Review questions — section 3

Review questions — section 3

Job planning

1. 
   What are the seven steps that make up the procedure to utilise numerical in manufacturing?
   
2. 
   What are the steps required for good job planning?
   
3. 
   What information is contained on an operators set up?
   
4. 
   What is the purpose of tool radius compensation?
   
5. 
   What are two methods of compensating for the tool's radius?
   
6. 
   How do you manually compensate for the tool nose radius when facing a job in the lathe?
   
7. 
   Complete the operators sheet for the job below:
   

8 List eight points to be considered when planning for NC manufacturing of a job.

Answers are at the end of CNC Machining text

Section 4: Write and edit basic CNC programs

Objectives

• 
  differentiate between absolute and incremental programming
  
• 
  define machine home and select an appropriate work piece zero
  
• 
  perform basic trigonometry operations to establish co ordinate positions D list methods of preparing a program
  
• 
  write a program to produce straight and circular tool movements to standard code format
  
• 
  enter and edit a simple program
  
• 
  explain the concept of canned cycles.
  

Safety reminders

• 
  In the workshop, always wear safety glasses, safety boots, hair protection and suitable clothing.
  
• 
  Avoid back injuries -lift the correct way.
  
• 
  Do not use a machine fitted with a Danger Tag.
  
• 
  Know where the first aid station is.
  
• 
  Don't run or play in the workshop.
  
• 
  Use ear muffs or plugs to protect your hearing.
  

4.1 Cartesian coordinate system

The Cartesian, or rectangular, coordinate system was devised by the French mathematician and philosopher Rene Descartes. With this system, any specific point can be described in mathematical terms from any other point along three perpendicular axis. This concept fits machine tools perfectly since their construction is generally based on three axis of motion (X, Y, and Z), plus an axis of rotation. On a plain vertical milling machine, the axis is the horizontal movement (right or left) of the table, the Y axis is the table cross movement (toward or away from the column), and the Z axis is the vertical movement of the knee or the spindle. NC systems rely heavily on the use of rectangular coordinates because the programmer can locate every point on a job precisely.

When points are located on a workpiece, two straight intersection lines, one vertical and one horizontal, are used. These lines must be at right angles to each other, and the point where they cross is called to origin, or zero point.

Figure 4.1 Origin or zero point

4.1.1 Three dimensional coordinates planes (axis) used in NC

The three dimensional coordinate planes are shown in opposite, X and Y planes (axis) are horizontal and represent horizontal machine table motions. The Z plane or axis represents the vertical tool motion. (The plus (+) and minus (-) signs indicate the direction from the zero point (origin) along the axis of movement.



Figure 4.2 3D co-ordinate planes

The four quadrants formed when the X Y axis cross are numbered in a counter clockwise direction. All positions located in quadrant 1 would be positive X (+ X) and positive Y (+ Y). In the second quadrant, all positions would be negative X (-X) and positive Y (+ Y). In the third quadrant, all locations would be negative X and Y (-X-Y). In the fourth quadrant, all locations would be positive X (+ X) and negative Y (-Y).

Figure 4.3 Two axis quadrants

Point ‘A’ in figure 4.4, would be 2 units to the right of the axis and 2 units above the Y axis. Assume that each unit equals 10 mm. The location of point A would be X (+) 20.0 and Y (+) 20.0. For point B, the location would be X (+) 10.0 and y (-) 20.0. In NC programming it is not necessary to indicate plus (+) values since these are assumed. However, the minus (-) values mus be indicated.

A=X2.00 Y2.00

B=X1.00 Y-2.00

Figure 4.4

4.1.2 Guidelines

Since NC is so dependent upon the system of rectangular coordinates, it is important to follow some guidelines. In this way, everyone involved in the manufacture of a part, the engineer, draftsperson, programmer, and machine operator, will understand exactly what is required.

1. 
   Use reference points on the part itself, if possible. This makes it easier for quality control to check the accuracy of the part later.
   
2. 
   Use Cartesian coordinates -specifying X, Y and Z planes -to define all part surfaces.
   
3. 
   Establish reference planes along part surfaces which are parallel to the machine axis.
   
4. 
   Establish the allowable tolerances at the design stage.
   
5. 
   Describe the part so that the cutter path maybe easily determined and programmed.
   
6. 
   Dimension the part so that it is easy to determine the shape of the part without calculations or guessing.
   

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