Filling aluminum extrusions has obvious appeal but epoxy granite is a decision you can’t really unmake and I’m not sure how much stepper motors like the weight it would add (on the x running rail) and also preserving threads and limiting your options to drill holes seems like a thing to worry about. Intuitively loose sand seems like it would actually be better for damping than epoxy granite but obviously worse for rigidity, but I’m also not confident extrusion flex is anyone’s bottleneck for precision. Sand seems like a hassle to keep where you want it and I had a much lazier idea that seems worth exploring, rubber mulch, which would take no effort to get it where you want and keep it there. It has some heft ~28lb/ft^3, but a lot less than sand ~96lbs/ft^2, this makes me think you could get away with a full fill which might be a plus because it seems like it would damp better when squished in. I am moving atm and new to cnc stuff so this so i haven’t tried it and I’m not an expert you should trust to evaluate how well it works. Just wanted to float the idea and see what people think, cheers.
I have always said fill with expanding foam to dampen. I am sure they tried this on the 5. Isn’t needed on pro.
It’s a very tempting idea, here’s some of the process I went through whilst thinking about it, which may be useful.
The vast majority of the stiffness comes from the outer wall of the extrusions. When I modelled the shape of the SO3 extrusions with their 5mm wall width, even making them completely solid only doubled the rigidity, with some fairly obvious impacts on cost and mass.
Regarding other materials, why machines use cast iron, epoxy granite etc. there’s a solid exploration of the materials properties on this page
Which includes this excellent chart
I’m assuming you’re considering this for one of the machines with linear rail bearings not V-Wheels.
If you’re looking at a belt driven machine, yes, you’d likely have to reduce accelerations to deal with the substantial additional mass in the X beam, the Y rails OTOH you could either fill, or just bolt down to the table the machine is on to reduce vibration.
Improving the damping in the frame initially seems highly desirable, but as was pointed out to me, the primary impact of adding mass to the system, without significantly impacting the rigidity, is to reduce the resonant frequencies of the system. This is likely to make for larger deflections under resonance modes in the machine frame.
I would only consider filling with a damping material at all on a machine with ballscrew drive, the deflections in the belt system (see my older posts on deflection and calibration) dominate those machines and I suspect the additional mass on the X rail would be as likely to make things worse as better, probably just giving a slightly different set of problems.
As for other ways to introduce damping into the Aluminium extrusions there’s quite a few things commonly tried
- Sand apparently has a strong initial effect, until it settles at which point it’s just additional mass making things worse
- Epoxy granite is hard to make well and generally seems more suited to a fixed gantry machine where the mass is also beneficial
- Foams are of little or no use as they are either rigid (therefore not absobing any energy) or too soft to usefully absorb enough vibration energy by converting it to heat (the same as when used in sound absorbtion they can only absorb treble)
- Visco elastic materials are promising, in the form of constrained layer damping or mass dampers, if you can get the correct visco-elastic material and the correct constraining layer or tuned mass you could knock out some key resonance modes, however this is beyond my simulation capabilities
HTH
Slightly off topic but managed to snag some of that 3M damping tape as surplus online for about 3% of the price from the useual reputable vendors. I am going to try some on some of the panels on my Nomad to try to quieten it down.
I’m confused again. Every time someone pops up with the idea of “dampening” the moving chassis components, I get confused as to what real problem, as opposed to a perceived problem, they are trying to solve.
It seems to me that the manufacturer has done all they can to reduce the mass of the moving components in the machine. I’m thinking that these basic hobby-type machines (that aren’t really) would not be suitable (because of other machine aspects) for any task that would benefit from these types of “dampening” processes.
Perhaps I’m just plain confused. 
I’m struggling to understand this, it seems like the deflection could only be less than the furthest swing of an undamped vibrating bit.
It’s more fiddling than problem solving, plenty of people polish firearm feed ramps without having a ftf, “if it aint broke fix it till it is.” Resonance is pretty much always bad even if it’s not a limiting problem and it can enter the equation to give worse surface finish and worse sounding cuts even when you stay well within the machine’s capabilities. Ofc you can adjust feeds and speeds to reduce this but i think it would be neat if dumping $10 of rubber inside made it cut nicely more consistently.
Durfill is another interesting material.
It’s a good question.
There’s an obvious desire to improve the machine to improve it’s performance, be that cutting speed, finish, precision or whatever aspect is bugging that user. Plus, a lot of us are just fiddlers who can’t help but try to improve things.
Some of the significant vibration modes which we see impacting cuts on these moving gantry lightweight machines are down to resonances of the moving masses on the flexing Aluminium frame. On the XXL machines it’s pretty clear when you’ve hit a resonance as you get a chatter that sets in quickly and needs a major change in feed rate, RPM or engagement to go away. These modes can be quite violent where the machine deflection is in a positive feedback loop with the cutter engagement, you know them when you see them.
It’s possible that in these cases, adding some sort of damper to the system would reduce the impact of these resonance modes and allow the machine to cut deeper, faster or with more consistent surface finish. Even relatively simple and cheap systems can work noticeably better with vibration damping, (or shock absorbers as we also know them)
As for reducing the mass, I’m not sure that’s really a design goal here, expect for components such as ballscrews where the motor torques required to spin them up and down can easily dominate the machine loads. Reducing mass for manufacturing and shipping is probably a larger concern.
In reality, as you suggest, how much improvement you’d get before hitting the next weakest link in the chain, well, there’s a good question.
Sorry, probably not well described.
