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Phaedrux

DBot 3rd Lead Screw Belt Tensioner (easier print)

by Phaedrux Nov 17, 2017
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I'm in the process of assembling a similar 3 lead screw D-bot, and am at the point where I'm lining up the lead screws. However, it seems that the rear bearing mount along the center of the rear rail doesn't align with the corresponding nut on the gantry; it's out by about a centimeter. (center of bearing is about 1 cm forward from lead screw threaded down from gantry)

I notice that you have a 3rd z-screw part in the files that I didn't notice which seems to account for this difference; is this the part meant to be at the back, rather than the flanged-bearing mounts used at the front? (I had assumed I'd just need 3x flanged bearing mounts) If so, how do the spacer / second bearing blocks work with it, given the difference in centers? (Also, it doesn't seem to take a flanged bearing as there is no indent like the flanged bearing mounts)

I'm out of ABS at this point, so I want to make sure I know if and what I have to print before I order any more (especially since I'd likely not use the rest of a new roll afterwards)

Thanks!

If you're referring to the extended lead screw bracket file, then yes, that accounts for the difference in alignment. The flanged bearing blocks are the same on all 3 at the bottom. The extended lead screw bracket takes the lead screw nut. I actually use the extended lead screw bracket for all 3, not just the back, though you could get away without using it on the sides since it's easier to slide the bearing blocks around.

Does that clear things up?

Not quite. The issue is that with the flanged_608_bearing_holder_plate and block are centered, but the rear lead screw has to use an offset hole. It doesn't seem that there are modified plates/spacers to account for this dimension change in the files. After printing the replacement Z mount (3rd Z screw), I can confirm that the holes do not line up with the other bearing blocks. So I don't quite understand how you're using the bearing blocks provided with the replacement Z mounts.

Did you modify the z plates/spacers at all?

I did not modify the bearing blocks. Perhaps this photo will help explain the arrangement? Hopefully you can see that the lead screw is on the inside of the bed frame. Perhaps that’s where the misalignment is for you?

Hi , I am going to make this mod on my d-bot can I ask you some questions?

  • After almost 1 year of use of your Dbot do you have some suggestions or something you would do differently ?
  • Are you using 40T pulley for threaded road?
  • It seems that you switched from 2 to 1 mm pitch for the lead screws is it right? Did you find improvment doing this?
  • I saw you have a lot of experience with this printer what do you think of this mod? https://www.thingiverse.com/thing:2365378

Thank you
Andrea

My D-Bot's Z-axis Setup
by 1sPiRe

Hi, overall the mod has worked well.

Things I may do differently, I might try using pillow blocks for the lead screw mounts at the base because they have a bit more flex to allow for self alignment, as well as some oldham style couplers at the lead screw bed mount to allow for more self alignment. The mod as it is requires very good alignment during assembly. The Rexipus mounts are very stable, but they don't allow for any flex, so the alignment is key.

For the pulleys I'm currently using 20T pulley on the Z motor and 40T on the lead screws for a 2:1 gear reduction and 1mm lead 1mm pitch single start lead screws and a 1.8 degree 2amp motor. I chose 1mm because it has the lowest torque requirements and is basically flat as a table so the bed will not drop at all when power is removed. I think 2mm would also work well, so if that's what you have on hand, go for it.

I'm not sure I like the idea of using outward pressure only to tension the Z axis wheels. Especially using printed parts. Eventually the pressure will deform the plastic somewhat and it will loosen. It's also not how v wheels are intended to be used. I much prefer the 3 wheel z axis mod from Nick Rimmer. It clamps to both sides and has a tensioner screw built in.

Good luck with your z axis modding.

I was thinking about pillow blocks too , it seems to me the simplest and tidiest solution , but I was in doubt about it because nobody seems to use them and I was worried about it, there is some contraindication in using pillow block that I am missing?
thank you

I think the tricky thing about the pillow blocks is that you have to find the right place to mount them so that the lead screws can line up with the bed. Then you also have to find a place to mount the motor, and line up the drive pulley with the lead screw pulleys. With the Rexipus bearing blocks they mount inside of a standard dbot motor mount, so everything lines up vertically and the way I have them arranged everything lines up with the bed mounts.

