I have a large gantry machine with two servos moving it. Well it will have two servos once they get here in a few days

I need to figure out a way to get the weight of my gantry so I can tune the servos independently.

Short of removing it from the machine and weighing it does anyone have a suggestions for a way to estimate it closely? I'll tell you it slides very easily back and forth with no motors installed so I'm not sure putting a scale on it to see at what pounds it starts moving would do it.

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Thanks in advance.