In your last photo, the tap is shown in a typical tap wrench with the quill of the milling machine above the work. Are you hand tapping using the quill as a reference? Also, what tapping fluid did you use during tapping? Was there any difficulty during the tapping process with the original threads being overcut?
Finally, alloy can be rather buttery or very difficult to machine. Did you ascertain the alloy used in the case before proceeding? What rod was used to provide the alloy fill?
Thank you. Continued good luck.