June 13, 2026
Description
This is a conversion from H-bot to coreXY for the ExtruH plotter which I designed a while back
Is it better? No, although it is different
Does it look cooler? Arguably, your mileage may vary
Should you print it? Probably not, the H-bot version does the job unless your machine is really large
This video shows my current driver board recommendation
I thought that it might be cool to have a coreXY version of ExtruH. Since I've used the earlier design I've noticed that the higher acceleration settings in the AxiDraw Inkscape extension produce very slightly wavy lines in some cases. This isn't very noticeable so it's not a huge problem but anyway is easily cured by using one of the lower acceleration settings. However I thought perhaps it might be improved by going from H-bot to coreXY since the two belts might allow a stiffer machine which might shift any resonances to higher frequencies which might be less visible or might happen only at higher accelerations
To cut a long story short: this didn't happen, while the resonant frequencies do seem to be different the overall behaviour isn't much better. As a result I don't recommend this design over the basic H-bot design unless for some reason you really, really, really want a coreXY machine or you want a really huge machine. This last point is important - in H-bot the main carriages keep the pen rail square to the main rails and while they are reasonably stiff they are only 3d printed parts so there's a limit to how stiff they can be. However in coreXY the belts keep the pen arm square when they're tensioned correctly and so with good belts a large machine might be better as coreXY.
To summarise if we think about the situation you're in:
I don't have a plotter and I think I might build one of these, which should I print? ExtruH
But I really want a coreXY machine and you can't tell me what to do. There might be better machines for that purpose, this one isn't bad but it's not noticeably better than the basic one
I have an ExtruH, should I upgrade to this? No, it's not really an upgrade and you'll probably need to buy more belt and pulleys
I want to build a really huge machine - will ExtruH be stiff enough? Maybe not, this could be a case where the coreXY design works
The parts in this model are only the ones that differ from the basic ExtruH design, this is not a complete plotter because I don't want to have to manage the common parts in two places. You'll need to go to the original to get the other parts
Parts that are used from the ExtruH plotter for this machine are marked with “ALSO USED ON ExtruXY” in the ExtruH parts listing, all other parts that are not so marked are changed and are not needed here. The general guide to which parts are changed or the same is:
The stepper mounts are the same for both but need height adjustment plates that are found in this remix
The plotter feet are the same in both
The pulley mounts & supports are completely different since this version has two belts and a different belt path
The same pen rail bracket lowers are used in both but the upper parts are different because of the changed belt layout
The pen carriage is almost completely changed, again because of the changed belts, this also include the belt clamps that are smaller because we now have 4 belt ends to clamp on the carriage in a fairly small space
The parts should ideally be printed in PLA for stiffness, PETG will probably work but I didn't test it. Use at least 3 perimeters for the main structure, more is probably better, and reasonably thick top/bottom solid layers
The parts are designed for 0.2mm layers and should print without supports at this layer height, if you use another layer height some upside-down counter-bored holes may not print correctly
Much of the assembly of the coreXY version is very similar to the H-bot plotter. If you want to check for the assembly and orientation of the parts the OpenSCAD file is included. There is a module in this file called assembly3() and this will allow you to see the arrangement of the parts used: please note that some of the parts are handed but are not just mirror images
Motor brackets cannot always be screwed directly to the 2040 rails on this machine, the motor driving the higher belt will need one or perhaps two motor height spacers, the lower belt motor might be able to use only one or zero. This is hard to define since it depends on dimensions of your motor shafts and drive pulleys. It's important to ensure that your belts aren't binding on either the top or the bottom of the motor brackets.
