[vger]

Mike,

Sounds like you have a handle on it. As to finding the natural freq of the rotor in combination with it's driven mass (screw). Some motors have the resonance listed in the specs, but as soon as you add the mass of the screw (and coupling) that resonance changes. The more rotating mass that is added, the lower the resonance goes. Take a look at this one...
http://www.cnczone.com/forums/genera...resonance.html

You would have to take the measurement on the unit of course. Best way to position the vibration picup would be on the side of the motor with its sensitive axis in line with the rotation plane. Put the analyzer in peak hold and have the motor step single steps very slowly (maybe 5 steps/sec) and watch the resonant freq pop up.

As to the resonant freq of the donut... that would depend on the elasticity of the matreial it is held with along with the mass distribution of the donut. If the donut weighs 5 oz and has a heavy hub, it will have a higher resonant frequency than a 5 oz one that has a light hub and heavy rim.

Walky

Your idea of the couplers is not too bad. Never thought of using them that way. Using a colar on the shaft that has 2 or more equaly spaced pieces of small threaded rod perpendicular to the motor shaft would allow you to use nuts and washers as adjustable weights (make sure the weight and radial distances match) to tune it.

Steve

[wildwestpat]

Impressed by the cut tab construction but I think the lack of strutting will leave the beams lacking in torsional stiffness. If you can borrow a high power strobe light maybe from some disco friends and look for movements in the structure under strobe lighting you will see what is buzzing and what is actually panting with a larger movement. The strobe frequency and or the stepper motor drive is altered to get the strobe to show the maximum movements.

A lower tech way is to use a light dusting of talcum powder on the structure. The powder will eventually concentrate on the points of minimum movement.

Until the structure is reasonably rigid you may tune the machine for one work table load / position with adding dampers / anti resonance tuned mass only to find that the resonance comes back under different table / arm positions.

A lot of work has been done on composite materials for light weight spars in the aircraft industry. There are aluminum balsa wood aluminum sandwich constructed boards or honey comb filling in lieu of the balsa wood. You might find a 'friend' in the DIY plane building hobby with some off cuts.

Hope you can borrow a strobe light - regards Pat

[cncnoob123]

I have a question according to the dampers. Do you think that this damper has also an effect if the motor is mounted vertically and not horizontally??...I recognize some torque variations which occur exactly with the frequency of my micro stepper drive voltage, and I want to suppress these vibrations with this damper.

[MillworkMike]

Should have the same effect either way. I have one on my K2cnc Z axis motor which is mounted vertical and it made a BIG difference!

[watsonstudios]

Yes. I have also used them on my Z and does great quieting the resonance at higher speeds.

[cncnoob123]

thanks for the quick replies...my problem is that i have very small speeds (1 to 0,01 rpm), I hope this thing will show some effect in this areas.

[Vogavt]

thanks for the quick replies...my problem is that i have very small speeds (1 to 0,01 rpm), I hope this thing will show some effect in this areas.

I'm not sure you should be expecting much vibration at those small speeds. If you need a dampner device at that rate, I'd say you've got bigger issues than vibration.:stickpoke

[mcArch]

thanks for the quick replies...my problem is that i have very small speeds (1 to 0,01 rpm), I hope this thing will show some effect in this areas.

Sounds like maybe you need to tune the stepper drive. Could that be the issue? With Gecko's you can tune the driver by adjusting a trimpot to tune the microstepping.

What kind of stepper driver are you using? I'd check the manual for any kind of trimpot to adjust.
Cheers!
-Calvino

[cncnoob123]

you're right, there is a possibility to tune the driver voltage waveform. That solved my problems. Thanks..

[andrzej144]

Hi there. I have had resonance issues both on my lathe and on my mill. I did manufacture several wheels with rollers in cavities and many other solutions that helped a little but not completely. Than i gave up and decided to do something really crazy. On timing belt pulley's face of the motor (2" dia) i glued 5 mm layer of polyurethane foam from some package fill. To the other side of the foam i glued the original dial ring from the dismounted handle. And.... bingo! It is a provisional solution that survived more than 9 months of using and still going strong! So on my lathe (direct coupling from motor to lead screw) i just turned a brass ring Ø 50 with a 25 mm hole, mounted it temporarily with a teflon spacer on my motor coupling and filled the gap with Dow Corning 738 silicone rubber. A day later removed the spacer and mounted the motor to lead screw with "super coupling". All my troubles are gone both in low and high rev range.

[the.sniper]

Some pics would be nice.

