Re: UHMW slides - and PCB routing

If you are doing routing for very small surface mount stuff with very
tiny traces you may need high accuracy, but if you are doing thru
hole, don't bother chaseing the devil.

mount your ways/guides/runners upside down, under your table.

make a vaccuum attachemnt to collect as much dust as possible and
HEPA filter it - twice.

Keeping the dust away from your presicision parts is much more
important than inital accuracy.

For the size of small 1 ft x 1 ft PCB table, it would be adviseable
to considder getting linear bearings and slide and not go exoxitic.

Accuracy is a devil to chase. you can use all-thread, with a single
threaded nut and get backlash of less than a tenth for a no-load
applicatin like you have. the problem is error over distance. but
even if it was off 20 thou a foot it would be REPEATABLE, the actual
goal if your type of project.

that way, you could rout a pad, then drill dead center of that pad,
even if your X/Y location was off like crazy.

but, if you do bouble side, then you would be off way crazy.

Hysteresys, or backlash is a different devil. you could always cuft
from zero to positive X and positive Y and eleminate the backlash
problem, but then for double side, you have to doo offsets and always
cut to negative X and negative Y to try to match those errors.

If you look at T-Tech where than route 5 traces between a pair of IC
pads, you gotta spend BIG bux for that kinda accuracy. (0.006mm) if
all you want is to route one trace beteeen pads, that would cost
bunches less.

if you want to use cheap parts, don't expect to get super precision.
A very simple Dremel head and cheap parts will do an exceptional job
on thru hole, single sided boards.

you would need to tighten it up a lot to do double sided.

and you would be extreelmy hard pressed to do SM double sided boards
with less quality parts, or lots and lots of corrections.

Dave

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> george_barr wrote:
>
> >I have a question. I am building a CNC (hobby) and need it as
> >accurate as
> >possible.
> >
> What does this mean? "Accurate as possible" has no limits. The
> semiconductor wafer
> fab guys use million dollar air bearings slides with laser
> interferometers for position
> sensing. Some of the wood router folks are satisfied with 1/16"
accuracy.
>
> > I am using aluminum extrusion from www.8020.net. They
> >have these
> >linear bearings which uses UHMW instead of ball bearings for
slides.
> >These
> >slides uses slippery plastic like pads, instead of ball bearings,
to
> >glide
> >along the aluminum extrusion. Does anyone have any experience
using
> >these
> >types of slides? They said a shim can be purchased to provide a
more
> >exact
> >fit with the slide and the extrusion beam.
> >
> >I don't want to waste my money if these slides are not worth it.
I
> >plan to
> >do PCB (circuit board) work with my cnc so it needs very good
> >accuracy.
> >
> >
> I doubt this will work well, or for very long. Aluminum is a
HORRIBLE
> bearing surface,
> although hard anodizing or nitriding makes it better. If you will
be
> milling out circuit
> board traces and drilling holes, you will be generating lots of
abrasive
> dust that will eat
> the slides and plastic sliders. I'm not sure what is the best or
most
> durable scheme for this
> use. I would think rolling elements would be much better at
tolerating
> this dust. Sealed
> ball bearings used as wheels that roll on some sort of way sounds
like a
> good design, as
> the way and rollers can be easily cleaned when necessary. A
toothed
> belt used like a
> drive chain would be a good choice for linear motion conversion, as
it
> won't need any
> lubrication, and lubes make dust stick.
>
> Jon