Detecting Chips in a Milling Spindle
Chips in a milling spindle can subtly deform a tool assembly until the cutting tool is out of alignment. Grob's chip-in-spindle detection system detect stresses that may indicate chip interference.
Almost by definition, a very productive milling machine generates a lot of chips. Moreover, today’s high speeds and feeds can really make the chips fly—and that is not just a figure of speech. As a result, unfortunately, chips can end up where they shouldn’t.
This can be an especially vexing problem during an automatic tool change, when chips clinging to the surfaces of the toolholder may get trapped between the toolholder and surfaces of the spindle. The unwanted presence of even very small chips in these areas can damage the holder and/or the spindle, as well as cause the tool to be out of position, leading it to cut a workpiece inaccurately.
An automatic chip detection system introduced by Grob for its machining centers is designed to prevent these problems and protect the toolholder, spindle and workpiece. The system uses a two-part collar integrated with the face of the spindle. Sensors in the collar detect tool clamping error and signal the machine control to issue a faulty tool-change alert.
The system checks for these errors while the spindle is at a standstill immediately after the toolholder is clamped following an automatic tool change. Because sensing occurs while the machine axes are moving into position before the spindle restarts, normal readings have no effect on cycle time. Detection of a faulty tool change, of course, triggers the normal safety response.
As a major supplier of high-volume machining systems to the automotive industry, Grob was particularly sensitive to problems that could result from possible chip-related interference during tool changes on these systems. However, the value of automatic chip detection for users of its line of standalone universal machining centers was also apparent. The system is now available for both in-line and standalone machining centers from this builder.
Designed for the short-taper (HSK A63) toolholder/spindle interfaces on Grob machining centers, the system consists of a stator and rotor assembly that is mounted within the face of the spindle. In this system, the machine spindle nose acts as the rotor, using imbedded strain gauges to sense face clamping force in multiple locations. As the spindle rotates, it uses wireless induction for power and signal communications. The stator, then, is a stationary ring mounted concentrically to the rotor as part of the spindle housing assembly. The stator ring is hard-wired to the machine control to supply the inductive current that powers the rotor sensors as well as energizes the wireless transmission of signals from the individual sensors. As these signals are received by the stator ring, they are carried by wire to the machine control for data processing.
Analyzing the stress distribution identifies and measures asymmetrical deformation caused by the presence of a chip between the tool flange and the spindle face or between the tool taper and the spindle taper.
The measured deformation, if any, is automatically compared to a preset limit. If the limit is exceeded, a faulty tool change response is triggered by the machine control, stopping the machine. Chips as small as 10 microns can create deformation detectable by the system. According to developers, this is a significant enhancement to accuracy, because detecting chips that small would prevent a change in tool concentricity as low as 0.01 mm at a point 150 mm from the face contact on a HSK A63 toolholder.
Automatic monitoring of tool clamping to detect chip interference between the toolholder and the spindle has multiple benefits. It increases process stability; protects cutting tools and the machine; helps optimize machining processes; makes automatic tool changes more reliable; and prevents runout discrepancies that might result in machining errors or defective parts.
Because the automatic chip detection system is integrated within the spindle unit, it must be ordered as an option on new machines from the factory. It is not retrofittable in the field.
While the mistakes listed here will not sound an alarm or cause a program to fail, they will cause confusion, wasted time and scrap parts.
Effectively commanding motion on most CNC machines requires more than describing how the CNC machine determines the end point position for each motion.
It is common to machine several identical workpiece attributes from within a single program. Consider the four identical circular counter-bored holes that must be milled in the workpiece shown in Figure 1.