12 IPM feed still seems a bit slow to me.
For my G-Wizard speeds and feeds calculator, I used Niagara's info. They're middle of the road. Exotic coatings, high helix, and so on will get you even more performance.
But, for this application, Niagara wants a little over 400 SFM and a chipload of 0.0020 to 0.0025 depending on your depth of cut on their basic HSS endmills:
http://www.niagaracutter.com/techinf..._mat/6061.html
If I crank that all through G-Wizard (actually, just tell it aluminum, 1/4" 2 flute, HSS Endmill, full slot, and 0.050 depth of cut), it says 6100 rpm and 29.5 IPM.
If it was me, I would run the higher chipload and feed and just monitor the depth of cut until I got something chatter free. That depth of cut is really going to be a function of the rigidity of your machine and setup. You can try up to the width of the endmill, which would be 0.250" deep cut. Not sure the Taig can hack that though.
As was mentioned, 1.5" is getting pretty deep. Chip clearance will be very important. I'd go with the 2 flute for sure cutting that deep. Get the flood going good on it.
Cheers,
BW
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