Chasing the nut if it's a bit too tight...
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I ground the screw to use it for chasing the threads like this:
This worked perfectly if you want it still slightly tight. Freezing or cooling it first should produce a perfect smooth fit. If you are using a screw that is from the machine then grinding just a small notch at one end should be sufficient although it will take more passes.
I then decided to crack it open to see what the threads look like. It took quite a bit of force to break it open. I wasn't expecting the threads to look this good.