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The CNC laser: accuracy and repeatability are among its outstanding values; smoother travel and controlled acceleration and deceleration need to be developed.

The CNC laser

The operation of a CNC laser is like that of any other CNC system, with a few exceptions. The material, its thickness and the operation to be performed (cutting, welding, marking etc) must be considered in order to program the proper feed rate and power (current, frequency, and pulse rate) of the laser. The laser must also be focused and tool length offset determined before starting the program.

The programming is similar to that of other systems. Included in the program are commands to turn the laser off and on, turn gas off and on, etc. The dimensions, hole locations, etc are then transformed into a language that the computer can understand.

The program is fed into the CNC and stored. The operator places the piece to be lased in the fixture, selects the program, and starts the operation.

Advantages of the CNC laser

There are numerous reasons for choosing a CNC laser. By implementation of a fixture to secure the piece and a program, a part can be exactly duplicated time and time again. This ensures the production of parts of identical quality, whether heat treating, cutting or marking.

This process is also time efficient. Since the laser is run by the program, the operator has time to perform other tasks. While one piece is being lased, the operator can prepare the next piece.

After the process is complete, the operator removes the finished part, locks the next piece to be lased in the fixture, starts the program, verifies the quality of the previous part, and prepares the next piece for lasing. This procedure, when repeated, saves time and money.

The CNC laser provides a continuous travel speed when tracing a line. This constant rate produces a more homogeneous weld. The precision of the CNC laser is perhaps its most impressive characteristic, however. A CNC laser can produce an ulimited number of parts that will be accurate to within 0.001 of the programmed dimensions. This degree of accuracy provides the opportunity for significant improvements in many areas of industry.

One such area is the manufacturing of dies. Laser-laminated dies are produced by the use of a stationary fixture, several sheets of metal, and a program. The holes cut in each plate are in exactly the same location. After cutting all of the pieces, the plates are aligned by driving dowel pins into the lased holes. These plates combine to form the die segment.

The pieces that were cut from the plates are aligned in a similar manner and placed on a die shoe according to the location of the die cavities. This piece becomes the punch.

An example of a laser laminated die is shown in Figure 1. The tight fit between the punch and the die, and the identical dimensions of each cut, yield a product of superior quality.

The laser-laminated die also produces definite cost advantages. By using cold-rolled steel (about 30^/lb) instead of tool steel (about $5.50/lb) for the back-up plates, you can realize a tremendous savings.

Figure 2 shows the condition of the punch plate after stamping 3000 pcs. The top two plates are made of 0.125 tool steel; the six backup plates are cold-rolled steel. Since the cutting action of the laser heat-treats the edge periphery, no subsequent heat treatment is required. The smooth accurate cut also eliminates the grinding process, one of the major costs in conventional die manufacturing.

In summation, a laser-laminated die costs approximately one fourth of the price of a comparable tool-steel die.

Built-in functions

The CNC has various built-in functions that simplify otherwise tedious operations. One of these is the "transformation' function that can enlarge or shrink the dimensions of a programmed piece. This transformation can be accomplished by inserting only one statement. Producing the same results on a system without this function would require revision of the entire program.

Another timesaving feature is the choice between absolute and incremental input. Absolute input is based on the X-Y-Z coordinate system. An origin (0,0,0) is selected and all points are interpreted as an absolute coordinate in reference to that origin. Incremental input is also based on the X-Y-Z coordinate system.

All entries are interpreted as commands to move the same distance as that programmed, from the present position, and not to an absolute point. Table 1 compares the commands that must be programmed in each type of input to achieve the same results.

The use of absolute input will prove to be more accurate in some cases. For example, suppose the CNC reads only to the thousandths place (0.001). Four circles are to be cut out, with the centers of the circles at (1.00,0), (1.1004,0), (2.2008,0) and (2.3010,0).

Assuming both programs start at the origin, Table 2, with the use of Figure 3, shows the steps taken in each program to reach the centers of each circle.

An error of 0.001 is detected in the incremental process. If this piece had required 51 holes at 0.1004 apart, the absolute system would put the last hole at (6.020,0). The same instructions using the incremental system would put the last hole at (6.000,0). This error of 0.020 could produce undesirable results.

There are some instances where incremental input is time efficient and produces accurate results. For example, suppose a customer orders a part with eight squares, each 0.200 apart. A loop can be constructed to cut out the square and move (0.200,0). If this loop is enacted eight times, the program is complete. This program would be considerably longer if the absolute system were used.

In other instances, incremental input will be a timesaving alternative. For these reasons, both are available to the CNC programmer.

Possible improvements

As research and development have always brought improvement to new products, such will be the case with the CNC laser. Two major revisions stand out as "top priority.'

A smoother travel that does not stop or slow down at the end of a programmed radius or line is necessary for many operations. When programmed to go to a certain point, A, the CNC lasers on the market today go to the point, stop, then turn the laser off. The laser dwells at point A for a very short time before it turns off.

This is particularly detrimental in welding and marking, since the laser tends to burn through the material. Ideally, the laser should go toward point A, reach point A, stop and shut off simultaneously.

A dwell is occasionally necessary when the first cut is made in the material. Dwelling should be the option of the programmer, however.

Since a particular piece may vary in thickness, an acceleration or deceleration may be required to perform a cut of decreasing or increasing thickness. For example, with today's CNC, it would be nearly impossible to produce a good quality cut on a wedge as shown in Figure 4.

The development of this option, along with others, will expand the market for lasers by increasing their uses and improving their current applications.

For information from Morton Metalcraft Co, circle E3.

Table: Comparison of programmed values, absolute versus incremental input.

Table: Moves required to reach centers of all circles.

Photo: 1. Example of laser-laminated die.

Photo: 2. Condition of punch plate after stamping 3000 pcs.

Photo: 3. Locations of centers of circles, and distances between them.

Photo: 4. Schematic, deceleration for good-quality cut on wedge.
COPYRIGHT 1984 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1984 Gale, Cengage Learning. All rights reserved.

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Title Annotation:computer numerical control
Author:Bruch, Kristi M.
Publication:Tooling & Production
Date:Jan 1, 1984
Previous Article:Drilling superalloy parts for the space shuttle.
Next Article:Aerospace components cut from the solid.

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