Sometimes the best information that you can get about your machining process requires your sense of touch to feel the vibration.
In many shops, the definition of a problem is “something that is not easily seen.”
Unwanted vibration in our machining processes can cause variation, unsatisfactory surface finish, and dimensional variability. Unwanted vibration is not easily seen.
A friend who plays fingerstyle guitar sent me a link to a video that got me to thinking about the role of vibration, harmonics, and chatter in our shop processes. The genius of the video is that it makes something not easily seen- easily seen!
The video is convincing that the sounds we hear are tied to a wave form on the string.
The sounds that we hear from our machining processes similarly are tied to a ‘harmonic’ or standing wave vibration in the workpiece or in the tool and machine. Sometimes, this sound is lost beneath the noise of other processes in the shop.
So when a machine operator complains about variation while machining, I ask him to lay his hands on a safe part of the machine and tell me what he feels.
If it is more like a Rock Concert than a smooth constant vibration punctuated with noticeable jolts from the indexing, I suggest that rather than looking at material, we look at the machine itself and the rigidity of the tooling as the likely cause of the variation being experienced.
- Mass in machines is used to attenuate this kind of unwanted vibration.
- Wear in machines or any kind of looseness can decouple the tool from that mass, promoting vibration, and thus variation.
In the guitar photo and video, the displacement of the string appears to be far greater than we could imagine possible.
And it turns out that it is an illusion created by the sampling rate of the CMOS camera ‘shutter’ in the iPhone Camera.
Regardless, it serves us a great picture of the unseen and reminds us that in noisy shops, feeling a vibration on a machine may tell us more about the process than our control charts or reading of the tools.
Arthur Zahl says:
Great post, Also sometimes over the course of time and incremental changes in pitch operators can been numbed to the changes in the sound of their machine and not realize there may be a problem on the horizon.
speakingofprecision says:
Great point Arthur. We were just talking about the longer term changes that you notice after being away for a week or so. And when we are reassigned, how it takes a while to get the “feel” of the machine that someone else has set up. (Or failed to set up properly, requiring us to make constant adjustments to stay in compliance with the print.)
Horace Tucker says:
Ideally, we should hear very little or nothing, other than the sound the metal makes as we are cutting it. This tends to be a very high-pitched “white noise.” Any steady pitch is typically in the material, and can often be a sign of incorrect feed rates or depths, or a lack of coolant. Remember that a violin string vibrates because the bow rubs across it without cutting. Friction causes orchestra concerts.
We all know the a follow rest reduces deflection on long thin stock. Deflection can be thought of as simply a very low frequency standing wave. Where the frequency is higher, in the audible range, it just means that the extent of the deflection is less, for the same energy input, because the frequency and the extent of the deflection are inversely related.
This holds true as long as the machine is significantly more rigid and robust than the stock. But if the stock and the machine are more equally “matched” then the machine itself may be the cause of the sound. Then things get a bit more complicated.
I hope this helps some.
speakingofprecision says:
Great comment Horace. Thanks for sharing your insight about deflection as a wave…