Monday, 30 September 2013

CNC MACHINE COORDIANTES

The CNC Machine Coordinate System is illustrated in Figure below. The control point for the Machine Coordinate System is defined as the center-face of the machine spindle.
The Origin point for the machine coordinate system is called Machine Home. This is the postion of the center-face of the machine spindle when the Z-axis is fully retracted and the table is moved to its limits near the back-left corner.

VMC Machine Coordinate System (At Home Position)

As shown in Figure above, when working with a CNC, always think, work, and write CNC programs in terms of tool motion, not table motion. 

  • For example, increasing +X coordinate values move the tool right in relation to the table (though the table actually moves left). 
  • Likewise, increasing +Y coordinate values move the tool towards the back of the machine (the table moves towards the operator).
  • Increasing +Z commands move the tool up (away from the table). 


CNC Motion Control:

Most CNC machines can position each axis within .0002 inches or less over the entire machining envelope. This accuracy is achieved in part by the use of a closed-loop servo mechanism, illustrated in Figure below. 

The machine control sends a motion signal, via a controller board, to a servomotor attached to each machine axis. This causes the servomotor to rotate a ball screw attached to the table or column, causing it to move. The actual position of the axis is continuously monitored and compared to the commanded position with feedback from a servo transmitter attached to the ball screw.

Ball screws have almost no backlash, so when the servo reverses direction there is almost no lag between a commanded reversing motion and corresponding change in table direction. CNC controls employ electronic compensation to adjust for any minor backlash that may exist. 

CNC Motion Control



Work Coordinate System :

Obviously it would be difficult to write a CNC program in relation to Machine Coordinates. The home position is far away from the table, so values in the CNC program would be large and have no easily recognized relation to the part model. To make programming and setting up the CNC easier, a Work Coordinate System (WCS) is established for each CNC program.

The WCS is a point selected by the CNC programmer on the part, stock or fixture. While the WCS can be the same as the part origin in CAD, it does not have to be. While it can be located anywhere in the machine envelope, its selection requires careful consideration.
The WCS location must be able to be found by mechanical means such as an edge finder, coaxial indicator or part probe.
  • It must be located with high precision: typically plus or minus .001 inches or less.
  • It must be repeatable: parts must be placed in exactly the same position every time.
  • It should take into account how the part will be rotated and moved as different sides of the part are machined.
For example, Figure below shows a part gripped in a vise. The outside dimensions of the part have already been milled to size on a manual machine before being set on the CNC machine.
The CNC is used to make the holes, pockets, and slot in this part. The WCS is located in the upper-left corner of the block. This corner is easily found using an Edge Finder or Probe.

Work Coordinate System
A typical example of WCS is show below with a WorkNC environment.
WCS Shown in WORKNC

Now as we became familiar with the various coordinate systems related to CNC machine we can get started with the WORK NC tutorials.. 
In next post we will learn about  setting a tool and work piece offsets so as to start machining with cam software.

Images Courtesy : CNC Handbook HSMWorks