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3kg plus print head anyone?

Posted by deckingman 
3kg plus print head anyone?
May 28, 2017 03:25PM
Here is a link to the latest evolution of my printer [www.youtube.com]

Alternative titles

"One way to mount 3 extruders on a 600mm x 600mm x 1,000mm frame and keep the Bowden tube length down to 170mm"


"Why you probably don't need Nema 23s as Nema 17s will likely do the job.

Re: 3kg plus print head anyone?
May 28, 2017 06:04PM
That's a serious bit of kit Ian. I love the passive following gantry.

Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Re: 3kg plus print head anyone?
May 29, 2017 12:25AM
That's an impressive machine! Does the constant tugging on the extruder carriage cause any print artifacts?

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: 3kg plus print head anyone?
May 29, 2017 02:21AM
Is it ropes or rubber band that drags the extruder- with the hotend platform? What's the best travel speed you can get?
Re: 3kg plus print head anyone?
May 29, 2017 04:49AM
Oh, lots of question.

No artefacts noticed as yet. If you look closely, you'll see that I deliberately left some slack in the wires that connect the two carriages, effectively decoupling them to some extent. This means that with short moves, such as short zigzag infill and small holes, the movement of the hot end carriage isn't directly mimicked by the extruder carriage, but largely taken up by flex in the Bowden tubes. The Bowden tubes themselves are always under compression, thus keeping them securely fixed in the extruders and heat sinks. One side effect of having a big mass is that you don't get "ringing" - it's just too heavy to resonate.

To connect the two carriages, I just used some 7/02 wire, stripped the insulation back about 40mm at each end and, poked it through the holes and back on itself, then twisted it together. No sign yet of it coming undone but a dab of solder would secure it if it did.

Speed wise, the calculations I did work like this - (but I'm not a stepper motor expert so may have missed something)

Stepper holding torque from data sheet = 40N.cm.
Using 85% current, this becomes 34N.cm
Take 71 % for micro stepping becomes 24.14 N.cm
Fudge factor for torque at speed of 50% gives 12.07 N.cm available
Pulley dia = 12mm so radius = 6 mm
So force (torque*10 to convert cm to mm)/radius = 20.12
X carriage(s) mass = 1470 gms (I weighed everything) apart from the populated cable chain which is supported at one end so added 200gms to give me 1670 gms
So acceleration (force/mass) = 12.05 m/sec^2 or 12,045.91 mm/sec^2.
Take 10% of this for a single micro step gives 1,204.59 mm/sec^2 for non print moves in X
Doing the same calculations for Y with a total mass of 3.05 gives 660 mm/sec^2

Using these accelerations, the maximum attainable speeds would be as follows.
Assuming maximum X and Y travel of 330mm (actually it's little more) the diagonal length corner to corner (and hence longest possible move) would be 467mm
using speed = sqrt(2*acceleration*length/2) we get
X speed 750.03mm/sec or 45,001.76 mm/min
Y speed 555.17mm/sec or 33,310 mm/min.

These calculations are flawed for a coreXY because for pure X or pure Y, it uses both motors so in theory, the available torque is double. But, I'm sticking with them because I mostly use diagonal infill and in that case, only one motor is used.

These are the accelerations and maximum speeds that I have put in my configuration file, so that's what it was using in the video. Of course, in the "real world" print speeds are very much lower. The best I can manage is about 90mm/sec due to the limitation of how fast the hot end can melt the filament. I use 350 mm/sec travel speed in my slicer but of course, the actual speed attained will depend on the length of the non-print move.

Hope that answers everything.

Re: 3kg plus print head anyone?
May 29, 2017 05:00AM
That's a serious bit of kit Ian. I love the passive following gantry.

Thanks DJ. It solves the problem of how to mount 3 extruders whilst still keeping the Bowden tubes short. Mounting the extruders directly on the hot end is a non starter just because of the amount of room they would take up. I did look into it but however I packaged them, I'd lose more than 100mm of travel in both X and Y. The downside of mounting them the way I have is that not only do I have to shift the mass of the extruders but I've now got the added mass of extra carriages and rails as well as bit more friction. However, I did the calcs and it looked feasible so built in anyway. As you see, it works.......

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