Having trouble deciding between leadscrew and rack and pinion for my 8x4 cnc just wondering you guys thoughts on it?
ok so after the first few comments i meant to say having trouble deciding between ballscews and rack and pinion.
Rack
Rack, leads screw need more power, and more backslash
But is you have more budget use balls screw
is there a difference between ballscrew and leadscrew? if so i meant ballscrew lol
thanks @Denis_Seguin that has enlightened me greatly and I checked the ones i was looking at and they are indeed ballscrews
Any reason why you have ruled belts out?
@Paul_Frederick I don’t think belts would be adequate for what I’m building I’ve ruled out belts and acme screws but not sure which out of ball screws and rack is better in regards to backlash and setup etc
I love ballscrew, more power, no backlash, better retention and smooth interpolation between axis only just problem… the money!
i used rack and pinion module 2 helical and i find great quality for low money.
Regards
The cost of balls crew in 9ft lengths is one thing, but I guess it depends on what you are intending to do with it.
I have an 8x4 with Gates belts and it’s perfectly adequate. I cut plywood almost exclusively.
@Mike_Thornbury i have 4 cnc 2 with ballscrews 1 rack and pinion and last with belt htd5, all have precision and use a servo motors belt drive its noiseless and smooth but dont have retention and have a little vibration on cut at high velocity about 3000 milimeters/minute.
Rack and pinion has problems with the pinion climbing on the rack gear faces and often requires some type of bogie to keep the gears engaged, but even at it’s best, repeatability is questionable for precision work. Precision gear sets are expensive and alignment from side to side is critical to avoid binding.
Toothed belt with eccentric tension adjusters are still a reasonable option for an inexpensive build with lower (but still reasonable) tolerances and many people forget wider belt configurations are available (don’t forget Harley Davidson motorcycles use a belt drive to put 80+ HP and ft-lb to the ground).
I personally prefer ball screws systems because I need the precision to place a 0.1mm wide engraving tip in the same place over and over again.
after trying to design my cnc in solidworks i’ve decided that ball screws are the way to go as it seems to me to be easier to implement
It´s the best way and the machine it´s for a long time use!
@Eugenio_Villaverde Try double-belting - glue one belt face-up (I used superglue on aluminium) and the driving belt face down. Then there is little stretch and no vibration. I can post a picture if you like.
@Gary_Wilson I completely disagree, but your mileage may vary 
Belts or rack/pinion (essentially they are the same, barring the material they are made from) are much, much easier - and without any of the massive inertia you are going to get with 9ft long 25mm leadscrew - because you will need at least 25mm - maybe 30mm for such a length.
2M of 25mm leadscrew weighs around 8.75kg, you need two of them for one axis alone. That means you have to overcome 17.5kg of inertia just to step your drive system. Then you have to reverse, from a reasonable speed.
Either you have massively deep pockets and cost is no object, or you have shares in a stepper motor company, because you are going to need really big ones of those, too. The slightest mis-step and your gantry is racked.
Obviously we have no idea what you are going to be doing with it - you could be cutting 1 inch steel plate, in which case, leadscrews are almost compulsory, but most 8x4 routers are used for sheet timber goods or light metal sheet.
If it’s mostly wood you will be routing, the downsides and cost associated with leadscrews far outweigh any assembly benefit.
FWIW, I made my machine without plans, only purchasing two pre-cut plates that fitted the ‘suspension’ wheels to the size of extrusion I was using. The rest I nutted out as I went. It wasn’t hard.
It is fast and repeatable and cuts timber to less than a tenth of a mm accuracy (that’s about as close as I can measure).
@Mike_Thornbury the short axis will have the dual steppers and the steppers I have are nema34 1600oz I was also considering some sort of follower to help keep the ball screw straight
@Mike_Thornbury
Yeah a timing belt is basically a rack with better pinion gear options.
@Gary_Wilson If you are dead set on ball screws you should at least try a timing belt first, as the cost would be nothing compared to the ball screw. You can simply throw the belt setup out if you do not like it. A belt may just surprise you though.
@Mike_Thornbury its good idea, i mill a rack in plastic and after in aluminium and have a good tension only in omega triangle (motor pulley/ and bearings camfollower) and have perfect repeteability! please put any photos, regards
@Paul_Frederick do you have any photos or links to photos/plans I could look at to give an idea of how I could implement it into my design?