Accuracy of ( was Linear Slide Components

Hi John,

there are a lot of factors, surface area of the bearing contact is
one.

for instance, a ball will only contact a flat surface at one point, a
roller will contact at one point but with much more surface area.


another factor is repetitive and continuous contact.

if your roller bearing has 3 balls, then depending on the current
locatoin of the balls, the center race would rise and fall. This is
the 'noise' your feel and hear on cheap bearings. if you get any
dust on your rails, you would have a table movement. either way, the
cutting would not be repeatable and accuracy would suffer, if the
error were in the direction of the cut.


Linear ball bearings on round shaft has a certain area of contact.
roller bearings on a flat have a different area. more area, more
rigidity.

then how many balls(roller) are in play at any time goes to how much
the bearing will move as a new ball begins to take part of the load.

precision bearings will spread the load on a closely machined race
increasing the pressure loading tremendously.


as for the comparrison between styles of application, I too am
looking forward to that thread. I've changed the subject to reflect.


Dave

--- In CAD_CAM_EDM_DRO@y..., John Craddock <johncrad@x> wrote:
> I have read this series of threads with great interest. Can someone
> enlighten me as to the engineering principle that establishes that
round
> ways and linear bearings or THK type rails are superior to Bishop-
Wisecarver
> V-Wheels and tracks given the same level of accuracy and tolerance
in their
> manufacture.
> Regards to all,
> Keep up the fine discussions
> John C
>
>
> -----Original Message-----
> From: ballendo [mailto:ballendo@y...]
> Sent: Saturday, 13 April 2002 7:21 PM
> To: CAD_CAM_EDM_DRO@y...
> Subject: [CAD_CAM_EDM_DRO] 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:
> > > FAQ: http://www.ktmarketing.com/faq.html
> > > FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
> > >
> > > OFF Topic POSTS: General Machining
> > > If you wish to post on unlimited OT subjects goto:
> > aol://5863:126/rec.crafts.metalworking or go thru Google.com to
> reach it if
> > you have trouble.
> > > http://www.metalworking.com/news_servers.html
> > >
> > > 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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>
> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html
> FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
>
> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto:
> aol://5863:126/rec.crafts.metalworking or go thru Google.com to
reach it if
> you have trouble.
> http://www.metalworking.com/news_servers.html
>
> 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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