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Making a large CoreXY printer

Posted by DickH 
Making a large CoreXY printer
April 04, 2018 02:53AM
Hello, last year I decided I wanted to try to get into 3d printing as a hobby. I started with a Prusa i3 and after a couple months I felt the need to have a larger printer. What I want to print requires a 30cm x 50cm print bed and I felt like I would get the most satisfaction out of making my own printer. I started out thinking I could just scale up a hypercube, but I found out the hard way that it didn't scale up very well. I currently have 2sqft sheet of 1/4 mic6 aluminum for a print bed sitting in a frame measuring 26" x 32" x 50" and am in the process of trial and error in making it all work. The frame is made mostly out of 2020 extrusion, and will be wrapped in wood. I was worried about the strength of the 2020, but it all feels fairly stiff when I bolt on the sides. I had problems with the frame twisting causing the x axis to bind up, but that seems to have been resolved after attaching the wood. I am at a point where I think I need help with my printer as I have spent more on this than I had originally hoped to. My current biggest problem is with the z axis. I had two 1620 ball screws centered on the sides with 10mm rods/linear bearings at each corner. This rocked back and forth too much so I had to scrap it. I do not have any room at the front or back of the print bed for a third ball screw and don't really want to have two on one side as I had a hard enough time finding a belt long enough for the two I have now. I have switched over to having the ball screws at opposing corners with hopes that that will minimize the rocking. It seems like I will have to make the z axis very rigid and with that I am curious if my drive setup for the z axis will be strong enough to move all the weight. I have a single nema 17, 92oz.in stepper with a 16 tooth pulley driving 40 tooth pulleys on the 1620 ball screws. Will that be enough to move what will probably be 20-25lbs without any print? I am also worried about the X/Y setup, I have abandoned the hypercube design and have moved onto mgn12 linear rails. I started to work on designing my own X/Y, but I am concerned that there are odd problems that may come up due to the size. In my initial testing (no belts) I found that PETG flexes too much for my liking and have a roll of carbon fiber PETG. I also have worries about the belt, and how I might keep adequate tension without creating new problems. Are there any decent setups that I could use with my current 2020 extrusion and mgn12 rails? I am getting a little tired of the trial and error stage I seem to be stuck in.

Re: Making a large CoreXY printer
April 04, 2018 11:32AM
I think it's better to keep the screws close to the guides, especially with as large a surface as you are trying to move. I think that 2 guides near the front on the sides and 3 screws- two next to the guides and 1 at the back center of the machine- should do the lifting and keep the whole thing stable.

That is a big project for a first DIY printer and you've made a lot of progress so far. When progress seems slow and gets frustrating, it's a good time to take a break and just think about things instead of building and testing. It took me a year and a half to get my first design/build printing.

Are you planning on heating the bed? What materials are you planning to print? That's a large bed to try to keep flat. Large machines are often used to print large objects in thick layers to keep the print time reasonable, so bed flatness may not be critical, depending on the layer thicknesses you expect to use. The Duet controller can map the motion system and compensate for bed and guide flatness/sag and guide rail sag. Smoothie might be able to do that, too.

In the XY mechanism the belts will be under tension and pulling pretty hard against the axles of the pulleys and motors. I would not use plastic parts to support the pulleys/motors unless they are very "beefy". It isn't hard to make metal pulley and motor mounts that will keep the axles from tilting under the tension. See: [drmrehorst.blogspot.com] Whatever you do, don't just stand them up on bolts with one end screwed into the printer frame. The bolts will flex under tension and whatever they are screwed into may flex, too. If you have to print them, don't make plastic parts that look like welded steel parts. The plastic will be too thin and flexible. Make the parts look like metal parts that are milled out of blocks of metal.

The mount on the left, if made of steel, is probably rigid enough, but not if it's made of plastic, even with some side webbing. The mount on the right can be attached to the printer on the side or the top surface and will be rigid, even if it's made of plastic. You can design and print similar mounts for the pulleys. If you intend to enclose and heat the printer, use ABS to print the parts.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Making a large CoreXY printer
April 04, 2018 12:44PM
I'm close to completing on a 400mm x 400mm CoreXY right now. All motion is V-Slot based. Part design is a mix of D-Bot and Hypercube.

I'm having very good success with the Carbon fiber filled PETG from Atomic Filament. Rigidity of parts is excellent and I'm not seeing any sag or creep on the printed motor mounts when stress testing the steppers at full operating temperature. It's some of the best printing filament I've used.

The Z axis is driven using two 1204 x 600mm ball screws coupled to a single Nema17 stepper through a 2:1 reduction using GT2 pulleys. The stepper is a 17HS24-2104S which is rated at 2.1A and I'm currently driving it at 1.6A @ 24V. I've taken it up to 1200 mm/min using a Duet wifi without issue. I suspect I can go faster yet, but simply haven't tried. I can hear the motor hum with idle power at 20%, but no issues at all otherwise.

