Starting off, printing on a larger bed printer is a little more tricky than on their smaller brothers and sisters. There are a few things that come into play here. We will try to identify and help solve the problems you may run into.
While bed levelling is a relatively irritating problem on a small bed printer, it can really push the limits of your patience on a larger bed machine. Every problem you have on a 220x220mm bed is truncated on a 300x300mm bed machine. This can be a non-trivial problem to solve. Starting with flatness.
1 Bed Flatness:
Fig 1, below, shows a pictographic representation of what flatness means in terms of engineering tolerances. So basically this is the trouble you are facing with the big bed printer. The larger the bed the harder it is to have a flat surface. No surface is truly flat, just have a look at the Rick and Morty skit on flatness.
Bowing or warping of the bed
Fig 2, below is showing the bed warping downwards, I have mostly seen the beds warping upwards. This can be for a few reasons. The main one we have come across is the bed levelling springs have been over-tightened. This causes the bed to warp as it is under pressure from the spring (Also note this will put undue wear on your bearings). The way to diagnose this problem is easy if you have a runout gauge, attach it to the head and then jog the head around the bed taking readings as you go and you can build a map showing the bed levels. Something like Fig 3, from Prusa printers below. This will show you where the bed is warped and give you an idea of why you have a problem. If you don’t have a runout gauge, like most of us mortals, you can print a single layer over the whole bed. This will show you where the bed is higher and lower by looking at the Fig 4 below of filament flow on badly levelled bed.
Damage to the bed
This one is self explanatory, if there are holes and gauges in the bed you probably need to replace it.
See below an image of how to see the flow pattern of your first layer based on your bed height. Again image from Prusa
2 Relative distance between extruder and bed
This one has a few factors. Starting with the gantry height of end stops, then moving to the gantry parallelism with the bed.
I highly recommend you add something like that in Fig 5, this gives you the opportunity to change the relative higher between the extruder and the bed by adjusting the homing position and without having to adjust the screws on the bed-springs. This can be very useful for avoiding the problem of over or under-tightening the bed springs that have caused many a 3D printing frustration.
The next one, gantry to bed parallelism, Fig 6 shows the relative parallel planes, ideally they are parallel planes. These can sometimes go out of alignment during a print. This can be caused by a few things, on a printer like the Creality Cr-10 the gantry is pseudo cantilever ie. it is only driven on one side and has a bearing system on the other side. What can happen here is that the bearings, if not tightened enough can introduce play or alternatively if too tight can introduce a sticking, in either case, it causes the gantry to appear to walk up the frame of the printer. When I say walk, I mean d1 and d2 are not equal to one another and they change their difference throughout the print. The relative distance is the critical thing. So if you are seeing strange levelling problems like this, have a look at those bearings and their eccentric nuts.
On a printer like the Anet E-16 which has duel leadscrews on the z-axis, the stepper motors can sometimes just not have the same step/mm ratio, if you have a dud motor or a loose wire. In this case you will have to figure out what the problem is so you have a matching pair. I have only seen the motor being the problem once and it was a manufacturing fault, we changed the motors and solved the problem.
3 Bent Bearing guides
Change them, this is probably something heavy having been put down on the box during shipping, or someone manhandling the bed while trying to remove a print. See Fig 7.
4. BLtouch auto bed levelling probe
We will do a blog on this shortly, we still need to get all the instructions and build notes together properly, but the auto bed levelling probes can only do so much. If your bed is completely out of alignment and your gantry is walking up the frame you are better off spending the time and money solving those problems before adding the probe. So auto bed levelling, great feature but not a magic bullet.
Another issue with larger bed printers is the stiffness of the frame. I recommend only going for an aluminium extrusion frame printer when you get larger than 200x200mm and shoring it up where you can. You can see some of the rods and additions some people have made in the links below:
Thanks for reading,