CNC Control - 1: Point to Point
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
A CNC system can be classified according to the structure of the controller , the mode of the control (point-to-point versus continuous path), and the type of control loop (open-loop or closed-loop). The mode of control determines how the tool is controlled or moved through its motion to position, or along the cutting path.

The point-to-point (PTP) control system is many times referred to as the positioning system. The primary function of the PTP control system is to move a tool from one point location to another specified point, generally to do hole operations such as drilling, boring, reaming, tapping and punching. The path the tool follows may be one of three paths, depending on the control being utilized:
A CNC system can be classified according to the structure of the controller , the mode of the control (point-to-point versus continuous path), and the type of control loop (open-loop or closed-loop). The mode of control determines how the tool is controlled or moved through its motion to position, or along the cutting path.

The point-to-point (PTP) control system is many times referred to as the positioning system. The primary function of the PTP control system is to move a tool from one point location to another specified point, generally to do hole operations such as drilling, boring, reaming, tapping and punching. The path the tool follows may be one of three paths, depending on the control being utilized:

Axial Path - The tool moves in a right-angle path parallel to the two positioning axes, X and Y. In this mode, the controller commands the tool to first move along the X axis and the Y axis would not move until the specified X coordinate position is reached. Then the Y axis would move to its specified coordinate position. This is the slowest method because the axes are moved one at a time.

45 Degree Path - The tool moves at a 45 degree angle until one of the axes is lined up with the new position. Then the tool moves along that axes to the specified coordinate position. Initially, both the X and Y axis move simultaneously until one of the specified coordinates is reached, and the tool then travels in axial path mode to the specified coordinate position

Linear Path - In this mode, the controller has the capability to synchronize the motion in both axes, X and Y, to generate a linear path. This method provides the quickest path from one point to another, but sophisticated equipment is required to coordinate the speed of each axis to maintain a straight line
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The 45-degree path nearly always has two components; one that is 45 degrees and one that finishes the move at 90 or 0 degrees. This is because, in essence, the control is not controlling position. Instead the control sends a signal to both motors to move at the same speed (RPM) towards a given point. Since the pitches of the feed screws are identical then this will result is a 45 degree move. If the given point lies at some other angle than 45 degrees then one motor must stop while the remaining motor drives a single axis to the final position. This is an important concept to understand since almost all CNC machines make rapid moves in this manner. Fixture crashes may occur if this concept is not fully understood. R.S.
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