Suburban Machinery Software Division 37777 Harlow Dr., Willoughby, OH 44094 (440) 951-8974 Fax: (440) 951-1412

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TOUCH-PROBE DIGITIZING

Another weapon in the toolmaker's arsenal

By Dan Fritz

President, Suburban Machinery, Software Div.

Recent advances in CNC controls and software have spawned a new process for duplicating 3-D curved surfaces. Many parts previously reserved for tracer mills can now be copied on a standard CNC mill, using an inexpensive touch-probe and a Personal Computer.

In theory, virtually any complex curved surface can be produced on a 3-axis CNC mill by making many overlapping passes with a ball-nosed end mill. In practice, however, NC programs for such surfaces can be very difficult to create. Also, since complex curves must be broken down into many short linear motions, these programs typically become so large that the average CNC control cannot accommodate them.

Such complex NC programs can be generated on a 3-axis CAM system if the surface geometry is well defined. Many shops, however, are faced with the task of duplicating models which have poorly defined geometry. These parts have traditionally been machined on tracer mills. Tracer mills generally have an extra long table, capable of holding both a model and the workpiece as they move in unison in the X-Y plane. The workpiece is machined with a conventional spindle while a separate tracing stylus moves over the surface of the model. Tracing machines are larger than conventional mills with the same work envelope, and use special CNC controls and software specifically designed for tracing.

Touch-probe digitizing, on the other hand, uses a spindle-mounted probe to "map" the surface of a model by touching it in a grid pattern. A Personal Computer connected to the CNC control records the data points, processes them into an NC program, and serves as a mini-DNC system so that very long NC programs can be run directly from disk memory. Since the addition of a probe and a Personal Computer does not inhibit the normal functions of the CNC control, such a machine can easily serve "double-duty" as a production mill and as a digitizer. Touch-probe digitizing cannot perform all the functions of a tracer, but for some types of work, it can be more cost-effective.

One basic difference between tracing and touch-probe digitizing is the device used to collect data. Tracers use a special tracing head which produces an analog signal proportional to the amount the stylus is moved. This signal is used by the tracer control like a position feedback device, causing the machine's Z axis to move in "locked step" with the stylus. A touch-probe, on the other hand, is simply a switch designed to make and break contact in a very repeatable manner. Touch-probes produce a simple "on/off" signal which is wired to the CNC control. Software in the CNC must be able to monitor the probe's status continuously as the machine moves.

Touch-probe digitizing is not new. Simple digitizing software has been available from several companies for many years. Early forms of touch-probe digitizing required that the ball-nose end mill exactly matched the size of the ball on the probe stylus. This resulted in an interesting paradox: If the probe stylus and end-mill were both small, the distance between cutting passes had to also remain small to reduce the "curf" left by the cutter. This made it necessary to digitize with a small point-to-point increment, and the process became very time-consuming. If, on the other hand, a large diameter cutting tool was used, the matching probe stylus would tend to occillate like a pendulum due to its increased mass. Software monitoring the probe had to be capable of "waiting out" these oscillations, which also slowed the digitizing process.

These two problems have recently been solved through the development of spline curve fitting and 3-D cutter compensation software. For surfaces requiring many passes with a small diameter cutter, the Personal Computer used to collect the data can also be used to fit spline curves through the digitized points, generating any number of intermediate points. For example, a digitized point grid with 100 points in the X axis and 50 points in Y can be "multiplied" into a 1000 by 500 grid simply by specifying a multiple of 10. This drastically reduces the time required to collect data points from models with smoothly curved surfaces.

For surfaces that permit the use of a large diameter cutter, the development of 3-D cutter compensation software has eliminated the need to digitize with a large probe stylus. Now, data points can be collected with a standard (small) stylus, and processed on the Personal Computer for any desired cutter size. This software can also be used to perform male-female transformations by simply entering a negative value for the cutter, and mirroring the resulting NC program in the appropriate axes. Mold shrinkage compensation and finishing allowances can also be performed by specifying a tool size slightly different from the actual cutter being used.

Even with the latest software, however, touch-probe digitizing has not become a replacement for CAD/CAM systems or tracer mills. Many types of surfaces which are difficult to program conventionally can easily be digitized, but there are many situations where a CAD/CAM system or a tracer is better. The following general rules should be considered.

1) Smoothly curved 3-D surfaces, or 2-D profiles having complex curves are ideal for touch-probe digitizing.

2) Surfaces with well defined geometry, or surfaces that can be defined as a series of planes, spheres, cylinders, and cones can usually be programmed on a good 3-axis CAD/CAM system much faster than they can be digitized.

3) Surfaces with a lot of intricate detail, or surface texture, can be digitized, but the digitizing increment must be kept very small or some detail will be lost. These surfaces are best left to the tracer mill, since an analog tracer head can duplicate even highly textured surfaces at a high speed.

4) When surface geometry is to be fed back into a CAD/CAM system for "re-engineering" work, it is usually better to use one of the new laser-scanning digitizing systems. When using a laser beam (instead of a probe or tracer stylus) the data collected are points ON THE SURFACE of the model, rather than at the center of the stylus ball. This makes subsequent processing of the data in a CAD/CAM system somewhat easier.

Many shops equipped with the latest in expensive CAD/CAM systems and tracing machines may find that touch-probe digitizing is still a useful tool for many jobs. Digitizing a simple curved shape can frequently be performed on a standard CNC machining center, freeing up the more expensive tracer mill for other, more suitable work.

The most time consuming aspect of touch-probe digitizing is the process of collecting data. In our experience, the average small machining center can collect data at the rate of about 2.5 seconds per point, or 1440 points per hour. This may seem fast, but a part surface 10 inches square, digitized in a 1/5 inch grid requires 2500 points, or about 1.75 hours of machine time for data collection. When the digitizing increment is cut from 1/5 inch to 1/20 inch between points, the number of points required increases by a factor of 16, to about 28 hours. Clearly, a smooth surface that can be digitized with a coarse increment, then spline curve fitted to the increment required for machining can be digitized efficiently, while a textured surface requiring a fine digitizing increment is best done on a tracer. Since tool wear and breakage are not a problem, as they are when cutting metal, digitizing can be safely performed overnight without human supervision, reducing the human cost of the digitizing process.

Machining a digitized surface takes about the same amount of time as that required to machine a surface programmed on a CAD/CAM system. The speed of the machining process is primarily regulated by the limits of feedrates, spindle speeds, and the "block-processing" rate of the CNC control.

Tracers can machine a workpiece as the model is being scanned, while digitizing requires two distinct operations. A two-step process may be preferred in some cases, since machine vibrations generated by the cutting spindle can adversely affect the accuracy of the tracing head. For this reason, several companies offer digitizing as an option on tracer mills. This approach works very well since the model can be digitized at a high speed with an analog tracer head, and later machined without the presence of the model. Such tracing/digitizing machines seem to have the best of both worlds, but the cost of a special tracer machine with digitizing software is higher than many small shops can afford for intermittent use.

Touch-probe digitizing has benefited greatly from recent advances in software and storage media for small computers. Disk drives capable huge amounts of data can now be installed in a Personal Computer at a very low cost. A few years ago such storage capacity was only available on large mainframe computers.

New digitizing methods using ultra-sonic probes or 3-D steroscopic vision systems are now becoming commercially available. These new systems may someday excel at digitizing soft surfaces, such as wax or styrofoam models, which are easily deformed by the pressure of a stylus.