Trouble with a tiny screw hole

[old martParticipant @oldmart]

I was presented with a really fiddly challenge at the museum. A top of a control column had a broken screw in it and someone had tried to remove it with a Black & Decker, by the looks of it. I wasn’t even sure what the thread was until I found a similar one on another part of the column, it turned out to be 4-40 UNC. After setting it up in one of the mills, I managed to get a hole close to the centreline with a 2mm drill and then used a solid carbide endmill to get closer to the centreline. At that stage, I decided to try hand picking the remains and unfortunately noticed under magnification that the thread was Helicoiled with a locking coil. Having fitted thousands into aircraft parts, the violet colour that the cad plated stainless coils which are used on aircraft gave it away. Picking did not work, neither did any of the  tapered trapezoidal coil extractors that I made, they were not hard enough. There was enough metal around the hole to try a 5mm solid carbide endmill but having never removed a helicoil using this method I proceeded very slowly down to 7mm deep and the hole was ready for a 4-40 tapped brass sleeve to be Loctited in. Next time, I will go for the easy method when the thread is tiny.

[Author]

Posts