CNC Programming Interview Questions & Answers

Question 1. Define Numeraical Control Machine?

Answer :

Numerical control machine cane be defined as a form of programmable machine in which the process are controlled by a program of numbers, letters, and symbols.

Question 2. What Is Nc Part Programming?

Answer :

NC part programming is the step by procedure of by which the sequence of processing steps to be performed on the NC machine is controlled by a program of numbers, letters, and symbols.

Question 3. What Is Apt Language?

Answer :

APT [automated programming language]is a computer program, it automatically calculates the tool path, generates program and controls the machine by receiving general high level languages.

Question 4. Mention Any 4 Post Processor Statement In Apt?

Answer :

COOLNT END FEDRAT/ 
MACHIN/ 
RAPID

Question 5. What Is Cnc?

Answer :

CNC is system consists of a computer, controller and a NC machine tool. Computer is used to store and edit the program. Controller controls the tool path based on the program.

Question 6. What Is Meant By Machining Centre?

Answer :

The machining centre is CNC system with automatic tool changing arrangement that is designed to perform a verity of machining operations, with large number of cutting tools.

Question 7. What Is Part Program?

Answer :

Part program is a high level language containing the instructions for machining a part to various standard words, codes and symbols.

Question 8. What Is Post Processing?

Answer :

Post processing is a computer program that takes a generalized part program output and adopts it to a particular machine control unit and machine tool combination. It is the basic intelligence required to change the program into computer language.

Question 9. Write The Order Of Instructions In A Part Program?

Answer :

End of Preparatory function Feed function Tool function block.

Question 10. What Is Manual Part Programming?

Answer :

Manual part programming is a process of writing programs which consists of a set of instructions [contains codes, symbols and numbers] to carry out the machining of the work.

Question 11. What Is Preparatory Function?

Answer :

It is word address format represented by the letter G, Followed by a numerical code for the operation of the control unit to instruct the machine tool.

Question 12. What Is Canned Cycle?

Answer :

Canned cycle simplifies and shortens the programming in such way whenever any one of the operation is required.

Question 13. What Are The Major Areas To Be Considered In The Design Of Nc Machine Tools?

Answer :

• Machine structure and frame
• Location of transducer 
• Slideways 
• Elements of transmission and positioning of sliders
• Spindlesvi.Tool holding arrangements

Question 14. How The Heat Effect On The Machine Bed, Tool Holder Can Be Taken Care?

Answer :

• Providing correctly designed mild steel structure with higherstiffness 
• Use of ribs, braces, angle plates to increase stiffness 
• Normal weight distribution over the entire frame 
• The hollow cross section for beds, bases and columns with a number of ribs welded with the walls cater for the rigidity as well as opening for inspection, lubrication and collection of chip coolants. 
• Thermo symmetrical design of all parts. 
• Providing large heat removing surfaces vii.Use of excellent coolants 
• Avoiding direct as well as local sources of heat such as sunlight and electrical motors.oil pumps respectively. 
• Reduction of ambient temperature by using air conditioning units 
• Proper alignment of the machine elements relative to each other while in operation as well as in stationary conditions

Question 15. Explain Slide And Slideways?

Answer :

In general machine tools are provided with tables, slides, carriages etc., to carry the work pieces or cutting tools etc., These parts are sliding in nature and mounted on the ways that are fixed on the other parts (column, housing, bed or knee) of the machines known as sliding ways.

Question 16. What Kind Of Pc Is The Minimum Hardware Requirement For Operate This Cnc-program?

Answer :

Use a PC 486 DX with 33 Mhz or higher. It requires a DOS Operation System installed, e.g. MS-DOS 6.22. MS-Windows 3.1/95/98 also includes an usefull DOS-Mode. 

1. The motion speed control works with program-loops, depending on the PC's performance. 
2. There are more than 4 Billion gears (speed-reductions) available. 
3. Pre-adjust the PC's groundspeed in the file "paramet", which is delivered with the package. 
4. For 6D applications we recomment use a Pentium PC or equals with min. 166Mhz.

Question 17. Which Steppermotor-driverboards Are Compatible To This Cnc-program?

Answer :

Those with 5V CMOS/TTL logic and 2 Bit (direction and clock/enable) standard signals.

Question 18. How Does It Work About The Reference- And End-switches?

Answer :

1. Pin11(reference-switch) must be on signal-ground when the program starts and ends. 
2. Pin10(end-switch) is normally closed to GND. If open, it can trigger a reference drive. 
3. When reference(Pin11) is open and end-switch(Pin10) opens also, then drive back to orgin! 
4. This saves time in case of interruption of processing. Pin10 is also good for overload use. 
5. All 9 axis will be driven back to orgin, one after the other. Order: W-V-U T-S-R Z-Y-X.

