View Full Version : About to dip me toe in the world of cnc- some advice please....

15-03-2012, 10:31 AM
Hi all- been lurking around the forums and have been impressed by the builds people are doing/ have done-!

liking the idea of a vertical space saving machine-!!

My build is an 8x4 cutting area- router at the moment-

looking at a cncusb card, steppers digital drivers, running on hiwin rails (25mm i think) driven via rack and pinion- maybe with a drivebelt reduction- with a decent spindle- maybe with tool changer in the future.

bed-wise- was thinking of a tnut affair with a 25mm mdf sacfrificial bed, routed to create a home made vac table.

on a welded steel frame, with the y maybe being an ali extrusion

main questions are-

are ball screws better than rack and pinion for the motion?

whats the best size steppers to use- was looking at nema 23's? http://www.zappautomation.co.uk/sy57sth763008b-nema-stepper-motor-p-38.html

is there a cheap but good source of hiwin rail?

does anyone have a list of ALL the electronics needed for a build like this- ie limits, tool height etc, or anyone fancy specifying the job?!! coz theres so many differing opinions/ suppliers out there- to be honest ts confusing me-!!!

think thats about it ...for now...!!


15-03-2012, 05:29 PM
Ballscrews are more accurate than rack and pinion, however they generally require a higher power motor to achieve the same feedrate. My machine is 1.7*0.74m, so a bit smaller and I used ballscrews all round. The main problem with using ballscrews on the longest axis (X) is preventing the screw whipping. The vast majority of people who do it and want good feedrates buy large ballscrews with a high lead (e.g. RM2525), and big stepper motors to spin the screws with good bearings at both ends. This will work perfectly well, however if instead of rotating the ballscrew you rotate the ballnut and fix the ballscrew you can use a much smaller motor since only the ballnut and a shaft is spinning. The moment of inertia of a ballnut with bearings/shaft etc for a 25mm ballsrew is small by comparison to the whole 3m (approx) ballscrew, hence it will accelerate faster and obtain a higher feedrate with the smaller motor. This is how I did it on my machine:


This also means you can now use 10mm pitch ballscrews, i.e. RM2510 which are significantly cheaper than the higher pitch screws as you can order them from eBay (linearmotionbearings2008) instead of using a supplier in England. The end mounts for the ballscrew are also simpler and therefore easy to make yourself. On my machine I used 3nm Nema24 motors with 70V (PM752) drivers, but if you spin the screw you'll need a two Nema 34 motors with high voltage (240V, 2m2280) drivers to get a good feedrate. So you save a lot of money on the ballscrews, motors, drivers and end supports but spend a little more on bearings. For me I reckon it worked out at about a third of the price to do it this way which should make it much cheaper than a rack and pinion. Compared to the standard way cable management is more difficult, and you add the weight of the motors to the gantry (negligible), however that's also the case with a rack and pinion.

25mmm profile rail is almost certainly excessive. Look at what size rails commercial machines use... 20mm or perhaps less would be fine so long as you space the bearing block out sufficiently.