Mouth of a Hand Plane

It can help reduce tearout in some cases, but it's not strictly necessary and it needs to be pretty small to be effective at this. See this article by Chris Schwarz.

It makes a difference, the why is a bit complicated.

Yes, a "tighter" mouth "can" produce finer shavings and a smoother cut.

If you have the time take a look at

You can also read

Regards.

Thanks for the good info, Chris (and the good blog site). Much appreciated.

S.

If planing doesn't interest you have a look at these dovetails from Jeff's site:

Good information, Tom. Thanks.

S.

The idea is that it a smaller throat should reduce chatter (and tear-out?) when going over knots and inter-locked grain.

In my experience holding the plane at an angle to the cut you're making and setting to an even finer cut is a lot more effective at stopping chatter than the most finely tuned throat. Maybe my technique is to blame...

-P.

Yes. When working against the grain the tendency is for the edge to get under the fibres. The backface lifts the fibres so they are rived instead of being cleanly cut. The riving (splitting) and eventual breaking causes the rough tearout.

A very closely set mouth acts so that its front lip presses down on the fibres, preventing them from being raised and allows the edge to reach the fibres and cut them.

Jeff