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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.
Re: Making a large CoreXY printer
August 06, 2018 03:58AM
I have started poking at my printer again and have made some fairly decent progress. I have it assembled far enough to have a functional corexy mechanism and the hot end situation is almost complete. I have verified a print area of 630x500mm, still need to work on the z axis. My plate of aluminum is only 600mm wide so I am thinking about dedicating that extra 30mm for purging and a brush or something to swipe the nozle across to clean buildup. I was worried about how loud the cheap mgn12 bearings and 16 pulleys would be, but its far quieter than my prusa. I took the advice about boxing in the pulleys into consideration, but my longest bolts were not long enough so I made a couple test pieces to see how much flex I had on the pulleys going to the gantry. I pulled pretty hard, harder than I imagine ill need the belt tension and saw no flex. Every final revision for my parts are printed with carbon fiber PETG and I am quite happy with how that material prints as well as the added stiffness compared to normal PETG. The hotend is a modified NF TC-01 off of ebay with a volcano block. I really liked how it looked but couldn't find anything online about it, for 30 bucks I took the gamble. I am going to run pretty high temps and it appears to have a teflon liner until about the middle of the block, I am hoping to avoid problems by increasing the cooling. I took a saw and cut into the block to add surface area to the cooling fins, followed by mounting a 6028 blower fan in place of the 30mm fan. I am also going to try running a direct drive and a bowden setup with my dual extrusion. I don't know if I will keep it, but I had the titan extruder laying around. Part cooling will be accomplished by a berd-air setup, whenever it shows up.







Edited 1 time(s). Last edit at 08/06/2018 04:12AM by DickH.
Re: Making a large CoreXY printer
August 06, 2018 05:14AM
Looks like you are using steel core belts?
They need bigger pulley diameters or they brake easily.
Re: Making a large CoreXY printer
August 06, 2018 06:29PM
I second that. Steel core belts need big pulleys or the frequent bending back and forth on the pulleys will break the steel wires inside the belts. That will cause them to stretch wherever the wires are broken and cause all sorts of weird print problems.

It also looks like there isn't nearly enough tension on the belts. If you don't pull them tight, the inertia of the mechanism will, briefly, every time motion starts/stops changes direction, like a big spring. You'll get exactly the opposite of what you wanted when you bought steel core belts.

I noticed that you crossed the belts at the back of the machine, but the belts appear to be stacked. In a stacked belt setup, there's no need to cross the belts, and it implies that each belt is starting at one level and ending up at another. Belts don't work well that way. The axles of the pulleys should all be parallel and the belt should run perpendicular to the axles. The only time the belts would have to cross is when they both occupy the same Z level. I wrote a blog post on corexy set up that might have some useful info for you.

Edited 2 time(s). Last edit at 08/06/2018 06:37PM 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
August 07, 2018 03:13AM
Thank you both for the feedback on my belts. What size pulleys are required if I run the steel core belts? Do I have a more suitable option for my application? The pictures were taken before I tensioned my belts, I have them fairly snug now. I have the belts crossover in the back due to how I thought I would try to deal with belt tensioning. If it turns out to be a problem I just need to redesign the carriage, the brackets for the pulleys can already accommodate the change. I am tensioning the belts by loosening the upper belt clamp on the back of my carriage and drawing the belt through, I am able to pull just one at a time without too much hassle. I kept adjusting each side until they rung the same when plucked. I measured afterwards and the movement on the y axis is nice and even, the x axis sits square with the y.
Re: Making a large CoreXY printer
August 07, 2018 07:54AM
Unfortunately, the chinese steel core cable makers don't ever seem to specify the minimum bending radius, and I have never found any documents Gates or DSP/SI or other makers that specify it because they don't seem to make steel core GT2 belts. In theory you could calculate it based on the number of wires in each little "rope" in the core and their diameters, but there are a lot of factors to consider and you'd only be guessing at most of them since there are no specs. Glass or kevlar core belts are much more flexible and provide all the strength needed for the XY mechanism.

