Today’s Machining World Archive: March 2006, Vol.2, Issue 05
Dear Shop Doc,
Recently, we accidentally dropped a Daewoo Puma, 12LC CNC lathe and broke the corner off the bed. We replaced the headstock, but we don’t want to pay for a brand new bed. The piece that was broken off is at the end of the travel, where the headstock mounts, and then the travel is along the ways, so that the scraped-in machined area has a hardened surface. We are worried about the reliability of brazing or welding it because it’s cast iron. It’s very difficult to clean all the oil and other stuff that has saturated in the casting over the years. We are also worried about what kind of distortion that would cause.
Sincerely, Headstock Case
Dear Headstock Case,
This is actually a very predictable repair, which can be done both in our shop or on sight. What our company does is a mechanical repair, so there would be no heat or fusion involved. We do metal stitching – or metal lock.
In past cases, similar to yours, we first clamped the piece back as close to the original alignment as possible. Then we installed two grade 8 Allen bolts, counter sunk with the heads torqued down to make sure the casting did not move while we made the repair.
Next, we began the actual process of metal stitching. Metal stitching uses keys, which are made out of invar, the equivalent of 4340 tensile strength, but the invar has a high nickel content so it contracts and expands at the same rate as cast iron.
Using a fixture, we drilled the holes for the keys on 1” centers. Then, we milled out the webbing of the holes and drove in the metal lock keys. The keys were then shot-peened and ground. This procedure causes an interference fit, which prevents the casting from moving.
This process is basically repeated along the whole length of the crack. Then, in between the metal lock, we put in what we refer to as metal lacing. Metal lacing is a piece of threaded invar typically .25” in diameter. We drill a hole in the crack line, we tap the hole, then we screw the metal lacing into the hole. Then, we shot-peen the metal stitching and grind it off. This does two things: In an area where there is leakage, and you’re trying to hold water pressure, it helps promote sealing, and it preloads the metal lock.
At this point, the metal lock will be in a ridged state. You don’t want any flexing or moving of the metal lock. We then would remove the clamps, and the casting will be ready to be finish machined. The ways are ready to be scraped in, and it will basically be as good as new.
Fred Lewis
Service Manager, Metalock Corporation. Willow Springs, IL
