Homebrew PCBs

Hi Charlie,

> take some thought as to how to do it cheap but good. If I can get my hands on a Bridgeport...or there are machine shops here in Manilla, I'm hoping to find out they're ridiculously cheap...a decent aluminum extrusion can be carefully milled to achieve a decent "straightness", something like that could be a great cost saver.
>    

By the time you get it machined, extrusion is likely to be more 
expensive than ground flat stock and less accurate. It is usually 
available in standard 12" and 18" lengths. When I was looking into 
designing a small router GFS was the cheapest solution I could find that 
was reasonably accurate. In the end I converted a CNC Bridgeport. More 
expensive and it takes up a lot more room but you can also do a lot more 
with it.

> I don't know what a ball drive is...but I was thinking a good toothed belt should work fine...I'll look up ball drives at McMaster-Carr's.
>    

Try this 
<http://cgi.ebay.co.uk/NSK-C1-ground-ballscrew-12-4mm-L469mm-BALL-SCREW-/140448845277>. 
Ball screws are very low friction and fairly accurate. Lower precision 
rolled screws aren't too expensive. If you want precision preloaded 
screws, your wallet is going to take a hammering!

> Les, I can achieve .001" (linear table motion) steps fairly easily with steppers.
>    

0.001" is fine for PCB work, as long as that is 0.001" per step, not 
0.001" per microstep. Under load you can only gurarantee a stepper will 
be within +/- 1 full step of the commanded position, no matter how many 
microsteps you use. Microsteps create smooth motion and reduce the 
chance of resonance induced lost steps. They don't increase accuracy.

> Depending upon the toothed drive belt, I would assume the precision would be at least as good as +/- 0.0001"
>    

No chance I am afraid. For that sort of precision you are looking at 
precision preloaded ball screws ($$$) and careful frame design. If you 
are lucky repeatability will be within about 0.002". Absolute accuracy 
could be as bad as 0.01" or worse over the whole travel. That sounds bad 
but it should be adequate for PCB work.

> PS: wowzer, that linear rail link from ebay you gave is a knockout!!! And only 10 pounds...that's a deal, in my book. :)
>    

Yeah but it is very short. Decent length rails cost a lot of money.

Les