METAL STITCH REPAIR SERVICES
General. For purposes of this report, the terms "locks," "lacing," and "masterlocks" will be used to describe the three repair procedures used in the metal-stitching technique.
Castings which have developed cracks or which have broken from being overstressed can be strengthened in the fractured area by inlaying pre-formed, high tensile strength, alloy steel bars referred to as "locks" or "metal locks". Locks are inserted into accurately prepared slots, which are
perpendicular to and located across the crack. Due to the serrated design of the lock, the casting sections on both sides of the crack are locked together. Since the locks traverse the
crack, the serrated
edges of the locks are placed in shear when loads are applied which act perpendicular to the crack.
Locks are cut to any desired length from preformed bar stock that is, overall, about as high as it is wide. Lock sizes used are 1/4" to 5/16" and may be stacked.
The first step in the installation of a lock is to make a slot for it that is positioned perpendicular to and across the crack. The major portion of the metal is removed by using a drill bit size that corresponds to the lock size; for example, a 5/16" bit is used for a 5/16" lock. With the use of a drilling guide, the first hole is drilled so the centerline of the hole and the crack coincide. A temporary pin is used to hold the guide in position while the far hole on one side is drilled
pinned. Then the remaining holes are drilled, after which the pins and drilling guide are removed.
Drilling depth must be limited to not more than 90% of the casting’s thickness in order to provide a base for the lock. Drilling depth is determined by the lock depth needed to provide the reinforcement required to match or exceed the original casting strength in the crack area.
Drill holes are normally balanced across the crack to provide equal strength on each side of the crack. The minimum number of holes drilled is 5, increasing to 7, generally alternating.
After the holes have been drilled, a web of metal remains between each hole. The webs are partially chiseled out using a pneumatic powered chisel and the metal chips are removed by hand or drilling, as necessary. Now, the slot is ready for insertion of the lock.
When liquid or gas containment is involved, a suitable sealant is put into the slot.
Locks have a slight interference fit in the slot and are driven into the slot with a wide, thin-bladed tool, bottomed-out, and peened to lock it into the casting. Locks are stacked in order to fill the slot. The height of the last lock installed is selected to allow the lock to protrude about 1/32" above the surface of the metal. Next, the lock is peened with a semi-pointed tool to interlock the casting and lock metal. The lock surface is then peened with a flat tool and finished ground. The
are usually spaced uniformly and at intervals that range from ¾" to 1-1/4".
Often times, breaks occur that fragment a casting into several pieces. These are additional examples of repairs that use extensive locking, lacing, and large-locking to stitch many broken pieces together and restore the part to operating condition in a relatively short time.
After the installation of the locks, "lacing" is placed along the entire length of the crack. The laces pre-stress the locks, prevent the leakage of liquids and gases, and prevent the movement of the metal casting on each side of the crack.
Standard procedure is to install a lace at each end of the crack to minimize further crack growth. After each lock is installed, one lace is place on each side of the lock and centered on the crack. Then the remaining crack areas between each lock, and at both ends, are filled with laces.
Laces are installed by drilling adjacent to the lock and allowing the drill bit to cut slightly into the lock. The hole is threaded in order to accept the lace material which is available in the form of soft bolts or threaded rod in sizes of 1/8", ¼", and 5/16". Where liquid or gas containment is involved, a suitable sealant is put in the hole ahead of the lace. After the lace is screwed to the bottom
of the hole, excess lace is cut off near the surface with an impact wrench. The next hole is drilled
adjacent to the lace with the drill bit cutting into and removing part of the lace. Thus the lacing overlaps, ensuring that the entire crack gets filled. This procedure is followed until all the crack has been plugged with lacing.
After all the lacing has been installed, peening and grinding of the lace metal protruding above the surface is necessary. First, a concave tool is used to upset (hammer) the lace and form a hemispherical head. Next, a semi-pointed tool is used to swedge the lace to interlock it with the parent metal. Next, a chisel is used to cut off most of the metal that protrudes above the surface. Finally, the surface is peened further with a flat tool and then finish-ground.
The purpose of large-locks is to provide a higher concentration of locked strength at points of maximum stress, particularly where it is necessary to dissipate a local stress over a wider area.
The area in which a large lock, or master lock, is to be installed is drilled along its perimeter using a drilling guide to provide uniform spacing and straight-line direction. Within the area to be drilled, additional holes are drilled to serve as pilot holes for a much larger drill bit, usually 1". Metal is drilled out to a predetermined depth and residual metal between holes is removed with a pneumatic chisel. The sides of the drilled out area must be chiseled and ground as necessary to
a reasonably close-fitting large lock. Where a master lock is to be installed to fill a hole in a casting, only perimeter drilling is needed.
With the use of a drilling guide, equally spaced, small holes are drilled on the line of contact between the large-lock and the casting. Where a master lock is inlaid in a casting, holes are drilled to the full depth of the large-lock. Where a large-lock is used to fill a hole in a casting, holes are drilled to a depth of not more than 90% of
of the large lock in order to provide a base for the pins that will be inserted in each hole. Holes are drilled so that half of the hole is
in the casting and the other half is in the large lock.
Hole depth is selected so that about 1/8" of pin will protrude above the metal surface after the hole has been filled with one or more pins. The exposed part of the pin is peened with a concave tool that forms a round head on the pin. Next, a semi-pointed tool is used to peen the pin further for the purpose of interlocking the pin metal with that of the casting and large-lock. Exposed pin metal still protruding above the surface is chiseled
off and the resulting surface is peened with a flat
before finish grinding.
The depth of a master lock depends on the strength needed and the type of metal being used.
Master locks may also be anchored into the casting by the use of locks only in a direction perpendicular to the crack and across the lines of contact between the casting and large locks. In some cases, locks and laces may be installed below a large lock.
LOCK AND LACE MATERIAL
The metal lock and lace material used to repair cracked cast iron and steel castings is a high nickel-iron alloy. One repair service contractor uses an alloy with the generic name, "invar", for 90% of all repairs. It is used for applications where dimensional changes due to temperature variation must be minimized. The mean coefficient of thermal expansion from 0°F to 350°F is 0.90x10-6in/in°F.
Repairs made to equipment handling liquids and gases will contain design operating pressures unless the equipment was under-designed for the application.