A set of stupid questions about screw plates and the matching taps

[Mark Elliott]

I just bought myself an old screw plate at a gun show.    I have been looking at it and have a couple of questions for those more knowledgeable than myself.

First, my plate has sixteen holes that appear to be divided into 8 sets by a stamped line between the sets.

1.   Does the little stamped line actually separate sets?

2   What are the sets?   Are they sets of a particular diameter or pitch or both?   They appear to me to be sets of the same pitch with one hole ever so slightly larger or smaller than the other. 

3.   If my observations above are correct,  do you use it by starting with the bigger hole/die and then finishing with the smaller hole?

Lastly,  a related question about the taps made from or used with such a plate.   The catalog plates I have studied of the taps sold with such plates, seems to show the starter end of the tap filed into a square reamer.  Presumably this is in order to open the hole to the correct size to be tapped.   Is this observation correct?   If that is the case,  do your really need to ream the hole with a separate square reamer before taping as long as the pilot hole is close to the correct size?  I was thinking that you probably still want a square reamer made for a brace for speed, but I figured I would ask so as to not make more tools than I need.   Although,  I don't guess a square reamer is really all that much trouble to make; a piece of drill rod a little bigger than you need, a few minutes at the forge to put four flats on either end.   

Thanks in advance for your indulgence. 

[James Wilson Everett]

Mark,

I just added a reply to an older topic on screw plate use, so it is now back on the "active" end here.

1.  The little lines help the user to match up the two hole thread sets.
2.  the two hole thread sets are for making two slightly different threads.  The slightly larger is for making the tap and the slightly smaller is for making the screw thread.  I am sure that you understand that the tap must be just a bit larger than the screw thread so the finished parts will fit together.
3.  You do not need to do a "two-step" thread forming, just go ahead and make the screw thread with the smaller hole.

I surmise that the smiths rarely used "store bought" taps from a catalog. Mostly the taps were made to match the individual screw plate.  You will find that, just like any small tap we use today, the expected life of a small tap can be rather short.  Also, the spade or fish tail drill bits were made by the smith, he just couldn't go out and buy a replacement #27 drill bit!  The tapered reamer was used to gradually enlarge the rough drilled hole to the proper size as measured by a gauge.  However, if the original drilled hole is of the proper size, it can be tapped without the use of a tapered reamer.

Jim

[Bob Roller]

Mark,

I just added a reply to an older topic on screw plate use, so it is now back on the "active" end here.

1.  The little lines help the user to match up the two hole thread sets.
2.  the two hole thread sets are for making two slightly different threads.  The slightly larger is for making the tap and the slightly smaller is for making the screw thread.  I am sure that you understand that the tap must be just a bit larger than the screw thread so the finished parts will fit together.
3.  You do not need to do a "two-step" thread forming, just go ahead and make the screw thread with the smaller hole.

I surmise that the smiths rarely used "store bought" taps from a catalog. Mostly the taps were made to match the individual screw plate.  You will find that, just like any small tap we use today, the expected life of a small tap can be rather short.  Also, the spade or fish tail drill bits were made by the smith, he just couldn't go out and buy a replacement #27 drill bit!  The tapered reamer was used to gradually enlarge the rough drilled hole to the proper size as measured by a gauge.  However, if the original drilled hole is of the proper size, it can be tapped without the use of a tapered reamer.

Jim

 How did these old gunmakers measure screw threads or the holes for them?
 Simple taps can be made now by threading round rod and filing it as square as possible
leaving threads on the corners and then after deburring,harden the tap. I had some years ago
made like that and I think they were French and made by a professional tool maker.

Bob Roller

[Mark Elliott]

Thanks Jim,  You answered all my questions.    You really should write a book.

[David R. Pennington]

Thanks Jim,  You answered all my questions.    You really should write a book.

Can I get on the list for a copy of the first edition?