Automated Rough Turning

Description And Advantages

The maximisation of material removal rate (MPR) is normally achieved by empirically selecting the feed rate and depth of cut based upon the size and material properties of the workpiece as well as the type of cutting tool employed. However, once a selection is made these two parameters are maintained at a constant level throughout the conventional turning. One strategy to about halve the cutting time requires the maximum cutting force to be maintained by adaptively changing the feed rate. A high cutting force reduces the cutting tool's life. A larger depth of cut increases the MPR but chatter is more likely to occur. The depth of cut is usually selected close to the upper limit determined from the lathe's maximum power. In rough turning, the metal removal process can begin with an unknown shape, which must be measured before appropriate cutting parameters can be determined. A laser is often incorporated in devices for mapping a profile.

The machine tool is a retrofitted lathe powered by two electrocraft servomotors. The laser device is attached to the lathe's tool post. The major components include a Uniphase, 155SL He-Ne laser, a sensor, gear train, rigid arm, two mirrors and a stepper motor. A reorientation mirror redirects the 1 mm diameter beam of the laser before hitting the stepper motor driven oscillating mirror, so as to be swept across a cross-section of the workpiece. The laser bream's movement is synchronised to that of the rigid arm by ensuring that the arm and oscillating mirror have the same centre of rotation and a common 2:1 gear train. Consequently the light beam when interrupted, is always captured by a sensor held near the arm's free end. The laser beam diverges at two concave lenses to a cover, a photocell inside the sensor. The photocell behaves as light controlled resistor that is connected to a voltage divider circuit to produce a voltage that is proportional to the intensity of light received. This intensity, of course, alters and depends instantaneously upon the percentage of the laser beam's cross-section that is blocked by the workpiece. The controller is an IBM compatible computer with three interface cards: one to control the servomotors, the second one to control the stepper motor and the last one is a 12-bit analogue-to-digital converter.

A laser device has been shown capable of automatically mapping the spinning profile of a workpiece, regardless of its surface roughness, colour or orientation. The accuracy of the profiled map was within 10% of the true profile and was adequate to implement the methodology. The cost of laser source and sensor is around $600. A comparable contact sensors utilised in CMMs is around $5,000. In comparison to a standard metal cutting strategy, a reduction in material removal rate of up to 36% was found using the developed stragtegy. The technique is also capable of monitoring chatter and the control program was found to be capable of automatically adjusting the depth of cut to maintain chatter free cutting.


Search, December,2000