Surfaces and Tolerances

Surfaces

In APT, the tool is considered to move round the workpiece to carry out the machining operation. Though in the actual machine tool, it is different, it is taken care of by the postprocessor depending upon the machine tool requirements. In order to generate the motion for the tool APT considers three surfaces to get the motion. They are termed as drive surface (dsurf), part surface (psurf) and check surface (csurf). The cutting tool is expected to move along the intersection of the drive and part surfaces till it is stopped by means of the check surface

The tool tip is supposed to be in continual contact with the part surface. This controls the depth of cut. The tool's peripheral surface will be in contact with the drive surface during a given motion. Check surface is the one which limits the given motion statement. These will become clearer when we discuss their use with respect to the other motion commands.

 Tolerances

Most CNC machine tools have no capacity to produce complex surfaces except when they are either straight lines or circular arcs. For all other surfaces, APT will have to approximates them by means of straight line cuts called as cut vectors. In order to do this, the part programmer needs to specify a tolerance with which APT should approximate the curved surfaces. The format of the tolerance specification is

INTOL/ dsval, psval, cs1val, cs2val

OUTTOL/ dsval, psval, cs1val, cs2val

TOLER/ dsval, psval, cs1val, cs2val

In the above definitions, the values refer to the drive surface, part surface, first check surface and second check surface respectively. The last three values are optional. Any value can be omitted from the right side in which case it is assumed as the one given for the previous one. For example, if only dsval is specified, the same tolerance value is used for all the surfaces.