Experimental Vibration Analysis of a WM280 Lathe

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... so a decent mechanical or electromechanical clock should be adequate for checking that

Seriously; it‘s interesting to consider what level of accuracy and stability is being achieved when a clock’s rate is within a few tens of seconds per year; and how one might reliably check/predict that in a short-duration test.

MichaelG.

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Edit: posted before I had seen Dave’s response.

Edited By Michael Gilligan on 19/06/2020 10:21:55

Dave, thanks for the reply.

Fascinating, I had not considered that level of measurement to check the operation of a clock.

Adrian

My accurate Clock reference - generates a 10MHz clock, typical accuracy to 10^-13.

Uses an 'old' HP voltage controlled ovenised crystal oscillator, with a phase locked loop, locked in essence to the GPS 1PPS signal, with a many tap IIR filter to help. The oscillator is from a defunct HP Cesium Beam standard - the Cesium tube now dead..

The GPS time can then be set to UTC, and then the 1PPS synchronized to UTC, derived from the stable crystal clock. Generally to better than 0.1 PPT...with jitter seemingly better than 1PPB - I cannot measure to better than than 1PPT, so...

With a Nucleo of course..

Waaayyy of topic now...

Joe

Very good! Us propeller-heads can admire the specification and the I and Q outputs while the rest of the world fall for the colour screen and Meerkat!

Loveable Russian Meerkat's sell insurance in the UK; I like to remind people that Meerkats eat their young!

Dave

Slow progress waiting for parts to arrive, time-wasting boobs and many 'Person From Porlock' visits!

Anyway, the first accelerometer module failed because it processes data to remove vibration, making it great for stabilising a quadcopter and useless for detecting vibration. So I bought a simpler module allowing its sensors to be read directly and transferred the whole project on to a RaspberryPi3B+, cost about £35. This is a small general purpose computer, considerably more powerful than an Arduino, not ideal for time-sensitive electronic control projects because it's multi-user/multi-tasking, but - unlike a PC - it exposes 40 General Purpose Input-Output (GPIO) pins suitable for electronic projects. (The closest a PC ever came to providing a hobby friendly interface is the obsolete parallel printer port.) The 3B isn't as fast as a Pi4 but it uses less power, better if the computer is run off a battery in a messy workshop.

The RaspberryPi GPIO is far more delicate than an Arduino so take care - over-volting or drawing excess current might brick the whole computer, which is why PCs don't let you do it! On the plus side, the Pi has a full Linux OS with all the tools needed to capture data, crunch the numbers, and display the answers.

The Pi3 and MCU6050 module are connected together thus:

The 10nF, 10k and 2k resistors are needed to suppress electrical noise which in my workshop are enough to crash the I2C data link (SCL and SDA). Arduino I2C is relatively bomb-proof!

In operation Pi & sensor are plonked on the headstock and powered on. Note the sensor is weighted down.

The PI connects to my home network over wifi so the program can be started remotely. It reads the sensor and - when a button is pressed - logs a stream of X,Y & Z accelerometer values to a time-stamped file. Pressing the button again closes the file. Many timestamped logs can be taken. A second program reads logfiles, does the maths, draws the graphs, and saves them as a jpg:

The graphs suggest my lathe doesn't have a vibration problem at the frequencies detected by a MCU6050! The signal graph shows the module detecting small movements when cutting metal, worst when I hit the shoulder at the end. The maximum frequency detected by this set-up is 100Hz because the module can only report 200 samples per second.

The FFT analysis shows no big frequency peaks. And the zoomed in signal looks like noise: random small amplitude vibrations in all directions.

Neil suggested balancing a penny as a simple way of detecting vibration and my WM280 passes this test too, at any RPM:

And on the saddle, even when cutting with power traverse.

Python3 programs here.

mcuaccel.py reads the sensor. The import modules used should all be pre-loaded with the Pi Rasbian operating system.  The log file is human readable.

mcugraph.py does the analysis. It requires scipy, numpy and matplotlib to be downloaded and installed from the command line with:

sudo apt install python3-dev
sudo apt install libffi-dev
sudo apt install python3-matplotlib
sudo apt install python3-scipy

(Installing scipy should also install numpy)

Next steps - follow up on earlier comments made by Andrew, Joe and Leslie and try some different sensors! Conclusions so far: it's possible but may not be worth the effort!

Dave

Edited By SillyOldDuffer on 29/06/2020 12:35:54

This project ain't dead yet though I may pay someone to put me out of my misery! It's not going well

In order to 'try some different sensors', I decided to build a vibration table. My lathe isn't an ideal test-bed because I don't how badly it vibrates, if at all, or at what frequencies. The vibration table is a loudspeaker in a tin box that can be fed whatever test frequencies I want. The complete set-up:

And in real-life:

First sensor up is this MEAS device kindly provided by Leslie:

From the factory this model is weighted to resonate at 200Hz so fun with maths to investigate lifting it.

Thanks to Duncan Webster sorting out the formula for me I can estimate the weight needed on Leslie's MEAS sensor for a particular resonance. Removing the original brass weights but leaving the rivet behind should give about 355Hz - about right.

