Troubleshooting solid-carbide boring bars requires the right diagnosis

Troubleshooting machining operations can be a daunting task, especially boring operations. This is because when machining the outside of a part, you can see what causes the tool to fail. However, when a tool is buried in a hole, as is the case when boring, you can’t see what is happening to the tool.

If you’ve done any boring—especially small-hole boring with a solid-carbide bar—you’ve probably experienced the following: The boring bar is set at or slightly above centerline and is not larger than the hole. The conditions look good. The program checks out. You feel comfortable about the setup and press the start button. Coolant sprays everywhere, and you notice nothing unusual until the boring bar holder retracts from the part.

At that point, your biggest fear has come true—the tip of the boring bar is missing!

Scientific Cutting Tools is introducing a toolholder with Coolant Ring Technology, which enables coolant to be applied around the circumference of a boring bar. All images courtesy Scientific Cutting Tools.

When this occurs, it is normally because of chip packing or the bar trying to feed past the end of a blind-hole. Upon examination, if the tip of the boring bar is still in the hole and difficult to remove, the hole is most likely packed with chips and you’ve found the cause of the catastrophic failure. In this case, the cutting parameters, such as DOC and feed rate, created a greater volume of chips than could be evacuated. A solution would be to apply a smaller boring bar, if possible. If not, the cutting parameters must be backed off to create a lower volume of chips.

If the broken bar is not in the hole, or is loose in the hole and being obstructed from exiting the hole, you may have tried to feed the bar deeper than the existing hole in the part. A boring bar does not make a good drill!

Rapid Wear

After you’ve made the adjustments necessary to allow the boring bar to finish holes without suffering catastrophic failure, let’s say you decide the production per shift is not acceptable and you feel you’re changing boring bars too frequently. Upon examination of the bar, you determine the tool’s cutting edge has excessive flank wear (Figure 1).

However, the cause of excessive or rapid flank wear can easily be misdiagnosed. Frequently, over time, interrupted cutting or chatter can cause small pits to form on the cutting edge that resemble flank wear.

Figure 1. The cause of flank wear is easy to misdiagnose when troubleshooting boring bars.

In addition, what looks like flank wear can start as built-up edge on the top surface of the bar (Figure 2). This is caused by metal from the workpiece welding to the top surface at the cutting edge as a result of heat and pressure during chip formation. Then, at some point, a chip will dislodge the buildup, along with a small piece of carbide from the bar.

To determine if one of these conditions caused the tool to look as though it has experienced flank wear, bore one or two holes and then check to see if any BUE or chipping exists (Figure 3 below).