D-bot conversion from 2 to 3 lead screw anyone made it?

claustro

Hi guys, I am studying some upgrade to my beloved modded D-bot.
New bed with three regulation point and 220v silicon heated bed are on the go .

Now I would like to convert the 2 lead screw structure to 3 lead screw.
On thingverse I found 2 mods
https://www.thingiverse.com/thing:2255835
https://www.thingiverse.com/thing:2651649
https://www.thingiverse.com/thing:2185911/comments
the first two use a lead screw support made by printed parts and 608 bearings the third one use kp08 bearing.

both use lead screw pulley of 60 tooth .
Is there anyone who made this mod and can suggest me the best way to procede? This is the first step if everything will be ok I'll stay with one motor solution if not I would like to go to three motor leadscrew with autolevel function

this is my printer now
https://www.dropbox.com/sh/7bvnlctmiqushv1/AAB_oR3lea8S2ZBIfeM5EsyUa?dl=0

thank you all
Andrea

Phaedrux

Well I'm partial to the second link, but I may be biased.

In case you haven't seen it there is a more detailed post here: https://forum.duet3d.com/topic/5282/my-custom-d-bot

To summarize the Z axis arrangement:

Three 400mm 1mm pitch 1mm lead single start lead screws.
1524mm continuous loop belt (though it could probably be shorter to give the tensioner more travel.
20T drive pulley and 40T screw pulleys for 2:1 gear ratio.
1.8 degree 2A stepper (17HS19-2004S1)
Printed bearing blocks with flanged 608 bearings. Though you could use KP08 pillow blocks with a little adjustment.

Through my experience with it, I have found 2 key things that you must keep in mind to prevent issues.

The quality of the lead screws and pulleys needs to be good otherwise you will see artifacts in the layer heights. This is true of any lead screw Z axis, but seems to be exaggerated since there are 3 lead screws with this arrangement.

The alignment of the bearing blocks, lead screws, and bed mounts must be nearly perfect, otherwise the tight pitch of the lead screws can lead to binding. This requires carefully loosening and tightening the bed frame in the correct order to isolate movement in either X or Y. This also applies to the Z V wheel carriages. Start with tightening the carriages and work from there. When done right, you can lift the entire bed frame with a single finger and it will ride smoothly.

Is it the best way of doing it? I don't know. It's tricky to align. But once set, it's stable. I can totally see the appeal of using 3 independant lead screws.

claustro

Thank you for your answer. I am quite worried about all the constraint a single motor lead. I leaning to a 3 indipendent motor solution ...
Another problem is : where to find good quality lead screws at reasonable prices? I always bought on aliexpress and I am afraid other sellers provide same stuff overpriced.

How you managed backslash?

Thank you
Andrea

Phaedrux

I am quite worried about all the constraint a single motor lead.

I don't understand what this means.

If you do decide to go with the 3 independent motor option, you will need to ensure there is some flex in your bed frame to allow for the tilt adjustment while the motors are being leveled. The DBot wasn't really designed to allow for that. It looks like you are using 4 Z wheel carriages with the extra front beam. I would recommend removing the front wheels and beam and using 2 of the lead screws to support the front and the single lead screw at the back in the middle between the wheel carriages. This should allow for enough flex.

I'm not sure where you're located, but I would try to find lead screws from as close a source as possible to reduce the chance of shipping damage. Ebay may be a good choice, but there are good sources local to Europe and North America.

There is very little backlash on the Z axis because you're generally moving in only one direction and gravity keeps it loaded in that direction. The 1mm pitch/lead lead screws also have such a fine pitched thread that there isn't much movement possible. This also keeps the bed from dropping when power is removed.

claustro

I was thinking about 3 single motor as the simplest solution in therms of fine tuning but maybe I was wrong.
I am using 3 wheel Z carriage but not in cantilever configuration.Do you think cantilever would be a better option in case of 3 single motors config or even with only on Z. Doesn't your printer use the extra front beam. I am from Italy
Regarding the backslash you are right, maybe I have to stop using vertical lift on retraction.
I confused

Phaedrux

For 3 lead screw independent motor leveling I would use the cantilevered bed frame arrangement so that there is some room for flex during the leveling routine. If there is no flex, there will be binding.

For the single motor setup, I would recommend using the front beam and all 4 corner wheel carriages for the rigidity. As I have done in my build.

I use Z hop occasionally. It does work, though I try not to use it because the bed is very heavy and it puts a lot of strain on the motor even though there is a 2:1 gear advantage and the 1mm pitch lead screws don't have much rotational resistance. Sometimes it is needed though, and I haven't noticed any issues from backlash or positioning errors.