Measured with the sig gen and oscilloscope, on the vibration table I actually got 415Hz.

Alas, woe and thrice woe, all is not well!

1. My vibration table (posh name for loudspeaker inside an old biscuit tin), has it's own resonances. Ideally the table should be acoustically flat and it isn't. Measurements are suspect.
2. Although delightfully sensitive to bumps the MEAS sensor is insensitive to regular vibration at audio frequencies, except at its resonant point and third harmonic. Not good as a sensor detecting vibration equally across a spectrum.
3. My oscilloscope's FFT function detects nothing. Must be doing something wrong
4. You would not believe the petty hassles I've had. One example, it took longer to sort out the audio amp's jack plug and socket than to build and box the amplifier.

Never mind, I can now compare various sensors in comfort to see which is best for this application.

Few other observations. The MEAS sensor vibrates faster than the input signal. The sensor detects a heightened response at at least 22 frequencies between 100Hz and 3.1kHz : this may be due to biscuit tin resonances. When vibrated with a square wave, the sensor detects square at low frequencies, and triangular waves at most others. Resonance and the third harmonic are both detected as pure sine waves, presumably because the sensor is vibrating in tune.

Taking much longer than expected with no useful result so far...

Dave

Sometimes too much information is useless because you don't know what to listen to.

25 years ago I was a machine tool designer making ultra-precision machine tools that produced optics where the material billet cost more than my annual salary. We were testing a brand new machine tool when a very clever colleague walked up, quoted a musical note, recited its frequency and walked away. I am sure our faces where something to behold.

Nowadays we all have instrumented hammers and the kit quoted in this thread.

The important frequencies are shown in your surface finish, try and estimate the number of wobbles in the surface around the circumference (or per mm), work out its frequency. That is the one that matters.

Almost perfect? Use a screwdriver to listen to the toolpost, compare to a frequency generator e.g a piano. the piano has frequencies from 27 to 4k Htz, its unlikely that you have a frequency driver outside of that range for normal machining, although possibly at the lower end. Then move around the lathe looking for where the sound comes from, either listening or tapping things to find their natural frequency.

I know all this stuff is very old-hat but it might at least verify which is the biggest "problem".

Good advice Roger! Ought to explain though that I'm not solving a particular issue with my lathe. Rather I'm exploring the possibility of plonking a magic box on the headstock which analyses the racket coming off any lathe and automatically points a finger at dodgy bearings, gear-train, lead-screw or motor etc.

Buying second-hand appearances can be deceptive. A grubby battered looking old lathe may be in much better condition than one tarted up by Coco the Clown for profit. Or the clean machine might be the real deal. If it can be made to work, my magic box would allow an inexperienced purchaser to identify potential lemons.

One of many obstacles to progress is I expected my noisy Chinese lathe to vibrate. As far as I can tell it doesn't have any significant vibrations, which makes it a poor test case, hence the need to build a Vibration Table.

The project is in four parts:

• Find a suitable sensor and capture vibration data from a machine, (data capture works but I'm struggling to find a suitable sensor without spending lots of money.)
• Analyse vibration data by applying a Fast Fourier Transform (working)
• Draw Graphs identifying abnormal frequencies and amplitudes. (working)
• Link frequencies to causes, such as motor speed, gear ratios, shaft rpm etc. ( not started )

Results so far suggest 'too much data' is a major problem. I'm now thinking it would be better to go for an electronic stethoscope that the operator applies to suspect parts like bearings one at a time. I knew it was ambitious before starting, but apart from the computer side, I'm mostly out of my depth. Quite Interesting though!

I agree completely about surface finish!

Dave

Edited By SillyOldDuffer on 01/08/2020 08:45:27

Good to see you progressing with this, Dave … but:

May I suggest that you re-engineer your loudspeaking shaker ?

The actual modification would depend upon the donor loudspeaker, but in essence you should replace the cone with a ‘spider’ ... this keeps things nicely linear, and filters-out most of the irrelevant noise.

MichaelG.

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Hint:

Yes, please do suggest re-engineering the shaker, and thanks for the patent drawing. Deciding how to make the cloverleaf will keep me amused for a while, and as for replacing the cone? Now where's my Stanley Knife...

Hi-Fi Buffs look away. At the moment the speaker is glued directly to the biscuit tin with no consideration of efficiency or acoustics. As a speaker enclosure, nought out of ten.

This is how engineering projects spiral out of control. To find a suitable accelerometer, I have to develop a Vibration Table / Shaker, which is yet more technology I don't understand...

Dave

LDS actually kept the cone in that one ... cunningly stifling its Audio output by gluing stuff to it.

**LINK**

https://worldwide.espacenet.com/patent/search?q=pn%3DUS5641910A

MichaelG.

How about a sawtooth wheel driven by a motor with a spike on the tin lid hitting the teeth? I think that's how klaxon's work, that should give you a dominant frequency with lots of noise (literally as well as electrically). I suspect ripsaw teeth are the way to go, but no evidence.