There are a lot of other added bonuses of using the Nomad compared to the Shapeoko, that have to do with the machine-design themselves, so as Rob asked, what are you trying to make?
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All of the control design and implementation has already been worked
out with the Nomad and it’s current spindle—so a custom spindle on a
Shapeoko is going to require you to do some homework to implement it, and you have to make sure your solution is relatively comparable in weight/constraint as @Randy pointed out so that it works with the existing Z-axis mechanics. -
The Nomad is carefully aligned and assembled with a rigid frame, so you can count on the precision much more “right out of the box” than the Shapeoko, which is going to depend in part on your assembly and alignment process. If you need that precision, that’s definitely something to consider as it’s much easier to be precise with the Nomad.
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The Shapeoko has a larger working foot-print, at 16x16x3" than the Nomad’s 8x8x3" working area, so weigh that into your decision too—how much cutting space do you need, and how much shop table-top do you have available for it to live on?
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Lastly, in each batch of the Nomads that get built, several of the spindles get tested to ensure that run-out is kept within a tight tolerance. If you’re implementing your own spindle setup, you’re going to be on the hook for your own craftsmanship and alignment in your mounting-bracket, etc… to ensure that your spindle is square and that your run-out is acceptable to whatever level of precision you want to hold yourself to
Hope that helps make your decision criteria clearer for which machine fits your needs!