Lab 4: CNC Programming For Milling

Transcription

1 Lab 4: CNC Programming For Milling OBJECTIVES 1. To make sure student understand CNC Milling programming. 2. To familiarize student with CNC Milling programming by doing exercises. A. MACHINE BASICS 1.0 MACHINE AXES CNC Milling machine has 3, 4 and 5 axes. For the Spinner VC810, there are only 3 axes. Y Z X Fig 1.1: 3 axes of CNC Milling Page 1 of 31

2 2.0 MACHINE ZERO POINT The Machine Zero Point, often referred to as the HOME position, or sometimes the Machine Origin is located at the top, away from the working table. This position has been set by the machine manufacturer. At this position, the reading for X, Y and Z axis for the machine position equals 0 ie. X=0, Y=0, and Z=0. There are 2 ways of returning the Spindle to its Home position. i. By pressing the origin button / switch, followed by the Start Switch. All the three axis, X,Y and Z, moves automatically to the Home position. ii. By using a program : G91 G28 Z0; G28 X0 Y0; All machining processes have to start from the Machine Zero Point. It is good to remember the two lines of the program because all programs that will be written will contain the two lines, one at the beginning of a program and another at the end. Page 2 of 31

3 Machine Zero Point Z Y X Fig 2.1: Machine Zero Point 3.0 WORK PART ZERO POINT The Reference Point of a workpiece or sometimes referred to as the Work part Zero Point is where the X, Y and Z coordinates equals 0. The distance from the machine zero point (Home) to the work part zero point is called the Workshift and the Tool Length Offset. Page 3 of 31

4 Work Part Zero Point Z Y X Fig 3.1: Work Zero Point Page 4 of 31

5 4.0 AXIS DURING MACHINING Fig 4.1: Axis during machining. Page 5 of 31

6 5.0 ZERO AND REFERENCE POINTS ON CNC MILLING a. Symbols used for points identification. M machine zero point W work part zero point R reference point E tool reference point B tool setup point A tool shank point N tool change point Tool length compensation and the tool radius compensation Page 6 of 31

7 Fig 5.1: Tool Length Compensation B - Tool Setup point L - Tool length compensation R - Radius compensation Page 7 of 31

8 6.0 TOOL COMPENSATION FOR CNC MILLING a) Types of tools available in CNC milling Fig 6.1: Tool Compensation for CNC Milling Page 8 of 31

9 B NC PROGRAMMING BASICS 1.0 STANDARD FORMATTING A NC program will be written in a standard format and consist of the following basic structure: The sequence of the words in an NC-block is as follows: Page 9 of 31

10 2.0 M-CODES Code Task Functions M00 Program stop This code is used to stop a program that is being executed. M01 Optional Program Stop This codes performed the same task as the M00 code but the programmer / machine operator still have a choice of whether to stop or not at the desired stopping position. Selections is done by pressing the Optional Stop Button. If the optional stop button is depressed or not choosen, machine will not stop even when the M01 code is displayed or reached. M02 Program End This code is placed at the end of a program. This code is also used to stop all axis movement, spindle rotation, coolant pump and also able to cut off the machine power supply automatically. Control will be reset to program start. M03 Spindle Clockwise Starts spindle rotation in the clockwise direction. When it is needed to rotate the spindle at 1500 rpm in the clockwise Direction direction, program is written as: M03 S1500; M04 Spindle Starts spindle rotation in the counter-clockwise direction. Counter- Clockwise Direction Program is written as: M04 S1500; M05 Spindle Stop Stops spindle rotation either in M03 or M04. M06 Tool Change This code is used to change to the tool required for cutting. Program is written as: M06 T02; M08 Coolant Pump To turn on the coolant pump On M09 Coolant Pump To turn off the coolant pump Off M30 Program End Has the same functions as M02. Page 10 of 31

11 3.0 G-CODES Code Task Functions G00 Rapid Traverse. This code is used to move the spindle head or the cutting tool to its destination from one point A, to another point, B with rapid movement of 15000mm/min. However, this code cannot be used for cutting purposes. This code is used when cutting in a straight direction from one point to another. Applies to vertical, horizontal or at an angle (slope) movement. The feedrate, F must be stated in the program. G01 Linear Interpolation Page 11 of 31

12 This code is used when cutting arcs or circles in the clockwise direction. X20.0 Y20.0 G02 Circular Interpolation - Clockwise R5.0 G02 X25.0 Y25.0 R5.0; X25.0 Y25.0 This code is used when cutting arcs or circles in the counter-clockwise direction. G03 Circular Interpolation -Counter- Clockwise G04 Dwell This code is used when to stop the cutting movement for a few seconds in order for better finishing or in the case of drilling, a better hole surface. Although the axis movement of the cutting tool has stopped, the rotation of the cutting tool remains active. Duration of the stop time can be define by using either X or P. G04 X4.0; (stops for 4 sec.) Or G04 P3000; (stops for 3 sec.) Page 12 of 31