Lace one of the belts, then the other loosely from the pen carriage and around the idler pulleys in a "P" shape. You'll need to make sure that the correct parts are used for those that are handed, check in the assembly if you're not sure but once done you'll have one belt running around the correct pulleys and idlers. When feeding belt around the motors be careful not to spin the drive pulleys quickly if they are connected to your driver board as the voltages generated by a spinning stepper can be a bit spicy
Once you have the belt around all idlers and pulleys and moderately tensioned fix the belt to the pen carriage at both ends, we will adjust the tension using the motor brackets. Ensure that you have enough length of belt before you cut the ends to length. Bear in mind that with the belts in their loose initial fix you should have the motor brackets in a position that will allow you to add tension by pulling them out, away from the rest of the plotter
Ideally use smooth idlers where the back side of the belt is contacting the idler surface and toothed idlers where it's the toothed side. You'll need 6 toothed and 2 smooth idlers (this is a different number of idlers than ExtruH, which needs 2 toothed and 4 smooth idlers). All idlers should be 20t size for 6mm GT2 belt. You might(?) be able to get away with smooth idlers everywhere if you want to but to me this feels icky
For an A3 machine 5m of belt might suffice, be careful not to cut your belt until you're happy that you have enough length in the two sections. For a coreXY machine belt tension needs to be adjusted to keep the pen rail square with the main rails
Fix the feet of the plotter to your baseboard before you tension the belts to ensure a good fit between the main rail rollers on each side. Once you tension the belts the forces will pull the feet inward and might cause the outer rollers to lose contact with the rails which will reduce torsional stiffness - this isn't a massive problem but might cause some imprecision
If you are converting from an ExtruH you will probably need to check your motor connections, they might not produce the same movements that you're used to, be cautious will all movements until you know what's going on
Because of allowances for the pulleys on the top of the main carriages this machine is a couple of centimetres wider than ExtruH for the same length of extrusion; coincidentally this made it fit better on my old base plate but you'll need to check
Normally it's best to have the same tension in both belts. This can be hard to achieve because there's currently no really nice way to set belt tension, the only option is pulling the motor mounts until the tension is correct. It's possible to estimate the tension by plucking the belts at the end (where the lengths are the same and both belts run next to each other) and listening to the sound or by feeling how easy it is to push the belts aside. However I've found it to be easier to use this print https://www.printables.com/model/523382-creality-k1-belt-tension-meter
Print two of the tension meter models and put them both in the centre of the belts at the end, right next to each other - they should not interfere with each other (or at least they didn't with mine). When the belts have the same tension the meters should hang at the same angle. You can't correlate the angle with any specific tension because this plotter does not have the same geometry as the Creality K1 printer so the important point is that the angle is the same
To set tension slacken three of the motor mount screws, leaving the one in line with and furthest from the motor tight. When you're ready to set the tension slacken the final screw and pull on the mount while watching the belt tension guides. It's often easiest to pull the belt tighter than needed and then let off the pressure as this can often be done more precisely. When you're happy tighten the screw that was undone last while holding the mount in position. Once this is done re-tighten all screws
As with ExtruH this machine is designed to work using the Inkscape extensions for the AxiDraw - it might work with NextDraw software also but I didn't test this.
The software will need some changes made to allow convenient use, these are small changes made using a text editor and are shown in detail on the ExtruH page. In summary you need to adjust the size of the machine and account for its “portrait” orientation
For a while I've been running on a high-voltage brushless servo which can't be supplied with power by the EiBotBoard directly because:
It only outputs 5V and the brushless servo really wants 7V+
It can only safely output perhaps 200mA at 5V and the brushless servo can easily take more than 2000mA which would probably make the EiBotBoard very unhappy - the stalled brushless servo would probably appear as a dead short which might blow the converter that supplies 5V for the servo
As a result I initially made a terrible power supply board from perfboard and a cheap DC-DC converter but I wasn't happy to share it because it's easy to get things wrong and this might break expensive things
Now instead of my perfboard abomination I've had a PCB made which has the circuitry included so all that's needed is to solder on some power sockets, a DC-DC converter board and some pin headers for the servo signal in and out connections
Example parts are linked below - these aren't recommended, they're just links to show roughly what you need:
Power sockets like these (£4.99 at time of writing; pack of 15, you only need two)
DCDC converter like these (£6.49 at time of writing; pack of 5, you only need one)
Pin headers like these (£5.69 at time of writing, pack of loads, you need only a couple)
The barrel jack power sockets are the same ones as I've seen on some Arduino boards - if you have any broken ones of this type you might be able to harvest your parts by desoldering some
The KiCad source & output files are in the project files for this project - there are two zip files:
The KiCad sources (servoPower2_sourceFiles.zip) are used to edit the project if you'd like to make changes, if not you don't need them
The Gerber files (servoPower2.zip) needed to get PCBs made are in the other zip file, you don't need to unzip though, you just upload it to the service of your choice as-is
A note about PCBs: I used PCBWay, I got 5 boards (I only needed 1 but 5 is the minimum order) which cost about $5 but they have a $5 first order promotional coupon so the boards were effectively free and I only had to pay shipping which was also about $5 because I chose a slow service (faster or better tracked services cost more and your postage may not cost the same as mine anyway)
[I do have 4 unused boards but it will almost certainly not be worth me sending them to you, because postage will probably cost more for me to send to wherever you are than for someone like PCBWay to send from China to you. No, I don't know why either]
You might be able to get the board from here
The printable parts for the full-size servo are different than for the micro-servo pen lift. I've added two parts:
servoMountBL.stl - this is the mount for the servo that holds it on top of the 2020 pen lift rail
penLiftPlateMod & lifter 05.stl - this is the new pen lift carriage and lifter that engages with the servo arm, the lifter is joined to the carriage with M3 screws
I used the aluminium servo arm that came with my servo, it already had a threaded M3 hole at the end and I used this to fix a 10mm bearing to the end to engage with the lifter (this is probably overkill, and you may not need it)
License:
Creative Commons — Attribution — Share Alike
pittance
