by Ruth Christopher and Harvey Patterson
Misdiagnosis can lead to tool failure and costly downtime
Troubleshooting machining operations can be a daunting task, especially when it is a boring operation. When machining the outside of a part, you may be able to
visually see what causes tool failure. When the tool is buried inside the part in a hole, it becomes impossible to see what is happening to the tool.
A common scenario, especially in small hole boring is as follows: the boring bar is set at centerline or slightly above. The bar
is not larger than the hole. The conditions look good. The program checks out. You feel good about the setup and press the
start button. The coolant is everywhere and you can see nothing until the boring bar holder retracts away from the part and your biggest fear comes true; the end of the boring bar is missing.
When this happens, it is normally due to chip packing or the bar trying to feed past the end of the hole. If the end of the bar is still in the hole and hard to remove, then the hole is most likely packed with chips and the cause for catastrophic failure. In this case, the cutting conditions created a greater volume of chips than could be evacuated. A solution is to use a smaller boring bar. If that is not possible, back off cutting conditions to create a lower volume of chips. If the bar is not in the hole or is loose in the hole, then the bar may have tried to feed deeper than the existing hole in the part. A boring bar does not make a good drill.
So now the boring bar is machining holes without catastrophic failure, but the production per shift is not what it should be and the operator is changing boring bars too frequently. Upon examination of the bar, it is determined that the tool has excessive flank wear. The solution for excessive or rapid flank wear can be misdiagnosed. Many times the cutting edge will develop small chips from an interruption or chatter that will, over time, look as if the bar has flank wear. Another condition that may end up looking like flank wear can start out as buildup on the top surface of the bar at the edge. This is metal welded to the top surface at the cutting edge from heat and pressure of the chip. At some point the buildup will be dislodged by the chip and will pull a piece of carbide with it.
To determine if one of these conditions caused the tool to look as though it has experienced flank wear, run the tool on one or two bores and then examine it to see if any buildup exists on the edge or if there is any chipping. To reduce BUE, you need to reduce the pressure or the heat. The pressure comes from the depth of cut and the feedrate; reducing these conditions reduces the pressure and the heat.
A more productive change would be to use a coated tool because the coating reduces the friction of the chip on the cutting surface, which reduces heat.
Generally, to reduce heat, you apply coolant. This is usually simple on OD machining, but can be difficult on small ID machining. There is a small hole partially filled with boring bar and metal chips that needs to be cooled. A coolant line close to the hole normally gets in the way, so the best approach is to supply coolant through the boring bar holder. A good supply of coolant all the way around the circumference of bar is ideal so that the coolant can find the path of least resistance and get to the bottom of the bore. If the tool, after a couple of parts, shows chipping, then it mostly likely came from an interruption in the part, such as a cross hole or keyway. If the part has an interruption, then a tool with a corner radius or a hone on the cutting edge may be needed. If the chipping is not from an interruption, then it could be from chatter. A solution that often works to solve this issue is to increase the feed rate. A 10 per cent increase in pressure can be enough to keep the tool in the deflected state so a shock from relaxing is not present.
There have been numerous studies of flank wear and all of them have come to the same conclusion: a small decrease in speed will make a big difference in minutes in the cut before flank wear becomes an issue: a drop from 400 sfm to 300 sfm could increase tool life from 12 to 40 minutes.
Solving production issues is a priority, but do not be too quick in identifying failure modes. The wrong diagnosis may trigger changes that may make the problem worse. SMT
Ruth Christopher is marketing coordinator and Harvey Patterson is an engineer at Scientific Cutting Tools.
