Hi Vern!
That "T" word you're looking for is "Trochoidal Milling" in all likelihood.
The high speed toolpath algorithms are not all Trochoidal, but they all involve looping the cutter to clear material instead of just diving straight in. They're doing this for two reasons.
First, cutter engagement in corners radically changes the stress on the cutters. So if instead of plowing straight into the corner, you take a series of smaller radius loops, you can clear a lot of that and reduce the amount of engagement:
If you consider cutting straight along a wall and the amount of material the cutter is embedded in versus when the cutter enters a corner, you will see there is a lot more. Consider the picture from above:
The corner exposes the cutter to about 2x as much stock as moving along the wall. Moreover, consider the fraction of the circle when the cutter isn't doing any cutting as cooling time for it to recover before going back into the material. If you're taking maximum advantage of that, you can run more SFM than the manufacturer would normally tell you to.
OK, second thing these toolpaths do is take advantage of radial chip thinning. When you use a width of cut that is less than 1/2 the diameter of the cutter, a funny thing happens--the chips are thinner than the normal speeds and feeds equations tell you they should be. You can see why from another diagram:
You can use the G-Wizard Machinist's Calculator to compute feedrates adjusted for chip thinning, but the idea is to increase the feedrate so the chip thickness is restored to the recommended chipload.
Lastly, one of my G-Wizard Beta Testers sent me a wonderful video that shows just how crazy fast you can go with this stuff:
[nomedia="http://www.youtube.com/watch?v=OzPpe8dI06o"]YouTube- Demo in a box, wide open![/nomedia]
Very cool stuff.
Cheers,
BW
Try G-Wizard Machinist's Calculator for free:
http://www.cnccookbook.com/CCGWizard.html