# What Is Coherence?

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## Introduction: 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.

## Step 1: 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.

## Step 2: Sending and Receiving

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.

## Step 3: Checking Coherence - Step 1

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

## Step 4: Checking Coherence - Step 2

Then click on the Cursor Tab on the far right of the menu, or press F3 to activate the cursor.

## Step 5: Checking Coherence - Step 3

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.

## Step 6: Checking Coherence - Step 4

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

## Step 7: Checking Coherence - Step 5

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

Click OK

## Step 9: Checking Coherence - Step 7

Click Yes to proceed to Y-Axis, rotate tool, tap-test and check coherence again.

## Step 10: Causes of Poor Coherence: Too Much Wax

Too much wax on the Accelerometer can absorb some of the force and decrease the coherence. Apply just enough wax to adhere the Accelerometer to the tool.

## Step 11: Causes for Poor Coherence: Coherence Just Below 80%

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 practice taps and then click on the Measure button to take the measurements again. Make sure you are tapping with a solid motion and land solidly on the tool tip opposite the accelerometer.

## Step 12: Causes for Poor Coherence: Wrong Tip Hardness

If the default white plastic tip doesn’t yield 80% or higher coherence, the tool is long and flexible (low frequency) then switch to the softer tips. Black first, if that doesn’t work, switch to the red. If the tool is short and stiff (vibrates at a high frequency) switch to the hard steel tip. Use care when switching to the steel tip and reduce your stroke force by at least half. The software will indicate if you need to tap harder or softer. Be especially careful not to damage the edges of the cutting tool. Conversely when switching to a softer tip you may find you need to increase the force. Again, the software will let you know.

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