4-32 Machine Screws

[dencins]

I recently stumbled across a very uncommon (and obsolete) 4-32 die to cut machine screw threads.  This is the size screw used to hold the bridge to the faceplate on solid back transmitters.  The 2XX type bridge uses a 4-32 x 1/4" brass flat head slotted machine screw and the 3XX uses a 4-32 x 1/4" steel round head slotted machine screw. 

My interest is mainly with the brass flat head screws since I have more difficulty finding them.  I have not been able to locate a source for brass flat head machine screw blanks so my thought is to take some 4-40 (UNC) or 4-48 (UNF) screws and recut the threads to 4-32.

Does anyone know of a source for #4 brass slotted machine screw blanks?  Has anyone tried this recutting approach on any size threads?

Dennis Hallworth     

[Bill]

Dennis -

I'm not a big fan of recutting threads on the same-size shank, because the resulting threads are so munged up that they don't hold very well, and the recut screw is a little wobbly in the hole. However, in this case, not much holding power is needed, so it would probably be OK.

This forum had a long discussion of oddball screw sizes about a year ago. At least one member found a site that supplies them, though the available quantity was small. You might try that discussion and see if there is anything of use to you there.

http://www.classicrotaryphones.com/forum/index.php?topic=6612.0

Bill

[dencins]

Bill

Thanks.  I had forgotten about that discussion.  The supply in the discussion was for the steel round head machine screws used to hold the 3XX bridge to the faceplate.  I am working on the brass flat head screw that holds the 2XX transmitter to the faceplate.

I was sent a good idea to use #4 brass flat head wood screws as the blank.  Wood screws have 2/3 of the length as thread so if I get longer screws and cut off the thread, I might have an unthreaded blank to use.

I noticed the original screws look odd so I measured some and found the major diameter of the thread is 0.112" which is a #4 but the head diameter is 0.175" which is a #3.  Those are not going to be available anywhere.

I did find I could file the #4 head height down to 0.055" then recut the slot to get a #3 head on a #4 shank.  The prototype was rough but it did fit the 2XX bridge.  I need to build some fixtures to get a repeatable process but this approach should work.

Dennis Hallworth   

[dencins]

I have completed a prototype of the reproduction 4-32 x 1/4" brass slotted flat head machine screw with an undersized head used to hold the 2XX bridge to the transmitter faceplate.  I will need to get a new hack saw blade to to clean up the slot.  I left the length of the prototype slightly longer than the original and the slot is a bit wider.

The first pic is shows the #4 brass wood screw, the blank before threading, the prototype and the last is an original screw.  The thread die is below.

I put an original and the prototype screws in a 2XX bridge and attached it to the faceplate.  The second picture shows the result.

Dennis Hallworth

[wds]

Very nice and quite ingenious.  Where did you get the thread cutter?

[dencins]

I came across the 4-32 thread cutter on ebay.  First time I have seen one.

I want to mention the credit for the idea of using the unthreaded section of a wood screw for the blank goes to Fred Haynes.

Dennis Hallworth

[cihensley@aol.com]

Dennis:

Good job on turning down the head size while maintaining the head to shank angle.

Chuck

[HarrySmith]

Great Job, Dennis! Can't tell the two apart. It is people like you that help us all continue in this great hobby!
Will you be offering them for sale?

[dencins]

Harry,

This is still at the prototype stage.  I like the thread and the head but still need to do some work with different ways to make the slot deeper.  The wood screw slot is too wide so when I cut the head down I would like to put in a narrower slot.

I did use a set to put a bridge on a transmitter and they worked fine.  Once I get the slot width resolved I will make some available.

Dennis Hallworth