Saturday, 9 March 2013

Linear rails for possible CNC?

A few weeks ago, at one of our Thursday BuildBrighton meetings, Matt suggested another cnc machine idea.   We've already completed one - a pcb drilling machine - and while the mechanics and software side of things were pretty good (for a £30+ cnc) the quality of the stepper motors was always a little suspect!

These cheap little stepper motors are all over the internet (and despite being rated at 5v as this one is, we found running them at 7v-9v the minimum, else they stall really really easily, but they can handle being driven hard at up to 12v. Beyond that, we've no idea - we didn't want to intentionally burn one out!)


What's not immediately apparent from this photo is that the shaft is actually offset from the centre:


And that's because, inside, there's a load of internal gearing which gives these little motors a pretty decent amount of torque for such a small package. In fact, the motors we had were geared by 1:64 on a 1/64 stepper motor - meaning they required 4096 steps for one complete revolution. By the sounds of things, perfect for ultra-tiny movements, and therefore precision accuracy.

Unfortunately, the internal gearing comes with a price - backlash!
When you send up to 100 step pulses to these motors, they can either move a great distance (depending on the size of the cog the motor is turning of course!) or they may not move at all, as all the slack in the cogs is taken up. This can be compensated for, a little, in software, but it's not ideal. So a while back, when some cheap steppers came up on eBay, we got hold of three decent motors and waited for the opportunity to present itself...


As often happens, when eating pizza and talking rubbish at BuildBrighton, discussions soon got around to making another cnc machine - this time with a multiple head attachment, for doing solder paste spotting as well as pick and place for SMT components. We already had the stepper motors, we've got access to equipment for cutting frames, inside of messing about with cheap-and-ok, we thought if we're going to do it, we'll make a pretty decent spec machine this time. And that means linear rails....

This morning, Mr Postie dropped off another peculiar looking package at Nerd Towers, covered in hieroglyphics from the Far East and we tore into it:


Yay! 12mm linear rails. Perfect for any decent-sized cnc machine (up to about A3 cutting bed size we reckon, certainly more than enough for up to A4). They're actually quite chunky but in a reassuring way. The mounting holes are already tapped, ready to take an M5 bolt.



Check out those tiny little ball bearings! These rails (we've got six) are going to be perfect, at least for our X/Y axis on the next CNC machine (whatever that is, whenever it gets made). They're actually quite big for a z-axis, unless Matt has ideas about making a massive monster-sized machine.......