Exam Tutor

1. What is BLU? Accuracy? Repeatability?

BLU (basic length unit) is the minimum length distinguishable by the control unit. Accuracy is the degree to which a measurement, calculation, or specification conforms to the correct or known value or standard. Repeatability is a measure of how closely a machine repeats a given position command.

2. Describe about the sources of error in CNC machining.

- Inaccuracies in the machine elements machine-tool assembly errors. - Spindle runout. - Leadscrew backlash. - Tool deflection. - Thermal error.

3. What is the difference between absolute and incremental encoder? Use sketches to explain the

main differences and similarities in their operations. Incremental encoder: - The output is a series of electric pulses. - There is no reference of the actual position. Absolute encoder: - An external device is needed to denote the reference on the encoder shaft (home position) - Actual position is got by keeping track of the output and accumulating the position count.

4. What is the difference between continuous path control and point to point control? PTP motion-control: - To move the machine table or spindle to a specified position. Continuous (contouring) control: - To control two or more axes simultaneously to get desired shape.

5. List the principle of operation of stepper motor and compare it with a DC servo motor and

describe its pros and cons. Stepping motor: - When the pattern of the signals shifts, the motor rotates one step angle. - The direction of rotation depends on the pattern-shifting direction. - NC sends one stepping signal and a direction signal. DC servo motor: voltage control - When a higher voltage is applied to the motor, a large current flows through the motor coil,

which in turn produces more torque and makes the motor run faster.

6. Explain the difference between the Acceleration before interpolation and acceleration after interpolation techniques and write the pros and cons of each? Acceleration Deceleration Before Interpolation (ADCBI): - Calculate the acceleration and deceleration of the tangent direction before calculating the

interpolation position of each axis. - The interpolation command is then sent to the control unit of each axis.

Acceleration Deceleration After Interpolation (ADCAI): - Calculates displacement distance for each axis, ΔX, ΔY, ΔZ for every interpolation time

interval using the rough interpolation module. Independent Acc/Dec control of each axis is performed with respect to ΔX, ΔY, ΔZ and fine interpolation.

- The total remaining displacement of each axis for every position control time interval is calculated

7. Why an S-Shaped rather than linear (Trapezoidal) curves are selected for acceleration and

deceleration controls in CNC machines? - Has a smooth and stable speed curve. - The acceleration at any point is continuous and the motor runs smoothly. - The quality of the product is high but requires high CPU.

8. An NC machine has a resolution (BLU) of 0.001. To move the cutter from coordinates (1,1) tto (3,4) at 18 ipm, how many pulses and at what rate would the controller send to the X and Y servo motors.? [ Note that the coordinate dimensions are given in inches] AB= 3 − 1 $ + 4 − 1 $ AB= 13𝐼𝑁𝐶𝐻 Time taken to move: +,

+- = 0.2min = 12 𝑠𝑒𝑐

Number of bullets required = +, 9.99+

= 3605.5 𝑏𝑢𝑙𝑠𝑒𝑠 x-direction = $ 9.$ = 10 𝑖𝑝𝑚

Y-direction = ,

9.$ = 15 𝑖𝑝𝑚