How fast can I turn a 54" long 1/2" lead screw before it starts whipping? I have arbitrarily chosen 100 IPM as a goal speed. I'm guessing I'll need to turn it much slower than the 500 RPM that this setup would require.

Step Pitch (degrees) 1.8
Step Multiplier 1
Lead Screw Pitch (TPI) 10
Lead Screw Starts 2

Step Accuracy 0.0010
Half step accuracy 0.0005

RPM @ 100 IPM 500
Steps per second @ 100 IPM 1666.67

If it's supported both ends I reckon it will work - suck it and see is my motto !

With direct coupling to the lead screw you can probably make a max rpm of around 1500 - but will it have any power to do anything useful?

Have a look at Nook industries engineering calculators; http://www.nookindustries.com/Engineering/Calculators.cfm

Critical speed calculator will give you some figures to work from. 1/2-10 root diameter is usually 0.359". Check the 'end fixity' or how the screw is mounted in bearings - the calculator shows descriptions of the end fixity. I have 48" 1/2-10 running at up to 800 rpm with type C. note thats 48" between bearing mounts. Is your 54" total length or between bearings?

'Lead Screw Starts 2' and 'Lead Screw Pitch (TPI) 10' ? Your screw is 2 start 1/2-10 and so 5tpi to give your 500rpm and 100ipm.?

The goal speed will be for rapid movement, not cutting.

The router dimensions will be 48"x24"x6"