What is mean up milling and down milling ? How to choose tapping drill size?

Question

Accepted Answer

Up milling and down milling refer to the direction of the cutter rotation in relation to the workpiece. Tapping drill size is chosen based on the size of the tap being used.

Up milling involves the cutter rotating against the direction of the workpiece feed, resulting in a smoother finish but requiring more power.
Down milling involves the cutter rotating in the same direction as the workpiece feed, which can lead to a rougher finish but requires less power.
When choosing a tapping drill size, it should be slightly smaller than the size of the tap being used to allow for proper thread engagement.
The tapping drill size can be calculated using formulas or reference charts based on the thread size and pitch of the tap being used.

Accepted Answer

In up milling cutter rotates against the direction of table feed. In down milling cutter rotates along the direction of table feed. 
Eg M8*1.25 tapping am selected drill at 6.8mm

What is mean up milling and down milling ? How to choose tapping drill size?

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