second issue: I create a 3D model of the screw and 3D subtract it from the object. You have to match the thread diameter and pitch so the grooves match the screw as close as possible. It usually takes a few experiments to figure out exactly what size screw to "subtract" for a good fit. Typically, it needs to be upsized by 5-10% in X and Y but not Z. Try several scale factors to see which one works best for your printer. This works well for coarse wood screws.
I created a screwdriver that had two pieces (the main body and a finger cot at the top). To hold them together, I subtracted the thread from a #6 screw (downloaded the model from McMaster-Carr), then used the actual screw.
I didn't have to worry about the tolerances as the screwdriver was printed in a Strong and Flexible plastic, so I knew I could fit the screw. I just had to be careful it didn't go so far in that it prevented the free rotation of the finger cot.
If you want to 3D print in metal, keep in mind that the repeatability of any steel pieces will typically be around 2%, so if your design is too precise, you may not be able to provide a safe buffer while still maintaining the operability you desire.
The takeaway is that you should review your design and the service bureau's capabilities. It would be best to use as many COTS (Commercial, Off The Shelf) parts as possible.
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