Remove the warped bow in heat bed  

RE: Remove the warped bow in heat bed

I am considering, whether I should mod my XL-heatbed in a somewhat similar way I did with my MK3S+. OOTB, my XL bed does not look not too shabby -- way better than 1mm deep according to my very brief examination with my 300mm straight edge. I'll have a closer look and see how that goes, once my XL is up and running. If my "frog" were bent more, I would consider getting a replacement by Prusa or dismantling it and straighten as much as possible. Preferably on a balancer if available. Meshbed-leveling or not: I'd like the undersides of my print to be rather straight.

On the MK3S+ I use the so called "springmod" on 3 printers and am happy with it. There I replaced all but the center distance cylinders with springs. This, similar to the so-called "silicon-mod" does not need any change to the frog. I use a terminalprogram to enter G80 to start the meshbed-leveling and G81 to spit out the table which I prefer over 3d graphics. Side note: I explicitly dislike the nyloc-mod.

On the XL OTOH, I think it has do be done differently due to the nature of the load-cell. Please take this with a grain of salt since I elaborate the idea while writing this:

General idea is, to add adjustement screws which push each stud of each tile upwards while pulling it down by a spring, so that the load-cell pushes against something comparably stiff, unlike the springs on the MK3S+.  Machine 1mm deep cylinders of 8mm diameter below each of the four 6mm diameter studs of each tile. I may have to flatter our chief of manufactoring to do this for me on his nice CNC-Machine, though. Alternatively, one could lower the 4 supports of the bed by 1mm. Yeah, seems way easier. One stud in the middle will get a 1mm washer to maintain nominal height since I do want to continue to use stock firmware which expects standard range of z-stroke. Cut a somewhat larger thread (M6, maybe even M7 or so) at each hole -- provided it weakens the "frog" not too much. Then use a regular hex-head screw (from below) but with a center bore of dia 3.3mm drilled (if I can) for the screw diameter (but longer M3 screws) and with a large 3mm washer. Put between this washer and the M6 hex-head a spring. Glue the M3 into tile studs (mental note: check which "loctite" will stand cooking).  Adjust height of each stud with hex-key. M6-counter nut, if necessary, seems feasible as is pulls screw downwards within the thread. Downsides: The "frog" does not come out of this unharmed. Ideally, I'd do this on a spare frog. I wonder, whether Prusa would make one available and if so, what they would charge for it. The switchboard has to be lowered if that is still possible. Adjusting the 64 studs of the XL is significantly more work compared with the 9 screws of the MK3S+. Fiddling below the heatbed, especially at the inner screws or those at the back is done best, after it is cooled down, so needs way more time. Should be necessary rarely though.

Any opinion?

RE: Remove the warped bow in heat bed

Is this a problem of the tile carrier being bent, poorly aligned tiles or tile tolerances?
If it's the carrier and you apparently have a proper machine shop around, why not just get a new one cut? Genuinely curious, I seem to have great flatness personally.

RE: Remove the warped bow in heat bed

I've heard of others getting their heat bed replaced and that solved the issue.  I don't know how the heat bed attaches on the XL, but if it's like the other Prusa printers then maybe the Nylock mod will work.

I'll look at it closer when I get mine built in the next few days.

RE: Remove the warped bow in heat bed

The heat bed isn't warped enough for Prusa to consider it a problem. Of course, just because they don't consider it a problem doesn't mean it isn't a problem. Starting off with a warp in every part is a mess.

I guess I'll have to create my own bed flattening procedure. I'll do that when I get my 2-5 tool upgrade.

RE: Remove the warped bow in heat bed

Looks like some sort of foam between the frame and the individual panels from looking at the assembly instructions.

Is that stuff compressible? 

RE: Remove the warped bow in heat bed

I haven't taken the heat bed apart yet. That foam, probably silicone foam, looks like insulation and isn't firmly attached to the heat bed. I will probably have to use shims to flatten out the bed. No big deal either as they can be printed from nylon or another high temperature plastic and inserted between the bed and the tile.

