REALLY Interesting Treatise On Thread Restoration

[ARCHINSTL]

From Hemmings:
http://blog.hemmings.com/index.php/2014 ... /#comments

I heartily endorse the thread file; it saved my bacon on a control arm bolt. See https://tercel4wd.com/forums/viewtopic.php?f=10&t=3267
I think this is a must-have tool!
Tom M.

[keith]

Unfortunately that article is missing all information concerning thread forming dies, also known as restoring dies. Most tap and die sets used in the US are cutting taps and dies, that is they work by removing metal. These work on all types of metals.

In Europe, thread forming dies are often used on softer metals such as aluminum and mild steel. they do not work on harder grades of metal. Let me explain the difference in the taps, the dies work the same way except for the gender switch. In any thread, you have the minor diameter and the major or nominal diameter. The minor diameter is the size of the largest roll pin that you can drop into the hole after the thread is cut. The major diameter is the diameter of the outsides of the thread, which you cannot effectively measure in the hole, you can measure it on the bolt though.

Typically you drill a hole about the size of the minor diameter, the tap then cuts away the material to form the threads. In a thread forming die, the hole is much larger, about half way between the minor and major diameters. The tap is made from a very hard material and it pushes the metal around to form the threads. This makes a thread that is quite a bit stronger than a cut thread. This can be critical in aluminum.

If you cross thread a hole, or strip it, sometimes a roll forming die will restore the threads enough to be useable. Another benefit is that the roll forming dies don't create many chips as it isn't cutting the metal. You can rent roll forming dies at O'Rielly's auto parts, actually borrow them with a big deposit which is 100% refunded on return. i would do this before using any of the other techniques in that article.

For mild steel and aluminum, sometimes a high grade (grade 8 sae or 12.2 metric) will work like a roll forming die, but they work best if the bolt has a "nose" on it, that is a non threaded starter or centering portion on the tip.

[dlb]

i've never heard of thread forming/restoring dies, keith. that's really interesting. the technique makes a lot of sense.

[ARCHINSTL]

Thanks, keith; I echo david's remark.
Tom M.