The start and the end points must be different, so that the center can be calculated. Radius programming can therefore not be used for programming a full circle. Centre point programming can be used for this purpose.

Circular interpolationprogram example

Centre point programming represents an alternative to the method that has just been described. The advantage of center point programming is that complete circles can also be described in this way.

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Circular interpolationin CNC

In radius programming the radius of the circle is programmed as well as the end point. Either of the letters 'B' or 'U' may be used for the radius.

So that the circle can be described unambiguously, it is necessary that the 3 points (the starting point is given implicitly) must not be collinear. It is thus not possible to program a full circle in this way.

Circular interpolationcode

Under the standard settings, the center point is always given relatively to the starting point of the arc. The parameters I, J and K are used for this purpose, with

In order to describe the circle unambiguously, further parameters are required in addition to the end point. A choice is available between center point programming and radius programming.

The 'set paramRadiusPrec' function is used to parameterize the required circular accuracy. This parameter affects circles programmed with G02 or G03.

It the start and end point are very close together, the centre point offset may be large. This may lead to problems with automatically generated G-Code (postprocessor). For manually written G-Code, the CPCON setting (centre point correction on) is recommended.

Circular interpolationprogram example PDF

Circular interpolationformula

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By programming an item of machine data it is however also possible to enter the center point absolutely. The command @402 is required for write access to a machine data bit.

The circles discussed so far can only be used in the principal planes. The CIP circle can also be used to program an arc anywhere in space. For this purpose it is necessary to program not only an end point but also some other point on the path.

Circular Interpolationcalculator

The function G3 describes a circular arc anticlockwise. The parameters and entry possibilities are the same as under G2.

Since the direction is prescribed with G2, the circle is also unambiguously described. The starting point is determined by the foregoing geometrical movements.

In centre point programming the circle is overdetermined. For data consistency, the centre point is usually corrected. Normally only a marginal modification of the centre point is required. After the centre point correction, the magnitude of the input radius equals the output radius.

I, J and K are available as path point parameters. By default their values are relative to the starting point of a circular path.

Circular arcs can be programmed in a number of ways. Two types must be distinguished. One of these is an arc in the working plane (e.g. the XY plane), and the other is an arc that can be freely located in space (a CIP circle).

Function G2 describes the path of a circular arc clockwise. This function requires the working plane to have already been defined (G17 is standard).