Thanks Hugo,
I have a project in mind for one of my UHU or HP_UHU boards. Hence some of the questions.
I use both R8 and an R8-TT collet on my mill. So most of the tools are in TT holders that slip into the 0.75" flatted face R8 collet.
To drop a tool requires maybe 1 to at most 2 turns of the drawbar.
To remove the R8 TT collet and say insert my R8 Face Mill requires about 12 turns of the drawbar which I don't want to do each time I remove a TT holder.

Specified torque for R8 tooling is about 20 ft-lbs although it's unlikely I have ever tightened them that much by hand. But I did build a butterfly impact wrench system along with a socket revolution counter to try and automate things.
Attachment 483730
It sort of works but not well. There are two issues.
1. The socket doesn't always end up lined up with the head and can even jam on crooked so I added a sensor for all the way down.
2. If the drawbar is not very tight and a short blip of the air to the butterfly wrench sends it spinning and once loose can literally over spin and drop the R8 collet.

Essentially the problem with the butterfly impact wrench is unloaded is way too fast to be able to properly control.

I'm using a controller I scavenged from a different project.
Attachment 483732

Clearly the butterfly wrench isn't suitable for automated behavior. So I want to drive the drawbar socket with a motor and reduction that at highest power will create 25 ft-lb torque. But I never want to tighten it with more than say 18 ft-lb of torque.

What I thought I'd try is turn a DC servo for 12 turns or until motion stops and say a 2.0A current limit is reached or a timeout. That's to tighten.

To loosen if it's a TT Holder the button to release is just tapped and the motor torque is increased to 2.7A and turned for 2 revolutions. (actually way more depending on reduction gear).

To loosen and remove the R8 collet or tool, the button is held for more than 1/2 second and again torque is increased to 2.7A but now 16 revolutions or as long as the button is held or a timeout.

So I thought I'd use a UHU and make it possible to adjust the reference so it always has reserve torque to loosen but can never tighten more than a preset value. Once the motor stops the continued steps into the UHU will create a following error. That's the indication that the drawbar is tight. A signal from the controller will toggle the reset line on the UHU to undo the fault.

So either a relay that switches in a parallel resistor into the reference setting or even a PWM generated filtered voltage to serve as the reference.

So that's the background. Where I'm undecided is which Vref to 'tweak'.