ExtruXY (remix of ExtruH)

Introduction

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

Background

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

Printing

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

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 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 geometry that's designed to avoid supports may not work properly

Assembly Differences

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 and 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 how big your drive pulleys are. 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 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. 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.

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 mostly needs 2 toothed and 4 smooth idlers). All idlers should be 20t size for 6mm GT2 belt

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

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

This is a conversion from H-bot to coreXY for the ExtruH plotter

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