Filling Aluminum Extrusions

[LiamN (Liam Newcombe)]

Yep,

@Chris5

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.

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

Backlash, Deflection and Vibration

Firstly, Thanks to @Julien @gmack and @spargeltarzan for their comments and assistance, quite a few poor assumptions of mine were corrected during the process of this investigation and write-up. Obviously the remaining mistakes are mine alone. Here’s all the content below as video on YouTube, some of these things are easier to explain with the video, it’s broken up into 3 videos A zip file with the machine deflection models, this text as a single PDF and a word doc with the maths I used fo…

As it has some illustrations of what we’re discussing.

@Chris5

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.

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.