More suitable for processing blind hole thread discharge machining chips back. Due to the helix angle of the reason, tap the actual cutting rake angle will increase with the increase of the helix angle. Experience tells us: the processing of ferrous metals, helix angle selected smaller, usually around 30 degrees, to ensure the strength of the helical teeth. Processing of non-ferrous metals, helix angle bigger election, may be around 45 degrees, cutting some sharp.