Spring 

1998

Volume 2

Number 5

 

The CAM Question: 
How Much Does Your Shop Need?


story by Benjamin Mund, CNC Software, Inc.

uring the past ten years, few aspects of manufacturing have progressed as rapidly as PC-based CAD/CAM technology. As with most innovations, the market presses for improvements, and eager hardware and software developers strive to meet the demands. But as PC-based CAD/CAM software grows more sophisticated, it becomes difficult for a shop to decide exactly what they require. As amazing as some of this software is, how much CAD/CAM capability does a shop really need?

To start with, the prospective buyer must determine the needs and desires of the company or department that he or she is working for. Get out into the shop! Take a look at your NC and CNC equipment. List the types of machines you are using, their abilities and the current work being produced on them.

It’s also important to consider what type of work you plan to do in the future. Many shops prefer a system that lets them purchase the capabilities they need now, and also offers additional functions they can add as their needs change. This lets them build on their initial software investment and avoids the need to learn a new system later.

Despite the complexity of many systems on the market, a shop may only need specific CAM functions. For the purposes of this article, we have broken these functions into four general levels of complexity. The following is a quick rundown of these levels and how they relate to different types of work.

1. The Basics

A talented programmer can stand at a control and program a part with basic shapes and angles. However, this becomes much more difficult and time-consuming for a part containing complex curves, odd angles or shapes requiring multiple passes.

Simple 2D or 3D shapes can be machined in planar fashion with most CAD/CAM packages. Shops that do primarily 2 or 2 1/2-axis work need a few basic machining functions and probably not much else:

  • Contouring – programs the cutter to stay at a constant Z-depth while following a series of lines or curves.

  • Drilling – instructs the machine to drill holes in the stock at different locations, depths and cycles. This type of function typically includes other plunge operations such as boring, counterboring, tapping and reaming.

  • Pocketing – removes material from the inside of a boundary to create a cavity in the stock. Good 2D pocketing avoids islands within the pocket, and does not force the cutter into an area that is too small for it. Pocketing can be done in several different ways:

  1. Zigzag pocketing moves the cutter back and forth across the cavity with the same step-over for each pass.

  2. Spiral pocketing starts at the inside or outside of a pocket and spirals out or in until the stock is removed.

  3. One-way pocketing creates a series of parallel passes in the same direction, allowing all cutting to be climb or conventional instead of a combination of the two.

  4. Morph pocketing creates a spiral toolpath that gradually changes from an internal to external shape, keeping a constant load on the cutter.

  5. Facing cleans material off the top of an area that may lie between depths, such as the top of an island.

  6. Pocket re-machining identifies areas left uncut from a previous operation and cleans out those areas with a smaller cutter.

  • Toolpath Associativity – This maximizes the above processes by linking toolpath and geometry. If either the toolpath or geometry are changed, a new, updated toolpath can immediately be generated. This means that a part only has to be programmed once, with any changes made to the model or machining process updated with a single mouse click. For example, a programmer may want to change drill size and hole location on a series of operations. Rather than reprogram the entire set of operations, he simply selects a new tool, moves the geometry and clicks a button. The result is a new, accurate toolpath reflecting those changes.

Many shops find these 2 and 2 1/2-axis CAM capabilities are well suited for a large number of their applications. More complex work can be done with these functions, but with increasing difficulty. In addition, shops often come across parts that are not extremely complex, but are difficult or impossible to program using 2 1/2-axis functions. An example is a spherical-bottom pocket. Since the bottom of the pocket does not lie exclusively in the X-Y plane, toolpath functions with greater control over the Z axis are needed.

2. The Next Step – Adding a Third Dimension

Most complicated parts with complex curvature can be defined using surfaces. A surface is a geometric entity that mathematically defines the curvature at any given point. Surfaces are applied to 3D geometry like a skin, and are trimmed or filleted together to fully define the curvature of the part.

 Surfaced geometry requires toolpaths that are flexible enough to follow sculpted shapes. One method of achieving this is through single-surface machining. This level of machining is suited to parts that can be defined with a few sculpted surfaces that are tangent to one another. Each surface can be programmed separately and the toolpaths can be combined into a single NC program.
Once projects become more complex and contain surfaces that are not tangent, machining with single-surface functions becomes somewhat difficult. 

3. Multi-Surface Machining

Multi-surface roughing and finishing are suited for applications such as complex prototyping or mold making. These functions allow a single toolpath to be generated across multiple surfaces of any type. All selected surfaces are considered when calculating the toolpath, thus delivering a consistent finish and avoiding gouging.

A good CAD/CAM system offers several options for roughing and finishing a multi-surface part. This allows the NC programmer to choose the most efficient machining strategy for a specific project.

  • Parallel machining – This is basic multi-surface machining. The tool moves back and forth across the model. Flexible parallel machining allows you to cut in zigzag or one-way motion.

  • Constant Z machining – This function cleans all the material from a given depth before moving on to the next depth. The result is less tool wear and a more consistent finish on some surfaces.

  • Scallop machining  – Scallop machining keeps a consistent tool step-over in 3D space. This provides a more uniform scallop height around the entire model and therefore reduces the amount of handwork required to finish a part.

