Has there been any testing done on the Altmill where sudden power loss to the machine or hitting the stop causes the z axis to plunge into the work piece or waste board from the weight of the z gantry ?
I ve read it was an issue on other cnc machines after the spindles extra weight was added , is why I’m asking .
I have not had sudden power loss yet 
but I have hit the E-Stop a number of times on my AltMill and have never had an issue with the Z axis plunging at all.
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Good to know . Maybe the beefier stepper motors the Altmill are what makes tge difference .
I know both onefinity and shapeoko have had a known issue with it but they also use weaker stepper motors .
I think you will find that the size of the stepper motor has very little to do with the spindle dropping. With no power applied to the motor, there shouldn’t be much difference in the amount of force needed to rotate the shaft. I think it is strictly the weight of the spindle, the pitch of the ball screw and friction that control the dropping.
This brings up an interesting point I had not thought about before. One of my future projects was to mount a small router in front of the spindle motor for vertical routing of dovetails and the like. It never occurred to me to find out what the maximum weight would be before the spindle would drop without power. This is also something to think about before mounting a beefier spindle motor.
I just did a quick test on the Altmill - with no power applied, It takes a surprisingly large amount of force before the spindle will drop. I have nothing handy to measure the actual force but would say a weight of between 10 and 20 lbs in addition to the already heavy spindle motor is required for this to become an issue.
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Thanks for chiming in , interesting to know that the Altmill doesnt appear to need a z- brake from your observation .
If it’s less to do with the more powerful stepper motors , the one thing you mentioned that’ is different is pitch of the ball screws . Atleast from checking specs in comparison to the other machines .
Best of luck with your trim router project . Sounds like a cool project .