Today's job is to remove that broken tap, but first I wanted a 'sanity check' that the circle of 12 mm mounting stud holes are accurately placed. There are 12 threaded holes in the lathe spindle, the chuck uses six of them in three pairs to avoid the jaw mechanism, my adaptor uses six, but evenly spaced.
Sitting my adaptor on the reverse of the original chuck I could check three at a time and I'm glad to report that they are bang on. This will allow me to temporarily mount the adaptor on the spindle to turn the outer diameter concentric to the PCD of these studs, and I can pick this diameter up on the manual lathe to get concentricity when turning the female A2-6 recess.
OK let's work out how to get that tap out. I need to make a hollow copper electrode 8.5 mm (M10 tapping size ) for the Sinker EDM machine. This is easy. But the next step isn't - that's getting it accurately set up co-axial with the existing hole. The hole is blocked by the broken stub of the tap that I will have to shear off to see the target hole. Or possibly I can make a jig using the co-ordinates used to drill the holes in the first place. All a bit of a pain in the nether regions. So . . . . .
Think outside the box - revert to car mechanics techniques - weld a nut on the broken tap and wind it out (but it IS in pretty solidly!). And even if this doesn't work it will probably shear off the top of the tap which I need anyway.
There is not quite enough room for a 10 mm nut to turn fixed to the tap and clashing with the adjacent shoulder - probably about 20 thou interference. But I reasoned that after welding the nut will be dead soft - the shoulder is the EN19T hard stuff, and we should be OK. A good spraying with anti-spatter juice, a good heavy current Mig weld and . . .
. . and sure enough - we are
