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Hbot torque vs corexy

Posted by Tinchus 
Hbot torque vs corexy
September 03, 2020 12:00AM
Hello. what would be advantages and disadvnatgaes of hbot vs corexy systems. Hbot looks like mor simple but also more difficult to have it running properly because of the torque induced to the frame.

Edited 4 time(s). Last edit at 09/05/2020 11:07AM by DeuxVis.
Re: Hbot linear guides problem
September 03, 2020 06:05AM
The problem is not the design, the problem is inherent to an HBOT gantry. This is exactly why people moved on to using CoreXY setups which produce much less skewing of the axes. I suggest looking at CoreXY, which will work wonders with Hiwin rails.

Re: Hbot linear guides problem
September 03, 2020 08:16AM
Thanks for the answer. There is a lot of debate here in de forums regarding wich system is better. Corexy has advantages but also disadvantages. Im trying to move towards a hbot becuase in my case I can setup all the hardware a good HBOT systems needs.
So I prefer in this setup to stay in the HBOT setup.
Any ideas how to overcame this problem?
Re: Hbot linear guides problem
September 03, 2020 09:46AM
The problem is that the bearings on the ends of the X axis are too short. Add a second bearing block spaced some distance from the first one. The spacing will depend on how long the X axis is. The extra bearing length will reduce movement in the Y direction unless you get longer guide rails and maybe build a bigger frame for the machine. You might find this interesting and useful: [www.machinedesign.com]

What was the claimed "advantage" of H-bot over coreXY that made it worth all this?

Edited 1 time(s). Last edit at 09/03/2020 11:10AM by the_digital_dentist.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Hbot linear guides problem
September 05, 2020 08:32AM
Well. I have been using mostly cartesian I3 style printers and made big ones using always the corexy system. Doing research, some calculations and observations, both corexy and hbot have advantages and disadvantages. Big and very precise printers uses Hbot. I have observed stratasys printers using it for example. That has to mean something.
Then, to my opinion:
.- corexy is somehow more simple to setup regarding mechanics because you are not dealing with the torque that hbot induces to the printer's frame. Corexy cancels many forces hbot doesn't.
.- in contrast, hbot uses a lot less belt, and I think this can give you more precision especially when going fast and with heavy printheads. The spring effect a log belt has con not be ignored, even if you use very expensive belts. If you have 3 o 4 meters of belt, it doesn't matter if you have a stell core on them, the belt will have come spring effect. Hbot will use half of that belt for the same printer size so you will get a benefit in this issue.
.- I have seen you articles regarding the use of steel cable as belt, and I have tried it, but using steel cables makes a new severe problem: the wear on the idlers. But since you tried it, I guess you will agree on my about the spring effect on long belts.
As always, Im super interested in your comments digital dentist.
Re: Hbot torque vs corexy
September 08, 2020 10:08AM
From what I have seen of amateur attempts, it is easier/cheaper/more compact to build a sufficiently precise corexy mechanism than an equally precise H-bot.

My limited experience with cable drives is that they are very troublesome. Of course, I only made one attempt to use cables so maybe more experienced people can avoid the probable many mistakes I made. That said, doing anything with cables is always more trouble than using belts. Cables are friction drive systems, and in order to get sufficient friction between the drive pulley and the cable, the cable has to wrap around the pulley multiple times (but how many times is enough?). That means there is offset between where the cable goes on the pulley and where it comes off- you can't just let the cable scramble-wind around the pulley. That means the system has to be designed to include that offset. Just like belts, specific cable segments have to be kept parallel to the guide rails in all 3 dimensions. The motor mounts I designed used multiple steering pulleys to ensure that the cable always wound and unwound from the drive pulley the same way. Two of the problems with that design were the complexity of weaving the cable through all the pulleys and the noise that all those pulleys made. Any adjustments that relaxed the cable tension caused the loops of cable on all the motor mount pulleys to come off those pulleys and it all had to be put back into position before I could run the machine again.

I used dyneema fishing line- it was very stretchy stuff. You can use steel cable instead, but then you have to ensure that all pulley diameters are large enough not to cause the cable to wear/break from having too small bend radius.

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