A crash course on gouge avoidance

Modern Machine Shop, March, 1994 by Gary Fulton

A vendor of CAM software presents the basic concepts of gouge and collision avoidance and discusses its importance to effective numerical control (NC) programming.

Manufacturers are always looking for ways to reduce the number of parts in their products. Fewer components mean a lower initial cost and, it is assumed, fewer problems in the field and lower warranty costs. The ironic result of reducing the number of parts in a product is that complexity and cost of the remaining pieces increases. Those remaining pieces may require the capability to machine 3D surfaces, and getting a good part on the first tryout is not as easy as with less complex 2D parts.

On average, four to five tryouts at the machine tool may be required to get a good first part. If there's a problem with the tool path, production is bottlenecked while the machinist waits for an update. The cost of these idle machine tools and machinists comes right off the bottom line. So reducing tryouts at the machine tool, minimizing downtime and eliminating scrap are important objectives for every machine shop.

The word most often associated with machine downtime is gouging. It's a word that rarely comes up during sales demonstrations; customers don't ask about it; and most vendors of CAM (computer-aided manufacturing) software fail to mention it. But the shop that machines complex workpieces, especially those with 3D surfaces, will hear about gouge and collision avoidance all too often. Here's some basic information that should help shed some light on a very complicated subject:

A Big And Growing Problem

Gouging a workpiece or surface is one of the biggest problems that machinists face today. It has always been a problem, but because the number of complex workpieces is increasing these days, gouging has emerged as the problem most often mentioned by machinists in at least one recent survey.

Gouging includes a host of problems, such as overcutting and undercutting the workpiece, and collisions. When too much material is removed from the part, it is said to be gouged or overcut. If the workpiece can be saved, the damaged area is built up with more material and machined again, but it's more likely the damaged piece will be relegated to the scrap pile.

The converse of gouging a work-piece is undercutting--not removing enough material. Undercutting may keep workpieces out of the scrap pile. However, if undercutting is not detected during NC programming, the problem probably won't be found until the workpiece is thoroughly inspected. Then, the cost of corrective action will now include additional CAM programming time, as well as setup and machining time.

Interference between the workpiece and components of the machining center such as clamps, fixtures, tool cutting surfaces, toolholders and shanks is called a collision. Gouging, undercutting and collisions are costly and result in a lot of scrap and major loss of productivity.

Until the past few years, sophisticated methods for gouge avoidance on personal computer-based systems were not very practical or effective. These small computers, fast as they were, could not make the extensive calculations for complete gouge and collision avoidance, and still meet the requirements of a production environment. But advances in hardware capabilities and processing speed are making gouge avoidance practical at the personal computer (PC) level. So, with the astonishing advancements of today's PC technology, why can't CAM software generate a gouge-or collision-free tool path every time? Not all CAM software offers gouge avoidance, and for those that do, just because the hardware capability is available doesn't mean that gouge-avoidance capabilities among CAM products are equal.

Why Gouging Occurs

If the tool follows the path defined by the CAM software, why does gouging occur? The generally accepted process for generating a tool path with CAM software, whether running on a workstation or PC, is the same. A tool path is generated based only on the cutter contact point (that is, the radius or entire geometry of the tool is not considered) and follows a line on the part geometry. What is immediately in front or to either side of the cutter contact point is not considered when the tool path is generated.

As the tool path is generated, various gouge-avoidance techniques are employed to alter this path and produce one that is gouge-free. These techniques are transparent to the user and are employed at the completion of each step. For example, the first step may move the cutter from point A to point B. Then gouge-avoidance techniques are implemented by testing the move to determine if a gouge will occur. If a gouge occurs, the tool path is altered according to rules of the CAM software's algorithm and then checked for gouges. Every step along the entire tool path must be tested before the whole program is ready to go to the machine tool.

For example, a gouge occurs when machining a square pocket with sidewalls perpendicular to the bottom. Most CAM software will use the tangent point (cutter contact point) of the ballnosed cutter to calculate the tool path for roughing and finishing. The ballnosed cutter runs along the bottom of the pocket and will overcut the side wall because the tool path has been calculated from the tangent point of the ballnosed cutter and not the leading edge of the tool. Another example of gouging involves machining of 3D (also called sculptured) surfaces. Most 3D surfaces are machined by point-to-point linear moves that approximate the curvature of the surface. Each linear move is calculated as a function of the chordal deviation (surface tolerance) as specified by the user. If the chordal deviation specified is too large, the result may be a gouge; if too short, the surface will be accurate but excessive machining will be required.