|
Machine Base
The machine base is the part of a CNC router structure that supports the system drive train, gantry, work surface and so on. It should be constructed in a
manner that provides a stable foundation which
yields superior rigidity, dampens
vibrations, resists flex and maintains dimensional stability.
Techno machine bases are designed for
the dynamics of high speed operation. They provide the necessary
stability and dampening to resist the deflection and
vibration caused by CNC movement and cutting forces. Additionally,
the Techno bases are dimensionally stable; they start out square,
level and accurate and stay that way.
Within the Techno line, all floor models (LC, LCX, RG, Panel Master
& Premium) feature the industry standard construction method for CNC
Router machine bases; precision ground steel components joined
together with stress relieved weldments. Joining two or more pieces
of metal together with weldments is the only way of making them act
as a single piece.
Competitive Consideration-
in contrast to a welded steel, stressed relieved machine base
construction method, a number of lower cost CNC
systems feature bolted assembly machine bases. Inherit in such
assemblies are several negative characteristics. In establishing
joints in a bolted assembly the contact area between components
(bolted area) is generally only about 15-20% of the contact area
resulting from a weldment. Obviously, this makes for a weaker
joint. Additionally, when the bolts are tightened, stresses are
introduced into the steel which cause the structure to twist and
creep on an ongoing basis. And, when a machine base shifts position
(creeps and twists) every linked component is pulled with it; guideways, bearings and so on. The result is a machine that’s
neither square, level or accurate.
Drive Train
The
term drive train on a CNC router refers to the group of components
that transforms rotary action into linear motion. It is comprised
of bearings (plane or rolling element) guide-ways and some means of
moving the system from one point to another, generally a screw or
rack. A drive train should be able to withstand extreme
environments, feature a low coefficient of friction, and reliably
provide precise, high-speed performance
with minimal user maintenance.
Techno’s CNC
Routers utilize THK precision ground steel rails, linear bearings
and ball screw drives. This combination provides smooth
high-speed performance; requiring minimal maintenance and excellent
accuracy and extremely long life. Techno drive trains feature the
following characteristics:
Critical Y-Axis Drive Train (drive train responsible for gantry
structure movement –front/back motion)
On all Techno
systems the ball screw is positioned in the middle of the axis, this
eliminates the possibility of racking (structural twisting).
Additionally, this drive is mounted under the system work table
which provides an effective shield against debris and
contaminants. The Y-Axis features
recirculating anti-friction linear bearings and precision
ground/hardened steel rails which are engineered to provide the
requisite stiffness and high load capacity to negate gantry skew
when heavy cuts are taken at the edge of the table. Precision
alignment of the guideway system during machine assembly guarantees
maximum feedrates, accuracy and excellent overall life expectancy.
The Y-Axis ball
screw ends are rigidly fixed and the ball nut is rotated
allowing Techno’s small diameter high-performance
screw/anti-backlash nut combination to operate well above critical
speed without encountering problems with screw whip.
X-Axis Drive Train (responsible for side to side motion) Z-Axis Drive Train
(responsible for up/down motion)
On all Techno systems the X/Z Axis drive trains are housed in a
rigid anodized aluminum extrusion profile which is scratch/corrosion
resistant and features an internal reinforced webbing to provide
maximum stability; the drive trains incorporates a fully enclosed
servo motor assembly fitted with plastic seals to guard against
contaminants entering the internal mechanism.
The Z Axis drive train utilizes a wide base which is well suited for
cantilevered loads (spindles, drill banks, etc.). Designed for
rigid, high load capacity the drives features: precision
ground/hardened steel rails,
recirculating anti-friction linear bearings
and Techno’s small diameter,
high-performance ball screws and anti-backlash ball nuts.
Precision alignment of the guideway system during machine
assembly guarantees maximum feedrates, accuracy and excellent
overall life expectancy
Rack & Pinion/Ball
Screw Comparison
The two most common
drives today are the rack and the ball screw. A rack and pinion is
one of the least expensive methods of generating linear motion from
rotary motion. The rack consists of a long steel bar into which gear
teeth have been cut. The drive motor turns a pinion gear that is
meshed into this rack. As the gear turns, it rolls up and down the
rack moving the axis.
While
most rack and pinion drives feature the
advantages of being simple and inexpensive, they also have some
notable limitations. The motor must
turn at a reasonably high speed to generate the required power. It
must therefore drive the pinion gear through a gear reduction box.
The gearbox required to drive the pinion gear is generally not more
than 75% efficient and in many cases far less. This means that a
significant amount of the available motor power is used to overcome
friction in the system. Friction increases wear, lowers a system’s
speed, acceleration and positional accuracy. If you loosen the
gears up to decrease friction you introduce additional backlash. If
you tighten the system to eliminate backlash you increase friction
and decrease performance.
The most common configuration for rack and pinion systems is a dual
drive train (see graphic directly below) on a machine’s long axis. Rack and pinion assemblies
are mounted
on each side of the gantry and each assembly is powered by its own
motor. This configuration is sometimes touted as providing superior
performance under large off center loads then can be attained with a
single drive train mounted in the middle of the axis. Fact is,
single ball screw drive trains that are middle axis mounted, when
engineered properly, provide the requisite stiffness and high load
capacity to negate gantry skew when heavy cuts are taken at the edge
of the table. They’ve been doing this successfully for decades.
Of course, when two
drive trains are utilized to run one axis, both drive trains always
have to move in concert. If the motors or the alignment of the two
rack drives do not stay in sync it
can put a significant strain on the system, causing a decline in
performance, cut quality and drive train service life.
A ball screw and
nut combination is the most common drive means for CNC routers. It
is also widely acknowledged as the best and most efficient CNC
drive. The nut is actually a bearing that turns on the screw with
very little friction. The ball nut is mounted to the part of the
machine that is to move and the screw is mounted to the base. As the
screw turns, it moves the ball nut and the axis it is connected to.
A variation of this configuration is to rigidly fix the ends of the
screw and rotate the ball nut instead.
All Techno CNC units feature
rigidity mounted ball screw drives and anti-backlash nuts
Ball screws have a
number of advantages over racks. A ball screw has excellent power
transmission (90-95% efficiency) due to the rolling-ball contact
between the nut and screws. So, ball screw based drive trains don't have the friction
problems that are inherit with rack based drive trains. Motor power
in a ball screw drive is used to move the system, not consumed overcoming
drive train friction. Keeping fraction to a
minimum contributes substantially to responsive operation and long
-term maintenance-free operation. Additionally, ball screws don’t
have the play or the requirement for adjustments that racks do; they
also do not wear as easily as racks, they are far less likely to get
debris in the mechanism than racks (debris in a drive train can cause skipping and errant
motion) and by the nature of the differences in mechanical engagement
ball screws run smoother.
|
