[ Bad English Warning : This post can include some bad uses of English because of the author's poor English ability. ]

Hi puzzlers! I'm an elementary designer of twisty puzzles.
The problem I want to talk about is the core design.
I always struggle with it when I design a puzzle because I can't make decision on what structure would be proper and stable to use.
Here's some reasons:

1. I've nearly zero information about screw. I don't know how many kinds of screw there is and what to use and where to get it.(Note. I live in South Korea.) So I can't use core with screw.(Maybe will be able to do if you teach me about it...)

2. And If I design without screw, I always make very unstable and hard design. I usually refer Pitcher's design, but because of my poor quantity of knowledge on Engineering and some more reason the result always disappoints me. It doesn't work or needs some other additional action to run.

So, That I want to ask you is to give me some information about screw or to suggest to me some nice core models which is stable and doesn't need screw.

I'd be so grateful if you help me. Have a nice day!

I would be tempted to say that David's puzzles are all brilliantly simple puzzles, so he can get away with using a snap fit structure. If you make more complicated puzzles, then the only way to get the precise tension you need is through screws.
The universal standard for screws is the ISO metric system. The Americans have their own (imperial) system but it is stupid.
The most commonly used screw is the M3 Machine Screw because it is about the right size to fit in most puzzles. Usually 10mm length is good.

Here is a picture of what you want the cross section of the center piece and core to look like:


The 2.75mm is the diameter of the hole in the core in which the screw will thread. 3.5mm is the diameter of the hole in the centerpiece through which the screw will go freely, but the head won't. 6mm is the diameter of the hole in the centerpiece through which the entire screw (including head) fits. 8mm is the OD of the centerpiece. 2mm is the thickness of the bottom of the centerpiece, though this measurement is not very important.

Finally, here is a picture of a M3 screw:

So, Don't I need to draw anything for the screw thread in the case of WSF material?
How a simple! Thank you a lot.
I'll try to find the m3 screw at the hardware store nearby me.

No, you don't. WSF is flexible enough that the screw will thread itself. In fact, I do not think the printing is detailed enough to even create the thread.