Triac VMC Servo // Z-Axis Dropping on power off

[danielbriggs]

Hi all,
I'm having an issue where the Z axis on my yr2000 Triac VMC - Servo (Baldor Servo + drives) 1ph... will drop the Z axis on power off or e-stop. It only does it sometimes, but when it doesn't... touching the head with fingertip pressure will cause the head to drop full [rotates ball screw] until it's at the end of the travel.

This machine has just been started to be converted; so I've rewired a few bits.
So... I understand why servos do this... (as opposed to steppers // holding torque etc.). However I am under the impression Triac's do no have a servo brake, or a counterbalance fitted. So was looking at do I need to fit a gas strut to stop my Z-axis dropping.

BUT: Why should I have to do this? I assume the machine worked perfectly without this, with the original DOS setup. What possible wiring could I have changed which now makes the Z-axis drop on power off? Is there anything I'm missing?
There were no brake / regen resistors fitted previously to the drives. Do I need to add these in, or are they only for decelerating at high speeds?

Also, does anyone have any advice on tuning the Baldor servo drives? The defaults were well off. Just wondered if anyone had any tip or direction on this.

All the best,
Dan

[battwell]

tighten the z gib screws slightly- to give a bit more friction? mine doesnt drop with no servo power.

[danielbriggs]

Thanks. Fixed it.

It was a combination of that and the motor mounting belt tensioner was incredibly loose.

Also fixed it tuning; it's now really smart.
I got up to 9,600mm/min rapids, but slowed that down to 5,000mm/min on the X/Y and 1,500/min as any faster was a bit silly!

[battwell]

does your machine orient the spindle? thats the bit im stuck on at the moment

[danielbriggs]

does your machine orient the spindle? thats the bit im stuck on at the moment

You mean if it knows the exact angular position of the spindle?

No...
Currently fitted is a SEM MT30U4-36 and Sprint 1800 drive.
That has a tacho on it; but anything better than that i.e. for rigid tapping etc you'd need a new spindle with resolver or encoder and a new drive.

[battwell]

-required for tool changer to align the drive dogs of the spindle.
original fanuc control did it- but im having trouble getting it to stop in the right position.
stupid thing.lol
mine has the control techniques lynx drive- not sprint- so already mains isolated :-)

[m_c]

Machines with the drive dogs, IIRC early ones used a drop in solenoid to locate the spindle, then later ones used a sensor (possibly alos with a programmed time delay).

Denford stopped fitting the drive dogs at some point though, as they aren't needed for the relatively low power spindle.