D-Bot 3D Printer

[tekkydave]

The D-Bot has been performing well since my last update. The only major change was to use optical X & Y endstops. I was using sensorless endstops which relied on detecting motors stalling as the carriages hit the endstops. It worked ok but was a bit noisy.

I have been thinking a lot about bed levelling recently. I have 3 independent motors so the Duet can calibrate the leadscrews by probing the bed next to each leadscrew. It then uses G32 to automatically level the frame which the leadscrews are attached to.

Despite this I still had 4-point levelling of the bed plate itself relative to the frame which sort of defeats the object.

So... I have decided to redesign the bed plate mounting to be 3-point. Each of the mounting points will coincide with the lead screw positions. The 6mm plate will be mounted rigidly to the same part that the leadscrew nut is fitted to. This way the G32 bed calibration will level the plate correctly. I hope that makes sense

I have bought a slightly larger plate 340x340x6mm as it needs to overlap the 20mm frame for mounting purposes. I haven't decided how I'm going to hold the 300x300mm glass plate in place yet but I want to get away from clips. I have also bought a new 300x300mm 750W 220V silicone heating pad. I don't fancy trying to peel the old one off the old plate - its well stuck down.

I'll post some pics once I've edited them.

 

Edited March 16, 2019 by tekkydave

[tekkydave]

Some pics of progress so far

Before: 3 leadscrew levelling of the frame with 4-point levelling of the bed (on weedy 3mm bolts)

 

The new frame with new 300mm cross piece. This supports the centre-back leadscrew and bed mount and also adds some extra rigidity. The bed mounts are equidistant.

 

Newly designed common leadscrew and bed mounting part. The bed will mount using M4 low-profile bolts to an M4 nut embedded in the part.

With leadscrew nut fitted

That's as far as I have got today. Next job is to fit the frame back into the printer and line up the screws. I'll drill the new plate once I'm happy with the screw positions and the levelling is working correctly.

[tekkydave]

Z carriage is back in the frame. All running up/down very smoothly at 10mm/s.

Next task is to measure & drill the 6mm plate.

[tekkydave]

I will have to rethink this design. Because of the extra cross-piece of 4020 extrusion there is no longer sufficient 'flex' in the z carriage for the 3 screws to level it correctly. Especially when the 6mm plate is bolted on it tends to pull the plum couplers apart rather than level the bed.

[Gina]

You're getting twice the Z axis speed that I get in my Concorde printer.  Mind you my print bed is 500mm x 500mm, weighs around 5Kg and driven by just two single start threaded rods.  OTOH the large 3D printed Z carriages do make the bed sufficiently level not to need bed levelling - the RMS error was 0.106mm over the 400mm x 400mm printing area with most even closer.  I was amazed!

Edited March 18, 2019 by Gina

[Dr_Ju_ju]

I also added another 2040 rail, as per the rear rail (full width, with rollers), so my bed sits on a moveable frame within the frame, but I only then need two motors for the Z axis. 

Then, using four corner bolts, I levelled & fixed the bed, and, touch wood, I've not had to re-do it...  

[tekkydave]

I tried having wheels in all 4 corners with 2 leadscrews in an earlier iteration. I still had some rocking of the bed which is why I went to 3 screws. Once I had that I found having 4 sets of wheels overconstrained the frame causing binding. So I removed the front wheels and everything was good.

I think I might be backing out my lastest changes, although I'll keep the bigger 340x340mm bed.

[tekkydave]

I gave up on having 3-point levelling of the bed. I couldn't get it to level correctly; one corner always seemed to be sagging.

[tekkydave]

I printed some M4 bed adjusters and reverted to 4-point bed support. I put the old leadscrew nut holders back too.

The low-profile M4x30mm bolts arrived in the post this p.m. so they will replace the bolts in these pics.

 

[tekkydave]

I'm getting much better levelling now.

 

[Dr_Ju_ju]

Instead of the M4 bolts with a head, I used countersunk bolts, with corresponding countersunk hole in bed, and like you, locking bolts, to tie everything down after levelling...

[tekkydave]

The nuts below the bed lock the bolt to the bed. There is a captive nut in the centre of the knurled wheel. Once adjusted they dont move unless I turn them

I could add locking nuts but so far I haven't needed them.

[Gina]

Now that is what I'd call good levelling!! 

[Gina]

I'm thinking of building another 3D printer which will be similar to the D-Bot.  I could do with a printer in between my Mini and my Concorde.  Mini has an aluminium extrusion frame and is working well.  Concorde was based on a very substantial wooden box and though solid has come out bigger than the replacement for the Titan printer it was supposed to be.  In fact it's more a replacement for my Giant printer with the same size print bed.  Whether I shall dismantle the Giant I haven't really decided - it depends how much "stuff" I can manage to part with to make space.

[tekkydave]

The standard D-Bot can be scaled quite well up to a point. Ooznest do rail kits for 300x200mm or 300x300mm.

The main weakness in the basic design is the dual leadscrews. It definitely needs 3. Using 4020 for some of the sections also gives it a good stability.