About the crossing belts... timing belts are intended to operate on pulleys with parallel axles with the belt running perpendicular to the axles. If you change the angle of the belt on the pulleys by changing the vertical displacement of the pulleys, you will create wear on the belt and the pulleys if they have teeth. You're loading the outer core elements of the belt much higher than the central core elements which will lead to rapid failure of the belt. There is absolutely no reason whatsoever to cross the belts in a stacked belt corexy mechanism. Tensioning the belts can be done by moving the motor mounts or corner pulleys in the Y direction, applying idlers at the outer segments or the back segments, or at the attachment to the extruder carriage (by pulling the belt in the X direction). You can't tension the belts individually- changing tension on one will change the tension in the other, no matter where or how you adjust the tension, even if you just move one motor mount, or pull on one belt at the extruder carriage.

So many people look at the original coreXY implementation and see crossing belts and think, "my belts better cross":



But if you look closely at the other photos, you can see that the belts in the original mechanism are stacked.



While the belts cross paths at the back of the mechanism, they don't cross in the XZ plane. Each belt stays either above or below the other at the motors, corner pulleys, and extruder attachment points. Look at the pulleys on the motors and the pulleys at the ends of the X axis, and the attachment points on the carriage. Two different Z levels, and each belt stays on its own Z level as it must.







The only layout that would require the belts to touch each other at the back of the mechanism is if the belts were both on the same Z level at the motors, the corner pulleys, the Y axis pulleys, and at the carriage attachments.

Edited 1 time(s). Last edit at 08/07/2018 07:56AM 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
August 07, 2018 01:12PM
You know, I started writing this big long drawn out reponse, read back up a little bit further and saw it's all been said previously.

Oh well...

You're on the right track, I could probably offer a few critiques, but I'm hardly an expert and am still tinkering with my own custom corexy.

Best of luck!
Re: Making a large CoreXY printer
August 12, 2018 08:58AM
The toothed pulleys I ordered for rerouting the belts showed up and I have made necessary adjustments to accommodate the new routing. I looked into the steel core belt issue and it appears that I would need something in the range of 30-40 tooth pulleys and that is just not going to work for me. I am now trying to working on getting some gates belts, the problem I am having is each belt needs to be 3m in length and almost everywhere I look online sells lengths in 2m or 5m sections. I am going to go with an added tensioner pulley in the corners. What is the consensus on spring loaded tensioners? How much do these belts stretch, how often do they need adjusted? Spring loaded tensioners would be easy for me to implement, but im thinking it might negatively effect the print whenever direction changes. This seems like this could be corrected by mounting the tensioners in the middle of the back of my printer, but I wouldn't want to over complicate this if the maintenance on the belts is minimal.

Something I am going to need to fix is that my X axis moves backwards. I assume this is due to my motors being mounted upside down. Is this something that can be corrected easily with the duet controller?
Re: Making a large CoreXY printer
August 12, 2018 10:08AM
Belts are designed for minimum stretch. That's why they have glass or steel reinforcement. In a power transmission system, if the belt acts like a spring it can be good to absorb shock. Using spring loaded tensioners is exactly the opposite of what you want in a high precision positioning mechanism like a 3D printer. The motion is all accelerated, so there's no shock to absorb (except when jerk is set to a high value). Normally you don't have to readjust tension if construction is solid. The belts will last for years.

I wrote a blog post that goes into tensioning belts in a corexy machine (along with other relevant topics). There are several places you can do it, but you must not mess up the parallel relationship between the belts and the guide rails when you adjust the tension.

You normally buy belt in whatever length is available and cut it to the size needed with scissors. You'll have some left over for another project.

The X axis runs backwards probably because you have told the controller the wrong location of the endstop switch -i.e. you've set it up for left hand rule coordinates. It doesn't matter how the motor is mounted. An individual motor can be reversed by reversing the connections to it or by a minor change in the firmware configuration. In a corexy mechanism, you can't just reverse the connection to change the X direction because both motors work together to define the motion. If you reverse the direction of one motor by flipping the connector over, you'll swap the X and Y motion.

