Leadscrew opinions please!
For an upcoming CoreXY box style printer where the bed moves up and down - 200x200mm aluminum bed say around 1/4" thick - need to decide whether to use TR8 with a lead of 8 or 2. The concern is - the finest layer height that the 8 would be able to do vs the speed at which the bed could travel with the 2.
I would prefer the 8mm lead, except you really want to do layer heights like 0.02.
You also have options to up your resolution like using a 0.9° stepper or even a belt with a matching pulley teeth ratio for driving Z.
Assuming you’re using 200 step/rev motors, with an 8mm lead you could still get .04mm layer heights without trouble. And under .1mm layer heights is, in my experience, a losing proposition without going to a smaller nozzle size.
If you’re planning on experimenting with small .2 or .1mm nozzles then greater z-resolution might be useful, but there’s diminishing returns there too in terms of visible improvement and printing time.
I think you’d be fine with an 8mm lead, really. Even in resin printers smaller than ~.035mm layer heights aren’t common, an FDM printer wouldn’t take serious advantage of such small layers.
I figured that someone would bring up 0.9 deg steppers 
- since I am looking to get a nema 17 with embedded leadscrew - 1.8 deg with lead 8 or 2 are the most available options. I would get a lead 4 except it seems to be less common and more expensive. (Like $35 instead of 20 for the above options)
@Stephen_Baird 0.2mm nozzles are on the table for this. I’ve worked with E3Dv6 and their 0.25mm nozzles before, and I have @Rene_Jurack airbrush hotend on the way which can do a 0.2mm nozzle.
I want to say that 50 micron layer heights would be good - I’ve never felt the need to go lower than that. Except for this marketing graphic from Raise3D advertising 10 micron… Not feasible but it’s the stuff of dreams.
I can’t seem to attach the image. It’s halfway down on this page: https://www.raise3d.com/products/raise3d-n2-plus-pre-order-fdm-3d-printer
Use Tr8x8 and a stepper driver that doesn’t have ripple issues, and you’ll be fine. 1.8 degree / 8mm lead is exceptionally common in Z-bed printers. (I’d guess >150,000 of them out there.) I routinely do 0.1mm and 0.15mm layers on my 8mm lead screw printers (full step 0.04mm) and have zero visual issues from the non-fullstep-multiple layer heights.
It’s really just the stupid Pololu 4988/8825 default decay settings that cause issues here.
We’re in Trinamic 2100 territory now
2100s are ok. 2660s are better
Are you on a Duet?
We’re working on getting 2130 support written into Smoothie - this way the Cohesion3D boards with Pololu driver sockets can take advantage of the same advanced features in the 2130 that the 2660 has - coolstep, native high microstepping OR microplyer for 1/16 --> 1/256 interpolation, and SPI configuration of all of this.
When the motors are powered down, the higher pitch leadscrews can let the bed drift downwards. A 2mm lead has much less potential of this happening. And do you really need the Z axis to move fast? The only time you make big moves are when a print is done and you return to home (well, for me anyway).
If you can move the bed up while it is being heated the speed isn’t that important either?
TR8 8mm/rev are fine, and have no issues printing at low layer heights, assuming you take into account the layer heights they can’t do properly, ie 300 microns needs to be 280 or 320 to get dimensional accuracy (see the Prusa Calculator), I do have a 1mm/rev leadscrew on the of the FB2020s and I’m pretty sure it can do stupid low layer heights, but it’s slow and really not needed.
Dropping is an issue with 8s if there is any appreciable weight. Anti-backlash nuts help a bit with that by adding some friction. How fast are you wanting to go? I suspect the 2- start screws will be plenty fast.
If it’s a single-screw or belt-sync Z stage, dropping isn’t a problem. In fact, I personally like being able to move the Z stage up and down by hand during maintenance.
The bigger the printer, the more a 2mm lead will piss you off.
@Ryan_Carlyle - with you there. The 1mm lead screw pisses me off when I need to move the bed down quickly and I can’t do it by hand. the fastest I can get with it is 5mm/s over 20mm/s with the 8mm
Printrbot is now shipping with an 8mm 4 start thread for Z axis
I would go 8x4