[bigtoad170]

I have a bridgeport series II with a heidenhain tnc 151 control.

Turning down .750 outside diameter to a .375 outside diameter .750 depth in 11L17 steel this is leaded free machining steel

Right now I'm using a .625 diameter four flute cobalt endmill running the helical passes around the diameter .0625 wide and .050 depth per rev 900 rpm 10 ipm with flood coolant them I come in .002 circle path around the diameter for cleanup bottom and side.

This makes a beautiful part only problem it only makes about 15 before I start seeing a big taper about .010 changing the endmill straightens it out what can I do to prolong the tooling besides buying a lathe

[Geof]

I think you are maybe running a bit fast and the cut is too small. On something like that using that cutter I would probably run 500 to 600 rpm and make a first pass down taking off nearly the full amount at the same 0.05 depth but maybe half the feed. Then bump the speed for a final clean up pass.

But my preffered cutter would be 1/2 coated carbide running at 2000 rpm, a much faster feed of course, and I would do the clean up with a second cutter. When my first cutter started going I would move the second one up to first and put a new cutter as second.

[bigtoad170]

Yeah that sounds like a good approach I'll give that a try.
Another thought is maybe getting rid of the helical and just side mill the whole depth and incrementaly moving in

[Geof]

......side mill the whole depth and incrementaly moving in

I think you might get much more deflection especially as you get close to size, and horrendous chatter.

[bigtoad170]

Are there endmills that are more suited for helical milling than a standard square edge endmill

[big_mak]

As long as the endmill is center cutting, then It can be used for Helical milling. Maybe if you had a small corner rad on the tool, the tips would last longer, but that's about it.

I really like using the Garr VRX mills for that sort of thing!!!!