I rigid tap small holes in aluminum often. Typically only a few holes per part and maybe a hundred parts per week. Normally, the threads are 4-40 and I drill with a .101 drill then use a form tap GH6 (the parts get anodized so I go big) held in a DA collet and spindle speed is 1000rpm. Flood coolant is always used.
If I go faster than 1000rpm, the threads get stretched and I increase the frequency of tap breakage. Running the spindle in air above the part and slowing the rpm, I can see that the rotation isn't exact by the "bottom" of the hole. Maybe off by a few degrees. I'm guessing a tension compression tap holder would be beneficial. Any recommendations on brands? I've never used anything but a rigid holder and have no experience with the flexible gizmos. Thanks.