As other articles in this series, this one is reprinted with permission of the author, who wishes to remain anonymous. After determining the correct size of the armature screws (Loewenherz L3.5×0.6), I mounted the armature vertically in the mill’s vise using the fixture described in a previous post that I had made for this purpose (shown in the third photograph, below). I then used a centering microscope to locate the positions of the three broken screws to +/-0.001″ and recorded the x,y coordinates of the mill’s Digital Read Out (DRO) so I could return to each of them after replacing the microscope with a carbide center drill. Meanwhile, I had ordered an M3.5×0.6 tap because this is an uncommon size and wasn’t in my set of metric taps and dies (more on the difference between an M tap and an L thread below). There was plenty to do, so waiting a few days for the tap to arrive didn’t cause any delay.
I didn’t take photographs at the time when I was using the centering microscope, so I set it up again a few days later using a 6-32 screw, which has nearly the same diameter as the L3.5 mm.
Each of the concentric rings on the microscope’s reticule differs from the next by 0.010″, so it should be apparent that it is easy to use this microscope to center the screw directly under the spindle of the mill to quite a bit better than this (specifically, to within 0.001″). Depending on how the photograph is displayed on your screen, you may be able to see the 0.001″ marks on the horizontal and vertical bars.
Having determined the screw sizes, and having located their centers to 0.001″, I then drilled out the three broken screws. To do this I used a carbide center drill to start the holes on the jagged surfaces followed by a 7/64″ drill, which is 5 thou. smaller than the tapping drill for M3.5×0.6. I used this drill because it’s easier to make a too-small hole larger, if necessary, than vice versa.
As can be seen from the next photograph, enough of the threads were exposed by the drill, followed by use of a dental pick, that a tap could properly engage the old threads and clean them out. At this point I could have taken the time to modify a 60-deg. carbide threading insert to the 53-deg. 8 min. of a Lowenhertz thread and used it to make my own “perfect” L3.5 tap. However, from what I had found thus far it was likely the actual internal threads were an imperfect variation of L3.5 anyway. So, although the small difference in thread form between L3.5 and M3.5 means a tiny amount of material from the armature itself (rather than just the fragments of broken screws) might have been removed, in the overall scheme of things I think my use of an M3.5 tap should count as a perfect fix of these holes.
To answer a question asked by several people offline, at this point we’re less than halfway through this magneto restoration, so there is quite a bit more to come. Send questions or comments to firstname.lastname@example.org.