Metric thread geometry and tolerancing

[ask34]

I am working on a project that requires thread milling so I have to master thread geometry. There is one thing that is unclear to me, let me explain it on an example.

I am going to thread mill M1.6 6H internal thread.

According to this website these are a few of my thread's characteristics:

Class: 6H
Pitch: 0.35 mm
Min. minor diameter: 1.221 mm
Max. minor diameter: 1.321 mm
Min. major diameter: 1.6 mm
Max. major diameter: 1.736 mm

Here is the page 206 from the book Tabellenbuch Metall:

There is an equation on this page that establishes a relation between major and minor diameter: minor = major - 1.0825 * pitch

I noticed that the equation is valid but at a certain point minor diameter will get out of it's own tolerance zone.

For example:
major = 1.736 mm (maximum alowable)
minor = major - 1.0825 * pitch = 1.736 - 1.0825 * 0.35 = 1.357 mm --> Not a valid result because max. minor diameter is 1.321 mm.

So what would be minor when major is between 1.700 and 1.736 mm?

 

[pylfrm]

ask34,

Those equations apply to the basic thread profile geometry, not tolerances or actual values. The allowed minor diameter does not depend on the actual major diameter.

I recommend consulting the actual specifications (such as ISO 965 or ASME B1.13M) that define the threads you are interested in.


pylfrm