Print bed leveling - corners level, center not

I am having a problem properly leveling my bed. I have been using the paper feeler guage method to level the corners and I can fairly easily get all 4 corners completely level. The problem is that the center of the heated build platform and/or the glass that sits on it, is bowing down in the center. The difference is pretty small and it wasn't even a problem until I switched to a new extruder that takes 1.75mm filament, then I started getting the filament curling up under the nozzle and sticking to the bottom of the nozzle, rather than sticking to the bed. I made sure that my screws are properly spaced so they aren't putting inward pressure on the corners of the bed. I also moved to bulldog clips from standard spring steel paper clips and that helped some, but the problem still persists and is making my first layer fail to stick now that I'm using 1.75mm filament instead of 3. Any suggestions?

Are there four leveling screws at the corners of the bed?

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Yes there are 4 corner leveling screws.

There are many factors at play here.
- the bed screw holes, if fitting snuggly can cause warping when heated
- the x smooth rods slightly bends when the carriage is in the center
- glass plate, heated bed and y carriage may not be exactly flat, and deform when heated up
- some 3 point bed leveling systems don't support 2 of the 4 heated bed corners allowing them to bend and flex freely.

I've just changed from a 3 point wood y carriage to a 4 point aluminium carriage and the bed is now much more stable and consistent. I've removed the springs and use big fenders to support the heated bed. It's much flatter and stable. I use auto bed leveling grid which removes the need for adjustment.

You could try finding a glassplate that's lower in the center than in the corners to compensate the x smoothrod sagging ie flip it around. Bending the heated bed may also help. After say 100 heatup/cooldown cycles the heated bed will conform to the glass plate flatness.
You could also try raising the centet by putting a few fenders under the heated bed center. The clamps should be strong enough to bend the glass plate or use a thin aluminiumone.

Try heating the bed, loosen the corner screws so the tension is removed and refasten them

Edited 1 time(s). Last edit at 09/14/2015 12:17PM by imqqmi.

Thanks for the suggestions. I will try the last one first, as it is the easiest to attempt. I was actually thinking along the lines of a screw pushing up on the center to push the center into shape, I just need to find something to spread the load out a bit. I could probably print something to fit on the end of the screw. I am able to print sometimes, I just have to set my first layer extrusion width up to 200%+, remove the stringy filament from the nozzle, just before the print starts, and sometimes try multiple times before the adhesion is good.
As far as the x axis smoothrod sagging, that doesn't seem to be part of the issue as the nozzle is further away from the glass in the center than in the corners, not closer, if the x carriage was sagging, it should be closer in the center. I'm just curious, does the auto-bed leveling help in cases where the bed is curved?

Edited 1 time(s). Last edit at 09/14/2015 12:53PM by thedoktorj.

I usually extrude 6mm or so in the startup gcode:
G28 ; home all axes
G29 ; auto level
G1 X0 Y0 Z5 F5000 ; lift nozzle
G92 E0
G1 E6.000 F900.000 ;pre extrude some filament to help start up.
G92 E0

3 loops around the object does the rest to get the extrusion up to speed. When it's actually printing the first layer of the object it's fully going. I don't use the 200% extrusion width. Just get the z offset properly set and use a slow speed like 20mm/s. I print on vinyl for PLA, ABS and HIPS, transparent vinyl table cover, heat up the glass plate to 60 degrees and put the vinyl on with a plastic dough scraper, works very well and ABS sometimes rip up the vinyl when trying to remove, that's how well it sticks. You can take off the print with vinyl and all and peel off the vinyl. For nylon I use a glue stick.

Unfortunately no, auto bed leveling does only rotation, not bed curvature. You really need to sort this out mechanically. It's not hard, you just need a good flat y carriage and flat piece of glass. After a while the heated bed will conform to the glass plate. Auto bed leveling shows the measurements, mine are within 0.1mm. It does make adjusting the z-offset much easier and prints will start properly 99% of the time you start a print instead of 33%.

I sometimes use a cotton cloth clamped down near where the nozzle starts in the corner to remove any filament just before starting. HIPS doesn't leak that much so I removed it for now. I'm going to use HIPS more than ABS unless I need something that's more heat resistant like a print for the extruder/hot end holder.

For some it's still not good enough but I don't have issues with filament not sticking to the bed and prints come out pretty well. In Some occasions the ABS warps a bit but seems to be also related to the design you use. If you use some holes to relieve material stress and account for shrinkage it can be managed. It just takes a bit of experience how certain materials behave under what conditions. I learn something new every day

That's a big piece of the problem.

Three points define a plane, 4 points define a potato chip. When you adjust one of the corner leveling screws, the bed attempts to pivot about the diagonal formed by the two adjacent corner screws. Since the diagonally opposite corner screw doesn't let that corner move up and down, the bed flexes instead of leveling. The problem is compounded by using an undercarriage and bed plates that are too thin. When you tighten the bed leveling screw, the undercarriage pulls up at the same time the bed plate corner pulls down.

The thin undercarriage isn't really a problem if the leveling scheme actually levels the bed. That means 3 points only, and if they are arranged right, only two of the screws ever need to be adjusted to level the bed.

