Quite often there postings about printing problems where it becomes obvious that the machine was never really calibrated.
How to calibrate your extruder and axis through the E-steps is well documented in more than enough places, so let's focus on those things not mentioned too often.
Everything that moves needs support for this, usually bearings of some sort.
But the rails, roller system or whatever is in use also needs calibrating, especially after transport or the first build.
Take a look at a standard bed mount and you see two smoth rods with linear bearing or slider on them.
It does not matter too much if those rods are a few mm out of the supposed position but it does matter that they are fully parallel and exactly at the same distance as the bearing mounts on the bed!
Imagine one rod would be slightly lower:
No matter how good you adjust your bed once you move it one corner will go down with the rail.
Imagine they are not fully parallel either:
The bed will move smooth up to a certain point where more force is required. This not only wears out the bearings but also messes with your prints.
This point becomes even more important for the axis with the hotend moving around.
This axis or both rods must be parallel to each other AND to the print bed!
So you imagine that setting up the bed "correctly" can mean you actuall set in an angle that corresponds to the axis with the hotend!
Some say it does not really matter as long as it is parallel, which is partly true, but I say it does.
Let me explain why:
See your moving axis as moving plates covering your build area.
Your print bed is located between them.
If now either the lower or the upper "plate" is at a different angle to the print bed you can adjust the bed to match.
But if the misalignment of the two "plates" is in more than just one direction it becomes next to impossible to compensate.
IMHO the only real way is to remove the build platform completely and to align all smooth rods directly to each other, first the bottom ones, then the top ones.
After that the bed levelling only has to take care about the bed itself or whatever is on the bed to be printed on.
The reward is very smooth movements allowing for higher speeds
Framing....
When it comes to things holding it all together we have come a long way from simple triangles made with long screws to acrylic, wooden and metal frames.
But there are even more design variation than materials, so what makes a printer good in terms of the frame?
Small, low budget printers are usually quite small and have little moving mass to fight with.
Heated beds, glass plates, dual extruders and more will change things quickly.
What the motors and electronics are capable, the frame must stand up to it without moving.
But do some really fast infill that goes parallel to your axis and you might notice some shaking.
This shaking can cause problem like prints with perimters poking out a bit, wavy lines or in sever cases prints popping off the bed mid print.
Modern printers should not have much problems in this department, at least at normal speeds.
But if you do notice movement in your framing there are things you can do to improve.
For example trying to redunce the moving mass - instead of a 5mm glass plate try a 3mm one, instead of two direct extruders try a bowden system...
But the frame itself can be made more strudy by adding angle brackets, either metal or printed in the areas that move, sometime even a simple wire under high tension will give support for "shoebox movements" - like the back panel on a free standing wardrobe.
One of the best improvements can be vibration feet like used for high end speaker systems - a sharp cone on the speaker rests on a disk, due to the tiny surface are the generated forces will cancel out most vibrations.
Dual motor systems...
There is a real benefit to using two motor over just one but it comes at a price.
Especially at high speeds or in case of some accident there is little to no way you can see if both motors and what they drive are still really parallel.
And of they suffer from lost steps like any other single motor system, only the here just one motor could miss a step causing a misalignment.
So it really pays off to check this every now and then, especially when suddenly the bed levelling seems to be out of whack for no reason
Belts and pulleys....
When it comes to belts and pulleys you have a near endless choice of combinations available.
But for us that does not matter here, what matters is how they work together and move things.
Belts with insufficient tension cause the axis to perform unwanted moves.
If you know Z-Wobble than you know what I mean as a similar thing happens with loose belts.
Too much tension is no good either, so it is best to use a tensioner if it is just made from a spring loaded clothes peg...
In many cases the pulley are on ball bearings and use big washers on their side to make sure the belt stays in place.
If your pulleys or motor mounts are not properly aligned the belt will always wander off, some just to one side, some to both depending on the direction of the movement.
Up to a point this movement is normal and poses no real problem but if you notice blank, shiny areas on your washers it is a good indication that the belt is rubbing.
This rubbing causes vibrations that can be transfered onto the print.
After the obvious alignment check you can still cheat a bit if you still have problems.
By using silicone grease in very small amounts on the sides of the belt you reduce the friction to a level that is no longer a concern.
Best way is to replace such pulley with types as used on the motors, you the kind where the belt actually sits in a proper groove?
What's left?
I guess not too much apart from the normal calibration procedures for your steps....