Early efforts and mixed results

Having assembled the bridge, worked out how to fix the bearings etc, seeing the system move was a dream come true. 

I replaced the router bit with a pencil and manually worked out the g code to draw a smily face!

It was evident that the table wasn't completely parallel with the x axis (only a mm or so off). 

The answer was beautifully simple after discussions with those who had been there, done that. I glued a piece of mdf to the table and then set it to mill the whole table at a fixed height. 

The result was a CNC machine capable of engraving and light milling. 

Unfortunately the z axis did have some rotational wobble and this results in circles coming out as diagonal ovals/egg shapes. 

After many failed attempts to improve this I resigned myself to accepting why people shell out for real linear bearings... Grrrrrr time to start saving and hit eBay....

Linear bearings - low cost alternative

The next problem was how to allow things to move smoothly but without wobble. 

I sketched a rough design using a conventional fixed table, bridge and z-axis on the bridge. 

In my design I look at the machine from the front so that the table depth gives the y-axis and the bridge provides the x-axis. 

Plan A: My idea for the y-axis was to use two diameters of aluminium tubing that fitted tightly but didn't bind. The narrower tube was approx 1m long and the outer tube approx. 15cm. BUT... The inner tube was too weak and flexed while the outer tube was too loose and wobbled while being very likely to fill up with dust and lock up. 

Plan B: find some cheap v-bearings. 

Unfortunately evenings searching the web found nothing :( 

V-bearings seem difficult to find and those that do exist are expensive. 

Plan C:  Eureca!

I discovered that in-line skates have really nice bearings however they do have some lateral wobble. Skate designers have solved this for me by combining two bearings in a single wheel and this means that general wobble is reduced significantly.... so... can I use skate wheels as complete units? 

The downside is that skate wheels are silly prices when you just try to buy the wheels on-line BUT.... if you look on the second hand market, you can pick up a pair of kids skates for as little as 6Euro with 8 really good wheels. 

12 Euro later and I has 14 nice wheels. 

I used the 8 best wheels to support the x bridge. 2 wheels each side bolt onto the vertical parts of the bridge and take the weight of the bridge. "C" shaped brackets then hold 4 more wheels rolling down the sides of the table. The horizontal wheels are spaced a few mm closer than the width of the table so that it clamps tightly and avoids wobble. 

The wood is 18mm ply. 

DIY Bearing Blocks

Leadscrews and the associated bits all seemed too expensive. Firstly you need the lead screws, then bearings and something to take the strain so that your precious motor bearings don't get ruined. Then come the anti-backlash screws etc etc....

I decided to use some M8 threaded rod instead of leadscrews and found the straightest ones I could in the local DIY shop but now how to mount them????

I had some 8mm bearings lying around (as you do!). Two problems had to be solved.... firstly, the bearings had to be held firmly so they couldn't shift but could rotate freely. Secondly, the M8 rod had to be help such that it could rotate freely but could not move laterally.

I found that the bearings were the same thickness as some plywood I had and made a sandwich with three plywood squares and labelled them '1', '2' and '3'. A simple adjustable hole cutter in a pillar drill cut out the hole for the bearing in piece '2'.

Now for the rod fixture... I found a steel M8 "T-nut" that fitted perfectly through the bearing.

The four folded "spikes" on the T-nut broke off very easily when I tried to fold them backwards. The barrel part of the T-nut was a bit longer than the thickness of the bearing so I filed it down so that it matched nicely. This allowed me to tightly clamp the centre piece of the bearing without crushing it.

The final stage was to recess enough room for the T-nut and on the other side have space for a self locking nut. The recess for the T-nut is a smaller diameter than the hole for the bearing so the outer back of the bearing remains sandwiched in part '2' between parts '1' and '3'.

To my great delight... it works! Cheap, easy and simple to mount to an overall wooden structure.

A simple view of the assembly (please ignore the "ears" still fitted to the T-nut)

Late in 2012...

In 2012 a chance discussion with a colleague made me realise that building a CNC machine at home was actually possible
I started to make myself a shopping list of all the bits and pieces that I wanted (motors, controllers, lead screws, bearings, spindle etc etc) and the total price is.... completely unrealistic!!!!!

Plan B... can I make this for next to nothing?
I stripped down an old inkjet printer to see what I could use... conclusion... not much :(

Plan C... try to buy a small number of key parts to allow me to build something sensible.

The one section that I couldn't work around was the set of motors and drivers.
Clearly these were going to use up a significant part of my budget so I wanted to ensure that I had at least enough torque this time... in the end I went for Nema23 425oz motors and a 36V 350W PSU with micro stepper controllers.

Excitedly I downloaded the trial version of Mach3 and plugged it all into to the parallel port on my old laptop. The motors jumped on power-up and then sat there not doing much.... a few tentative movements but not much else.
I read the Mach3 info including statements about not using a laptop (due to the power saving features if I'm correct).

Several discussions later and indeed the conclusion was.... don't use a laptop!
Since my fastest non-laptop was a Pentium 2, it was time to hit the second hand market for an old PC. I found a great looking Dell and rushed round to pick it up.
Next problem - no parallel port!!!! AGGGGHHHHHHHHHH!
A few days later a PCI LPT card appeared through the post and..... it 

works!!!!

I had a dream....

Many years ago I had an idea to use a computer to control a linear positioning system. I bought a couple of stepper motors from Maplin in the UK and started to hook these up to my 8 bit PIO card. Harnessing the full power of Window 98 and Turbo Pascal, I excitedly hooked up some simple gears to the stepper motors and a piece of M8 threaded rod... it twitched slightly and NOTHING!!!.
It turned out that I had massively underestimated the torque of the tiny motors.

To be clear, I'm not saying there was anything wrong with the motors - they were really easy to get going and quite reasonably priced. With no load, they trundled nicely clockwise and anticlockwise. I had a lot of fun with these learning about stepper motor sequencing etc but my expectations regarding what they could drive were just all wrong - I had set the bar way too high for myself and so, the motors went back in a box, the M8 threaded rod and bearings went back in the garage.

Over time, the bits were recycled but I kept finding the old piece of wood with my "linear bearing" (a piece of aluminium tube with another slightly larger diameter section fitted on the outside + the M8 rod.... there must be a better way of doing this.