Quick background: This is Mark III 1/2 of "The Auto-Art Machine".
I started with an MDF one(MkI) which was way too large for my needs so I built a smaller Al. one(MkII). That had an A3 sized cutting area. Much more practical size for my needs. It did, however have some issus with 'play'! I made a few modifications to it with linear bearing on the longest axis and a few other changes. Hence the 1/2 mark. I still wasn't happy with the results.
So onward and upward to MkIII 1/2. I decided to go with 20mm rails for my X and Y axis. I bought the hardened chromed steel rail. 2m for just under 40 euro, not bad. I got it from Reliance Bearings in Dublin.(Very nice fellas, easy to deal with and helpful. 'Customer service' is a concept many companies in Ireland have yet to grasp! Not Reliance!)
But then I discovered that 20mm dia' rail will sag over my planned 800mm.(In fact, it will sag under its own weight!) So I needed to support it. I found that 1.6m of supported rail would cost 180 euro or more. So I bought 20mm square Al. bar and a 120 degree router bit, and went for it myself!
Thats the bearing that had to go on the rail. I got the bearings on Ebay 30 euro for the 4. The slot in the bottom of the bearing is 12mm but its diagram reads 10mm and standard supported rail systems have a contact of 8mm. So thats what I went for.
I used the 120 degree router bit for the shaping of the supports. I used my cheepo Aldi router table again! Using plenty of cutting oil, I did maybe 14 or 16 passes on each rail. V groove first then the sides. And got this:
I drilled them every 100mm or so to attach the rail. And I learnt that drilling case hardened steel is difficult... Untill you learn the trick! Which took me 10 bad holes. (of 16)
So here is the way I did it:First, you will need to grind a flat. If you can get all the way through the case hardening at this point all the better. But the thing is, the case hardening was 1.5-2mm thick. This means that even if the centre of your ground area gets through, the edges did not. Very important. Next mark your centre points with a centre punch. As you are through the hardened steel in the centre it should mark just like mild steel. If not grind a bit more.
Next you will need a tile drill. Choose one with the external diameter of your thread. M6=6mm tile drill. The tip of the bit tapers down to a point so it will only be the case hardened bit which recieves the full 6mm dia., as you will only drill a little way in. The interior of the hole will be ok to continue with a 5mm, normal HSS drill bit to finish the hole. I went in about 15 mm in total. Cutting oil all the way...
Then you can get going with your tap. In theory it will never touch the hardened steel at all. I found the cone shape left by the tile drill led the tap in really well. So the last 6 holes were a dream! I broke a tap very early on, maybe hole 3 or 4, it just touched the hardened steel and snapped right off. 
I counter sunk some cheese head m6 bolts and that was that.
A quick note: Aligning all the holes to be square to one another was tricky. What I did was drill and tap the first hole. Screw a nice long piece of threaded bar in and it can be used as a referance to make sure the bar is always rotated to the right angle. You could use a spirit level against the threaded bar. I eyeballed it against the pillar of the drill. That's not a good idea.
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