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Sheet Metal Brake and 3d Printer.

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nrml:
A 3D printer does not need huge amounts of structural rigidity. Vibration and resonance are more of a problem. It would be far more productive for you to use your time in vibration control rather than building a substantial chassis. Make the gantry as light and vibration free as possible.

Some of the kits on Banggood and Aliexpress are not a bad place to start IMHO. Just keep the bits you need and beef up or change what you don't like .

Joules:
Table sag is an issue, especially with larger heavier prints.  As can be seen in some of my tall prints, if the setup isn’t rigid you will get banding as the print/machine flex.  I get good results partly due to the small size of my printer and the steel welded chassis.  Scaling it up and keeping the bars same size, as seems to be the norm in the hobby class machines, is not a good recipe for precision.

You also need to think about the nozzle being in the centre position when extruding.  The pressure at the nozzle is easily able to lift/bend a lightly constructed XY mechanism, never mind push down the table.  That leads to mid print bulge and outer edges, corners getting squashed or distorted.  It only takes a few thou movement to go from a good to bad print if you are trying to hold close tolerance on parts and crisp sharp corners.  Heavy design is good, where it’s needed, vibration will show in your prints as banding or echo of printed detail.

Have a think about keeping the table fixed and moving the XY mechanism up and down for a large printer.  Those eBay rails look tempting, but I would want to do some serious testing on them first.

nrml:
The MGN12 rails are very good for the price. I have about 10 of them (all half finished projects). There is very little play between rail and carriage in any direction. They are well made and finished. The grinding (on all mine at least) is flat and parallel. The only discernible flaws on mine are that some of them feel slightly gritty when they run on the rails. It doesn't affect movement much when there is a bit of force applied to the carriages but the difference in friction is evident under just gravity. I tried washing them out with paraffin and re-greasing but it didn't make a huge difference. I think the problem is the finish on the bearing surface of the carriages rather than dust or grit.

There are countless builds on the reprap forum using these rails (some of them very high spec indeed) and hardly anyone has issues with their actual performance in real life. The performance of other mechanical components like stepper motors, timing belts & pulleys and the physics of molten plastic become an issue well before these rails reach their limit.

They are noisier than igus bearings  or delrin wheels.

S. Heslop:
I never knew about the pressure from the head flexing things. From what i'd read it was people just chasing vibrations. That said I have seen videos where people had really long nozzles that you could see twisting about as the motors changed direction.

Keeping the table fixed and moving the XY up is also one of the first thoughts I had. I forget why I dropped it for the Z axis bed... the only reason I can think of right now is trying to reduce the overall size of the machine. I think I also had a hard time finding examples of such machines to copy.

I've got alot of 12mm bar. It's just cold rolled, I figured that'd be good enough. Or I at least wanted to try see if it's good enough since it's not too expensive and still useful if I switch it out. And 4 12mm linear bearings i'm keen to use. So right now i'm thinking a coreXY deal built on a 2mm steel sheet, since i've still got loads of that stuff, bent up around the edges that haven't got a linear guide on it for stiffness.... and then that's lifted by 2 or maybe 3 leadscrews.

Y'know if I made the vertical guides stiff enough then everything could be supported on the base. I wouldn't even need sides except to keep the breeze out.

PK:

--- Quote from: nrml on September 06, 2018, 05:25:01 AM ---A 3D printer does not need huge amounts of structural rigidity.

--- End quote ---
My lip hurts.....
A 3d printer needs LOTS of structural rigidity in X and Y because it runs at high accelerations which generate large forces even with lightweight heads.
A 3d printer needs LOTS of mass because it moves repetitively  in almost every direction so your only hope is to have the resonant frequency of the structure below every conceivable excitation frequency and the best way to do that is make it massive.

The reason this sounds wrong is because you never see commercial 3d printers built this way and *that* is because a). it's such a price competitive market, and b). they have to be shipped.
Instead you spend you time and money on 32bit controllers trying to tune 3rd order jerk parameters to get the thing to stay still.


There... I feel better now!

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