Avoiding A Disastrous Situation

When verifying a program, the most crucial time is when each tool approaches the workpiece for the first time. This is true for both machining centers and turning centers.

Columns From: 1/3/2005 Modern Machine Shop,

When verifying a program, the most crucial time is when each tool approaches the workpiece for the first time. This is true for both machining centers and turning centers. Most programmers use a 0.100 inch (about 2.5 mm) rapid-approach distance, meaning the cutting tool is going to rapid to within a small distance of the workpiece before it stops. This tiny distance is difficult to see and verify during a dry run.

If a setup-related mistake has been made (possibly with program zero assignment or with the cutting tool measurement or related offset entry), the approach movement might cause the tool to rapid into the workpiece–even if you’re running a proven program. Knowing how to take full control of this approach movement can ensure that the cutting tool will stop before it contacts the workpiece. So, by one means or another, you must be able to take full control of the motion rate during approach movements.

Machine tool builders vary when it comes to how much override control they provide for rapid traverse. Some provide a multi-position switch that may have as many as 25 settings. Because the rapid override switch is inadequate for controlling rapid motion during approach movements for most machines, one alternative is a dry-run switch, which most machine tool builders provide. When the dry run switch is turned on, most machines will allow full control of all motions (rapid and cutting) with a multi-position switch—usually the feed rate override switch. This enables you to slow any motion to a crawl—including rapid motion.

Single block—During a program’s verification, you need the ability to make the machine stop after each command. When the single block switch is turned on, the machine will stop after each command, which eliminates the need to use dry-run for rapid approach movements.

Feed hold—The feed hold button is your panic button. When pressed, it causes all axis motion to stop. During a program’s verification, you should have a finger ready to press this button at all times.

Distance-to-go—This display screen function will show you how much farther the cutting tool will move in the current motion command.

The following procedure allows you to take full control of the approach movement for each tool. It is simple in nature; use it whenever you are worried about a cutting tool’s first approach. It assumes the correct program is active; the cursor in the program is positioned at the beginning of the tool that will be approaching (commonly an optional stop command when verifying a new program) and also that you have included an M01 (optional stop) at the end of each tool.

1) Set the display screen to show a portion of the program, along with the distance-to-go function.

2) Turn on the optional stop switch.

3) Turn on the single block switch.

4) Gain control of all motions with the feed rate override switch. With most machines, this means turning on the dry-run switch.

5) Set the feed rate override switch to its lowest setting.

6) Repeatedly press cycle start until the tool change occurs.

7) Repeatedly press cycle start until the tool begins its approach movement. Because the feed rate override switch is set to itslowest position, the motion will be quite slow.

This is the crucial movement. Use the feed rate override switch to control motion rate. Turn it up to go faster. Be careful as the tool gets close to the workpiece. Stop the motion when the tool gets within about an inch of the workpiece (by pressing the feed hold button). Look at the distance-to-go display. Confirm that the amount of motion left in the command is less than the distance between the cutting tool and the workpiece. As long as it is, press cycle start to reactivate the cycle. Let the tool come to its final approach position and stop.

8) If you turned on the dry-run switch, turn it off. (Never let cutting tools machine a workpiece under the influence of dry-run; it tends to slow down rapid motions, but it speeds up cutting motions.)

9) The tool has safely approached. What you do next will depend on whether you’re running a new or a proven program. For a new program, you must step through the cutting tools’ cutting motions. So, leave the single block switch on, and repeatedly press cycle start to step through the tool. For a proven program (be absolutely sure that you’re running the current version of the program), turn off single block and press cycle start to let the tool machine the workpiece as it did the last time the program was run.

10) When the tool is finished, the machine will return to the tool change position. Repeat the procedure from step three.

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