Tapped a single 6-32UN hole. The real issue was how to hold a large odd shaped part and locate, drill hole, and tap the 6-32 hole. My solution was to use the same fixture made to hold the part horizontally on the mill, to be held vertically in the 6 inch vise on the mill. One of the problems with this was the tapped hole goes thru one of the three 3/8" holes used with Mite-bite XYZ expansion pins that hold the part. I used a small Sherline mill hold down clamp to replace one of the XYZ pins. Also used 2-4-6 blocks to beef up the jaws on the 6 inch mill vise. The other issue was with this setup had very limited Z height with holding the tap and drilling the holes. I wound up using a 6-32 Balax form tap in a Starrett 93A T tap handle with a Fisher micro tap guide. I normally hold the Fisher micro tap guide in a 1/2" TTS end mill holder however that was too tall to fit the limited Z height available. Even using a 1/2" R8 collet didn't clear up enough Z height. I solved this issue by machining a 3/4" low profile 1/2" holder which allowed the Fisher micro tap guide to fit up inside the holder and free up Z height.

Don

Using an edge finder held in a TTS ER 20 holder to locate the center using a 3/8" dowel pin
Attachment 421120

Spot Drill
Attachment 421122

1/8" parabolic flute drill held in a TTS ER 20 holder
Attachment 421124

partial depth hole using #27 drill for clearance
Attachment 421126

6-32 form tap held in T tap handle with micro tap guide
Attachment 421128

Modified 3/4" tool holder with micro tap guide
Attachment 421130