I recently bought off Amazon an R8-MT1 adaptor to enable me to use my increasing stock of taper shank drills in my milling machine.

Unfortunately, unlike an R8-MT2 adaptor I have that I bought from a UK-based supplier before they ceased trading, which had only a few microns of runout up the taper, this MT1 adaptor has about 0.1mm runout up the taper and is never going to let me do good drilling with it.

Question is, how feasible is it to re-bore the internal MT1 taper in a way that improves concentricity? I can hold the top section of the adaptor in a 3- or 4-jaw chuck but what would I indicate off to keep the 7/16-20 threaded section held in the chuck jaws as perfectly in line as possible with the bore I want to create anew? The black central section of the adaptor isn’t straight-sided but has a very slight and uneven taper on it, and the male taper doesn’t look like a very good reference surface either, especially as it may be contributing to the overall eccentricity of drills placed in the adaptor, which typically show considerably more than the 0.1mm runout measured in the bore.

As an alternative I could make it into a side-lock holder for end mills, but there isn’t really anywhere suitable to drill for a set screw as, unlike on dedicated end-mill holders, there is very little in the way of a straight-sided section down below the male taper.

After receiving a partial refund, the adaptor cost me less than £5, so whatever I do with it it won’t be a great loss.