calibration was first carried out. To provide a force on the machine table (Fig. 1), upright 2 with screw 1 was rigidly fastened; the screw exerted pressure, through a control elastic-mechanical dynamometer DS-1, on a clamp and plate 4, fastened respectively either in the tool holder of the dynamometer or on the dynamometric table. Beforehand, the spindle and, consequently, the dynamometer were fixed against rotation by means of a special mandrel.

During calibration of the dynamometer, a linear relationship was obtained between the magnitude of the circumferential force and the readings of the microammeter. Specially conducted investigations showed that the action on the dynamometer (through the cutter) of a force of up to 500 kg in the axial and radial directions affects the readings of the instruments recording the circumferential force by up to 2%.

During calibration of the dynamometric table, a linear relationship was also obtained between the magnitude of the feed force and the readings of the microammeter or oscillograph. The calibration curves are shown in Fig. 3.

The advantages of the described dynamometer are: high sensitivity, low inertia, the possibility of changing the cutter radius, simplicity of manufacture, absence of internal losses, and universality, allowing it to be used in work on vertical and horizontal milling machines. In cylindrical milling it is only necessary to change the tool holder. The use of interchangeable tool holders with jumpers of different thicknesses (depending on the magnitude of the circumferential force being measured) makes it possible to record forces with high accuracy.

REFERENCES

1. V. Ya. Rassokhin. Circumferential force in face milling with carbide cutters. TsBTI, 1952.
2. A. M. Turichin. Electrical Measurements, GEI, 1961.
3. Metal Cutting and Tool, ed. A. M. Rozenberg. Mashinostroenie, Moscow, 1964.
4. A. M. Rozenberg. Dynamics of Milling. Izd. “Sovetskaya nauka,” Moscow, 1945.
5. M. F. Poletika. Instruments for Measuring Cutting Forces and Torques. Mashgiz, Moscow, 1963.