When you set up a corexy machine, the first thing to understand is right hand rule coordinates. CAD and slicers use RHR coordinates, so your printer must also use RHR coordinates or prints will come out mirrored. I recommend you set up the center of the bed as the machine's origin so that all slicers will drop prints at the center of the bed without having to make any custom gcode for each slicer you may use. The second thing to understand is that you must set the hardware and firmware so that the extruder carriage moves toward the endstop switches when you send a "home" command. The switches can be put at either end of either axis, just put them where it's convenient for you to either mount them or wire them. Once the extruder hits the switches, you assign ordinate values at each axis that will put the printer's origin at the center of the bed. I wrote two blog posts on setting up corexy machines. Start with this one, then read through this one. The first post deals with getting things moving in the right directions using RHR coordinates. The second with putting the printer's origin at the center of the bed. I used a SmoothieBoard and Duet in the examples, but the concepts are the same and apply to any controller. I should probably combine the two into a single post...

Edited 4 time(s). Last edit at 08/12/2018 10:45AM 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
August 13, 2018 05:19AM
I actually have not setup the controller yet, its just running on defaults. I also have not yet installed end stops, I want to finalize the corexy stuff before moving onto anything else.

I think this is what I am going to go with for belt tensioning. This will replace my corner pulley brackets and will be bolted to the 2020 extrusion and to the wood backing. The tensioner will be adjusted on the back of the printer by adjusting a nut that draws the tensioner pulley in. This gives me about 1.25" of belt tension before it bottoms out. I think this is my best option as it doesn't effect print volume, it would have been easier to adjust tension with the steppers but they are mounted remotely to give a little extra room on the x axis. I have ordered the needed bolts, I wont be able to try it out until they show up,


Re: Making a large CoreXY printer
August 13, 2018 07:33AM
The belt tension on that adjustable pulley is going to also put tension on the little printed fork that the pulley is mounted in, with the force trying to pull the layers of plastic apart. That is the weakest direction for the printed plastic to be loaded. That's not going to last long.

You're going to need a metal bracket/fork to hold the pulley itself because there's no way to remove the tension on it. If you're going to keep putting tension on the adjuster parts (the pulley frame, etc.) you'll have to print them rotated 90 degrees (and use lots of support material) so that the tension is pulling along the layer lines. Everything will have to be bulked up, too. It would be better if you put the adjuster parts outside the enclosure so the belt tension puts some of them into compression.

You aren't going to need so much adjustment range to tighten the belts. You should be able to simplify the design a lot- like remove some of those extra pulleys that are going to greatly increase the load on the motors.

You can tension the belts at any of the locations shown in the image below:



If you push on the belts at the back and/or sides of the machine, you don't have to bend the belts around multiple pulleys at the corners. Pushing against the belts can be done using printed parts that are in compression, so they'll be much better able to handle the stress, and the belts won't have to wrap around the pulleys you use to do the pushing. It might be easier to redesign the clamps at the extruder carriage to allow tightening the belts there.

It seems you've sort of designed the machine in a backwards sequence. You designed/built the enclosure first, and now struggle to make the corexy mechanism fit and still preserve the desired build plate area. Maybe in your next printer, you can design the corexy mechanism first to achieve the motion you want, then design and build the enclosure around it. You may have to give up some of your print area to make a mechanism that fits in the enclosure. One thing I would recommend is that you try to reduce the number of pulleys that the belts have to wrap around.

I recently put together a large corexy mechanism driven by cables and designed the motor mounts and corner pulley mounts to fit in the Y axis parallel frame members. That arrangement lets me tighten the cables simply by sliding the motors or the corner pulley mounts back, then locking them in place using t-nuts that fit in the slots in the frame members. It would be even easier to apply that technique to a stacked belt driven system, and in fact, I may convert the cable driven system to a belt driven system due to some difficulties I'm having with the cables.