13 By using only code G01, G02 and G03, write a program for Exercise 1 and Exercise 2 shown below, on the program sheet provided. You are now at point A. Proceed to the next point B, then C,D,... following in alphabetical order until you reach your start point A. Exercise 1: Exercise 2: Figure 1 Figure 2 Page 13 of 31

14 Program Sheet O Exercise 1 N10 N20 N30 N40 N50 N60 N70 N80 N90 N100 N110 N120 N130 N140 N150 N160 N170 N180 N190 Page 14 of 31

15 O N10 N20 N30 N40 N50 N60 N70 N80 N90 N100 N110 N120 N130 N140 N150 N160 N170 N180 N190 Program Sheet Exercise 2 Page 15 of 31

16 C. THE ABSOLUTE SYSTEM (G90) AND INCREMENTAL SYSTEM (G91) When writing a program, it is necessary to choose either one of the following coordinate system: i. the absolute system, G90 or ii. the incremental system, G91 Even though a program may consist of both the systems, it must be separated at different sections, and not mixed. 1.0 The Absolute System - G90 When using this system, the distant from one point to another in the X or Y axis must be taken from the reference point and does not depend on the distance of travel. The reference point is where the X, Y and Z coordinates equals zero, ie. X0,Y0 and Z0 (see example below). Figure 1.1 Program: G90; (Absolute System) G01 Y25.0; (Point B) X25.0; (Point C) X60.0 Y50.0; (Point D) Y60.0; (Point E) X75.0; (Point F) Y0; (Point G) X0; (Back to Point A) Page 16 of 31

17 2.0 The Incremental System - G91 Reference point is not necessary when using this system. When programming, it refers to the distance of travel either in the X or Y direction or in both. In the X axis, movement to the right is positive and movement to the left is negative. In the Y-axis, movement upwards is positive and movement downwards is negative (see example below). Figure 2.1 Program: G91; (Incremental System) G01 Y25.0; (Point B) X25.0; (Point C) X35.0 Y25.0; (Point D) Y10.0; (Point E) X15.0; (Point F) Y-60.0; (Point G) X-75.0; (Back to Point A) Figure 2.2 below shows a component to be machined. Using the absolute system, write a program starting from point A. Take note that the reference point is located 10mm from point A on the X and Y axis. Page 17 of 31

18 Reference Point X0, Y0, Z0 Figure 2.2 Program Sheet Exercise 3 O N10 G90; N20 G00 X10.0 Y10.0 ; N30 N40 N50 N60 N70 N80 Absolute System. Point A (Start Point) Point B Point C Point D Point E Point F Back to Point A Page 18 of 31

19 D. COORDINATE SYSTEM SETTING G92, G54 G59 The work coordinate system refers to a point on the workpiece where the reference point or the work part zero point is located, and is measured from the Home position ( machine zero point). These distances, consisting of the X-axis and Y-axis is referred to as the Workshift and the Z-axis is referred to as the Tool Length Offset. Since the machine is unable to trace the location of the work part visually, these 3 values of X, Y and Z are guidance for the machine to know exactly where the work part has been placed. To obtain the values for X, Y and Z, the machine operator has to first clamp the workpart in position and to return the tool to its origin or the Home position. Using the Jog mode, the tool is brought slowly to the workpart zero point and its position is taken at the machine axis position from the monitor. For example, the reading taken are: X Y Z There are two ways of notifying the machine or the computer the location of the work part. i. Using code G92 and writing the X,Y and Z values in the program. This method is suitable when programming with a single cutting tool. In the program it will be written as : G92 X Y Z ii. Using code G54 and G43 in the program and installing the X and Y values in the Workshift (G54 G59) and Z values in the Tool Offset (G43) on the monitor. This method is suitable when programming with a single cutting tool or using more than one cutting tool. In the program it will be written as: G54; G43 H01; Page 19 of 31

20 E. TOOL LENGTH OFFSET - G43 A CNC machine with Automatic Tool Change (ATC) facilities, ranging from 8 tools to 200 tools are normally equipped with different types of tools such as End Mills, Ball Ends, Drills, Taps, Reamers, Fly Cutters, Edge Finders, Slot Cutters, etc. Each of these tools has different lengths and diameters and there must be a way of recording their values. The diagram below shows an automatic tool change (ATC) device that holds 10 tools of different sizes and shapes. The are many designs for the ATC, depending on the machine manufacturer. Some ATC have tools placed vertically, while others are placed horizontally. No matter what the design could be, these machine manufacturers always try to reduce the time taken for a tool change. Every second counts. Tool No. 2 End Mill Dia.10mm Tool No. 3 End Mill Dia.6mm The Automatic Tool Change (ATC) with 10 Tools mounted on a circular turret For example, two end mills of diameter 10mm and diameter 6mm located at the tool turret at slot no.2 and no 3 respectively, are to be used to machine a component. Before Page 20 of 31

