Box ways was re:Re: Accuracy of ( was Linear Slide Components

Elliot,

Tim already covered this some...

Box ways are PLANE bearings (note the spelling), and are called such
because they are usually rectilinear (rectangle shaped, like a box).

Almost all modern box ways have Turcite or a Turcite/Teflon
engineered plastic compound as "liners". These are in the form of
thin plastic sheets, which are placed between the sliding surfaces of
the ways. The use of these high tech liners reduces the friction
substantially over the older types. Older box ways were often made
of cast iron sliding against cast iron or steel (sometimes bronze).

Since box ways have such a large surface area (with its resulting
high load capacity), friction is nearly always higher than with
modern linear bearings. (Even with the liners.)

The length of the box? Same as a linear bearing; the length of one
component of the slide system is typically at least as long as the
travel of the associated axis. The other side may be as long as the
first, or much shorter (again like the linear bearings "blocks".) The
size of the "box" is dependent upon design loads, and will be larger
in the direction of most load (The rectangles' long side will face
the heaviest load; we're talking cross section here.)

Most box ways are not pressurised. A pressurised box way is called a
hydrostatic bearing. In this type, a thin film of fluid (oil) is
CONSTANTLY fed in between the two surfaces of the slide. This creates
a film of fluid upon which the moving part(s) slide. Failure of the
pump, or of the seals maintaining an adequate thickness film, will
make for disastrous results, as these are not designed to touch at
all.

In machining centers, box ways are usually part of the main castings.
They are milled, ground and scraped into alignment and accuracy. If
liners are used, the liners are added and their surfaces are made
true and accurate. Nearly all box ways have GIBS (The G is pronounced
like "Glad", not "gypsy"). These are adjustable surfaces on one or
more sides of the "box". Their job is to take up wear. They also make
it easier to assemble the components of the box slide.

Nowadays it would be tough to give the box OR the linear bearings
the "Win" in overall accuracy. Both can be (and are) made with more
than enough accuracy for most machine tools. You DO find well-stated
arguments claiming the superiority of one or the other type. IMO,
Like everything else, a decision of which type is better depends upon
what you are trying to accomplish. And you can get to some pretty
fine "hairsplitting".

However, because box ways nearly always have more surface area than
linear guides (in a similarly sized machine); they are better able to
cope with short repeatable motions. The down side is that they
require at least a very small motion to "ramp up" on their fine oil
film (If they have been sitting). They will also require stronger
motors and drives.

Hope this helps,

Ballendo

P.S. Some of what's above is pulled from my book one (which I haven't
received yet from the mfr.)I do go into a fair amount of detail about
box slides (and other plane bearings) versus recirculating ball
bearings in my books one and two. As I mentioned above, there are
very good arguments for each type.

--- In CAD_CAM_EDM_DRO@y..., "Elliot Burke" <elliot@h...> wrote:
> Ballendo writes:
>
> >Exactly right. This is why nearly ALL truly heavy duty CNC machines
> >use BOX ways, rather than linear bearings. Intermittant cuts are
very
> >hard on any piece of machinery, and the linear bearings do not
handle
> >these types of loads the way the full sliding contact of box ways
> >can...
>
> Tell us more about box ways.
> What sort of sliding surfaces do they have turcite/teflon/babbit?
> size and length of box?
> pressurized lubrication?
> how are the ways supported?
> If they have more friction that roller bearings, does this affect
the
> ultimate precision of the machine?
>
> Thanks.
>
> Elliot