There was also a link to a page on chip thinning: http://www.sandvik.coromant.com/en-us/k ... fault.aspxThe reason you "arc on/off" or "lead in/out" is simply to minimize the amount of gouging that happens when the cutter bites into the material.
When climb milling (especially with HSS cutters, but carbide as well to a smaller degree) the cutter is actually "sucked" ahead of it's intended path and therefore will overcut if you approach a piece in a perpendicular manner.
When you approach in an arc tangent to the intended toolpath, the deflection is redirected into the material ahead of the cutter parallel to the toolpath, and not into the material that makes up the part.
Additionally, when using compensation you need "room" to apply it, and therefore a typical approach is a linear move parallel to the path followed a 90 degree radial arc tangent to the path. As a rule of thumb the linear move and the arc radius can be twice the cutter diameter to ensure everything happens correctly.
(who is hoping to be cutting this weekend)