The bed is a 6.4mm thick cast aluminum tool plate that is 406mm x 406mm with a borosilicate glass plate on top and a silicone heater mat on the bottom side. I've not calculated the weight, but it should help give you a reference point.

I'm not having any issue with back-driving the ballscrews with power off. Between the holding torque of the 17HS24-2104S and the 2:1 reduction, it holds firm.

Edited 2 time(s). Last edit at 04/04/2018 06:12PM by gen2eng.
Re: Making a large CoreXY printer
April 05, 2018 04:11AM
The print bed will be heated. I have a 20"x20" 800W silicone heater, and plan on printing a lot of nylon so it will be completely enclosed and probably insulated. I am a little worried about temperatures inside the printer so I am mounting all of the electrical stuff separately from the print area. It looks like this might have to print for days at a time and I am assuming that at that point an important factor for success is maintaining the temperature for the electronics. With that concern with is how well the steppers will work in the sustained higher temperatures from being in an enclosed printer. The z axis stepper will be isolated from the print area and possibly actively cooled. I am debating mounting the xy steppers remotely up with the rest of the electrical and running a shaft down for the pulleys. This seems preferable as I can quite comfortably secure a 10mm shaft with bearings and not have to worry about it moving on me. On the topic of xy, have I made a mistake in getting 6mm belt? Since ordering my belt I have second guessed this decision thinking that I should have gone with 8mm or 10mm belt.

Thanks gen2eng, it sounds like your z axis is similar to mine so I have gained some confidence. What is an appropriate stepper/pulley setup for the xy? I don't remember exact details but I have a pair of 84oz.in steppers and a pair of .9 degree 62oz.in steppers. I initially purchased the .9 degree steppers after reading they were supposed to be more precise and quieter, but I am not too worried about that anymore. I kind of just jumped right into buying stuff without realizing the differences and complications in making such a large printer.
Re: Making a large CoreXY printer
April 05, 2018 05:23AM
My steppers are 1.8° @ 92oz.in. I've got a 16 tooth on the stepper and 32 tooth directly attached to the ball screws for the Z. I'm using 20 tooth gears for X&Y. I'm using 6mm belt as well, and so far, don't see any issues. I may upgrade to a real Gates belt later on.

I'm using the same steppers for the X&Y right now, but may go to some 0.9 if I see any print artifacts. I've see a few comments to the effect that 0.9 won't provide a significant advantage on a CoreXY printer. The current steppers are overkill for X&Y, but the price was good at the time.

Edited 1 time(s). Last edit at 04/05/2018 05:24AM by gen2eng.
Re: Making a large CoreXY printer
May 12, 2018 04:52AM
Small update. I have taken a break from the printer for a little over a month and have come back with a little progress. I really liked the idea of relocating the steppers which should keep them cooler and in doing so I actually can gain a little extra print volume. This also makes mounting the pulleys directly to the linear bearings possible which also makes easier for me to get the belts routed nice and square. I need to buy more pulleys and design proper corner brackets for the back side. I am debating adding some complexity to the brackets in the back to wrap the belts around the corner of my 2020 extrusion to give me a little more clearance for the carriage. I think more importantly this will allow me to implement some sort of tensioner along the back side that I will probably need with these long belts.

Edited 3 time(s). Last edit at 05/12/2018 01:34PM by DickH.
Re: Making a large CoreXY printer
May 12, 2018 08:36AM
I think you're going to find that when you tension the XY belts, the plastic block that has the pulley standing up like a fence post on the Y axis bearing block, and the skinny little M3 screw that is the pulley's axle, are going to flex inward. You might want to try to redesign that plate to support that pulley from top and bottom. It will require longer screws to hold the plastic mount on the bearing block but will be much less likely to flex. It's the same concept as the blocky motor mount. Start with a solid block and carve away only as much as needed to allow the parts to be assembled and the belts to pass through. The same idea applies to the other pulleys in the system, too.

And yes, you will definitely need some means of adjusting belt tension. In the XY stage the belts should be at about the same tension. After you tension the first belt, pulling the second belt tight increases tension in the first belt, so don't set the first belt too tight initially. If the tensions are not close to each other, the X axis will tend to shift so it is not orthogonal to the Y axis, so after you tension the belts, you will want to verify the orthogonality of the X and Y axes. You may be able to do that using a machinist's or carpenter's square, or you can just print a square and measure the diagonals, which should match if the X and Y axes are orthogonal. If they are not orthogonal, you'll adjust one of the belt tensions a bit to bring them back to square.

Edited 2 time(s). Last edit at 05/12/2018 08:47AM by the_digital_dentist.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Making a large CoreXY printer
May 12, 2018 08:53PM
Oh the white pieces are temporary, they were made to help test fit everything. All of it is actually temporary, I have some carbon fiber PETG filament I specifically bought to make the final pieces after I know that it works.
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