21 the tools could be use, the machine operator will have to record the tool length offset measured from the machine origin to the work part zero point for each tool. He will have to first call the first tool with the code below: M06 T02; When the particular tool has appeared on the spindle, and in the Jog mode, bring the tool down to where the work part zero point is located. The Z-axis value is recorded and installed in the Tool Offset (see chart below) at no. 2 and its radius at no. 12. TOOL OFFSET The whole procedure is repeated with Tool No. 3 and its Z value is recorded and installed in the Tool Offset (see chart below) at no. 3 and its radius at no. 13. TOOL OFFSET Both the tools are now ready to be used in the program to machine the component. In the program, the first tool will be referred to as Tool No.2 and its tool length offset will be written as G43 H02; Its tool radius compensation will be G41/G42 H12; H, being the offset and the two digits after H is the location of the offset value. The second tool will be refered to as Tool No. 3 and its tool length offset will be written as G43 H03; Its tool radius compensation will be G41/G42 H13; NOTE: G43 can be cancelled by a G49. G49 is considered as a Safety Code Page 21 of 31

22 F. TOOL RADIUS COMPENSATION G40, G41 and G42 Tool radius compensation refers to the distance a tool has to compensate in order to obtain a product with correct dimensions. The amount of compensation will depend directly on the radius of the cutting tool. When writing a program, the coordinates for the X-axis and Y-axis are taken from one point to another based on the drawing of a product, regardless of the size of the cutting tool to be used. Without the offset values, the tool will be cutting directly on the line of the product causing the product to be smaller or a hole to be bigger than the actual size. Therefore it is necessary to include the tool radius compensation in the program. There are three codes for the tool radius compensation. i. G40 : TOOL RADIUS COMPENSATION OFF / CANCEL PRODUCT CUTTING DIRECTION CUTTING [ TOOL When this code is used (or tool radius compensation is not used at all), the cutting tool will move on the object lines or the contour of the product causing it to cut into the product by half its diameter. Page 22 of 31

23 ii. G41: TOOL RADIUS COMPENSATION LEFT CUTTING DIRECTION PRODUCT CUTTING TOOL When using G41, the cutting tool will move to the left of the product contour lines depending totally on the direction of cut. In the program, it will be written as G41 H15; iii. G42: TOOL RADIUS COMPENSATION RIGHT PRODUCT CUTTING DIRECTION CUTTING TOOL When using G42, the cutting tool will move to the right of the product contour lines depending totally on the direction of cut. In the program, it will be written as G42 H15; (The offset value, H, has been mentioned earlier on page 17) NOTE: G41 and G42 can be cancelled by a G40. G40 is considered as a Safety Code Page 23 of 31

24 G. PROGRAMMING Programming can be simplified into 4 sections. It is easy to understand the sequences according to the sections. Program Sheet O1122; Program Number N10 G40 G49 G80 ; Safety Code Section 1: Homing, Tool Change and Spindle Speed N20 G91 G28 Z0 ; Home position. N30 G28 X0 Y0 ; N40 M06 T07; Tool Change N50 M03 S1700; Spindle Speed Section 2: Workshift, Tool Length Offset, Absolute Sys. Section 3: Machining, Radius Compensation N60 G54; N70 G43 H07; N80 G00 G90 X0 Y0; N90 G42 H17; N100 X10.0 Y10.0; N110 G01 Z-2.0 F200.; N120 X40.0; N130 Z10.0; N140 M05; Workshift: X,Y position Tool Length Offset: Z position Absolute System Tool Radius Offset Spindle Stop. Section 4: Program Ending N150 G91 G28 Z0; Home Position N160 G28 X0 Y0; N170 M02; Page 24 of 31

25 From the program sheet, a program practically consists of 4 sections. Section 1, Section 2 and Section 4 are basically the same for all programs and could remain the same when writing a new program. Section 3 changes according to the shape of a product to be machined. Exercise 4: Figure 6 shows a component to be machined. Material: Nylon Material size: 85 x 85x 40mm Feedrate: 400mm/min Cutter Diameter: 10mm Location: Tool Number 4 Depth of cut: 3mm Based on the above information, write a program to machine the component. Program Sheet Figure 6 Page 25 of 31

26 O Exercise 4 N10 N20 N30 N40 N50 N60 N70 N80 N90 N100 N110 N120 N130 N140 N150 N160 N170 N180 N190 Page 26 of 31

27 N 2 00 N 210 N 220 N 230 N 240 N 250 N 260 N 270 N 280 N 290 N 300 N 310 N 320 N 330 N 340 N 350 N 360 N 370 N 380 N 390 N 400 Page 27 of 31