I'm sure that a combination of KP08 and KFL08 pillow blocks could be arranged to work just fine. It would be a lot easier if you have some additional aluminum extrusions to work with to get the placement right. It can also be a challenge to find good quality pillow blocks for a reasonable price. You'll also want to use some sort of self aligning bed mount like an oldham coupler like this https://www.thingiverse.com/thing:2858698 or like this https://www.thingiverse.com/thing:2907060

You'd also have to check that switching to pillow blocks doesn't change the required length of the lead screws too much.

Small Oldham Coupler for Tevo Tarantula
Mini Bearing Oldham Coupler

Thank you for your help , I checked your thread on Duet forun also :)
I am thinking about following this line https://www.thingiverse.com/thing:2185911/comments
even if he connected the T8 bar with 2 pillow holder , one at the base and one at the top of the frame. Isn't it over constrained?

Why you used the same 40t pulley on the motor? Isn't better to use a lower pulley then the lead screw for increasing torque. We don't need speed on Z axis.
Thank you

Three lead screws and one nema 17 motor remix for C/D/J-bot
by oeyhaga

I definitely would not put pillow blocks on the top and bottom, because it would be overconstrained and you'd be fighting with alignment again. At that point you might as well just use the double bearing blocks like I have since it makes the vertical alignment easy. Better to leave the top free of constraint and use a self aligning bed mount like the Oldham. Also, If you go with the pillow blocks consider adding a thrust bearing to take the downward load of the bed and add locking rings above and below the pillow block.

Originally I used 40T pulleys on both the drive and lead screw for a 1:1 gearing, which was ok, but my bed is heavy and at the bottom of the travel extent where the belt pulls the lead screws together it would sometimes bind/skip steps when reversing direction. So I switched to 20T on the drive and 40T on the lead screw for 2:1 reduction. The additional torque from the gear reduction helped to fix that. I also reduced acceleration for homing moves which helped for those occasions when homing after a very tall print and the bed was at the bottom. The gear reduction also means the motor has to spin twice as long which allows more time for a smooth acceleration curve. The 2:1 reduction also kept the math simple for the steps increase. The Z axis resolution is total overkill now though, with the 1mm lead screws. 2mm lead would probably be ideal.

Thank you for all your suggestion and times I surely go with thrust bearing setup. I need to study how to assemble all this stuff together :-)

This can really help with getting things laid out if you use Fusion360.

https://www.thingiverse.com/thing:2430436

Fusion360 D-Bot Core-XY 3D Printer
by axelsp

Yes I already downloaded it TANK YOU for all your precious suggestions you helped me a lot.
I am posting my solutions here feel free to post what you think about it :-) and excuse me for having spammed with all my stuff your "make" here .
https://forum.duet3d.com/topic/10260/best-way-to-manage-with-vertical-forces-on-lead-screw

THX!

My build is coming along nicely...do have a question...did you use the original z threaded rod length of 400mm or did you lengthen them for the z-mod. I have not yet mounted my hot end, but it seems (visually) that I might not reach it.

Bart

I used 400mm lead screws. I have a titan aero which isn't as long as a regular V6 style.

https://cdn.thingiverse.com/renders/6b/eb/a9/56/9f/1d95779a4b00b3e65e15e656b0a4e63b_preview_featured.JPG

In this image you can see the side of the printer with the lead screw mount on the bed. Orienting it this way gave enough reach. When the bed is raised to the nozzle the top of the lead screw is just inside of the nut.

500mm lead screws were actually much too long and ended up getting in the way of the print head.

Thanks for the response, my screw mounts are oriented in the same fashion. I'll have to mout the hot end, that way i'll know for sure.

Really like the layout of your machine.Building mine almost identical...
By chance do you have the files or links to the files for your flexible camera mount and the cable chain/mounts?

