3D-Printable cover for 7x10/14/16 Minilathe Apron Gears

[Elizabeth Greene]

I pulled my lathe apron apart for maintenance and made this 3-D printable cover for the gears in the apron.





I put the download up on Thingiverse.
https://www.thingiverse.com/thing:5871566

Anyone that needs one and doesn't have a 3d printer, PM me. I'd be happy to print you one.

Question: How do I know how much clearance to put around a screw hole when designing a part? I've read about press fits and slip fits in the Machinery's handbook, but I'm struggling to connect that hypothetical knowledge with real-world designs. Specifically, this part uses M4 screws for mounting and I added (I think) 15% of the diameter for clearance around the fastener. Still, I had to drill out the cover get the screw to fit. What am I missing?

I can (and will) brute-force the problem by printing an array of holes of various sizes for my next project, but I'd like to understand the "right" answer too.

Thanks all.