In regards to taking-out/changing the vibration freq/ resonance [sp] of a structure. Has anyone ever tried putting something inside the structure to stiffen or improve/change this? I'm thinking of my situation in particular with hollow tube structures. Would adding some type of pourable, expanding foam increase or change the resonance?


I don't have my machine up and running yet but, in my current construction phaze, it would be easy to either add it, or make the nesc allowances to add it later. If I do it later, however, it would have to be some type of pourable product to get it in there..

Of course, adding weight is not hugely desirable..

Jerry [just curious more than anything at this point..]

Low shrinkage grout, packed sand, lead shot, concrete, low shrink epoxy - all have been used for vibration dampening in industry. Using foam would not have much impact on dampening vibration.

Scott

I'd thought of all of those, however.. the weight factor comes into effect w/ each of them. I was thinking foam simply because it will add some stiffness yet its light. [I assume it would help w/ the stiffness simply from seeing other products use it.. in baking furnace wall panels for example..]. I know that typically to change the vibration or resonance of a structure they usually add weight, but.. adding a couple thousand pounds to my gantry isn't something I'm too excited about :)

Jerry

As Scott says various things have been used. You are never going to get rid of resonant frequencies in your structure but you want to avoid exciting them under normal conditions. One approach is to make things really heavy so they are below normal operating frequencies; this to a large extent is why cast iron works well but it means lots of weight. This is primarily the effect of grout, concrete or very tightly packed sand; the weight is just being increased but a secondary effect is the packed in material dampens the drumming of flat surfaces. Lead shot also increases the weight but when it is loosely packed it can also dampen vibration because the shot rattles around. Maybe rattle is not exactly the correct word because it doesn't bounce off the walls but it moves and absorbs and dampens vibrations over a range of frequencies.

With all these you cannot avoid significantly increasing the weight but there is one vibration damping method that does not increase weight as much and that is to attach weights using rubber mounts with the stiffness of the mount and the mass of the weight chosen for the location it is mounted. Crankshaft dampers on engines which have a metal ring mounted on an elastomeric ring are an example of this. These work because the resonant frequency of the attached weight in conjunction with the stiffness of the rubber is differs by a fractional multiple from the resonant frequency of the structure at that point. I don't know if this method is used for the structural as opposed to rotating parts of machine but cannot think of any reason it should not be. It is used on a large scale for damping the vibration of buildings in earthquakes and the smallest application of the principle I know about is on a pedestrian bridge over the Thames in England.

Interesting.. well.. I think for now I'll just have to continue the route I'm on. I'll see when its running what is what.. If it becomes a problem in the future.. I'll address it then..

Jerry [btw Geof, I've got all my stuff in order to get the rails potted on now. I've got all the jig's D&T'd, the substructure is all drilled.. I've just got to hatch up the mounting surface so the epoxy has something to stick to on the sub structure..& finally.. wax the bottom of the rails and potting fences.. then its setting the rails in place.. adding the potting fences and jig's and start putting the epoxy in.. might even have it ready for epoxy this weekend..

Making progess slowly.. :D]

Jerry -

Please photograph this procedure (potting) for others to see!

Scott

.... If it becomes a problem in the future.. I'll address it then..

That is the best approach.

Regarding your potting with tons of gooey stuff. I agree with Scott - pictures would be nice. Mount your camera on a tripod and buy a wireless remote for it and every 30 seconds or so push the button. I have a vision of a camera thoroughly potted in epoxy; not a nice vision.

I have potted many things over the years - prior to my involvement with this forum. I have potted mounts for large diesel engines, prop shaft bearings, machine bases and structures and even made prototype molding tools from potting compound but I have not ever been in a position to take photographs nor have I had the need to until my involvement here. I have never seen a good pictorial on here about how it is done, the steps taken, it's purpose, etc, etc and I just thought maybe this would be a good time to make something happen - thanks to Jerry, that is!!!

