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Stop me from making rookie mistakes - CoreXY

Posted by Storckenfeldt 
Stop me from making rookie mistakes - CoreXY
July 09, 2020 10:16AM
Hi Guys!
Been lurking around here for a while and want to get some input on my build.
I've worked with printers, lasers and other NC-machines for a while, but this is my first 3d-printer build.

My main goal is to build a rigid quality machine. Hopefully it will be able to maintain quality at high speeds (hopefully 200-300mm/s).

To give some perspective, the extrusions are 30x30mm and the aluminium bed measures 500x500 (printable area ca 400x400mm)

I wont take credit for the design, it's heavily inspired by 3d distributed [3ddistributed.com] but I also looked at railcore, BLV-Cube etc. for inspiration.

Any opinions, really don't want to make any rookie mistakes since it's my first build.

Thoughts and questions:

1. Belt path? I chose to place the 9mm GT2 belts inside the frame (instead of the more common approach to put it above the frame) for two reasons - easier to enclose, and primarily because I imagine it will be more rigid since all moving parts are in the same plane (?) The cost is of cause a bit of print are. Would any other belt path design be more rigid?

2. Z-axis mount? How would you fasten the aluminium tool plate in the angle mounts? I'm thinking of placing a spring washer or a short spring between the screw and the mount to allow some movement for heat expansion and allow the independent Z axis motors to do their job.Would that be sufficent? Is it preferable to have the linear guide and lead screw in axis as on the pictures, or should them be placed side by side or any other way?

3. Torque? Im planning on using these 59Ncm steppers, but with a custom ordered longer shaft so I can fit the big pulleys and a bearing supporting the long shaft [www.omc-stepperonline.com]
How much does the pulley weight affect the speed/acceleration/quality? Lighter is better ofc, I've looked at the Zesty Nimble as a extruder, but how much would 150g stepper motor affect the performance?

4. Rigidity? The motor and idler-mounts will reinforce the corners, and I will also add corner brackets on the outside and the enclosure will also provide some rigidity. the motor mounts are 6mm and the U-profile used for the gantry is 4mm. Is that enough?

Do you see any design flaws?
What would you say the "weakest link" in the design is?

Please let me know what you think and please ask questions if the renderings are unclear (I haven't placed out all the screws etc. yet).
open | download - Main_-_Test_2020-Jul-09_01-18-22PM-000_CustomizedView52797084757.png (312.7 KB)
open | download - Main_-_Test_2020-Jul-09_01-20-51PM-000_CustomizedView2601860678.png (522.9 KB)
open | download - Main_-_Test_2020-Jul-09_01-26-18PM-000_CustomizedView44000927882.png (588.7 KB)
open | download - Main_-_Test_2020-Jul-09_01-24-00PM-000_CustomizedView23701994987.png (499.5 KB)
Re: Stop me from making rookie mistakes - CoreXY
July 09, 2020 01:10PM
You already did the first mistake.

Didn't browse through the forum, didn't notice there is a CoreXY thread and therefore posted at the wrong place.

Plenty of reading and many requests as yours.

"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)
Re: Stop me from making rookie mistakes - CoreXY
July 09, 2020 04:05PM
A few things...

1) 3030 t-slot for a 500mm square bed seems a bit small, but if you're enclosing it, the enclosure panels will stiffen things up nicely.
2) get the belt layout right and you'll be save yourself a lot of headaches. See: [drmrehorst.blogspot.com]
3) if you use multiple motors to lift Z, you have to use autoleveling because you used multiple motors. If you use a single motor, and connect the screws with a belt, it will not go out of tram and you won't need autoleveling. I read a lot of forum posts and see about 1/3 of them are about getting autoleveling to work. People have traded all the screwing around trying to manually tram their printer's beds for screwing around trying to get autoleveling to work and keep working. The sensors seem to be pretty unreliable.
4) If you put a 400mm square heater on a 500 mm square plate, the central area of the plate will be trying to expand while the much cooler edges won't. That is likely to lead to bowing of the bed when it's heated. It is much better to heat the entire bed surface- if you're only going to print on 400 mm area, then use a 400 mm square plate and heater.
5) The motor torque is fine, but those are 200 step/rev motors. You can use 400 step per rev motors to maximize XY resolution. Similar sized 400 step/rev motors are rated for about 64 oz-in torque which is plenty for X and Y. Some people swear by 400 step per rev motors in the extruder, too.
6) using a bowden type extruder will keep moving mass down but will prevent you from printing flexible filament. TPU can be really useful stuff to print
7) If you don't mind the footprint getting a little bigger, and since you're putting the belts inside the frame, you can make the motor mounts fit in the t-slot and then just slide along the slot to tighten the belts. I did such a system in my sand table and it works well. If I ever build another printer I may use a similar arrangement. See: [drmrehorst.blogspot.com]
8) use stacked bearings for pulleys. The crappy little bearings in 3D printer pulleys don't last. The small diameter of the pulleys can lead print artifacts as the belt teeth hit the smooth pulley surfaces. That doesn't happen with larger bearings used as pulleys. The mass doesn't seem to be a problem.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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