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. 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.
If you want to really go to town then make some felt wipers to wipe off
any dirt that may accumulate on the rails. Oiled felt is very effective
at excluding dust.
Les
David McNab wrote:
>Hi all,
>
>I'm about 80% of the way towards getting my CNC mill up and running,
>will eventually be able to feed it a JPEG/GIF/PNG file of the PCB
>artwork, and it will cut the tracks, auto-detect the holes from the
>artwork and drill those too, using a narrow-tapered conical diamond bit.
>
>The completed unit will have a total desktop footprint barely larger
>than a sheet of A3 paper.
>
>All reasonable IMHO for a final parts cost circa US$150 all up.
>
>The X-Y tracks have been a pain though. As someone who's very skilled
>with software, reasonable with hardware but largely clueless with the
>mechanical engineering, this has been a pain.
>
>What I've got for each X and Y direction is an aluminium platform, with
>lengths of aluminium angle bracket along the short edges, riding on top
>of aluminium angle bracket, shown below as cross-sectional view, excuse
>the rough ASCII graphics:
>
> ∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗ PCB
> ///////////////////////////// chipboard to absorb drill
> ================================ upper platform
> +---- ----+
> | _ |
> | | / \ | | upper rail
> | | \_/ | |
> | |
> ----+ +----- lower rail
> ===================================== lower platform
>
>(circle in middle is threaded rod, driven by unipolar stepper motor).
>
>With liberal spraying of CRC lubricant (will grease it later), the top X
>and bottom Y platforms are moving freely enough for the stepper motors
>to be able to turn the threaded rods (@ 1:1 gearing) through the full
>movement range in both X and Y directions. Getting reasonable movement
>speed, rotational/translational stability, plus precision of around 0.04mm.
>
>But at some time, I would like to replace this crude abomination with
>something similarly simple but better.
>
>Can someone recommend better ways to do the tracks, using inexpensive
>and easily available materials that don't need specialised tools to
>deploy? Preferably materials which can be easily sourced in a small city
>without 8 hours searching the Yellow Pages?
>
>(The other extreme is ready-made precision linear bearing parts, but
>these are hideously expensive within NZ).
>
>All ideas appreciated.
>
>
>