I was wondering what exactly should be the gap between slider parts so that they will slide smoothly but stays in place.
Like Per Enberg said - it depends on the type of 3D printer you are using. is it SLA, SLS, ObJet or FDM?
Also, what type of fit do you need - what is the application? Provide a screen shot so that I don't have to d/l and open your parts. I will give you my opinion on a clearance if you answer these questions. Roland mentioned a food grade silicone. I have not used that material for lubrication, but I have used butcher's wax which also works well. Will that parts be finished (sanded smooth) before assembling? Sanding will remove material and the parts can be fit to you liking even if the parts are modeled line-to-line.
You'll have to look at the tolerance from the 3D printing process. Then leave enough gap so that if the slot is undersize by the max amount and the sliding part is oversize by the max amount they will not interfere.
How much is "enough" though? I am planning to convert this to an STL file and 3d printing it in plastic.
You also need to worry about the texture. Make sure parts are not too coarse, either in the file (STL triangles) or the printer settings.
Clean and polish the material. Make sure parts are dried an cured. A little food-grade silicone lube works wonders.
Depends on the 3D-printer. Many different types with different tolerances are available today.
You'll never know until you make your first piece. Personally, I'd start with 0.1mm and expect a couple more iterations before it's correct. If it's a big piece I'll make a small portion of it first as a test.
A 'guess' comes with time from using your printer(s) a number of times and depending on the part orientation for some printers. If possible, I would treat the part as if it's got the clearance as a manufacturered part or undersized a bit then hand fit the parts if it's that important.
I would start a little bit tight at like .002" to .003" clearance. As you slide these back and forth a few times FDM printed parts will "loosen up" a little bit. If you are wanting these in ABS you could acetone smooth the surfaces as well, which would help. If you need help let me know.
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