Scott

I'll most definatly be taking some pictures of the proceedure. I don't kow if the 30sec interval thing will happen but some will be taken anyway.. The results will also be documented.

I've attached a PDF that is a simple "How To" of what I'm planning on doing.

Jerry [How did we get from vibration to potting rails!?? :p]

You started it, Jerry!

:D

Scott

BUT MOMMMMMYYY......... :p

J

Hey Jerry , I,m in the same boat as you except my milling machine is already built. My column is a 2x3x 1/8 hollow tube. This I plan to fill with a grout/cement. I used to work construction along time ago and we did this job of removing the grout off theses steel column feet(HUGE) anyway I remember swinging a sledge hammer(big one) to break up the grout and it just bounced off the grout, you could not chisel this stuff, it was as if it melded with the steel column and turned to steel. We ended up grinding it off. I never knew what its actual name was but thats the stuff I,m hunting down to fill my column. What do I think its impact on performance will be by filling it? I figured a 3-5% increase in performance of the mill as far as rigidness goes and every % counts in my book. Heres a link to my never ending project

http://www.cnczone.com/forums/showthread.php?t=18992&page=5&pp=15
CNCzone.com-The Ultimate Machinist Community - My Manual small mill project

Hey Jerry larry here.
I like the foam idea.Dont know about the resonance but it will stiffen the tubes a lot.You built an aircraft so I assume you have an understanding of what lite weight cores can do. Here is a site selling composite materials{really cheap epoxy]They have polyurethane foam in 2lb to 16lb denisity 8lb is similar to softwood lumber 16lb is extremely strong.
Be ing nubeee I dont know how to link so here is the address WWW.shopmaninc.com
Larry

I have something to add to the dampening issue. I used to Machine Automotive Torque Converters for a place that made better than Stock (OEM's) and High Performance Applications. After the Core (body of the converter that is actually seen) was Cut open using a manual Lathe, I would bore a set of jaws to grab the HUB. While holding the Hub the Face [only .095 to .175 in the best cases] and the O.D. were machined. This forms a problem because it makes a very nice Bell. The first idea was to pack it with a foam insert using the Tail Stock [specially machined by me because we needed one that was 18' extended]. That worked for a little while during slow production. When things picked up this method was no longer productive.

The next plan was even better. I wound up using the CNC Mill as a CMM to plot points of the Various Converters. Using the information I created spline contour shape. The various shapes were cut into the jaws with the wonderful G73 Rough Forging Type Cycle [First and only time of use to date]. Then a new Tail Stock Attachment was made with a Large Bull Cone Nose and extension of 18". Using both of these Plus a Rubber Bungee Cord taylored to a specific diameter Worked Like a Charm. No More Vibration and I increased the Production almost 250%. As a note I had to set the Chuck in the Reverse Clamping Mode to allow the Core to Float to it's True Center for Machining. In other words you didn't have to open and close the jaws because the Tail Stock held everything in place for machining.

I'll see if I can dig up some video footage. That place was a great learning experience and allowed one to really open the Imagination.

Wow.. lots of help here.. Zip: I'm in a bit deeper in that, adding some type of concrete to my machine would add in the neighbourhood of 1500lb's of weight.. thats a bit more than I'm comfortable w/, it would reduce performance by at least 50%.

Larry, thanks for the link.. I've actually looked at this stuff for making some airplane parts.. it works great for making engine cowl's and the like. It was what I had in mind when I started this thread :)

Toby, I'm not totally following you on your description.. and video would really clear things up!