There isn't much you can do with four screws to make it better except to keep messing around with it until you manage to get the bed into a shape that works well enough to print. I have no idea why the printer came with 4 leveling screws. Maybe the designer never had a geometry class.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

@imggmi: Yes, I am using the extrude at least 10mm setting in slic3r and 3 skirt layers to force the nozzle to get cleared out first. This definitely helps, but when that initial skirt extrusion doesn't stick to the bed, it winds up wrapped around the nozzle which in turn grabs random pieces of properly stuck pla and pulls them loose. I tried using custom g-code to extrude out into open air, but that also winds up stuck to the bottom of the nozzle and winds up pulling properly stuck pla off the bed. I may give the piece of cloth on the bed a try though, that could help. I've been using 15mm/s for the first layer, that helps some, but due to the curvature of the bed, it still doesn't stick properly, unless I push my 1st layer extrusion width up to 175 or 200 %. Right now it seems to be doing pretty well with 200% extrusion width on the first layer, 15mm/s speed on the first layer and .3mm first layer height. .25 mm first layer height or 150% or below on the extrusion width for the first layer and it completely fails. It works off and on at 20mm/s for the first layer, but not reliably. Strangely, I didn't have this issue with my previous 3mm hotend, despite the bed being even more bowed than it is now, and the nozzle sizes being the same.

@the digital dentist: This printer is from a kit I bought about 2 years ago. The bed is a mk1 PCB heater which has 4 holes instead of 3, unlike the newer ones. Job related issues caused the project of building this thing to be put on hold for more than a year and I only got it out of storage a couple months ago and finished building it. At some point in the near future I've got plans to build a prusa i3 and then go back and update this thing to a reprappro style i2 with a newer model hbp and all, but for now, i've got to make due with what I've got.

When the time comes to rebuild or build another printer, look into 3 point leveling. It actually levels the bed without flexing it or the undercarriage. If you follow the link in my sig, below, go to step #7 to see how a 3 point leveling system is done right.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

One question: if you have an aluminium heatd bed, useing the 3 point levelling doesnt allow the alluminium to warp in teh edges? I think I read something like that somewhere and that is why the 4 points in prefered in those cases?

Using thin, flexible aluminum sheet for a printer's bed is an error. Using 4 leveling screws guarantees that it will not be flat.

Here's an explanation of 3 point vs 4 point leveling: [drmrehorst.blogspot.com]

After you read that, take a look at this: [drmrehorst.blogspot.com]

And finally, take a look at this: [drmrehorst.blogspot.com]

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

What I have is a mk3 aluminiun heaed bed, is that considered thin? It is 3mm and as I know is an alloy of aluminiun

The aluminum is just extruded sheet and won't be flat, and the 4 screws "leveling" system will further ensure that it isn't, and because it is extruded or rolled, the grains will be oriented mainly in one direction so thermal expansion won't be uniform (read warp). You're expected to put a piece of glass on it (though they won't say that, or provide the glass, because then they have to support all the problems that go with it) to try to make it flat enough to print.

Glass is a thermal insulator. If you put a piece of glass on an unflat piece of heated aluminum, the places where the glass touches the aluminum will heat up and the places where it doesn't touch will be significantly cooler (the air space is an insulator, too). The result is that you just traded an evenly heated, unflat surface for an unevenly heated flatter surface. Neither is particularly easy to print on. People try to fix the uneven heating problem by putting "thermal pads" between the glass and aluminum. It might work if you put a single large thermal pad under the glass that covers the entire bed, but if you do it the cheap way that most people do, with little squares of thermal pad spaced around the bed surface, you may actually make the problem worse.

Cast tooling plate is typically 1/4" or more thick which makes it quite rigid. It is cast so the grains are randomly oriented, which makes thermal expansion uniform. The top and bottom surfaces come milled flat. Put it on 3 screws, add a piece of thin piece of PEI, and it will heat evenly, stay flat when heated, and prints will stick.

Why don't printer kit makers supply it this way? Would you pay an extra $20-50 for a printer? That's why. People have a negative reaction to paying more for something at the time of purchase, and have no reaction to spending future time, energy, and "separate" money for glass and thermal pads, and hairspray, and whatever.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

ok, so i should be ok useing 3 levelling points and the MK3 heated bed since that plate is not extruded (is a cast tolling plate with heated elelments integrated into it)

I don't think it's cast, but it's certainly worth trying their 3 point leveling.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Looking at the specs of the mk3 aluminiun heated bed... it already comes with a hole intended to be used with the 3 point levelling system, so I guess thermal expansion was already taken into account?
By the way: the info in your site is awesome.

Thanks.

No, they weren't thinking about thermal expansion, just leveling. 3 points makes it easy to level the bed. A 300 mm bed will expand about 0.6 mm when heated, and since you do the leveling when the bed is heated, and the carriage plate is usually pretty flexible, and the leveling screws aren't very rigidly mounted, it works OK. You might notice a difference when trying to print on a cooler bed if you leveled it hot. The kinematic mount I used in my printer won't care about the temperature at all.

One thing I should mention is that your original post about the bed being higher in the center may be true or it may be that the guide rails flex when the extruder carriage is near the center, lowering the nozzle near the center of the bed, making it look like the bed is higher, or more likely, a combo of the two. The Y axis rails can flex, too, further complicating things.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]