Than i gave up and decided to do something really crazy. On timing belt pulley's face of the motor (2" dia) i glued 5 mm layer of polyurethane foam from some package fill. To the other side of the foam i glued the original dial ring from the dismounted handle. And.... bingo! It is a provisional solution that survived more than 9 months of using and still going strong! So on my lathe (direct coupling from motor to lead screw) i just turned a brass ring Ø 50 with a 25 mm hole, mounted it temporarily with a teflon spacer on my motor coupling and filled the gap with Dow Corning 738 silicone rubber. A day later removed the spacer and mounted the motor to lead screw with "super coupling". All my troubles are gone both in low and high rev range.

[andrzej144]

If you promise not to lough.....
here they are: the coupling with damper and timing wheel wit provisional damper

[BAMCNC.COM]

Wouldn't digital drivers fix this?

[the.sniper]

Great job! and thx for the pics!

If you promise not to lough.....
here they are: the coupling with damper and timing wheel wit provisional damper

[109jb]

Digital drivers are known to be better at combating resonance than non-digital drivers. However, there is a cost involved that some might consider substantial, especially for a hobby machine. The cost is not as great if you start with digital, but upgrading you basically have the digital cost plus the cost of the original analog drivers. I actually opted for the analog drivers on my new machine from the start because the digitals were over 2x the price. I'd rather spend the difference on tooling, and for my application there are plenty of people using the analog drivers with great success.

The dampers provide a cheap way to improve things for just a little time and some scrap material. On my first machine I just made some 3/4" thick 2-1/4" diameter aluminum disks to mount on the back end of the dual shaft steppers and it greatly improved things.

[Bartos]

Hi,
I'd like to refresh this post and put it on a more theoretical/experimental path.
First of all I'd like to refer to one of patents, that I've dug out recently, covering this kind of a damper - Patent US5295411 - Torsional vibration absorber system - Google Patents (pdf available)
Most important IMO part of it, is an equation N^2=rw^2*r3/[(rg^2+rw^2)*(r1-rw)] where
N- "number of torsional disturbances per revolution" or (I believe) simply speaking harmonic frequency
rw - radius of rolling element
r1 - radius of the cavity
r3 - radial distance of the center of cavity from the center of the rotating shaft
rg - radius of gyration of the roling element - for a full disk like mostly discussed here, it is 0,5*rw^2

if you go to column 6, line 49 in pdf there is the equation with exemplary dimensions for N=4 (pay attention to the fact that some variable are given as diameters and not as radii).

I'd do the research myself and share my findings but I'm still couple months away from having my own machine (design is mostly done).
Does anyone have an accelerometer and an oscilloscope to measure harmonics of an normally operating stepper and a resonating one?

Maybe people that are using this damper could share their dimensions and calculation to what harmonic is it tuned and to what extent it helped

Best regards
Bart

[Vogavt]

I haven't look at the PDF yet but those that have smart phones can download a free accelerometer app and somehow affix their phones to the stepper's side. Some apps will let you store the data which can be imported to a spreadsheet for graphing/plotting.

This resurrection comes at a good time for me!

Sent from my DROID RAZR using Tapatalk 2

[lasersafe1]

Bart,

There was a time that I thought vibrational dampers were needed. Now that I have switched to 32bit DSP digital stepper drivers, vibrational resonances simply do not exist. I would have never believed it if I hadn't seen it first hand on my own mill. I had very high quality Oregon Scientific stepper drivers capable of 7 amps. These cost several hundred dollars a piece in their day. They were based on 1980's technologies. Needless to say, my mill was slipping poles when during X rapids so I purchased a new driver and I was going to upgrade the stepper motor to a 500+ oz-in NEMA 23 motor. I purchased the new KL-8056D Digital driver from Automation Technology. The first thing I did was swap from my Oregon Scientific driver to the new digital driver. ABSOLUTELY AMAZING RESULTS! The slipping is GONE and the motor is practically silent. It no longer whines and hums when it moves. It sounds like it's a servo. The DSP in the driver is measuring reflected voltages from the motor and actively compensating the drive current during each step (or microstep). To make a long story short. No need in the world for a vibration damper in my opinion.
John

[Vogavt]

Makes sense if funds are available. Not so for some.

I too upgraded my cnc4pc c11 to a Gecko 540 and saw a difference but not as much as adding the rotational dampers.

Gecko 540 is smoother though.

Sent from my DROID RAZR using Tapatalk 2

[Bartos]

lasersafe1
1. as Vogavt said - not everyone have funds
2. some people might want to just play with this idea and fix it themselves - and from the amount of posts you can see that a lot of people did it
3. as I wrote in previous post - I want to put this thread on a more theoretical/experimental path just for fun and learning experience

Anyways, I appreciate your information on digital drivers - I didn't know about them and now I will consider it for my machine.
I've checked their webpage and it looks that they are selling same thing as kelinginc.net which means that it's IMO Chinese stuff, which I'm slightly afraid to buy - but I will read about it.

For the dimensions of a damper given in one of the first post, the damper is tuned for around 7th harmonic

regards
Bart