Re: Linear Slide Components

Doug,

A very fine post. I'd like to add that many of these accuracy "claims"
are just that; claims to an accuracy which "really" doesn't exist...

I'm putting together a bibliography of some articles in the
metalworking trade mags about accuracy, inmagined and real... And
I'm planning to put a video on my site which will graphically show
just why many machines are inaccurate. There are some pics in the
books as well. I had hoped to take photos at Westec of some
commercial machines being "deflected", but most vendors didn't think
too much of the idea!

Besides the woeful lack of stiffness inherant in nearly all of the
machines generally discussed here on CCED(including the
Bridgeports!), there are also the issues of thermal expansion,
geometrical errors (such as Abbe), servo following errors, software
rounding errors, etc. And THEN we get to address the accuracy of the
individual components, and the tolerance stacking which occurs as
they are put together in axis and machine systems...

But the main point of all this is: "So What!" Bragging rights are
less important than parts which fulfill their intended purpose.
And once you allow yourself to work from what you NEED, instead of
what you "think" you need (or WANT; or what someone else tells you
you need), you just might find that you CAN get "there" from "here".
And you might even do it on skatewheels!

Hope this helps,

Ballendo

P.S. Several years back, Lon Haldeman rode across the USA on
his "stock" bicycle in about 9 days, eating McDonalds hamburgers
along the way. A competitor in this race, Michael Shermer, was
adament about his macrobiotic diet, and fancy bicycle. He lost to
Lon, and when Lon was questioned; he replied, "Muscles win races."
Michael Shermer later DID win the same race another year it was run.
Many different paths...

--- In CAD_CAM_EDM_DRO@y..., "Doug Harrison" <prototype@c...> wrote:
>
> ----- Original Message -----
> From: Lee Wenger <wenger2k@y...>
> >People regularly bash any use of anything other
> > than commercial grade linear slide components? Why, why is it
that a
> > V-wheel on angle iron or skateboard wheels against a hard flat
edge is
> such
> > a horrible idea?
>
> Actually, it is a very good idea. Skate wheels and angle iron will
make a
> very good linear rail for a hobby router. Some commercial plasma
tables
> even use this arrangement, though the wheels they use are inferior
to skate
> wheels.
>
> Your table won't get .005" accuracy but then neither do most of the
> ballslide tables the rest of us build. Accuracy is in the design
and
> construction, not so much in the components.
>
> >
> > I'm not being defensive but rather truly trying to determine the
> difference
> > between these approaches.
>
> Linear slides are easy to apply - drill some holes and bolt them
down.
> Skate wheels and angle iron will require more work and more
thinking to get
> good results.
>
>
> > Another question I have is how does one go about squaring a large
table.
>
> If you put a cheap dial indicator on the end of a bar (or even a
stick) then
> you have a very accurate distance comparator. Use it to get two
rails
> parallel within a few thousandths.
>
> A carpenter square will work fine if you take readings from both
directions
> and average the difference. You can easily obtain .005 in/ft
> perpendicularity this way.
>
> >
> > Last question, I don't own a mill and will need to make some
parts for my
> > machine. Do you all have suggestions as to lower cost ways to
produce
> some
> > of the machined parts I need made outside of the traditional
commercial
> > machine shop?
>
> My first lathe was a drill press and file. My first mill was the
same drill
> press and a $100 XY table. A little ingenuity goes a long way if
you temper
> it with patience. You can build a very nice router table with wood
and
> glue. Just don't get to hung up on this accuracy stuff. It's
mostly
> bragging rights anyway, like having 550 horsepower in the car you
drive to
> work every day.
>
> Doug