Monday, December 8, 2008

interpolation understanding

CNC control manufacturers try to make it as easy as possible to make movement commands within the program. For those styles of motion that are commonly needed, they give the CNC user interpolation types.

Understanding interpolation

Say for example, you wish to move only one linear axis in a command. Say you wish to move the X axis to a position one inch to the right of program zero. In this case, the command X1. would be given (assuming the absolute mode is instated). The machine would move along a perfectly straight line during this movement (since only one axis is moving). Now let's say you wish to include a Y axis movement to a position one inch above program zero in Y (with the X movement). We'll say you are trying to machine a tapered or chamfered surface of your workpiece in this command. For the control to move along a perfectly straight line to get to the programmed end point, it must perfectly synchronize the X and Y axis movements. Also, if machining is to occur during the motion, a motion rate (feedrate) must also be specified. This requires linear interpolation.

During linear interpolation commands, the control will precisely and automatically calculate a series of very tiny single axis departures, keeping the tool as close to the programmed linear path as possible. With today's CNC machine tools, it will appear that the machine is forming a perfectly straight line motion. However, Figure 3.1 shows what the CNC control is actually doing during linear interpolation. Figure 3.1 - Actual motion generated with linear interpolation. Notice the series of very tiny single axis movements. The step size is equal to the machine's resolution, usually 0.0001 in or 0.001 mm.

In similar fashion, many applications for CNC machine tools require that the machine be able to form circular motions. Applications for circular motions include forming radii on turned workpieces between faces and turns and milling radii on contours of machining center workpieces. This kind of motion requires circular interpolation. As with linear interpolation, the control will do its best to generate as close to a circular path as possible.

Other interpolation types

Depending on the machine's application, you may find that you have other interpolation types available. Again, CNC control manufacturers try to make it as easy as possible to program their controls. If an application requires a special kind of movement, the control manufacturer can give the applicable interpolation type. For example, many machining center users perform thread milling operations on their machines. During thread milling, the machine must move in a circular manner along two axes (usually X and Y) at the same time a third axis (usually Z) moves in a linear manner. This allows the helix of the thread to be properly machined. This motion resembles a spiraling motion (though the radius of the spiral remains constant).

Knowing that their customers need this type of motion for thread milling, CNC machining center control manufacturers offer the feature helical interpolation. With this feature, the user can easily command the motions necessary for thread milling. 

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