Re: [CAD_CAM_EDM_DRO] Accuracy of ( was Linear Slide Components

More rigidity (a.k.a. stiffness) is better because the
accuarcy when the machine is not uder power or load is
only part of the battle. When the cutter is working
it is pushing on the linear guides with X large
amounts of force. The more flexible the bearings, the
more they will deflect under load and cause a position
error. The bearing stiffness also effects the natural
frequency of the the system. The stiffer the high the
nat. freq. and the less likely you are to go through a
resonance which will really throw things off.

Chris


--- "J.Critchfield" <js3mc@...> wrote:

> Dave,
>
> I'm very new to all this but by your argument
>
> "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"
>
> a solid surface bearing would be the most accurate
> of all. I would reason
> that the bearing is only as accurate as it is built
> and how it is applied. A
> ball bearing race with an accuracy of .001" is just
> as accurate as a roller
> slide with an accuracy of .001" If you are not able
> to maintain the design
> accuracy then maybe the bearing is being used beyond
> its design parameter.
> To me surface area contact has nothing to accuracy
> but with load and drag.
>
> Jay
>
>
>
>
> ----- Original Message -----
> From: "dave_ace_me" <davemucha@...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Wednesday, April 17, 2002 5:46 AM
> Subject: [CAD_CAM_EDM_DRO] 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
>

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