When the bolt spins freely inside the hole or the part features threads that were formed during a plating process, most people believe that the part should be scrapped. However, most shops dont choose to discard the part because they have a set of oversize taps that will fix the thread issues on the part. The reason that oversize taps exist is to fix the issue caused by plating processes in which the zinc, chrome, or rust wears away at the threads.
In some cases, the zinc coat shrink the clearance in the hole such that the standard bolt cannot fit into the part. In other cases, the chrome or rust removes enough material from the bolts such that the bolt will no longer hold the torque that is required to secure the fastener. In these cases, the original threads can be lost.
Fixing Stripped Threads with Oversize Taps
However, the metal part is otherwise intact. In these instances, an oversize tap can be used to cut the new threads slightly larger than the original size of the bolt. This allows for either the use of an oversize bolt or a thread insert to bring the bolt back to it’s original size.
The chart that depicts the oversize classes includes drill sizes for the taps. There are three oversize classes, each corresponding to the least amount of stripping of the thread. The first oversize class is the least amount of stripping of the threads and the lightest in weight.
The middle oversize class includes the middle step and is used for medium stripping of the threads. The third oversize class is the largest and is used in instances of severe stripping or when installing a solid bushing into the part. The smallest oversize tap should be selected for stripped threads such that there is more remaining wall thickness for the threads and more engagement length for the bolt.
Each oversize tap includes a drill size to match the tap such that the metal part will not have any more material removed than is required to deepen the tap. The decision of the oversize tap that is to be used will determine the repair method for the part. If the oversize tap is used alone, this is the cheapest repair method.
Using the oversize tap alone will work if the larger bolt that is created with the oversize tap is acceptable for the application. However, if the bolt size must be preserved, then a wire-coil insert or solid bushing can be installed in the part through the oversize tapped hole. The insert will bring the size of the bolt back to it’s original size.
There are five common repair methods for stripped threads as depicted in the chart. The relative strength of each repair method is also depicted in the chart in order to allow shops to make an informed decision regarding the repair method that will be used. The thickness of the plating will indicate which oversize tap is to be selected.
If the plating is zinc or electroless nickel, the lightest class of oversize taps will be used. However, if the thickness of the plating is hard chrome or anodizing, then the middle class of the oversize taps may be used after measuring the thickness of the plating. The thickness of the plated metal must be measured before any oversize taps are chosen.
The hole can then be measured with a plug gauge after the oversize taps are completed. The step by step process that is included in the chart will guide the shop in the correct order of the steps. First, the number of threads that remain on the part must be determined.
Based on the number of remaining threads, the oversize class of the tap must be selected. Following the selection of the oversize class, the part must be drilled to the size that is indicated in the chart. The size of the tap will be selected and used to tap the hole in which the stripped threads existed.
After the hole is tapped, it must be cleaned. Following the cleaning of the hole, the fit of the threads should be checked for the installation of a thread insert. Any torque that is used to secure the bolt will be applied with care because the oversize tap will alter the engagement area of the bolt.
The original torque specification will need to be adjusted because of the altered engagement area. The drill sizes for the oversize taps are determined with a certain formula. The formula results in a size that will contain approximately 75% of the thread for the part.
The 75% of the thread is all that is required for the structural joint for the part. If the metal part is softer, it can take less of the threads. With less required threads, the drill size can be slightly larger.
However, the difference in size will be to the hundredths of a millimeter in size. The cobalt drills and tapping fluid will be used in the tapping process because cobalt drills will not work harden the metal. By ensuring that the metal does not work harden, the tap will not dull or tear the newly tapped threads.
A number of errors are common when tapping with oversize taps. If the process starts with a bottoming tap into the damaged threads, the tap will misalign with the threads of the part, and the entire repair will be ruined. Skipping the reverse turn with the tap will pack the flutes of the tap, which will cause the tap to snap.
Ignoring the plating table for the oversize taps will waste the thickness of the wall of the part. The chart includes all the information necessary for the workers to perform the oversize tap process correctly and consistent. Using the oversize tap is a compromise on the part.
The oversize tap will require the bolt to remove a small amount of the original diameter of the hole. However, it will save the shop the effort and cost of replacing the part. If the shop uses the chart to determine the size of the oversize tap that is to be used, the trade-off that the shop makes is deliberate.
In this situation, the strength of the repaired part will be greater then that of the original damaged part.
