Mechanical Design Info
There industrial standard practice when designing machine or mechanical parts. This is to facilitate a better coordinated world. Imagine if manufacturers produce different screws standard (length, thread, pitches etc), designers uses their own preferred method of putting shafts and holes... the design world will be more choatic. Hence, for certain common used items like bolts and nuts, screw and washer... there are certain agreed standard people follow.
However, there are also customised designed screws for specific purpose ... of course then the designer would need to pay more for such special request.
This page provides information on some industrial practices .
Things to note when designing
1. Hole ( thru vs blind, countersunk vs counterbore, depth, diameter)
2. Chamfer?
3. Rounded corner?
4. Tap diameter
5. Depth of tap
6. dimensioning - (dia 4.5, C' bore) or (dia 4.5 x 12 DP)
7. Ream hole ?
8. Tolerance for fit ( H7 - refers to Hole & g6 refers to shaft)
9 Quantitiy needed