[Gina]

The problem with the D-Bot Z system as I see it is that there is insufficient height in the Z carriages to hold the bed level.  Here are a couple of diagrams from the design of my Concorde printer where the bed has proved to be very level after adjusting the Z drive one side manually for a level X axis.  The Y axis was level within a tenth of a mm in 400.  Of course, the Z rails needed to be accurately vertical to start with.

[tekkydave]

Agreed. To be fair the D-Bot original design only carries a PCB-style heated bed which weighs next to nothing. Putting a 6mm cast ali plate and 3mm borosilicate glass on it like I have breaks the design a bit

I have resolved that one way, others have other solutions. Having 3 screws, whether independent or driven by a single motor + belt(s) sets the bed very accurately in the X-Y plane. The wheels at the rear just stabilise the position and support very little weight.

Your solution for the Concorde would work on the D-Bot but would also steal a significant amount of the Z height. I suspect using the v-slot wheels will always produce a certain amount of 'give' in any system. The next evolution would be to go to linear rails. Maybe next year ?

 

[Gina]

Ah, the continuing pursuit of a better 3D printer!  ?  I'm certainly not there yet.  The space taken up by the Z carriages I'm using for the filament spool but I agree, it does reduce the Z print range unless you increase the overall height to compensate.  I shall continue thinking and researching before starting a new printer - I don't intend to start another printer build just yet. 

The print bed from the Titan seems alright - just the bed, the Z drive is another matter.  This bed has a 300x300 mains voltage silicone heater pad stuck directly onto a 3mm borosilicate glass plate of the same size, 12mm thick polyurethane foam underneath and all supported by 6mm plywood and surrounded by a frame of PETG.

Edited March 23, 2019 by Gina

[Gina]

Read this thread again and I'm confused (it's not unusual!).  Am I right in thinking you have :-

1. manual bed levelling with four adjusters - one in each corner. 
2. three threaded Z drive rods.
3. three separate Z motors.
4. individual control of the Z motors.

?  When I feel like a break from current project (ASC) I'm thinking about my next 3D printer with 300x300 bed and wondering about how to arrange the Z drive.

[tekkydave]

17 minutes ago, Gina said:

Read this thread again and I'm confused (it's not unusual!).  Am I right in thinking you have :-

1. manual bed levelling with four adjusters - one in each corner. 
2. three threaded Z drive rods.
3. three separate Z motors.
4. individual control of the Z motors.

?  When I feel like a break from current project (ASC) I'm thinking about my next 3D printer with 300x300 bed and wondering about how to arrange the Z drive.

Yes, spot on.

The 3 Z motors are defined individually in config.g, but act in concert as one z axis. The Duet Wiki explains how it works.

Setup is a 3-stage process which is all run by a single macro:

1. Calibrate the 3 leadscrews by probing adjacent to them and using G32 to set the z axis frame level i.e. parallel to the x & y axes. The Duet will move 1 or more leadscrews individually to make them all the same height.

2. Manually adjust the 4 corners of the heated plate so it is level wrt the z axis frame. This can take care of any slight sags or twists in the plate. Once done this is rarely adjusted.

3. Run G29 Mesh Grid Compensation to create the heightmap.csv file. Mops up any remaining imperfections in the plate & glass.

 

 

[Dr_Ju_ju]

I have a 300x300 6mm ally plate bed, SSR driven & a 0.2 mm PEI sheet on top, with 2 Z-motors driving the 2040 frame within a frame setup....

[tekkydave]

5 minutes ago, Dr_Ju_ju said:

I have a 300x300 6mm ally plate bed, SSR driven & a 0.2 mm PEI sheet on top, with 2 Z-motors driving the 2040 frame within a frame setup....

Do you have any pics? Sounds interesting.

[Gina]

How about :-

1. Have the three Z motors controlled as one say by connecting the windings in series and running from 24v. 
2. Adjust the bed height by turning the motors by hand before any power is applied. 
3. Then level the bed manually with the four corner adjusters, if necessary. 
4. Finally, if you're really fussy, run the Mesh Grid Compensation.

TBH though, I would have thought a glass plate would be flat enough.  I have a 500mm x 500mm x 4mm float glass plate on my Concorde printer and having manually levelled the bed I found it showed only 0.1mm RMS error on Mesh Grid Compensation, probing a 400x400 area.  I consider 0.1mm accurate enough.

[Gina]

Just thought of where the error may be coming from, Dave.  You have a thick aluminium plate with glass plate on top.  I guess if the ali plate is not perfectly flat it could bend the glass to its profile.  Why not have the heater pad stuck directly to the glass?

Edited March 24, 2019 by Gina

[tekkydave]

20 minutes ago, Gina said:

How about :-

1. Have the three Z motors controlled as one say by connecting the windings in series and running from 24v. 
2. Adjust the bed height by turning the motors by hand before any power is applied. 
3. Then level the bed manually with the four corner adjusters, if necessary. 
4. Finally, if you're really fussy, run the Mesh Grid Compensation.

TBH though, I would have thought a glass plate would be flat enough.  I have a 500mm x 500mm x 4mm float glass plate on my Concorde printer and having manually levelled the bed I found it showed only 0.1mm RMS error on Mesh Grid Compensation, probing a 400x400 area.  I consider 0.1mm accurate enough.

No reason why not. I just like everything being automatic - that's just me really