Probably a Stupid Question.

[Electroconducive]

Thanks m_c

I'm finding it hard to keep the weight down with an epoxy granite filled steel gantry of a suitably rigid shape.

I'm probably over compensating quite a bit. I'm going to try and bring the weight down.

I'm also failing to understand how to calculate which motor I'll need for a given linear system.

I'm basing the servo on Boyan's massive purple machine. Which I believe used a 400w servo for a 120kg gantry.

[Ger21]

Weight is not an issue, as long as you have the power to move it.
We're looking at a new industrial router that weighs about 4-5 tons, and rapids at 96m/min.

There's no reason to fill the gantry beam with epoxy granite imo. A steel beam with a 10-12mm wall thickness is plenty strong enough, if you add some internal bracing.