Numerical Control and CNC Questions and Answers

Answer: C

A point to point system suits Drilling.

In point to point (positioning) control, the machine moves the tool or table to a programmed position, stops, and then performs an operation there. The path taken between points does not matter, because no cutting happens during the move. Drilling, reaming, tapping and punching patterns of holes are exactly this kind of work.

Milling and turning cut while the tool is moving, so the path and feed rate along it must be controlled continuously; they need straight-cut or contouring systems.

Answer: C

It is Servo Control.

In a closed-loop servo system, a feedback device such as an encoder or linear scale measures the actual position of the slide and sends it back to the controller, which compares it with the commanded (input) value. Any difference, the error signal, drives the servo motor until the slide is in the correct position.

Hydraulic and pneumatic describe the power medium used to drive a machine, not whether its position is compared with the input; either could be used with or without feedback. An open-loop system, as with stepper motors, sends commands without checking the result.

Answer: C

The keyed answer is to increase the tool offset by 0.05 mm.

The diameter is 0.1 mm too large, so the tool must go in by half of that on the radius:

\[ \Delta r = \frac{50.1 - 50}{2} = 0.05 \]

A correction of 0.1 mm, as in options A and B, would change the diameter by 0.2 mm and over-correct it. That leaves options C and D, which differ only in direction. The key takes the offset as the distance the tool is set in, so that a larger offset takes the tool further in and the diameter falls, which gives C.

Doubt: the direction depends on the sign convention of the control and on whether the offset is entered in radius or diameter, which the question does not state. The official key gave C, and it is kept.

Answer: A

The basic length unit is 10 microns.

The BLU is the distance the table moves for one pulse, that is, one step of the motor. A 1.8 degree step is \(1.8/360 = 1/200\) of a turn, and one turn of the lead screw moves the table one pitch, 2 mm:

\[ \begin{aligned} \text{BLU} &= \frac{1.8}{360} \times 2\ \text{mm} \\ &= 0.01\ \text{mm} = 10\ \mu\text{m} \end{aligned} \]

20 microns forgets that the motor needs 200 steps per revolution, not 100; 40 and 100 microns do not follow from these figures.

Answer: C

The interpolator in a CNC machine controls the feed rate.

To cut a straight line at an angle or an arc, two or more axes must move at once at continuously related speeds. The interpolator works out the intermediate points and sends each axis drive the pulses it needs, so it coordinates the axis feeds and holds the programmed feed rate along the path.

Spindle speed, coolant flow and tool change are switched by the machine's logic and auxiliary (M) functions; none of them is part of interpolating a tool path.