I'm kind of surprised that Prusa blew me off and didn't have a procedure. Their support really and truly sucks. Thanks for nothing.

RE:

It would be a lot of screws but I imagine you could make the Nylock mod for the other printers work.  The problem with shims will be is that they would have to be super thin.

They do make steel shim stock in . 001" increments.  You could make little shims.

The reason making it adjustable is nice is because you need to level it when it's heated because it will expand and warp as it warms up.  So something like a shim might be tough to implement as it would require disassembly to change.

I think 1mm is too much as well, but in all reality a bed that size your not going to super flat.  It's going to grow and warp a bunch as it heats up.

Many industrial printers with heated chambers print on a raft for this reason as it's not realistic to get the bed completely flat.

Now you could engineer it in a way that would allow it to grow without warping, but it would need to be a much more precise part than what is used and it'd raise the price even more.

Are you checking the bed flatness hot? 

RE:

The parts are coming out warped by 1 mm so it's probably warped just as much hot or cold. I will have to heat it up and put a straight edge on it to tell for sure. I think the aluminum plate got bent during shipping as that silly huge thing of candy was on top of it. Who designed that shipping container? And why include a kilo of that silly candy? I'd have rather had a cheaper price. 

As for shims, since I have such a large deviance, printed shims would be just fine. I could use metal. Certainly have plenty lying around.

I am unfamiliar with the "Nylock Mod". Can you explain?

RE: Remove the warped bow in heat bed

The Nylock mod involves replacing the spacers under the heat bed with a Nylock nut.  This allows you to use the heat bed screws to adjust the bed level.  On an MK3 you could use the pronterface program to get the bed mesh numbers from the printer and the based on those numbers adjust the heatbed screws to raise or lower points on the bed to make it flat.  Doing this you can easily get the a MK3 sized bed within .03mm of flatness.

For the Prusa XL I imagine you could do the same thing by attaching an indicator to the gantry and use it to measure the flatness off the bed.

It's going to be a little different for the XL since there are many tiles vs. 1 larger tile, but I imagine you could do the same thing by having the center tiles be the master and adjusting the rest to match.

Here's a link to the MK3 modification.

https://www.rearvuemirror.com/guides/nylock-mod-for-the-mk3s

RE: Remove the warped bow in heat bed

A similar method that many use on the Prusa Mini uses short segments of silicone tubing   instead of nylon spacers. They allow adjustment without needing to add/remove spacers.

The pictures of the bottom side of the XL bed suggests there may be a sheet  of flexible material between the carriage and the heatbed tiles. I don't have an XL to check, but is it possible that material might afford adjustment?

RE: Remove the warped bow in heat bed

That said, I find adjusting eight points on my Mini can get a bit 'fiddly' - tuning an XL with 64 screws would be quite an exercise...

RE:

As already stated: My advice is not to use the nylock-mod -- neither on an MK3/4, let alone on an XL! On an MK3/4, prefer instead the spring-mod (see e.g. when-my-bed-isnt-flat) or silicon-mod. The principle of adjusting each screw is the same. I tried nylock-nuts once on my MK3S and took them out again right away. They have backlash, are fiddly and above all: The risk to break the headbed is quite elevated.

At the XL, the studs of each tile are screwed directly onto the frog. Unlike at the MK3S where distance cylinders provide an exploitable gap between heatbed and frog, on the XL there is no gap whatsoever.

If your bed is bent down in the middle, shim washer can work but figuring out the excact combination will be quite tedious, considering the 64 points. I would be reluctant to lift the heatbed level as a whole by even a single mm. Usually, Prusa firmware checks for available stroke of each axis and its tolerances to accept a range are somewhat tight in order to spot assembly errors.

If your bed is bent u-shaped or like a flat bowl, disassembly and trying to straighten it a bit as a whole is probably way easier an an order of magnitude faster than using shim washer.

