There are many factors that influence the dimensional accuracy of 3D-printed components. It is advised that you print a calibration cube or a "Benchy" to test the accuracy of your printer. If your printer isn't calibrated, you may be able to find tutorials on YouTube to help with calibrating your printer. If not, contact your printer's manufacturer for assistance. A printer that has not been calibrated, may not produce dimensionally accurate prints. This is very important when it comes to printing functional models such as firearms.
Even if your printer is calibrated, it may be necessary to make slight modifications to your 3D-printed frame/lower receiver to ensure proper fitment and alignment of the rails. Depending on your printer settings and the way your frame was oriented during the printing process, there may be support material, burrs, blobs, or stringing on surfaces that the rails use for location and alignment. If that’s the case, you must be sure to remove all of these imperfections. You also want to check that the screw holes in the 3D-printed frame are not too tight or too loose. Usually, the holes are undersized. You should open the holes just enough so the screw can pass all the way through without much resistance but not so much that they can wobble around excessively. Check the product descriptions for recommended sizes to ream/drill holes.
Fine-thread screws are very easy to cross-thread. To avoid this, always install the screws and tighten them by hand first, then torque them down with a tool. You are likely tightening down on plastic, so be careful to not over-tighten.
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