Machine Zero Return

The ability of a control system to return a cutting tool from any position to the machine reference position is a critical feature of all modern CNC systems. Programmers and operators understand the term machine reference position as synonymous with the home position or machine zero position. This is the position of all machine slides at one of the extreme travel limits of each axis. The exact position is determined by the machine manufacturer and is not normally changed during the machine working life. Return to that position is automatic, on request from the control panel, in MDI operation, or via the program.

MACHINE REFERENCE POSITION

The existence of machine reference position is for referencing purposes. In order that the CNC machine is accurate, we need more than just the high quality components,
we need some unique location that can be considered the origin point of the machine - a zero position - a home position. Machine reference position is exactly such a point.

Machining Centers
Although the design of CNC machining centers varies for
different models, there are only four possible locations for
the machine zero, within the XY view:
o Lower left corner of the machine
o Upper left corner of the machine
o Lower right corner of the machine
o Upper right corner of the machine
It is quite common, in fact normal, to start the first part of a new program from the machine zero position. Often, it is also necessary to make a tool change at machine zero position and return there when the program execution is completed. So, several of the four alternatives are not very convenient for setup of the part on the machine table and its removal when the machining is done.
The most common and standard machine reference position for vertical machining centers is at the upper right corner of the machine, looking perpendicularly towards the XY plane

So far, any reference to the Z axis in the description was quite intentional. The Z axis machine zero position for a vertical machining center is always where the Automatic Tool Change (ATC) takes place. This is a built-in location,normally placed a safe distance from the machine table and the work area. For most machines, the standard machine zero of CNC machining centers is at the extreme travel limit of each axis in the positive direction. There are exceptions, as may be expected.

As Figure above illustrates, some CNC vertical machining centers have the machine zero position at the upper left corner of the XY plane.

In both illustrations, the arrows indicate the tool motion direction towards the work area. Moving the tool from machine zero into the opposite direction will result in a condition known as overtravel - compare the two possibilities:
♦ Tool motion from machine zero, if machine zero is located at the upper right corner:
X+ Y+ Z+ ........tool motion will overtravel
♦Tool motion from machine zero, if machine zero is located at the upper left corner
X- Y+ Z+ ....... tool motion will overtravel
The other two corners (lower left and lower right of the XY view) are not used as machine zero.

Lathes
The machine reference position for two axis CNC lathes is logically no different from the reference position of the machining centers. An easy access by the CNC operator to the mounted part is the main determining factor. Both, the X and the Z axes have their machine reference position at the furthest distance from the rotating part, which means away from the headstock area, consisting of the chuck, col-let, face plate, etc.
For the X axis, the machine zero reference position is al-ways at the extreme limit of the travel away from the spin-dle center line. For the Z axis, the machine reference posi-tion is always at the extreme travel away, from the machine headstock. In both cases, it normally means a positive di-rection towards the machine zero, the same as for the ma-chining centers. The illustration in Figure shows a machine zero for a typical CNC lathe.

In the illustration, the arrows indicate the tool motion direction towards the work area. Moving the tool from the machine zero into the opposite direction will result in over-travel in the particular axis:
♦Tool motion from machine zero of a typical rear lathe:
X+ Z+ .....tool motion will overtravel

♦ Setting the Machine Axes
From the previous sections, remember that there is a direct relationship between the CNC machine, the cutting tool and the part itself. The work reference point (program zero or part zero) is always determined by the CNC programmer, the tool reference point is determined by the tool length at the cutting edge, also by the programmer.
Only the machine reference point (home position) is determined by the manufacturer of the machine and is located at a fixed position. This is a very important consideration.

In order to physically reach the machine reference position (home) and set the machine axes, for example, during the part or fixture setup, there are three methods available to the CNC operator
->Manually - using the control panel of the system
The machine operator will use the XYZ (machining centers) or the XZ (lathes) switches or buttons available for that purpose. One or more machine axes can be activated simultaneously, depending on the control unit.
->Using the MDI - Manual Data Input mode
This method also uses the control panel. In this case, the machine operator sets the MDI mode and actually programs the tool motion, using the suitable program commands (G28, G30).
->In the CNC program - during a cycle operation
Using the same program commands as for the MDI operation, the CNC programmer, not the machine operator, includes machine zero return command (or commands) in the program, at desired places.

When the operator has performed the actual machine zero return, it is always a good idea to set the relative and absolute positions to zero on the display screen. Keep in mind that the relative display can only be set to zero from the control panel and the absolute display can only be changed through a work offset, MDI mode, or the part pro-gram. This topic normally a part of CNC machine operation training, directly at the machine.
For the last two methods of a machine zero return, the CNC system offers specific preparatory commands.

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