RE: Remove the warped bow in heat bed

I think I'll first try and figure out where the warp is and if it's in the aluminum machined plate, I'll put it in the press and take it out. That would also help prevent warps when the machine is hot. Straight parts just fit better and perform closer to the ideal.

RE: Remove the warped bow in heat bed

Posted by: @blauzahn

As already stated: My advice is not to use the nylock-mod -- neither on an MK3/4, let alone on an XL! On an MK3/4, prefer instead the spring-mod (see e.g. when-my-bed-isnt-flat) or silicon-mod. The principle of adjusting each screw is the same. I tried nylock-nuts once on my MK3S and took them out again right away. They have backlash, are fiddly and above all: The risk to break the headbed is quite elevated.

At the XL, the studs of each tile are screwed directly onto the frog. Unlike at the MK3S where distance cylinders provide an exploitable gap between heatbed and frog, on the XL there is no gap whatsoever.

If your bed is bent down in the middle, shim washer can work but figuring out the excact combination will be quite tedious, considering the 64 points. I would be reluctant to lift the heatbed level as a whole by even a single mm. Usually, Prusa firmware checks for available stroke of each axis and its tolerances to accept a range are somewhat tight in order to spot assembly errors.

If your bed is bent u-shaped or like a flat bowl, disassembly and trying to straighten it a bit as a whole is probably way easier an an order of magnitude faster than using shim washer.

 

The silicone mod can work too, but it seems to drift over time, whereas the Nylock mod seems to hold it's position.

Silicone like any plastic will creep over time especially when subjected to elevated temperatures.

I'm not saying the silicone mod is bad, it's just a different way to do it. 

RE: Remove the warped bow in heat bed

The silicone mod can work too, but it seems to drift over time

That's why I still prefer and applied the spring mod to 3 MK3S+. Just last week I checked one of them after months of operation. The bed still was spot on. It was the x-gantry which was slightly tilted. After an automatic z-calibration bumping into the (modded) upper z-stops the x-axis was again parallel to the bed and good to go.

RE:

So I just took a heat bed panel off my brand new bed.  They are spaced off the frame by risers attached to the panel.

So in theory it could be your bed frame is not flat, and it could also be that the heat bed tile itself is not flat. 

The panels do not attach rigidly to the frame as every screw has an o-ring under the head.  I imagine this is too allow for thermal expansion.

There is some variation on my new bed, but no where near 1mm.

2 things to check. 

First I'd check the screws under the affected area to make sure they are not loose. (*Keep in mind that you shouldn't tighten them much because you'll just squish the o-ring under the head, just make sure they aren't loose*)

Second you can pull off the affected heat bed tile and see if the risers on the bottom are flat.

RE: Remove the warped bow in heat bed

Adding a picture of the screw with the o-ring. 

RE: Remove the warped bow in heat bed

Do you have a picture of the bed without the tile?

Posted by: @brian-12

Adding a picture of the screw with the o-ring. 

 fc60678b-20240213_161242.jpg

RE: Remove the warped bow in heat bed

We're currently looking into a similar problem with prusa at the moment with our own XL.
One thing I've discovered is a way to measure the bed as a mesh via probing

• Attach a USB-C cable from the printer to a PC
• Install / Download Printrun and use it to open up a serial console connection to the printer
• Run the following G code

G29
G29
G29 T0

For some reason sometimes I have to run G29 more than once, but it should probe the bed
The G29 T0 will then output some text you can copy and paste which lists the mesh level points

After that you then need to reformat the text a bit so it will work with the below visualiser
Basically re-order the lines so it's from 0 to 35 instead of 35 to 0
Remove " |", Remove any blank lines etc
Generally get it to look like the demo text in the below web page, then paste it in and visualise it

https://i.chillrain.com/index.php/3d-printer-auto-bed-leveling-mesh-visualizer/