When we have a resonance mode that is some mass, say the Z axis and spindle, oscillating back and forth on a spring, say the X beam. In resonance what happens is the force exciting the motion is cyclic and at a close enough frequency (or harmonic) to the natural resonant frequency of the mass on the spring that the system actually stores energy in this vibration. Think person on a swing, you don’t stop swinging the instant you stop moving, that’s the stored energy you put in to the resonant system.
For any given energy input to this system, if we increase the mass but keep the spring stiffness constant, the resonant frequency will reduce. As an example, thicker, heavier strings on a bass guitar, same length and similar tension to a regular guitar, but heavier and so vibrate at a lower frequency.
The other effect we’ll tend to notice is that the maximum displacement will increase with the lower resonant frequency, again, the bass guitar strings tend to move further when vibrating.
So, unless the additional vibration damping (energy absorbtion) capability is sufficient to very strongly damp the resonance, the added mass may well just move the resonant frequency down, causing the chatter to occur at a different cutter RPM, feed rate or engagement, and when it does happen, the amplitude of the vibration may well be significantly larger and a worse problem.
Where you’re dealing with non-resonant vibrations, such as those directly from the cutter, yes, extra mass helps a lot, but those don’t seem (in my experience) to be the vibrations that really limit what the gantry router can do.
In more traditional machine, say a bridgeport, with a head that only goes up and down on a column, additional mass tends to be highly desirable as it adds inertia and stability, whilst the ‘spring’ of that giant column is very stiff and resonance modes in the machine frame don’t really show up.
That makes sense, i appreciate you taking time to help me understand. One thing now occurring to me is that I’m not sure how much of chattery cuts is the aluminum extrusions and not say the spindle drive shaft (or whatever else), something that doesn’t transfer movement to the rest of the system completely enough to keep it from resonating at the frequencies it wants to. I’m not sure if you have any thoughts there.
An aimless aside; Im wondering about the possibility of part geometry that creates destructive interference at the resonant frequencies of the material its made of. I believe geometry aimed at creating destructive interference is used for sound control in some applications. An obvious limitation is that even consistent sound will usually be composed of multiple frequencies (i think*.) Aluminum has an exact frequency it likes to vibrate at so design could be based around only that. I would assume you don’t need to hit that exact frequency to get incomplete resonance but it would make sense to me if our (hypothetical and perfect) destructive geometry would proportionally oppose and still adding up to nothing for resonating vibration (in isolated perfect conditions ofc). I don’t think this would never make sense to seriously investigate applying on a $2-5k cnc but i think it’s an interesting thought.
It would seem to me that if the resonance is in the spindle added mass to the extrusion would reduce chatter/deflection because inertia would have it more stubbornly resist the partial motion that’s transferred, it seems like weight wouldn’t be a liability except in cases where the extrusion wants to resonate which i have no idea how often is the case
Yep,
NP, smart questions are always good, they frequently change your previous understanding.
There’s a nearly endless list of reasons why the machine cannot keep the cutter edge perfectly on the target track.
One is deflection in the cutter itself, the cutter, collet and spindle assembly can also resonate, on large industrial machines people make tests and measurements to find the RPMs, feed rates and cut depths that excite those resonances and avoid them in programming (generally with the help of software)
The spindle won’t be perfectly rigid and will have deflection, in the shaft, bearings and it’s mounting. Note that there will be axial as well as radial deflection here.
Then you have every linear motion bearing in the machine and the beam it is attached to which will all deflect under load.
Continuing, the spoilboard, workholding and workpiece will all also deflect during cutting, so the chain between the workpiece and the cutter is extensive.
If the machine is simply deflecting due to cutting forces then this would typically result in small issues in surface finish or cut accuracy. It’s really where some sort of larger vibration sets in, either through positive feedback between the cutter and the deflection, or an excited resonance mode, that things get bad and you don’t want to keep cutting in those conditions, as you’ll typically damage something sooner rather than later in addition to having a horrible cut quality.
This post may be useful
As it has some illustrations of what we’re discussing.
Even in a single Aluminium beam there are many resonance modes, front to back, up and down, rotational etc. When we then add a moving mass such as the Z assembly to the beam this changes the resonance modes by it’s presence and again by moving it’s mass around on the beam.
Unfortunately, when we assemble a stack of these things into a machine we find a complex set of modes, any of which may be excited and become the dominant vibration in the machine.
That’s what led me to thinking of putting a mass damping collar around the spindle attached with a visco-elastic damping material, to convert vibrations into heat as close to the cutter as I could get. I never got round to that experiment but would be interested to see if it worked.
We did some tests on an HDM a while back - to be honest, it made no notable difference to the quality of cuts.
Also, as a FYI it’s worth digging into the product spec sheets if you do decide to fill extrusions - some of these products are not suited for use with aluminum especially those concrete-based.
My tenative understanding was that uniform materials have a specific resonant frequency which i think of as a starting point. Then “geometry” affects how it presents, depending on how vibrations travel through the shape and meet each other (opposing or compounding) with things like location and impact of damping and rigidity being added variables. What i meant was starting was something that would start with fixed value the material resonance (and maybe conditional damping), and using some program to iteratively modify a simulative shape and project resonance, some kind of crude version of what people call ai (for some reason.) I’ve taken the shape as a modifying/compounding factor rather than an initial factor (that is you couldn’t meaningfully say what its resonance patterns are without specifying a material,) this might be naive but i dont know what i dont know.
Damping at contact points seems like it would have to always sacrifice rigidity, is this the case? Obv even if it is the benefits could outweigh minor losses there.
(I look forward to digging into the post you linked)
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