general discussion of plus-sized Kossel-based printers

I finally have my Kossel Mini printing stuff and it's too small! (and my FSR solution is issue-prone, making good calibration a difficult process.)
I'm in the process of designing a printer around some elements of the dc42 midi-Kossel, including the duet pcb.

However, as I converged on the dimensions for a 300mm bed and 300 mm build-height, I realized that it was more than just longer extrusions, wider bed assembly, and longer effector arms; in fact I have to rebuild almost everything. Here are some of my observations, nothing shocking but certainly a reality-check:
--Effector rods are almost twice as long
--Vertical extrusions are much longer
--Belt lengths will be much longer
Given these increases, the load on the motors will almost double if I want the effector to move at the same speed as the Mini. And this is with a much larger bed. If I want to speed up the print for larger parts I have to speed up the motion, also more load on the steppers. It seems I (may) have to upgrade to heavier steppers which may require bigger drivers than off-the-shelf electronics (duet, ramps) can handle. I would like to move to .9deg steppers but that forces the GCODE bandwidth to double which I hear is not supportable with the 8-bit designs, although methinks the Duet board can handle it no problem.

--With all the longer dimensions (and speeds), accuracy and precision should also suffer. I can help this some by keeping the prints slow but that conflicts with the need for MORE speed, not less.
--The bowden tube must be longer, resulting in more filament friction and heavier load on the extruder motor. It also adds more sponginess to the filament compression. I am already working with 3mm so it aint as bad as 1.75 but it still causes lower performance. (especially since I want to keep my option to use Ninja-flex-like materials.)

--Higher print speeds with larger objects will surpass the 40W heater flow-rate for sufficient melting. That means a non-standard heater size too! I've been running a .8mm nozzle with .4mm layers on the Mini to make fast objects on my and I can't go over 30mm sec with PLA without getting under-melted plastic issues.

At this point I seems like I might as well finish up the Mini and shelve it and buy all new parts since only the cold-end assy and the metal vertex parts are re-usable. I don't really want to keep the Mini because of calibration headaches that drove me to invest in the Duet/DC42 firmware in the first place.

At this point I am wrestling with all the trade offs and any experience/observations would be appreciated.

So who has built bigger ones and what are your experiences? (details!)
thanks

Edited 2 time(s). Last edit at 07/10/2015 03:36PM by shadowphile.

Scaling up the size does not of itself increase the load on the motors significantly, because the only moving parts that get bigger are the belts and diagonal rods, and both have low mass. The motors on my Kossel are rated 1.68A but I only need to run them at 800mA, or 1A if I increase the acceleration in order to do speed tests. So there is still plenty of margin left.

If a 40W cartridge heater proves insufficient for a 0.8mm or larger nozzle, then if you have spare power capacity in the PSU you can turn up the voltage to get more power. The usual LED power supplies have a voltage adjistment pot. Increasing PSU voltage by 10% increases heater power by 21%. A higher supply voltage also makes it easier for the drivers to move the motors at high speeds.

Probably the biggest issue with increasing the size is making the frame rigid enough. The increased Bowden tube length is also a concern.

When I print at high speeds, I increase the hot end temp by 5 or 10C to reduce the load on the extruder drive.

Edited 1 time(s). Last edit at 07/10/2015 04:25PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].