  • Flow-line machining – Flow-line machining uses the natural shape of a set of surfaces to determine tool movements, resulting in a  more efficient toolpath.

  • Radial machining – This type of toolpath radiates out from a center point like spokes on a wheel. It is ideal for spherical parts.

  • Containment boundaries  – Definable containment boundaries allow the programmer to define a specific area to be cut, even if it contains only parts of surrounding surfaces. This is useful when a specific area of a multi-surface part needs a different machining strategy than the rest of the part.

After a finish pass is run, there is often material left in small or hard-to-reach areas. A good CAD/CAM system provides automatic options to remove that extra material.

  • Multi-Surface Leftover Machining – This function identifies areas that are left uncut by a previous multi-surface operation, and programs a smaller tool to clean out those areas. 

  • Pencil Tracing  – Pencil tracing walks a small cutter along surface intersections to achieve the best possible finish in hard-to-reach areas.

  • Shallow / Steep Machining – This function identifies and machines steep and shallow areas that have scallops left from a previous cut

 

These 3-axis functions provide most of what a complex mold, prototype or production shop needs. There are, however, additional machining options available.

4. Machines That Do More

Some operations not only require software that is capable of generating the toolpaths, but machine tools that provide the appropriate capabilities.

  • 4-axis machining  – This adds a fourth dimension of simultaneous movement, and requires a machine with a rotary table or tilting machine head.

  • 5-axis machining  – This adds a fifth dimension of simultaneous movement, and requires a machine with one or more rotary tables and/or a tilting machine head. Good CAM software automates 4- and 5-axis requirements such as calculation of leads/lags and surface normal vectors.

  • High-Speed Machining  – Machines that support high-speed cutting need CAM that delivers features such as tangential entry/exit arcs, smooth tool direction changes and plunge roughing.

Additional Tools to Consider

There are several NC programming features which are important in all 2-, 3-, 4-, and 5-axis applications. These include:

  • Post processors – In most systems, the post processor translates the toolpath information into NC code for the machine. Therefore, good post processors are essential in any level of machining software. CAM vendors typically have a library of these to run with most machines. Many CAM systems include user-customizable post processors, allowing programmers and machinists to make adjustments themselves or with a quick phone call to their vendor’s tech support.

  • Toolpath Verification – Solid model toolpath verification runs your toolpath on-screen on a piece of “virtual stock.” The result is a solid model of the finished part that can be inspected from all angles to ensure the toolpath produces the desired results. This helps eliminate dry runs and test cutting.

  • Data Translators – If a shop plans to receive files from other CAD systems, good translators are vital. If a shop can accurately accept a wide variety of data formats, such as IGES (Initial Graphics Exchange Specification), they do not have to spend time recreating geometry that is already available. Many vendors provide these translators with their software; others charge extra.

  • CAD Capabilities – Regardless of the type of work shops do, most find it necessary to have CAD as well as CAM capabilities. Even if a shop receives all its work electronically as CAD files, the programmer often needs to edit the geometry to make it machinable. Many shops prefer a package that includes both CAD and CAM. Using a tightly integrated CAD/CAM system eliminates the need to translate files between separate CAD and CAM packages. Since an integrated CAD/CAM system uses a common database for the CAD and CAM information, complete data compatibility is maintained at all times. In addition, this type of CAD/CAM system shares a common interface, avoiding the problem of training programmers on two separate systems.

Know Your Needs – for Today as well as Tomorrow

When selecting the level of CAM capability you need, keep in mind your machines’ capabilities, the type of work you produce now, and the type of work you plan to produce in the future. Many shops prefer a system that lets them add capabilities as their needs change. This allows them to purchase only what they require and lets them plan for the future by providing an upgrade path to more complex functions.

Choosing a system that grows with your business also helps reduce your learning curve. If your CAD/CAM package provides a growth path, you won’t have to learn new software when you want to expand your capabilities.

Many CAM developers provide a family of software for milling, turning, wire EDM and other types of machining. If you decide to expand the category of machining you do, this lets you get a system with a familiar interface, further reducing training time.

Choosing a good CAD/CAM system with the correct functions helps you improve the quality, productivity and profitability of your shop. 

CNC Software publishes a booklet titled “What Every Shop Should Know About Choosing a PC-Based CAD/CAM System”. The booklet provides useful tips on software and hardware selection, and discusses how to get the most from any CAD/CAM package. For a free copy of this booklet (a $4.95 value), call 1-800- 228-2877, send an e-mail to info@mastercam.com, or write to:

Free CAD/CAM Booklet
CNC Software
344 Merrow Road
Tolland, CT 06084.

CNC Software, Inc.   860-875-5006

 

[ Home ] [ Sizeable Cuts ] [ Industry News ] [ Race Report ] [ PrecisionBikes ] [ Speedway Eng ] [ Swift Racers ] [ Aircraft Engg ] [ CAD/CAM ] [ Cutting Fluids ] [ New Products ] [ Back Page ] [ Spring98.pdf ]

 

SearchOnline!