Dimensioning

Examples of chain and parallel dimensioning are above below.   The advantage of parallel dimensioning is that there is no build-up of tolerances.

Dimensions of diameters are shown on view providing greatest clarity

   

Thread Designation

The complete designation of a screw thread gives

• the thread symbol e.g. M for Metric
• the thread size e.g. 6
• the thread pitch e.g. x 1
• the tolerance class e.g. 4g6g
• the length of thread if not dimensioned separately eg. x 30 LNG

Notes:
If the thread is standard course then the pitch need not be shown.
If the tolerance grade for the pitch diameter and the major diameter is the same then only one needs to be shown.
Threads right handed (clockwise turn to screw in) unless a -LH suffix is added to indicate left hand thread.
The Class of Fit is a measure of the degree of fit between mating internal and external threads.  

Classes of Fit

Three main Classes of Fit are defined for metric screw threads :

• FINE: This has a tolerance class of 5H for internal threads and 4h for external threads.
• MEDIUM: This has a tolerance class of 6H for internal threads and 6g for external threads.
• COARSE: This has a tolerance class of 7H for internal threads and 8g for external threads.

The typical designation for a thread on a drawing is as follows

• M8 - 6e..This is a M8 course male thread with a 6e external(male) tolerance  (before coating)
• M8 x 1 - 6e.. This is a M8 course male fine thread with a 6e external(male) tolerance  (before coating)
• M8 - 6H... This is a M8 course female thread with a 6H internal(female) tolerance  (not coated)
• M8 - 6H - LH This is a Left hand M8 course female thread with a 6H internal(female) tolerance  (not coated)

The typical designation for a Pipe Threads

• R 1/2    External Taper - Sealing on Thread (BS 21)
• Rc 1/2   Internal Tape - Sealing on Thread(BS 21)
• Rp 1/2   Internal Parallel- Sealing on Thread (BS 21)
• G 1/2 A,B or ext    External Parallel - Not Sealing on Threads -Additional seal required(BS2779)
• G 1/2(F = full thread)   Internal Parallel Not Sealing on Threads -Additional seal required (BS2779)

Indication of Surface Finish

For notes on surface roughness use link Surface Roughness Notes

Direction of lay