Re: Linear Slide Components

Lee,

You keep right on believing that the cheaper methods will work! They
do... (but there ARE drawbacks)

Everything in this CNC hobby is a balance, IMO. And FINDING the
balance which meets a given set of needs is where the real joy lies,
again IMO.

As you say, inline skate bearings are as hi-grade as you care to buy.

The real issue with the lower cost as used by shopbot (bishop
wisecarver dual-vee wheels on angle iron) is longevity and wear-in.
The rail and wheel "form" to each other (as did the earlier shopbot
technique of unistrut and glass patio door bearings). During this
time a machine built from these components will produce changing
parts, as the components of the linear rail system wear-in... Next,
they will wear-out. And generally much sooner than a well
implemented commercial design. But that will typically take long
enough to not present a huge problem (again, most worthwhile where
cost is a major factor).

What you get with the commercial linear systems is a "known
quantity"; as there is engineering and testing behind the products
which you can "stake" your business upon...

Another issue is simply the time to set these "home-configured" type
of components up, when compared to the "real" linear bearings. And
finally, YES, the real linear bearings ARE stiffer/straighter than
these "hacked" parts, when overall size is considered. In fact, I
have cautioned before that those going the "ebay" route be careful
that the bearings they purchase are not so hi-grade as to require a
structure beyond their ability. Some of these hi-end parts will
rapidly fail if this is not considered and dealt with.

You will/may need to factor this over-sizing of your "cobbled" parts
into your design. But as Miyagi told young Daniel, Balance is the
key. And balance of your cnc machine design can be attained in many
different ways...

Hope this helps,

Ballendo

P.S. I talk a little about these sorts of things in my "book one" of
CNC, Getting into Motion, CNC Tools, Techniques, and Traditions. I
speak to these issues DIRECTLY in book 2, which is a designers guide
to building cnc machines.

--- In CAD_CAM_EDM_DRO@y..., "Lee Wenger" <wenger2k@y...> wrote:
> Scott, et. al,
>
> You make the point that rollerskate and/or roller blade bearings
aren't up
> to the challenge but my experience is exactly the opposite. Do you
honestly
> think that your commercial linear component bearings would hold up
to your
> 12mile/30% grade challenge? A 12mile/30% grade works out to about
a 20,000
> ft vertical drop btw? And to the contrary, I know several people
that have
> roller-bladed an entire marathon - and lived to tell about it. I
suspect
> several of you have rollerbladed several miles at a time and you
would have
> to attest that the bearings held up just fine. Most bearings of
these type
> have significantly higher speed and life-expectancy requirements
than
> anything in the cnc world would require. So these bearings seem to
me to be
> designed for exactly what you stated was desirable for CNC - heavy
work-load
> and designed for non-stop use. I know you can buy a can(12
bearings) of
> abec-7 roller-blade bearings for $10. Isn't that more than
sufficient for
> the needs I am talking about?
>
> Quite honestly, I don't see where all of the inaccuracy is coming
from in a
> home-made (read non-comercial linear components) solution. As an
example, I
> would refer to something like thk slides which must be
screwed/bolted to a
> substrate surface every 6-8 inches. It seems to me that the
stiffness of
> the substrate and the accuracy of the placement of those bolts will
have an
> enormous effect on the accuracy of this system. Having never
actually used
> thk or similar slides - am I wrong? Obviously a hand made solution
has the
> same issues; however, my point is that the implementation of a
given part
> looks to me to be at least as important as the part itself.
>
> As for the alternatives to having components made at commercial
machine
> shops, I was hoping that there existed things like mail-order job
shops that
> were more cost effective and people might have names numbers etc.
>
>
> ----- Original Message -----
> From: "Steve" <robo_man@c...>
> To: <CAD_CAM_EDM_DRO@y...>
> Sent: Sunday, March 31, 2002 4:01 PM
> Subject: Re: [CAD_CAM_EDM_DRO] Linear Slide Components
>
>
> > From: "Lee Wenger" <wenger2k@y...>
> > > One of the major elements is obviously the linear slide
components and
> to
> > be
> > > perfectly honest I don't understand why the need (in other
words, the
> > > quantifiable difference) to use commercial quality components
that
> > increase
> > > the cost so dramatically. 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?
> >
> > We have a 30% grade road that runs for about 12 miles where I
use to
> > live. A friend thought it would be fun to skateboard down it. He
made it
> > about 1/3 of the way down before the wheels got soft and the
bearings came
> > loose, landing him in the hospital. Other then the fact that they
aren't
> > built to handle heavy loads,and non-stop use, there is no reason
not to,
> > unless accuracy matters. If you aren't going to place heavy loads
on it,
> not
> > going to use it for more then a little while non-stop and don't
need to
> hold
> > a tolerance then there is no reason to spend the extra money.
> >
> > > From what I've seen, as soon as you use any commercially
> > > available linear components the cost of those components alone
is well
> in
> > > excess of $1000 for a 4x8
> >
> > It's not a cheep hobby.
> >
> > > I'm not being defensive but rather truly trying to determine the
> > difference
> > > between these approaches.
> >
> > It all depends on what you want to make.
> >
> > > Another question I have is how does one go about squaring a
large table.
> > I
> > > certainly know how to do traditional construction type of
squaring and
> > what
> > > a 3-4-5 triangle is and all that but how on earth do you
measure these
> > > things to sufficient accuracy on such a large scale. I'm
assuming my
> > > framing square will be completely useless as it would only be
suitable
> for
> > > initial setting but I would need a far more accurate way to
finalize my
> > > squaring.
> >
> > It totally depends on how accurate you need to be. A framing
square
> might
> > well be as good as you need. An option for large home built
machines is to
> > build it with some adjustment in it. Put something large on the
table with
> > the back edge as square as you can. You then cut the back, front,
right
> and
> > left. Check size to make sure you cut the front/back, left/right
even.
> Then
> > flip the object on the table over and line up the back edge. When
you
> check
> > the right or left side it'll be off twice as much as the table is
out of
> > square. Adjust the table until you've taken out half the distance
and
> start
> > over to make sure it's right.
> >
> > > 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?
> >
> > A hammer, a chisel, a file and a micrometer. There isn't a
huge amount
> > you can do with a machine that you can't do by hand. The
difference is
> that
> > the machine might take 20 min and doing it by hand might take
hours, days
> or
> > even months. What you are paying for at a shop is mostly their
time, but
> > it's also what they know and the machines they have to do things
faster.
> IE
> > I have no interest in hand scraping in ways, it's worth the price
to send
> > them out to some one with a big grinder. If it's not worth it to
you, then
> > use what ever tools you do have and do it your self.
> >
> >
> >
> > Addresses:
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> >
> > OFF Topic POSTS: General Machining
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reach it if
> you have trouble.
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> >
> > http://groups.yahoo.com/group/jobshophomeshop I consider this
as a
> sister site to the CCED group, as many of the same memembers are
there, for
> OT subjects, that are not allowed on the CCED list.
> >
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