Homebrew PCBs

Les Newell wrote:
> Hi David,
> 
> As others have mentioned, al on al is bad. It is ok if it is lubricated 
> but the lubricant will pick up abrasive dust and turn into grinding 
> paste. If you don't lubricate it, it tends to stick and 'gall'. Drill 
> rod/silver steel (depends where you are) with plastic bearings works 
> well but even large diameter rod will flex an amazing amount if it isn't 
> supported.

Yes that's true and ideally the rod should be
mounted via a continuous support or discrete
supports at fixed intervals.

Commercial solutions typically use a continuous
extruded AL support.  If I had to fabricate an
approximation I'd probably use sections of 1/4"
AL bar of suitable height and mill the top to
receive the drill rod.  It would likely be preferable
to pre-bore the rod mounting holes in the bar
before milling the top relief.

> My favourite is rectangular bright mild steel with roller 
> skate bearings running on it. Bright mild steel is obtainable from most 
> metal stockists and is cheap. You end up using quite a few bearings but 
> they are dirt cheap so it isn't a problem.

What type of tolerances are you able to hold with this
approach?  At least with the mild steel of which I take
delivery the surface really can't compare with ground
drill rod.

-- 
uhmgawa@...        www.gnu.org