Home built rigid couplers. I could never get those stupid spiral couplers to center properly on a threaded rod. I figured I’d make some custom couplers and instead of set screws , I tapped one side for M8x1.25 threaded rods. and drilled a 5mm hole in the other side for the stepper shaft. It was all done in one step on the lathe so everything should be perfectly centered.
Your defects are probably from the m8 thread pitch. It’s not divisible by 0.1, 0.2, etc for normal z heights. This causes rounding errors, the slicing software assumes you have a perfect machine that is capable of meeting any dimension it specifies. The firmware rounds to the nearest step.
In addition, 8mm rods cannot withstand the 8mm leadscrew if it’s not perfectly straight. No screw is perfectly straight (unless you pony up for a ballscrew), meaning that as it turns, the bend in the rod is moving your entire x axis, fighting against the z linear rods.
Watch out for resonance, rigid screw couplers tend to have more issues than slightly flexible couplers.
Stephanie’s comment is good. You basically need the smallest threaded rod with a pitch that’s divisible by the standard layer heights you’re using.
You have vertical, quality rods to keep the alignment.
The threaded rods only need to move a few nuts up and down(preferably only in 1 direction because of backlash) - even if they are bent a little, the vertical movement will be 99+% accurate if the pitch is good.
The threaded rods should not be over-constrained. They should be fixed only at the coupling with the stepper - the opposite end must be loose so the bending is relieved there, instead of in the middle.
You have a lathe, and instead of just reworking the end of the threaded rod to fit perfectly into the flex coupling, you make a new custom-made-coupling to blame the old standard one? Just lathe down the M8 to like 6 or 5mm and use a standard cheap coupling: https://youtu.be/X2P7b766e5Q
It’s easy to blame a part, but its easier to use a part in a way it is designed to.
@Rene_Jurack , That was my fist thought, however, I could not get an M8 rod to center in my spindle without some sort of already machined surface off of which to index. Attempts at dialing in on the threaded surface did not work. I ended up with the machined part of the rod off center and more runout than before I started. Tapping a new coupler right on the lathe ensured perfect concentricity with the rod.
@Stephanie_A , your comments make sense. This is my first foray into the 3D printing realm. My hobbies are mostly in metalworking where such things rarely come into play. Perhaps I should replace the threaded rods with trapezoidal lead screws. I see that they come in a 2.0mm pitch.
If your threaded rod is resting directly on the motor shaft, meaning its touching it, place a ball bearing (just a single ball) between the motor shaft and the rod. That way you will have a single smaller point of contact.
Second, i did that on my own printer, is to float the X axis, meaning it is lifted by an aparatus, but not screwed into it. The X axis the is driven by vertical shafts keeping it straight.
And to solve the Z banding, i got 1204 ball screw from china, turned down to 1/4, and got a bellows coupler between the motor and the shaft.
The screw weight (thus the X carriage weight) is resting on a bearing, not the bellows itself.
Look at my design there, it might help http://www.thingiverse.com/thing:313095
You’re right, I was thinking of the old inch screws. Still will have problems if the screw is not perfectly straight.
Oh @Stephanie_A just made me think of something. if the Z axiz is rigidly mounted so it moves perfectly linear to stop interference from the threaded rod what about mounting a 2 differently sized washers the smaller one on a nut that’s on the screw and the bigger one on the Z carriage. it will allow side movement so it doesn’t affect the Z axiz. with a flexible coupler it may not work but with a rigid one it might.
@Michael_Scholtz You’d need something to keep the Z nut from spinning along with the screws…
Oldham coupler is the best but a bit expensive. There are a few ideas out there that let the nut float by decoupling the lateral movement, definitely possible.