Leadscrews
There are numerous ways to move an axis on a machine like
this, most commonly in milling machines either a rack and pinion system is used
or a leadscrew, belt drive systems generally aren’t strong enough to cope with
the loads induced or the repeatability requirements. Of the two common systems, the leadscrew is
by far the cheapest and easiest way of moving the axis. The leadscrew is placed along each axis and
is turned by the motor (or by a hand handle) to move the axis along by the
desired amount. I’m using M8 * 1.25
threaded bar as my leadscrew, so for a single full turn of the bar the axis
will move by 1.25mm, this is called the “pitch”. Therefore, to move the axis 100mm the motor
will need to turn the leadscrew (distance/pitch = 100/1.25) = 80 turns. The threaded bar used isn’t designed to be
used as a leadscrew, it’s designed to be used as a fastener so it has a low
pitch compared to a thread form designed to transmit power such as an ACME
thread. This means to achieve the same
traverse speed on the machine we have to spin the threaded bar faster, which
will cause more friction and then need more power to overcome these
losses. Ideally a ball-screw should be
used, this has re-circulating ball bearings built into the nuts and allows
power to be transmitted with minimal frictional losses. Industrial machines will use these
ball-screws, along with DC Servo motors to achieve extremely high traverse
speeds along with excellent repeatability and accuracy.
Axis drive motors
The lead screws are driven by stepper motors, unlike regular
motors that spin when a current is applied to the terminals, a stepper motor
has multiple wires and by applying current to them in a certain order it is
possible to turn the motor by a certain number of “steps”. Stepper motors are classified by how many
“steps” they take to complete a full rotation.
Most commonly found are 200 step motors.
This means they will make steps in increments of 1.8 degrees (360
degrees / number of steps = 360/200).
The physical size of the stepper motor is based around NEMA
classifications and most commonly found used are NEMA 17 and NEMA 23 motors.
There are some formulas that can be applied to selecting the
correct motor that take into account the inertia in the design of the machine,
the inertia in the drive system and then required power to overcome these at
certain speeds and direction changes.
The friction in the bearings and the lead screw can also be calculated
and included and a reasonably accurate motor torque figure can be calculated. I’m not going to go into the formulas here as
I don’t have time to dig them out but they are fairly straight-forward and give
a very good indication of what motor to use, if it’s too small you will
struggle to get a reasonable traverse speed and might well have a loss of repeatability
due to not having enough torque to generate the steps quick enough. If the motor is too big you will end up with
wasted money for the large motors and large amounts of inertia in the rotating components.
