MK2: moving gantry router for metal cutting - comments sought!

[driftspin]

Hi folks!

I have not designed anything yet as I still struggle to decide which design route to take. Your help here will be really appreciated!

What I want:

- a machine capable of milling aluminium with a decent speed and surface finish
- a cutting area of about 600 x 900 mm
- to be lightweight, with a small footprint. To be able to move through a 80 cm door (partly disassembled).

I was starting from Jonathan and Routercnc designs. Then I tried to find ways to add damping and gantry stiffness without adding a lot of weight.

The easiest and fastest design to build is Jonathan's frame with Routercnc's gantry , all welded RHS steel and epoxy levelled. But those RHS left empty look so... 'not professional'. The stiffness isn't great and they ring like a bell.

For the frame, filling it with sand (after moving/assembling) looks like a good idea.
The gantry needs to remain lightweight. What could be used to fill the RHS to add damping, stiffness without adding to much weight?

This aluminium foam looks so sexy.. But how is it produced? How complicated/expensive? I couldn't find an answer yet.


And here's an article about it: Vibration Damping Analysis of Lightweight Structures in Machine Tools

Another idea is filling them with foam concrete.
But in both cases the filler will shrink by cooling/curing and maybe detach from the steel walls.

There are other damping techniques/materials but do we have access to them? http://www.aspe.net/publications/Ann...IS/BAMBERG.PDF

Then I compared the stiffness of rectangular hollow section steel vs. aluminium t-slot profile.
Adding a 500N force in the middle in a supported two ends configuration, 160 x 80 x 5 mm x 1 meter RHS steel (18 kg) will deflect 0.1 mm and the 160 x 80 mm x 1 meter aluminium t-slot profile (13.5 kg) is deflecting only 0.012 mm. I have no idea which of the two is better at vibration damping. The only advantage that I see for using steel for the gantry is the price, 5-6x cheaper than aluminium extrusion.

Most of the 'real' metal cutting machines are using grey cast iron which has 20x or more vibration damping capacity than steel. And it won't bend but break.
Iron casting with lost foam technique isn't that complicated/expensive. But how strong should a 1 meter gantry be? How heavy will it be? I couldn't find any information on cast iron gantry design...

I am just wondering what material is Datron using for the gantry of their M8Cube CNC?

Here is another way to add damping but the cost is far beyond the budget of a DIY cnc. https://www.machinedesign.com/bearin...on-linear-axes

What do you think? Any comments are welcomed as I'm lost in so much technical information/alternatives!

Paul

Hi Paul,

Are you sure about the stiffness you describe above.

As a rule of thump, steel is more rigid by a factor of 3 for a given shape at the same weight.

Box section should be more rigid than tslot profile..

You explain above the tslot alu 13.5kg is more rigid than the steel box at 18kg....

I am not saying you are wrong... it just doesn't sound right.
Please check to make sure.

Also weight is your friend preventing vibrations.


I know it is hard to make the choices.

If you have access to tools and skill to go the steel frame / epoxy route i recommend it.

Grtz Bert.








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[m_c]

Hi Paul,

Are you sure about the stiffness you describe above.

As a rule of thump, steel is more rigid by a factor of 3 for a given shape at the same weight.

Box section should be more rigid than tslot profile..

You explain above the tslot alu 13.5kg is more rigid than the steel box at 18kg....

Aluminium is stronger/stiffer than steel for a given weight. By volume steel is stronger, but not necessarily stiffer, as Steel generally has more elasticity than aluminium.

Off course that's a very general observation, as strength will depend on the exact profile/loading/application.

[AndyGuid]

. . . . . you should be very interested by this build series https://www.youtube.com/playlist?lis...BoMfvQ8Ttsq-57 (as well as by all other videos from this guy!)

Thanks for that link, I spent most of Sunday watching that guy's videos!!!

[paulus.v]

Thank you all for your input!

I'm intending to add a layer of foil-faced Tecsound damping sheet inside my gantry extrusion, I've found it very effective in the past on metal enclosures. I may also try some expanding polyurethane foam, either on it's own or in conjunction with the Tecsound: just need a bit of time to experiment!.

