Re: [CAD_CAM_EDM_DRO] Re: Z axis drive

Many machines even with mechanical counterweights use an electric brake
on the Ball Screw that is tied to the Servo Drive "Drive OK" output.
This brake is engaged in its default state to prevent the weight of the
head from back driving the ball screw and free falling and requires
power to be applied to disengage. This power is controlled by the Servo
Drive "Drive OK" and sometimes by the CNC "Drive Enable" outputs in
series. If either output goes low the brake voltage is dropped and the
brake mechanically engages preventing free fall. Usually this will occur
so quickly that the brake is set before the drive output has fully
decayed and the head will nt drop at all. Similarly when the machine is
restarted the drive is fully energized and in control of the axis before
the brake is released.

On large machines that I have retro-fitted where the head could be over
7,500#, we generally use a hydraulic counterbalance. One example of this
was a 24 Foot Farrel Vertical Boring Mill. The z-axis ram was 20"
square, have a 50 HP Spindle inside of it and would extend out 9 feet,
it weighted 16,000#. This used a simple ram circuit with the cylinder
mounted parallel to the ram and connected to the end pushing up on the
ram. We apply a variable hydraulic pressure to the hydraulic cylinder
ram to take a major part of the head weight and allow the cylinder to be
back filled by the pump as the head rises maintaining constant load.
When the head stops the cylinder ram maintains a constant load on the
head. When the head is driven downward a check valve closes the supply
line from the pump cylinder and the ball screw drives against the
cylinder increasing the hydraulic pressure. A separate pressure relief
valve in he circuit then opens to vent the cylinder back to the tank.
Again there is usually a normally closed solenoid valve in this circuit
behind the relief valve that is held open by the Drive OK/Drive Enable
Signal which closes if the signal goes low maintaining counterbalance in
the event of a power failure or machine health problem.

The biggest reason that we use a counterbalance however is to balance
the dynamic reactions of the machine in the up and down direction. On a
heavy head without a counter balance it is not unusual for the servo
drive to be firing upward all the time even when the machine head is
going down or at rest. It is easy to understand that when the head is
being raised that the drive has to fire upward, what is not so obvious
is that as the head feeds downward the ball screw essentially offers no
resistance to back driving and the head tends to free fall. The servo
has to act like an engine brake on a truck to maintain control. You can
see this if you monitor the current up and down. The counterbalance
allows us to balance the drive current and performance independently for
both up and down travel. The pump pressure is varied to assume a large
portion of the head weight to keep the up currents and accelerations
reasonable. The pressure relief valve in the return circuit is then
adjusted separately to prevent head free fall and offer enough
resistance to match the down current and accelerations to the up travel.
This allows us to run a much tighter error limits and higher acc and dec
ramps with faulting the machine with a following error.

For lighter weight heads we have also used a hydraulic ram connect
directly to a nitrogen accumulator where the nitrogen pre-charge
maintains the load.

gary

skykotech wrote:

>>All the commercial VMCs I've worked on use hydraulic or pneumatic
>>counterbalencing on the Z axis, usually hydraulic and with the
>>cylinder in a 2:1 compound arangment with chains like on a forklift.
>>Reduces the inertia problem since the counterballance is some chain
>>and a piston as opposed to a giant weight.
>>
>>Pete C.
>>
>>
>>
>
>Hmmm, my Shizuoka B-3V bedmill does not use a counterweight for the Z
>axis, and the z axis alone probably weighs 1000lbs. I wonder why?
>
>I think it has some clutch arrangement to prevent the z axis from
>falling when the servo is off.
>
>I wonder if adding a counterweight would be beneficial to my
>mill...hmmm....
>
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