Les, thanks, I see your point about flat ground stock.
I do remember being amazed at how inexpensive flat ground stock really is when I used to order it for various milling tasks, especially compared to trying to make some oneself!!
What would you recommend for bearings to run on the ground stock? Will regular plain old sealed ball bearings from McMaster-Carr do?
yeah, unfortunately, the only ball-screw drives at McMaster-Carr are horribly expensive, seriously expensive, that's what got me retreating to toothed belts. I don't suppose....steel chain will work? It does wear over time and "stretch", but it seems to me this is a pretty light duty application for chain, and perhaps if the chain was a bit oversized for the app, it would wear very well?
Best, Charlie
--- In Homebrew_PCBs@yahoogroups.com, Leslie Newell <les.newell@...> wrote:
>
> 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
>