What is Coherence?

When someone is talking and you cannot understand them, you would describe their speech as “incoherent”. They are trying to say something, but you are not hearing or receiving it correctly. If you could understand 80% of the words they are saying you could probably make it out and they would then be "coherent".

The same applies to the “coherence” of the tap test.

We are applying a measured force ("speaking") to the tool with the hammer. We want the accelerometer to receive the vibration and see only vibration from that force ("hearing") as much as possible. A reading of 80% is considered “coherent”, below is “incoherent”.

Coherent measurements above 80% are essential to produce accurate Dashboards.

After completing the measurements for the X-axis, click on the Coherence radio button in the menu on the right side of the screen.

If the graph in the upper window stays clearly above the 80% threshold (shown in red), Click OK.

If it goes below the 80% line, then click on the Cursor Tab on the far right of the menu, or press F3 to activate the cursor.

You can then simply left click the cursor over the peaks of the waves in the lower box (locate it inside the center of wave, the cursor will automatically find the peak, you may have to make a couple of tries to get it to the peak) then read the coherence percentage in the menu. If it is above 80% you are taking good measurements.

There may be several peaks to check. The tool vibrates in more than one mode wave (the source of that vibration). The cutting tool is vibrating at a certain frequency and that is a mode you will always see as the first peak. The toolholder vibrates at another mode as does the spindle and the machine itself. If the tool assembly is short and stiff these other modes may appear as additional peaks to the right of the dominant tool point peak. You must be above 80% in all of them.

Click F1 or the TXF Tab to return to the main screen.

Click OK.

What causes poor coherence? CLICK HERE

SpeedCast

What is Coherence?