Tecsound looks promising. Not sure how different could be compared with the ISODAMP C-1002 mentioned in the MIT paper. A friend is soundproofing cars for 'dB drag racing' with StP. Not sure how good it is at vibration damping but I'll test it.
I don't think the PU foam has any damping characteristics. If you knock it the sound is very well propagated.

The answer is easy, the implementation of it is not, you set up a factory to make the Aluminium Foam filled parts of your choice, it is not an "Aftermarket Filler" choice.

I see a few books cited on wiki about ally foam manufacturing. I need to get my hands on one of these books.
Here's an experiment but the result doesn't look that great. https://www.youtube.com/watch?v=8FHTK2LZNTY

In the end, for this size of machine, the weight vs dynamics can be managed with appropriate motion component choices.

Exactly here's my problem. I need a lightweight machine, and being light allows me to use steppers and the price difference to servo I can use to improve damping and stiffness without adding to much weight.

Wow, never heard of this! Veeeery interesting. You just added another option to investigate to my list.

About shrinking/retraction, I don't think this such an issue, or at least that it can't be solved easily, as this is a construction method used daily in buildings. Quite sure there should be something in Sika or other product range for this problem.

This is actually my champion ATM! I find this Constrained Layer Damping technique very interesting by its use of multiple materials, smart mechanical principle, and affordable materials and build techniques.
Visco-elastic mat used in the thesis can be found and is not insanely expensive, and I'm sure can (and most probably will) be replaced by other cheaper and/or more easily available products. I'm currently thinking to asphalt mats...
They also use (IIRC) aluminum oxyde in concrete mix as an expansion agent, which should also be an answer to the previous retraction issue.

I doubt the expansion agent will work with foam concrete as it will tend to put pressure on the air bubbles in the concrete not against the walls of the mold. This chart shows that the shrinkage increases as concrete density lowers.

Even if I'm a little surprised by your stiffness numbers... Price remains one of the top decision factors, at least because all cents saved in structure will be to increase quality and performance of other components.

i have checked the deflection only with online calculators, two for each and they gave similar values. But now I tried a lot more calculators fot the RHS and the values were different... from 0.007 to 0.11

Could any engineer here help me with a proper calculation?

Other usual suspects are epoxy concrete/granite casting, and of course, solid granite stone. These are my second and third options.
You'll find tons to read about the first, and high end machines are also built this way.
About the second, you should be very interested by this build series https://www.youtube.com/playlist?lis...BoMfvQ8Ttsq-57 (as well as by all other videos from this guy!)

Interesting granite build.
The epoxy-granite is another story. There's the famous thread on cnczone with almost 5000 replies. But as far as I understand it, the most difficult part is the vibration for compacting. Vibrators are not cheap and not that easy to build and for good results they need to be tuned to the right frequency using inverters...

But a epoxy-granite cnc wouldn't be lightweight so it is out of the question for this build.

I won't even look for pricing infos just to not feel the pain!

They say 5-6 times the price of roller rails. But the price of their rails not the hiwin ones... And requires a dedicated oil pump.

Hi Paul,

Are you sure about the stiffness you describe above.

As a rule of thump, steel is more rigid by a factor of 3 for a given shape at the same weight.

Box section should be more rigid than tslot profile..

You explain above the tslot alu 13.5kg is more rigid than the steel box at 18kg....

I am not saying you are wrong... it just doesn't sound right.
Please check to make sure.

Also weight is your friend preventing vibrations.

I know it is hard to make the choices.

If you have access to tools and skill to go the steel frame / epoxy route i recommend it.

Grtz Bert.

I'm not sure anymore about the hollow section stiffness... If the calculation was wrong and stiffness is not a problem I'll probably take the steel/epoxy route with some sort of vibration damping inside them.

[m_c]

Hollow sections work well if designed correctly. The original FADAL's were mostly weldments, and performed well.

If you've not seen them yet, check out the David Decaussin (one of the original FADAL owners) videos on youtube showcasing his small mill and lathe, as he mentions a bit about it. His channel is at https://www.youtube.com/user/DAVIDDECAUSSIN

And having just searched for that, I've just noticed he has some newer videos which I'm now off to watch!

