Automatic Electric Plug Replacement

[Westrex]

Here's a picture:

[TelePlay]

That's a self threading screw used in plastic mounts, considered a limited use screw in that the insertion and removal too many times may end up stripping the plastic threads.

It has a special thread pitch which is different than screws used on a pre tapped or metal insert hole.

This is a topic on the forum about them.

http://www.classicrotaryphones.com/forum/index.php?topic=8650.0

And this is an external discussion of self tapping screws for plastic which is linked within that topic.

https://engineeringproductdesign.com/knowledge-base/self-tapping-screws-for-plastics/

The WE 500 uses these for the cup mounted under the cradle, under the hand hold. If a plastic post/hole does not have a metal threaded insert, it uses that type of screw.

It is easy to strip one of these plastic mounts of over used (taken out and put back in) and/or over tightened.

If stripped, inserting a filler item, as mentioned above, can be a one time fix. A more rigorous fix is filling the whole with a setting material and drilled out.

Another way, which can be tricky, is the "glue" the screw into place (not assembled) and when the "glue" is set, remove the screw leaving new threads in place.

[TelePlay]

Here is the thread creation method that works on metal or plastic:

"When removing the internal components several of the rusty screws broke off flush with the anchor points. I had to drill them out, destroying the threads. I 3/4 filled the holes with Araldite epoxy and inserted new screws coated in 3-in-1 oil. When they dried, I had restored threads."

http://www.classicrotaryphones.com/forum/index.php?topic=3233.0

[TelePlay]

Here is one of my 4 prong plugs made in Japan.

Both screws are plastic self tapping screws.

[TelePlay]

Here's a comparison of 3 four prong plugs, a WE round (probably the first produced) with a Bell icon but no part number, a square WE plug with no icon but a 283B part number and a square plug stamped "Japan."

The WE plugs used slotted screws, the Japan used Phillips head screws.

The WE screws we're not self tapping but did have a bit of a taper at the thread end probably to make it easier to get the screw in the hole on the production line (the plastic posts must have been threaded, tapped, for those screws to work).

The Japan plug used self tapping screws.

All plug screws had a coarse thread pitch consistent with that needed to best grip the plastic post.

The last image shows the plug screws with standard machine screws to show the design and thread pitch difference (there is no way the machine screw pitch would work inside a plastic post). Machine screws were used on posts that had metal inserts but the engineering needed to have a screw bite and hold inside a plastic post alone is quite different (not that anyone may care anymore).