Try shimming your tool so it is above CL. If you are doing high production I agree a custom grind would be best, however the custom tool will most likely look just like the threading tool , just rotated a bit. by moving your tool above CL it will rotate the relief angles, and change the rake as well. you can customize the rake easily enough by hand. If the diameter tolerances of your groove are fairly tight, you will find that .001" offset doesnt equal .001" offset on the part. You might have to offset .0013" to obtain a .001" change in your parts size( This would be easy to calculate once you find the right shim height)
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