[paulus.v]

Thanks m_c!
I have not heard about Fadal and David. I like the vertical toolchanger he has built. And how he explains his ATC drawbar.
But steel remains steel and cannot be compared with grey iron in machine tools :) . When he pushes the machine a bit harder it starts ringing...

Yes the online calculators I've used for the RHS deflection were crap. I have checked with a FEA and it gave 0.0081 mm for the 160x80x5 1 meter RHS with 50 kgf in the middle. Now compared with the 160x80 aluminium extrusion with 0.012 mm deflection, if it had the same weight (18 kg), it would deflect 0.009 mm.
In conclusion the rectangular hollow section steel is a bit stronger than the aluminium extrusion with the same external dimensions and weight.
And I think that steel has a little bit better vibration damping than aluminium but I'm unable to find figures.

[m_c]

Everything rings ;-)
The question is how much is due to the weldment construction, and how much due to the linear rails?
There's a very good reason why lots of the high end machine builders are still using box ways.

Aluminium has better vibration damping than steel. A very general rule is, the more springy something is, the more it rings.
Simple experiment is take a couples bit of steel and aluminium bar, and give them a tap. The steel will ring far more than the aluminium. Even cast iron bar will ring more than aluminium. Which reminds me, only certain grades of iron have self-damping properties.


This is something I've occasionally looked at, as I have a machine idea bubbling away in my mind, but I think to build it properly, would involve a foundry. But then I come back to linear rails acting like springs, and think would a weldment be just as good, with a lot less expense involved?

[Nick]

Interesting granite build.
The epoxy-granite is another story. There's the famous thread on cnczone with almost 5000 replies. But as far as I understand it, the most difficult part is the vibration for compacting. Vibrators are not cheap and not that easy to build and for good results they need to be tuned to the right frequency using inverters...

But a epoxy-granite cnc wouldn't be lightweight so it is out of the question for this build.

That cnczone thread is a mess... I have read through it twice and i'm not sure it is helpful. Ideally it should be vibrated and I was put off due to that thread before. But I have found you can get reasonable results with just compacting it with your hands/tools. If your very concerned about vibration I think something like this might be worth trying

https://www.amazon.co.uk/Nordstrand-...FCCYN1BBXNSFA0

The other discussion topic on that thread is about aggregate size, the members of the "cncecke" forum tend to use pre-mixed fullers approximated aggregate such as Silimix-282 with good results. If your interested there is loads of good information on these forums but you need to register and use google translate.

Filling the RHS section with epoxy granite or UHPC concrete would be a reasonable way of vibration damping. It won't add as much weight as you might think, for example the 160x80x1m section filled with EG would only add 25kg.

You would be using the RHS and just using the EG for dampening so you could use an easy flowing sand mix such as this one

https://www.adambender.info/single-p...e-Frame-How-To

I am currently making a mill from EG so i'm perhaps a bit biased.

[paulus.v]

Hi Nick, I'm following your build with interest. Looking good so far. I like the manual scraped cast iron parts :)

Reading the Silimix-282 description I found that the company did exactly what the forum guys were trying to achieve. To have a very compact aggregate to use as little resin as possible. Less resin means higher stiffness. But if you have an internal metal structure or use the resin to dampen vibration in a cast iron machine then stiffness is not very important and more resin means better vibration damping.
It is good for DIY that such aggregate mix is available on the market but what if you want real granite aggregate instead of quartz? :) You have to go back to that messy thread trying to find answers/recipes...

I'm not sure if/how I'll be using epoxy-granite or cast iron in this build. I cannot add to much weight. For cast iron it should have at least 10 mm thickness for a decent stiffness and for epoxy-granite about 25 mm which translates to roughly the same weight.

Like m_c said, I need to find the best compromise. I'm aware that I cannot have great material removal rates, surface finish and tool life with a lightweight machine.

[silopolis]

https://www.amazon.co.uk/Nordstrand-...FCCYN1BBXNSFA0

Nice find! Had only found bigger ones. Thanks for sharing

There are also electrical and air powered vibrators of all kind of sizes. Affordable one can be found also.

Finally, good infos on DIY vibrators and vibrating tables can be found on YT


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