The Home Machinist!

I restore antique Western Electric transmitters used on telephones prior to 1913. Each transmitter uses 6 odd size brass flat head machine screws and sometimes the head will break when taking the old screws out. Up to now I have been making replacements in limited quantity from wood screws using files and a #4-32 thread die that I was able to locate.
I am looking for someone to make a quantity (maybe 300 – 400?) of screws so I can have a supply on-hand. I made the following drawing using measurements from the old screws. Please excuse any errors but it has been nearly 50 years since I have done mechanical drawing. I believe the end of the screw should have a chamfer for threading but I did know how much so I left it off for now.


The screw does not match any current standards. The #4-32 thread is obsolete, the head OD is like a #3 while the shank has a #4 major diameter and the angle of the head is 61º rather than the current 82º or 100º. If helpful, I would consider loaning the #4-32 thread die to make the parts.

Dennis Hallworth
Seekonk, MA

An update to the post.

I do not have an optical comparator so I have been using a feeler gauge to measure the width of the slot. Since the screws I am measuring are over 100 years old and have been removed at least once, the slot has probably been made wider. I did a search on slotting cutters and it seems a 0.028" is a standard size. That should work fine for the slot rather than the 0.031" on the drawing.

Dennis Hallworth

That thread is a tough one, there is both a 3/16 -32 and a 7/32-32 available for a diehead, I don't believe that I can adjust the 7/32 down to .112 and for sure the 3/16 can't go up that much. So using a fixed die is the way to make them. Do you have to flip the die to do the last bit of threading, or is that the starting chamfer on the threads under the head?

Let me hear back and I'll give you a quote.

Are you positive on that countersink angle, the photos look more shallow (towards 100°) than towards 60°

No I am not sure about the angle and certainly would want to have the dimensions verified.

The way I have been making the reproductions is I start with a 1 1/4" wood screw and cut off the threads. I then screw it into the thread die until it hits the head. When that is done I unscrew the die then file down the head until it is about the right diameter then recut the slot with a hacksaw. Not good but the screws work. Not sure if that answers your question but I do not flip the die and I certainly did not put in a chamfer anywhere.

I have some of the original screws that can be used to verify the dimensions.

Dennis Hallworth

tetramachine wrote:That thread is a tough one, there is both a 3/16 -32 and a 7/32-32 available for a diehead, I don't believe that I can adjust the 7/32 down to .112 and for sure the 3/16 can't go up that much.

You're headed the wrong direction. The 3/16" is still larger than the required .112" diameter by .075".

Harold

Duh, thats what I get for doing math in my head. After 5pm

Seems like you might squeeze a Geometric die head with 6-32 chasers down enough to do 4-32. If not, 4-32 dies are about eleven bucks from Victor Machinery.

Frank, the chasers are $109 from Victor for a 9/16 head, $96 for a 5/16 geometric head. I was looking at ebay as well, since those tend to be nos/used. Boy that $11 sounds nice, what decade was that list.

This site looks current and shows the price at $10.40.

http://www.victornet.com/subdepartments ... /1350.html

The other option is I could send you the die I have been using.

Dennis Hallworth

For a run in quantity, a die would raise the cost considerably. Die heads allow for rapid threading, with the added feature of not having to reverse the spindle, or operate with a powered tailstock with variable speed capabilities. Instead, they snap open, so the chasers can be be withdrawn. They also tend to yield superior threads, as an added bonus.

Harold

Thank you for the explanation. I actually do not understand it but appreciate you taking the time to explain. I am definitely out of my league when it comes to this.

Dennis Hallworth

The harsh reality is that the thread would most likely take no more than four seconds (if that) to create, using a die head. The big issue here is if chasers can be obtained that will create the required pitch diameter. Simply closing down a set of 6-32 may or may not solve the riddle, as the helix angle doesn't remain a constant going from size to size. The difference may or may not create a problem---I'm unable to say, as I don't have that degree of sophistication of math skills, but I can clearly see the problem.

Harold