Thanks guy's

Jerry

Jer
I am not an engineer but have had 30 years expierence with fiberglass epoxy and foam all relating to loudspeakers.speaker cabinets have lots of resonance and i control it with high denisty foam in hollow cavitys.16lb foam in steel tube would have no resonance but who cares,the increace in stiffness would be incredible.25 years ago we put 2lb foam in a volkswagen door.IT WAS INDUSTRABLE.{ROLLUP WINDOWS NO GOOd.Also rising foam is about 20psi if confined eg foaming a pannel 4'x8' would require a 40 Ton press to hold it together. Steel tube would have no problem with 20 psi, what happens is the urethane bubbles break on contact with the steel resulting in a thin layer of solid urethane,greatly in creasing stiffness.Again ME NUBEEE know alot about nothing.Why are you still up?It is 12:00 here in toronto what time zone are you in?Already had 2 beers , time for bed, sorry for spellinc errorz.
Larry

Wow.. lots of help here.. Zip: I'm in a bit deeper in that, adding some type of concrete to my machine would add in the neighbourhood of 1500lb's of weight.. thats a bit more than I'm comfortable w/, it would reduce performance by at least 50%.

Larry, thanks for the link.. I've actually looked at this stuff for making some airplane parts.. it works great for making engine cowl's and the like. It was what I had in mind when I started this thread :)

Toby, I'm not totally following you on your description.. and video would really clear things up!

Thanks guy's

Jerry


My post is just to show that even though things look impossible right now, that there is a solution, and I'm sure you with the help of everyone here will find one. ;) Kind of a boost of sorts. I'll post the video when I find it. That's 300 hours of video documentation :eek:

Jerry one footnote
DONOT know much about plasma or whatever your machine does but be warned urethane foam,if subject to burning is highly toxic,gives off cyanide gas. Incase of fire___
____RUN like HELL
LARRY

Hey Larry, thanks.. there was some points in there that I didn't know about :)

I'm temped to just leave my structure the way it is, w/ some ports from which I can induce the foam, then in the future I can put it in and see what differences it makes. Might be a fun experiment. My machine is primarily intended for milling/routing so head shouldn't be an issue. Even if I do use a plasma or torch of sorts on the head at some time.. The structure will be sealed so it shouldn't really matter if there is an open flame source nearby. Interestingly enough, at work we use a spray on foam for insulating buildings [which we build for oilfield stuff] and we tried a small pc to see how fast/well it would burn up.. we couldn't even get it to sustain a flame.. used a propane torch.. as soon as the torch was removed the flame went out.. I'm not sure that it's urethane based but.. I guess not all expanding foam's are the same.. just some food for though..

Toby, I can't wait to see it, I appreciate the vote of confidence, I've over come enough obstical's that I'm sure, somehow, someway I'll get it done the way I need it..

Jerry [A little behind out west here.. but not as behind[er] as out by Geof :p.. I'm in Sask Larry..:D] {sorry Geof.. I just COULDN'T help myself :cool:}

Well.. it's only been well over a year since I posted in this thread. Lots has happened in the last year but what I suspected would be a problem, has come to pass.

I have a bit of a vibration problem.

Now, it's not where I thought it would be [the gantry] instead it's in the Z axis post. I say 'post' because my z axis is a 6 x 6 .250 wall HSS section. It's ~50" long and makes a GREAT trumpet... or was it a trumbone..? Anyway... it will go into and out of resonant/ harmonic notes at various router RPM's. I'm tossing around a couple ideas and figured I'd bounce them off ya'll..

Hu mentioned to me about possibly using some rubber and using that to dampen things.

Here's my plan.. [Not to far off what Geof mentioned really]

I'm thinking I'd use some hockey puck's and suspend a pc of 3/4 <-->1" plate between them, up inside the tube. I's just wedge it all up there and retain it w/ some wire through a hole at the top.. It's not the most ellegant.. but I expect i'll be getting a cast iron Z axis in the near future... so it just has to work in the interm.

Does this sound like something that would actually dampen vibration? I'd also thought about making a foam core [3" PVC pipe filled w/ foam.. and then remove the foam after it harden's into a "round bar" of foam] and put that inside the post. Then pour in some non-shrink grout around it and let that harden in there.. it'd be heavy but...

I also wondered about filling the bottom 1/3 or so w/ lead shot.. that also would be heavy, but might work best of all?? At least it's removable if it doesn't work..

