Fighting deflection milling 2-12" deep through hole in 1020 steel

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We are using a 1/2"carbide end mill. Our stick out is 2.750. We are getting a lot of harmonics and the hole ends up under sized by .002 - .003". Our tool OAL is 4" so we might not have enough up in the milling chuck. We also are using a collet set up with similar results. We have 5" OAL cutters on order to see if that helps with the deflection. Our finish diameter hole is .875. We start our drilling the hole to 9/16" and interpolate out to size with the 1/2" 5 flute. Any recommendations?

We are running at a .500" depth of cut with a .015 per side step over. We have tried multiple feeds and speeds. Started with about 2700 rpm at 37 ipm.

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Do you have enough Z to drill and ream or a boring tool? If Z is a problem, I would recommend using a 3/4 endmill, if you have a centercutting endmill, you could use it like a drill following the 9/16 drill thru hole.

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