Question 19. How Is The Order Of Signals Sending Out To The Printerports?

Answer :

1. First the direction signal send out, e.g. if X-forward and spindle enabled [binary 64]. 
2. At second the direction signal inclusive the clock signal send out [binary 65]. 
3. A step is executing with pos. edge of clock! Code in C: outp(0x378,64); outp(0x378,65); 
4. The databits 1,3,5 are the direction bits. If forward set on [0], if backwards set on [1]. 
5. The signal of current boost should be connected to the clock signal in parallel. 
6. Make sure that the primary current (30% to 70% of boost) is enabled on the driver-unit to hold the motor in constant position while don't have motions.

Question 20. How Do I Have To Connect The Step-encoders And How Does It Work?

Answer :

1. When the program starts Pin 10,11,12,13,15 must be on ground (Low)! 
2. Pin 12(X),13(Y),15(Z) will be checked every 4th fullstep whether they are on GND (Low). 
3. Otherwise it should exist a High-Level. 
4. If the Low-Level won't be reached at 4th fullstep (because of overload), then the program try to push more steps (microsteps) on the printerport until the Low-Level will be reached. 
5. The max. amount of failed microsteps until automatic program stop and beginning of auto-reference-drive can be adjusted in the file "paramet". 
6. An immediate reference-drive also can be triggered if any key is hit on the keyboard. 
7. You may design your step-encoders by yourself with a disk and photogap. 
8. The position of the photogap must be adjusted precisely that it does pass the current if the fullstep is reached. Then it pulls down the High-Level to Low-Level. 
9. If the design is an open design, please protect from daylight or other light on photogap.

Question 21. Some Producers Of Steppermotor-driverunits Offer A "parallel-interface". Do I Need It?

Answer :

Not required! 

1. If you want to drive more than 3 axis, then you have to install a 2. or 3. port in your PC. 
2. The order of parallel ports are: 378h 278h 3BCh (special requirements on request).

Question 22. If Switch S1 (bit7 = Milling Spindle On) Is Enabled, Why Does Bit8 Pulsing While Motions?

Answer :

1. Bit8 puts an impuls out, depending on length of motion (adjustable), while s1 is enabled. 
2. Good for drive an electromechanic coolingpump, working with diaphragm or piston. 
3. With s2 (Bit8) you can put an extraimpuls on coolingpump, e.g. short before drilling a hole.

Question 23. Is It Possible Milling From Cad Data And Drawings?

Answer :

Yes! With the Fullversion we deliver a tool for DXF- and HPGL-Import. 

You may also use the integrated Teach-In function for draw figures and then plot with your machine. It is generated a programcode in the file "teachdat". 

Call of Teach-In: "cnc teach 100 bs" The "100" means 100 percent screenscale! 

1. With this function it is possible to scale the screen. So you can put a foil on the screen and then use the mousepointer to move along the figures. 
2. Call "cnc teachdat 0" in DOS Mode and your machine will mill the figure in 1:1 or other scale! 
3. The "0" is the reduction of speed, means fastest gear (most slowly == "4.000.000.000").

Question 24. How Do I Realise Effective Programming, Axis-transportation And Serial Works?

Answer :

1. Choose a cutter with suitable diameter for your most common works, e.g. Dia 4mm. 
2. Do coding only for this specific tool-diameter. Use the integrated cycles! 
3. The application of the cycles avoids often tool changes and cutter radius compensation. 
4. Axis-Transportation with self-coded assisting programs will transport the Start/Endposition. 
5. Serial works with batch procedures (*.bat) is possible!

Question 25. Why The Z-axis Is Counting Positive When It Comes Down? (different To Din-iso Standard).

Answer :

• A logical improvement! You save time not always type the "minus" in front of the figure. 
• The basic scale system is absolute, means that the scale is measuring from a zero-position. 
• All measurements of the triangle-, square- and sparkerosion-cycles are completely increment. 
• From the circle-cycle just the Radius is increment, the other data is measured absolute. 
• The startposition of a cycle is the actual position (automatic zero-point compensation). 
• After finish a cycle the automatic zero-point compensation is reset to the old position.

Question 26. Do The Axis Need All The Same Mechanic Gear And Spindle Slopes?

Answer :

Each axis can have a different gear and spindle slope. 

Question 27. How Driving To More Precise Positions Than The Standard 0.1mm Raster?

Answer :

Full values in the CNC-code are 1/10mm-Motions. This makes coding quick and easy. 

Decimal values, e.g. g 50.12 22.4 11.7 are in real X=5.012mm Y=2.24mm Z=1.17mm.