Which d'ya'll think would work better/best??

Curious..
Jerry

Forgive me for mixing threads. What if you put a thin tube inside the structural tube. The "inner tube" would have clearance between it and the inside of the structural tube. Inject silicon rubber caulking (or?) between the tubes thru a pipe fitting (s) using something like a grease gun till filled to required amount. Could keep weight/inertia down to acceptable level.

Just a thought.

Forgive me for mixing threads. What if you put a thin tube inside the structural tube. The "inner tube" would have clearance between it and the inside of the structural tube. Inject silicon rubber caulking (or?) between the tubes thru a pipe fitting (s) using something like a grease gun till filled to required amount. Could keep weight/inertia down to acceptable level.

Just a thought.

Go back to the other current thread about vibrations I posted a pdf. You are describing constrained layer damping.

Geof, Excuse me for not giving your pdf credit but that is why the mixing threads thing was there. I read that paper some time ago when first mentioned in another thread. I was only thinking of a practical way to accomplish it here.

... that is why the mixing threads thing was there....

I don't mind mixing threads; us old guys have good memories and can keep things straight.:D The pdf I posted was a different one actually.

Just curious, has anyone ever tried using "dynamat" (typically used in car audio applications). It's a fairly heavy rubberized adhesive mat that you apply to floors and doors. It really takes any vibration out. Metal that used to "ding" when you tap it would have a distinct "thud" afterward. I realize this is relatively light compared to the machines used here...but it might help.

I'm having a similar resonance problem with a cheapo lathe I'm trying to make work so I've been considering this for a while.

I also remember watching some "bike build off" show where someone who ran a large CNC lathe to turn grips of all things (metal grips - wierd) - anyway he was distributing his own product that was some type of rubber-type material that went around the stock on the outside of the bore (opposite of the cutting side). It removed the resonance and allowed them to turn at twice the previous rate. Anyone hear of this?

I don't mind mixing threads; us old guys have good memories and can keep things straight.:D The pdf I posted was a different one actually.

Old Guys???? Your not Old, LOL but you have a ton of information stored. Thanks for that PDF

Cheers!!!!:D

Thanks for the comments guy's I'll close this off here and post my replies on the other thread.. this will keep things all going through one channel.

Jerry

A great way to dampen tubes without increasing the weight too much (which will lower the resonant frequency, which is generally bad on a machine tool) is to create a "tube within a tube" and separate the tube with a viscous fluid such as oil. The principle is as the tubes bend they shear against eachother and the oil takes out the vibrational energy.

Search for Alexander Slocum Constrained Layer Damping. He is an MIT professor who has come up with some good design tricks along these lines.

Another way to provide damping, as previous posts have eluded to, is to fill the tube with polymer concrete. Try ITW Philadelphia. I have done this quite successfully.

Let me know if I can be of further help.

Slocum rules!

If you want a quick epoxy mixture, find some barium sulfate powder on eBay, then go to you local Home Depot or Lowe's and buy some pool filter sand.
Hop over to http://www.uscomposites.com/epoxy.html and buy the 635 2:1 THIN EPOXY SYSTEM.

The barium sulfate adds mass making the machine heavier; you can leave it out if not. ITW uses it in there polymer concrete as well.

Figure out how much material you need to fill the bottom of bed by attaching some forms, or perhaps there are cavities that can be filled, or do what sc_brian suggests.

Mix up the sand (and barium) with pre-mixed epoxy until it's about peanut butter, then scoop it in and let it set. Little vibration would help settle things nicely as well.

That's a quick polymer concrete recipe and it doesn't shrink.

If you really want to get heavy use tungsten powder instead of barium sulfate. But that depends on the resonant frequencies you're trying to eliminate.

Scbrian&Jhudler we were running two parallel threads and moved to thread "eliminating vibrations".
Larry

Yeah I saw that after I posted! DOH!

thanks for the information given.