Regards, Bart

Sure, for the camera arm I used various pieces from these things. I've added them as remix sources as well.
https://www.thingiverse.com/thing:2837458
https://www.thingiverse.com/thing:2734389
https://www.thingiverse.com/thing:2194278

My original intention was to use a cable chain for the extruder, however I couldn't find mounts that I liked or that were large enough to hold all my wires, and the added weight and noise eventually dissuaded me from pursuing that path and I went for a flexible wire conduit wrap around the extruder harness bundle instead. I still used the cable chain mount as a guide on the frame because it was already mounted and seemed to work to hold the bundle in a nice radius.
https://www.thingiverse.com/thing:2354196

For the bed cable chain I used the chain pieces from Spauda01 and these end mounts.
http://www.thingiverse.com/thing:1688921

Glad you like the arrangement. It's turned out to be a very capable and reliable machine.

Modular Mounting System Extensions
by Inhibit
V-Slot GoPro mount with extension arm
Modular Mounting System
by yyh1002
15x25 Cable Chain for D-Bot
Cable Chain Remix for Pro Operation

Going to follow your lead on this for my 300x300 build. Great design should work smoothly
I figured out the bearings/belt/pulleys by what you posted in the details.
By chance do you have a hardware list for all the nuts/bolts/washers used for this mod?

Regards, Bart

No, sorry, I don't have one condensed list with all the parts from all the mods, I do have a BOM for the electronics and such. Just not all the nuts and bolts.

However, I used the 300x300 base BOM from the stock dbot, then each mod is listed, and they each list their own BOMs complete with nuts and bolts.

I have uploaded the BOM for the electronics and other parts though.

Love the design, just wanted to know what size bearings this uses? Thanks

Which bearings specifically do you mean? There are a few to choose from. If you mean the bearing blocks for the lead screws they use flanged 608 bearings.

Did you have to change the stepper motor for this ? When I try something similar one stepper can not lift the bed as its starts missing steps / slipping. I have
Bipolar Nema 17 Stepper Motor with high torque
Each phase current is 1.7A (at 12V) & phase resistance 1.5ohms, and the holding moment is 40Ncm (56.2oz.in).

I can stop the motor moving with two fingers holding a coupler on the shaft which does seem like to little torque.

The driver is set to 1v on the ramps board, microsteps 16 I think. Any other ideas ?

Thanks

I made a few design choices to try and minimize problems like that.

  • DuetWifi with powerful drivers
  • 1.8 degree 2.0A motor with 65oz.in.
  • 24v power supply
  • 2:1 gearing on Z axis. 20 tooth drive and 40 tooth on the lead screws.
  • 1mm pitch lead screws for low torque requirement to move
  • The bed is evenly tensioned on all 4 corners and it raises smoothly by hand
  • The lead screws are perfectly aligned from double bearing block to bed mount so there is no binding.

It should be very easy to raise and lower the bed by moving the belt by hand. If it's hard to move by hand you've got binding and the motor won't be able to move it.

Thanks. I think it is a binding problem as I moved things about a little and the bed is now easier to move by hand. I think I will also do the 2:1 gearing as well as a first step.

I should have also mentioned that low speeds, jerk, and acceleration values also help. For instance, I probe at 1.5mm/s, travel at 5mm/s. Acceleration 0.5mm/s and jerk 0.25mm/s. The low acceleration helps the most. The lead screws spin quite fast due to the low pitch angle, so they can carry quite a bit of momentum at the end of a top speed travel. The low acceleration gives it a nice ramp up and down. I could travel at 10mm/s without any real problems, but you can really tell the lead screws want to over rotate at the end of a move. So I use pretty conservative speeds just to reduce the chance for backlash.

It may sound slow, but it doesn't really come into play very often. It's rare that you ever travel very far in Z. Dropping the bed by 50 or 100mm for easier access to the print head takes maybe 10 seconds. I have a Z max endstop so I can rehome and resume a print after a power loss, and even that full travel all the way down and back up isn't that bad.