CAD and CNC Integration

Impact of CAD surface modeling on CNC machining

Golden E. Herrin

There is little debate among the manufacturing community that the surface modeling techniques provided in today's CAD systems are an essential design tool and will play an ever increasing role in part designs of the future. There are a number of modeling methods designers can choose from including: interpolating splines (cubic splines); basis-spline (B-spline) of which there can be rational B-splines and non uniform rational B-spline (NURBS); and Bezier mathematics. Some forms of splines appear to offer significant benefits to manufacturing when applied to the interpolation method in the CNC.

Splines in the most general sense can be viewed as a certain type of mathematical representation of curves and surfaces in 3-dimensional space. One of the key properties of splines is that they can be used to represent smooth free form surfaces. In the design phase, the designer incorporates spline representations to generate sculptured 3-dimensional surfaces that define the shape of a part. Before the surface can be machined, it must be converted to tool centerline (CL) path data. This path data traditionally is presented to the control as a series of short straight line segments for execution by linear interpolation. The shorter the lines, the more accurate the part surface definition - but with better definition also comes enormous amounts of data for the control to process which often causes even the fastest controls to become data bound.

Additionally, when executing a contour using straight line segments with linear interpolation, a corner exists at every segment (or block) boundary. These corners are normally smoothed thanks to the affect of corner rounding and following error until feed forward is applied. Feed forward is a servo feature that is gaining popularity with users doing high speed machining. Its function is to close up the following error, thus causing the machine to track the programmed path more accurately. When feed forward is active, the segment boundary becomes more pronounced in the machined surface and unacceptable levels of vibration are generated in the machine structures due to discontinuities in axis feed rates.

Circular interpolation, the other traditional interpolation method, provides an excellent way to define a very accurate circle or circular segment with a very little amount of data but unfortunately it is limited to a single plane that can not effectively address 3-dimensional freeform shapes. An alternative method to both of the traditional types of interpolation (linear and circular) is to utilize spline interpolation in the control. This will invoke a continuous smooth curve - no segment or axis velocity discontinuities.

The following are two approaches to utilizing spline interpolation in the CNC:

* The first method starts with the part being developed as a spline representation on a CAD/CAM system. After the part is designed, a spline representation of the tool centerline path is made.

The cutter path curve is then transformed from the part based coordinate system to a machine tool based coordinate system while maintaining the spline representation of the curve. This is accomplished by an advanced postprocessor. The part program will contain axis positioning information in terms of spline parameter representation.

As the part program is run, the CNC unit interprets the spline representation of the cutter path and uses this to interpolate a sequence of discrete time axis position commands for the given feed rate.

* A second approach is to use a part program generated the traditional way, processed for linear segments, but have the control interpolate between the points using interpolating splines. This would allow smoother axis command sequences at the segment boundaries. This method could be further enhanced by additional off-line processing to eliminate unnecessary segments, thus reducing the amount of data while maintaining the smoothness.

As the manufacturing community moves to higher and higher feed rates and demand more accurate cutter paths for competitive reasons, machines and controls are both being taxed to keep pace. For controls, the data throughput has become a major technology challenge to control builders as well as the ability to command the machine to accurately follow the programmed path at high feed rates. To address these issues some CNC builders already support spline interpolation, others are considering it.

There are good reasons why users want CNC builders to provide interpolation methods based on spline representation but which method is most appropriate? The cubic spline method offers the advantage of a smooth curve passing through a series of points and good matching at the intersection of each curve - a characteristic needed to meet the requirements of accurate path following at high speeds. NURBS on the other hand offers all of these advantages plus it also allows the designer to create parts with fewer surfaces than other modeling systems making it more efficient for the designer, It is also viewed by many in the CAD/CAM field as the de facto worldwide standard for representation of curves and surfaces. Users are in a good position today to steer the CNC builders in the direction that best meets their future manufacturing needs.