Taking the plunge

Milling a round pocket with no islands in it. I think it's a special case, on account of it being a common requirement and easy to spot and cater for. Need to choose a method.

There's the centre, plunge, expand, centre, plunge, expand, over and over again method.

Not bad, but there is also the spiral straight down to the bottom method which gives a deliciously constant cut, and, often as not can work with a non-centre cutting end mill.

Plunging without zero backlash is not friendly, the faintest slop on the table can make for ghastly vibration as it tries to cut a slighthly out of round geometry. Plus the cutting force gives a powerful downward component on the screw least well positioned to take it.

Tempting to slow the feed rate near the bottom, but then I run the risk of getting in to cut/no cut territory, a rub/snatch cycle due to flex in the system.

Is a roughing out plunge with a horizontal component a bad idea? I've never tried it.

Anyone seen a super smooth round pocket cutting algorythm in action that I can stea... er, 'borrow' from?

Robin