I'm new to thread repairs and I was just wondering when using a helicoil, why does the helicoil need to be inserted if you have already tapped the new threads into a hole? Can't you just put a bolt in once you've tapped new threads?
Why does the helicoil need to be inserted if you have already tapped the new threads into a hole? Can't you just put a bolt in once you've tapped new threads?
Consider the case of an M5 x 0.8 tapped hole. The helicoil will need the same pitch (0.8 mm) with a larger diameter hole. Note that the M6 is a 1.0 mm pitch so even if the helicoil tap was the same diameter the pitch wouldn't work. I haven't checked but I imagine that all the helicoil taps are non-standard threads.
The next issue is that the part being held in place has a hole for a 5 mm screw so tapping to M6 would require a modification to the part to be held. A related issue to this is that you might now have one odd screw in a set holding a part on.
In my (very limited) experience helicoils are mostly used to repair stripped threads in aluminium or to toughen up threads in aluminium that require frequent un/fastening.
Short and simple, if you want to use the same size fastener again, you have to use a helicoil. By drilling out the old thread with the new pilot hole you have made it larger thus the fastener size will go up unless a helicoil is used to take up that change in diameter. Further more, helicoils also prevent the threads from needing to be repaired in the future, because the helicoil can be replaced and it is made from (usually) a stronger material than the base material.
The simple answer is that using a helicoil allows you to repair a thread while keeping it the same size.
Say you have an M8 thread which is completely stripped, you now have an approximately 8mm diameter hole. If you want to restore the original thread size you need some way to reduce the size of the hole again.
You could just drill out the hole to the next size up and re-tap but that is not always desirable, for example if it is one of a set of tapped holes and you want to use the same size bolts for all of them, or if the male thread is part of a more complex part not readily available in other sizes eg a spark plug.
Helicoils can also be useful in materials which are a bit too soft to accept threads or for dissimilar materials. Because of manufacturing tolerances most thread standards tend to put the load on quite a small area of the thread and some materials are prone to galling threads.
Because helicoils introduce a bit of flex they tend to distribute the load a bit better. For example aluminium engine blocks often use some sort of steel insert for to take bolts where they are needed and helicoils are a convenient way to do that.