A neutral interface language for numerical control programs is a boon on the shop floor, but it also has substantial benefits to users--and vendors--of CAD/CAM systems.

Postprocessing has often been considered a necessary evil, an unpleasant fact of life for everyone involved with numerical control (NC) programming. Postprocessors are the software programs that "customize" a file of tool path data and related machine instructions for a particular machine tool. Once an NC program has been postprocessed, it can be used to produce parts on the designated machine.

The same program may or may not be usable at other machine tools unless the machines are identical--same configuration, same control unit, same home position, same clearance planes, and so on. This is rarely the case for most shops since they are likely to have a variety of equipment from a variety of machine tool builders and computer numerical control (CNC) unit suppliers.

It is likely that a company will purchase machines and/or control units over a period of time. In many cases, these machines are intended to perform similar tasks. They may even be identical. However, identical machines may be equipped with a variety of controls, each requiring its own postprocessor. The greater the number of postprocessors a shop must have, the greater the magnitude of associated problems.

The Challenge

Most programming departments must have several postprocessors to properly prepare an NC program for whichever machine tool will be producing the workpieces. The programming departments at larger shops may have fifty or sixty different postprocessors to support a hundred NC machine tools. A small job shop may have a unique postprocessor for every one of its machines.

For suppliers of computer-aided design/computer-aided manufacturing (CAD/CAM) systems with NC programming capability, postprocessing represents a tremendous challenge. Gerber Systems Technology, for example, has over 1300 systems installed at over 600 shops and plants around the world. To meet the needs of these customers, Gerber has developed a library of more than 500 postprocessors. Gerber considers the availability of reliable and efficient postprocessors an essential part of its commitment to computer-aided manufacturing and one of its key strengths in the marketplace.

However, maintaining and enlarging this library is costly. It diverts resources, complicates customer service, and slows product development.

This proliferation of postprocessors results from a lack of standardization that creates problems both on the shop floor and in NC programming. The trend to computer-integrated manufacturing, which is driven by the pressures of global competition, will only intensify the difficulties created by this lack of standardization.

It would be irresponsible for CAD/CAM vendors, machine tool builders, CNC suppliers, and machine tool consumers not to pursue a solution.

The BCL Concept

BCL was designed to help avoid the delays and inflexibility that can result when production of a workpiece is moved from one machine to another. This situation can occur because of equipment failures, scheduling requirements, or a need to increase production by using several machines simultaneously. BCL is short for Binary Cutter Location, which comes from the title of EIA standard 494, "32 Bit Binary CL Exchange (BCL) Input Format For Numerically Controlled Machines."

The BCL concept is simple. It shifts the postprocessor function from the NC programming system to the machine tool control unit. Whereas a postprocessor usually resides in the computer along with the NC processor and is specially written for a particular machine/control unit combination, BCL replaces this conventional postprocessor with a "converter" that generates a standard RS-494 format.

The fundamental principle guiding this standard is that data used as input to the machine tool remains oriented to the part coordinate system, not to the coordinate system of the machine tool. Use of the BCL format requires that the control unit itself be equipped to perform all machine-geometry dependent functions, which may have been previously performed by a postprocessor.

The BCL converter is a part-oriented postprocessing program, which converts the contents of a CLFILE (what the NC processor normally feeds to the postprocessor) to the BCL format file. The BCL format as specified by RS-494 represents NC machining input data as a series of records that are groups of 32-bit binary integer words.

The primary functions of the BCL converter are to retrieve CLDATA records, select the relevant portions of the data, convert the floating point data to 32-bit integer data, format data into records and produce a BCL program file. It performs no machine-dependent functions such as orienting data to the origin of the machine's coordinate system or computing tool length values.

The BCL format was originally developed for use with the APT (Automatically Programmed Tools) NC processor. It is designed to utilize the integer codes defined for major and minor words from the ANSIX3.37 specified CLDATA (X3.37 is the national standard that governs the APT processor language). All of the postprocessor commands are represented as integer codes.