Question about screws and screw tunnels.

[alphacat]

Hey, i know its not that related to electronics, but since i find this forum very helpful, i though asking it here.

A plastic screw tunnel is actually like a plastic cone.
The bottom of the cone, where the metallic screw enters from, is more wide than the top of the cone, where the metal screw gets out from.
(The reason for this is called "draft angle").

My question is:
If I want to use a 3mm diameter screw, what should be the diameter of the bottom of the plastic cone, in order for the screw to get in well?

2.5mm?
2.7mm?
A different value?

Thank you.

 

[PatM]

Plastic anchor sizes are normally rated by the entry hole where the screw enters.
Drill a hole the same size as the outer body of the anchor.
When the screw is inserted it will expand to hold the anchor in place.