As you know, CNC machines have extremely fast motion rates. When you're verifying a program (new or proven programs), you also know that each tool will rapid to within a very small distance (usually 0.1 inch) of the workpiece during approach.
Remember - even with proven programs - there are several setup-related considerations related to whether the tool will stop where it is supposed to. Was the tool measured correctly? Has its offsets been entered correctly? Has program zero been correctly assigned? And with new programs, of course, the program commands for the approach may be incorrect.
You must find a way to take full control of each tool's approach movement. By this we mean that you must be able to slow the motion rate to a crawl in order to judge whether the motion will stop at the appropriate position. Don't give up until you find a way to fully control this motion.
Some machine tool builders make it easier than others. With one we know of (Okuma), it is as simple as turning on the single block switch. When single block is turned on, the feedrate override switch controls the rate for all motion (including rapid). And you can slow rapid motion to a crawl.
With others, its a little more difficult. Some machine tool builders provide a very good rapid override switch. During rapid motion, you can use this multi-position switch like a rheostat, and again, you can slow rapid motion to a crawl.
Unfortunately, some machine tool builders provide a poor rapid override switch - having but three or four positions (like 100%, 50%, 25%, and 10%). With this kind of rapid override switch, you don't have fine control of rapid motion - and you may not be able to slow rapid motion to a crawl.
For machines with limited rapid override switches, dry run can be used to take control of all motion. When dry run is turned on, a multi-position switch (usually feedrate override) is used to control motion rate - and again, you can slow rapid motion to a crawl when dry run is turned on. But you must remember to turn off dry run prior to allowing the machine to begin cutting. While dry run nicely slows rapid motion, it accelerates feedrate motions.
Use single block to go step-by-step
Since the safe approach procedure can be a little nerve-wracking, you'll want to be able to stop and take a breather after each motion. Single block, of course, provides you with the ability.
The distance-to-go display really helps
All Fanuc controls have a distance-to-go display. You find it on the program check page. As the name implies, this function provides you with a real-time display of how much further the tool will move in the current command.
Keep a finger ready to press feed hold
Pressing the feed hold button, of course, will stop motion. When the tool is within about 0.5 inch of the surface being approached, you can press feed hold and then look at the distance-to-go display. It must, of course, be less than 0.5 inch.