28 H. DRILLING Before starting with the drilling processes, we have to first understand the differences between the start plane (G98) and the withdrawal plane (G99). G98 - After reaching the drilling depth, the tool retracts to the start plane. The value of R need not be stated. G99 - After reaching the drilling depth, the tool retracts to the withdrawal plane defined by the R parameter. R must be programmed. 1.0 Chip Break Drilling Cycle - G73 Program format: N.. G98(G99) G73 X. Y. Z. (R.) P. Q. F. K. The tool dips into the workpiece for the infeed Q, drives back 1mm to break the chips, dips in again, etc.. until end depth is reached and retracts with rapid feed. Applications : Used for deep borings, material with bad cutting property. G98(G99) - Return to starting plane (withdrawal plane) X,Y - Hole position Z - Drilling depth R(mm) - value of withdrawal plane P(msec) - Dwell at the hole bottom, P1000 = 1 sec. F - Feed rate Q(mm) - Cutting division infeed per cut K - Number of repetitions 2.0 Drilling Cycle - G81 Program format: N.. G98(G99) G81 X.Y.Z.(R.) F.K. The tool traverses down to the end depth with feed speed and retracts with rapid feed. Applications: Short drilling, material with good cutting properties. G98(G99) - Return to starting plane (withdrawal plane) X,Y - Hole position Z - Drilling depth R(mm) - value of withdrawal plane P(msec) - Dwell at the hole bottom, P1000 = 1 sec. F - Feed rate K - Number of repetitions Page 28 of 31

29 3.0 Cancel Drilling Cycles - G80 The drilling cycles are modal and the have to be cancelled by G Drilling Cycle With Dwell G82 Program format: N.. G98(G99) G82 X. Y. Z. (R.) P. F. K. The tool traverses down to the end depth with feed speed, dwells turning to clean the hole ground and retracts with rapid feed. Applications: Short boring, material with good cutting poperty. G98(G99) - Return to starting plane (withdrawal plane) X,Y - Hole position Z - Drilling depth R(mm) - value of withdrawal plane P(msec) - Dwell at the hole bottom, P1000 = 1 sec. F - Feed rate K - Number of repetitions 5.0 Withdrawal Drilling Cycle - G83 Program format: N.. G98(G99) G83 X. Y. Z. (R.) P. Q. F. K. The tool dips into the workpiece for the infeed Q, drives back to the start plane (G98) or to the withdrawal plane (G99), to break the chips and remove it from the hole. It then traverses with rapid speed until 1 mm over the previous drilling depth, dips in again for the infeed Q etc. until end depth is reached and retract with rapid feed. Applications: deep borings, (soft) material with long chips. G98(G99) - Return to starting plane (withdrawal plane) X,Y - Hole position Z - Drilling depth R(mm) - value of withdrawal plane P(msec) - Dwell at the hole bottom, P1000 = 1 sec. F - Feed rate Q (mm) - Cutting division infeed per cut K - Number of repetitions Page 29 of 31

30 6.0 Tapping Cycle - G84 Program format: N.. G98(G99) G84 X. Y. Z. (R.) F. P. K. A tapping chuck with length compensation must be used. Spindle override and feed override will be set fixed to 100% while machining. The tool moves turning clockwise with programmed feed into the workpiece down to drilling depth Z, dwells (P), switches to counter clockwise turning and retracts with feed. G98(G99) - Retraction to to starting plane (withdrawal plane) X,Y - Hole position Z - Tapping depth R(mm) - value of withdrawal plane F - Thread pitch (feed per revolution) P(msec) - Dwell at thread ground Q (mm) - Cutting division infeed per cut K - Number of repetitions 7.0 Reaming cycle - G85 Program format: N.. G98(G99) G85 X. Y. Z. (R.) F. K. The tool traverses down to end depth with feed speed and retracts to the withdrawal plane with feed. G98(G99) - Return to starting plane (withdrawal plane) X,Y - Hole position Z - Drilling depth R(mm) - value of withdrawal plane F - Feed rate K - Number of repetitions Page 30 of 31

31 8.0 Drilling cycle with spindle stop - G86 Program format: N.. G98(G99) G86 X. Y. Z. (R.) F. The tool traverses down to the end depth with feed speed. At the hole ground the spindle stops and the tool retracts with rapid feed. G98(G99) - Return to starting plane (withdrawal plane) X,Y - Hole position Z - Drilling depth R(mm) - value of withdrawal plane F - Feed rate K - Number of repetitions I. OTHER G CODES G Codes Code Task Functions G94 Feed per All F (feed) values are in mm/min minute Eg. N G94 F400. ; G95 Feed per All F (feed) values are in mm/rev. revolution Eg. N G95 F0.5 ; G97 Revolutions S values are in rev/min per minute Eg. N G97 S2000 ; Page 31 of 31