Edited 1 time(s). Last edit at 08/13/2018 12:06PM 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
August 14, 2018 07:25AM
Yeah I know I should have figured out the movement first and then built the machine around that. I actually started by trying to do this as a larger tech2c style hypercube and found that it just wasn't a good approach for the printer this size. This actually kind of haunts me as I am not a fan of the 2020 extrusion but keep it anyway knowing it will be stiff enough with the wood mounted on the frame. You mention that the tension might cause failure in the parts, how much tension is needed? I had the belts tight enough to pluck with the previous routing and can't imagine that being enough to break the printed parts. What are the side effects of not being tight enough? The PETG I am using seems to hold up well when I try to twist or break the parts. The final prints are in carbon fiber PETG with 45% infill, and I have seen the 2020 extrusion twist by the time before the pulley bracket deflects. Speaking of twists, is there any harm in twisting the belts 180 along the run in the back? I preferred being able to use the belt clamps and I don't think they will hold the smooth side adequately if the tension has to be so tight I should worry about potentially breaking the pulley mounts. Also, if I go back to being able to clamp the belts like this I can tension the belts like I did the first time which was at the resolution of one tooth increments. This was good enough to get the machine to move square the first time.
Re: Making a large CoreXY printer
August 15, 2018 12:41AM
A loose belt would end up in flat spots while trying to print a circle. There will be a dead time at the turning points of movement.
Re: Making a large CoreXY printer
August 15, 2018 05:42AM
Quote
the_digital_dentist



I want to rebuild your CoreXY construction but think about a modification, adding two pulleys:


The stepper can be a bit more right (left stepper) or more left (right stepper).

The additional pulleys are good for:
- easier align the three pulleys which must be in one line (4 identical housings, no belt thickness calculations, one pulleys axis line)
- tensioning of stepper is easier, as the position is not critical in X direction

What do you think?

Edited 6 time(s). Last edit at 08/15/2018 05:53AM by JoergS5.
Re: Making a large CoreXY printer
August 15, 2018 07:24AM
It should work as long as the left motor is moved only to the right and not to the left, and the right motor is moved only to the left and not the right.

To me, adding more pulleys is a lot more trouble than measuring the belt thickness. If you're going to tension the belts by moving the motors anyway, why not put them in the correct left-right positions and make sure the adjustment only moves them in Y (use the slots in the frame as the guide for the motion)? I've seen some builds where people actually put in two pulleys and bend the cable path 90 degrees so that moving the motors in X tensions the belts. If you did that, you could change pulley sizes on the motors, and change belts without regard to thickness because the extra pulleys will keep everything that needs to be aligned in place.

I think it's like making screw-adjustable tensioners. You can do it, but you only have to tension the belts once, so is it worth the trouble of a complex design? If the parts that mount the motors and pulleys are all printed, it isn't difficult to change the dimensions and just print another part if you decide to change from a 16 tooth drive pulley to a 20 tooth pulley, for example (but why would you ever do that?)


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Making a large CoreXY printer
August 15, 2018 07:50AM
Thank you for your opinion. I am still not sure what I will do.

I want to be flexible with pulley sizes on the stepper because I don't know which speed is possible maintaining a good quality print. It will be trial and error, I did not see a mathematical model anywhere to decide this. For example, is it possible to use 60 tooth pulleys on the stepper and run full speed?

Same uncertainty is with pulley diameter also: I thought about 40 mm diameter pulleys and using steel belts. But I don't know whether this is better than the glass fiber solution.

This uncertainties are the reason why I try to build a system which I can change fast, to allow experiments. So my goal is to use a highly modular system (Z totally separated from XY, and both from he frame).

I agree with you that if a solution does not fit, then construct another one. That is the reason why I have to build a small CNC machine first now. I have problems boring drilling exact holes in aluminium...

Edited 2 time(s). Last edit at 08/16/2018 01:30AM by JoergS5.
Re: Making a large CoreXY printer
August 15, 2018 09:06AM
I think you'll find that moving the mechanism, even if you use 16 tooth pulleys and 400 step/rev motors, is not going to limit print speed. Extrusion is the limiting factor. Trying to make a quality print above 200 mm/sec is a real challenge because the stuff coming out of the extruder takes time to cool. Getting that stuff to come out of a small nozzle fast enough to keep up with the mechanism is also a problem.

