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. 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.
Why is there sometimes more than one peak? 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.
After completing the measurements for an axis, click on the Coherence button in the menu on the right side of the screen. Then click on the Cursor Tab on the far right of the menu, or press F3 to activate the cursor. You can then simple left click the cursor over the peaks of the waves in the lower box (get close and inside the wave, the cursor will automatically find the peak) then read the coherence percentage in the menu. If it is above 80% you are taking good measurements.
If the coherence is just below 80%, the first thing you should do is try the tap-test again (“can you repeat what you just said”) by clicking on the re-practice button. Go through the 8-10 practice taps and then click on the Measure button to take the measurements again. Make sure the accelerometer is firmly attached and that you are not using too much wax. Too much wax absorbs some of the force received from the hammer (“please move closer to the microphone”). Make sure you are tapping with a solid motion and land solidly on the tool tip opposite the accelerometer. If it fails to reach 80% coherence again change to a harder tip. Typically go from the white plastic tip (most popular) to the steel tip to increase the force applied (“please speak louder”). The plastic tip absorbs more of the force than the steel tip, but does have any potential to damage the tool. That is why we always start with the plastic tip and go to the steel tip only when necessary.
You can also increase coherence by taking more measurements per axis and deleting any measurements that appear as outliers (measurements that may have been accepted, but show amplitudes far less or far more than the others). To delete a measurement simply click on top of it and X's will appear. They won't count as part of the average and will increase the coherence percentage.