Spindle Load Meter Fluctuations

For various reasons, spindle load meters may display momentary very high readings during normal operations. March 28, 2008

Is it normal for the dial on the spindle load meter to briefly swing over to yellow or red during CNC routing? I'm talking for 1-2 seconds at the most.

Forum Responses
(CNC Forum)
From contributor M:
There is a very scientific and technical response to this - "It depends!"

Some machines have just a spindle load meter on them measuring the current supply to the spindle. Other machines have axial load meters measuring the current supply to the axis motors. The latter are more prevalent on metal machinery, however I do not know what your machines have, so I try to briefly cover all bases. Also, there are analog gages with color coded bars (yellow probably marking from 80-100% and red marking 100%+) and a sweeping needle design. There are also digitally embedded load meters within certain controls. Accessing the load meter screen is typically a soft key function relevant to the specific control.

Normally, the spindle load meter will read a heavy load when the spindle initially begins to spin. From a dead stop, it takes significant current to accelerate to the programmed rpm in a specified time frame. Once the specified rpm is reached, the load meter should stabilize at a low range percentage (say 15-25%). Once material is engaged there should be an increase in load relevant to the type of material being machined. Again, upon initial engagement, you should see a noticeable increase, then a recession back to a stable load. The load should remain stable as long as all cutting conditions remain stable.

The last condition that will cause a noticeable increase in load, then a decrease down to no load, is when the spindle brakes to stop turning. All of this is relevant to what is being machined, cutting conditions, machine condition, spindle condition and cutter condition! But there is a definite pattern of predictability as to what you should see occurring on the load meter.

Some controls are parametrically governed to halt if the load meter remains at over a predetermined percentage for a specified time (ex: 150% for 6 consecutive seconds).