There's nothing stopping you from using 40 mm pulleys with glass core belts. The only problem there is how do you make the pulleys? You could try to print some pieces that fit over bearings, but if your print isn't really good, it may produce defects in the prints your machine produces. Imagine a seam that puts a little extra tug on the belt every time the pulley rotates, then multiply that by at least 8 for all the pulleys in the mechanism.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Making a large CoreXY printer
August 15, 2018 01:58PM
I read at the DICE description about 800 mm/s speed and wondered how he achieved that.
Edit: ok, I checked the video again: the speed test is without extruding :-)

I found a ball bearing fitting could be a 6301: 12x37x12, so you need only one at each place, even with 9 mm belt. I would use a 12 mm shaft directly with locking rings.

In a CNC forum I found an information about so called "ballige Kugellager" (english: spherical/crowned/convex bearings?). They are thicker in the middle and self centre the belts. But they are not cheap.

Edited 1 time(s). Last edit at 08/15/2018 02:15PM by JoergS5.
Re: Making a large CoreXY printer
August 15, 2018 02:12PM
Were there any photos of prints made at 800 mm/sec? In a small printer, the only way to get the mechanism to hit that sort of speed is to have extremely high acceleration. But that requires very high acceleration in the extruder, too.
Let's say you manage to print at 800 mm/sec. In a small machine the extruder is going to be putting hot plastic on top of hot plastic, so even if the extruder can keep up with the acceleration/speed, the resulting print is going to be poor quality.

The problem with big bearings for pulleys is the mass. Every time the belt moves, you have to spin the mass of the pulleys, too, and 4 of them plus their 12 mm axles will be on the moving Y axis.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Making a large CoreXY printer
August 15, 2018 02:19PM
I reviewed Dice: it achieved 1000 mm/s and 10000 mm/s², but without extruding!

Sorry, I was not observing enough. And he uses small pulleys, which supports your words.
Re: Making a large CoreXY printer
August 16, 2018 12:31AM
Quote

I have problems boring exact holes in aluminium...

@Joerg
boring = langweilig
drilling ( holes ) = Löcher bohren
smiling smiley

Schönen Tag noch
Olaf
Re: Making a large CoreXY printer
August 16, 2018 01:29AM
Danke! (thanks ;-))

P.S. drilling is NOT boring ;-)

Edited 1 time(s). Last edit at 08/16/2018 01:34AM by JoergS5.
Re: Making a large CoreXY printer
September 01, 2018 06:15PM
I have swapped out the steel core belts and ditched a few pulleys. I changed a few things to accommodate the belt change and tried adjusting belt tension on the carriage. I was able to get the tension close enough for me this way, I measured a difference in movement on my Y axis of ~.2mm across the 700mm or so between linear bearings. I also swapped out the linear bearings that came with the rails with some Hiwin bearings. I didn't think the bearings I had were noisy so I was hesitant to pull everything apart to swap them out. I will say that the Hiwin bearings are significantly quieter, especially when things move fast. I messed with the alignment my Y axis rails until light taps would send everything sliding down the length of the rails, this reduced noise further. I called it good enough with a difference of .15mm variation in rail placement from one end to the other across the 600mm travel. I tried for a while to get this to be better but when I would set it dead on I would have some slight binding and the bearings wouldn't ride as smooth. I am fine with this and am pretty sure its just my frame not being perfectly square.

I am now back to messing with the Z axis, I am just unhappy with everything I have tried so far. I want to say that the 10mm rods I am using are just undersized for what I want, but at this point I don't really feel like buying more stuff again. At the moment I am trying two 10mm rods with longer linear bearings on the outside with a 1620 ballscrew in the middle. This setup is mimicked for each side. The problem I have with this is that I can get this to rock up and down a couple degrees and I think it is due to some flex/give in the 10mm rods. This does take a little bit of force, maybe more than the weight of anything I would print. I have not tried coupling the left and right sides together to see how hard it would be to move, maybe it would be fine. I intend to make a 2020 frame to couple everything together, with a 1/2" thick piece of MDF attached for added rigidity. This is all getting pretty heavy for the z axis, easily 30lbs of weight. On the topic of weight, I am realizing that this printer is going to end up weighing ~200lbs by the time I am done.
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