CNC Trade Secrets. James Harvey

the machine will stop executing the program when the controller reads the M01 commands. Optional Stops are generally used for debugging and proofing programs because they give the machinist a chance to see how the last tool ran.

Figure 3-5 Large remnants can get jammed between the table and the back column of the machine. It’s a condition that must be avoided.

      9.Program drilled pilot holes deep enough so reamers won’t jam.

      The pilot hole for a reamer has to be drilled deep enough so that the reamer won’t jam against the bottom of the pilot hole. When you have blind reamed holes, it is a good idea to check the program at the machine to make sure pilot holes are drilled deep enough to avoid this interference.

      10.Be extra cautious when using tie-down clamps. (see Fig. 3-6)

      Except for simple setups, I try to avoid using tie-down clamps unless I know there is little chance of running into them. Besides just sticking up in harms way, they are too often placed inconsistently by operators. Instead of using tie-down clamps, I prefer bolting parts down. I try to avoid special programming that may be needed to “jump” over clamps. These special programs can be nightmares for someone trying to set up the job at a later date.

      11.Be cautious of protruding bolt heads. (see Fig. 3-7)

      With bolts, a default rapid plane of one inch is usually adequate to jump from one location to the next without having to concern yourself with interference — as long as the bolt heads don’t stick up higher than the rapid plane.

Figure 3-6 Tie-down clamps are something that can easily be run into. When using tie-down clamps, program using generous rapid planes.

Figure 3-7 Bolting parts down for machining is preferable to using tie-down clamps because bolt heads don’t stick up as high and are used in consistent locations.

      12.If a tap breaks while you’re at the machine, hit either the “Emergency Stop” button or the “Reset” button.

      Once the machine starts executing a G84 tapping canned cycle, pressing the “Feed Hold” button will not stop the spindle from continuing its journey to tap the hole. This can make a bad situation worse as the broken tap, which is still in the tool holder, grinds into the portion of the tap that is broken off in the part.

      13.When mounting cutters in tool holders, make sure all cutters extend far enough below the tool holder so the holder doesn’t interfere with the part when the program is run. (see Fig. 3-8)

      This is a common crash, one that can easily be avoided if the machinist takes a little time to understand what features a tool is going to cut and how deep the cutter will be going. Most setup sheets provide this information.

      I would be a nervous wreck running somebody’s unproven program until I fully understood what feature a tool was going to cut. Sometimes cutter depths are obvious by looking at the drawing. Other times — especially when there are many tools and the part is complicated — it’s not so obvious. If you have the option and the experience, it is best to check how deep the cutters will be going by referring to the CAD model and the CAM simulation.

      Standard center drills are relatively short. Be especially careful with standard center drills in order to avoid tool holder interference. You can purchase long center drills; they work well for reaching down to lower levels of a part.

Figure 3-8 Tool holder interference is a common crash. This type of crash can easily be avoided if the machinist knows what the tool is going to cut and also how deep it will cut.

Figure 3-9 Long tools and tall parts are a recipe for crashes during tool changes. In this case, an edit to the program was necessary to move the part out of the way before conducting a tool change.

      14.Manually edit programs to avoid crashing long tools into tall parts. (see Fig. 3-9)