How to Duplicate Factory Machine Screws Found On Firearms

NYECOGunsmith

Obsessed Member
Staff member
Moderator
#1
Making Machine Screws for Guns





A friend of many years standing, who is a good gunsmith machinist but not a real machinist, called the other day with a question.


He needed to know how to duplicate a couple of screws for a gun, for which Brownell’s was out of stock, (as was he) of screw blanks.


Now he knows how to cut a thread on a lathe, and how to thread a piece of stock with a Die and Die Stock, and he knows what size to turn a piece of stock to for a particular screw diameter and pitch, what he didn’t know was how to properly make the head of the screw so that it would match the existing screws.


You see, the head of a screw has proportions relevant to the shank, and no one ever told him this, but he could see that there had to be some way to make the new screws so that the heads would end up as perfect matches for the factory-made screws.


Lucky for him I remembered how this is calculated so I gave him the math and drawings of how it is applied.


Then I got to thinking that since a few folks here now have metal lathes at home, there might be some interest in how to make a screw beyond the simple single pointing of it on the lathe or using a die and die stock to make one out of a screw blank.


That’s pretty easy to figure out, and I will detail it here if someone wants to know the process, but you can find lots of You Tube videos that will show you how to cut a male thread with a lathe or by using a die and die stock.


The four screw types generally found on firearms are the Flat Head Filister, The Oval Head Filister, the Flat Head, and the Round Head, with that last one not often being seen.


The accompanying drawings show each type, along with the math to make one of them in any size and have it come out the same as a factory screw, matching proportions that is.


Just in case the text below the pictures isn’t legible, here it is again.


For a Filister Head Machine screw:


A = Diameter of the Body


B = 1.64 times A, then subtract 0.009 from the result, this is the diameter of the Head


C = 0.66 times A, then subtract 0.002 from the result, this is the height of the head


D = 0.173 times A, then add 0.015, this is the width of the slot


E = ½ times C, this is the depth of the slot





For Oval Head Filister Screws:



A = Diameter of the body


B = 1.64 times A, then add 0.009 to the result, this is the diameter of the head and the radius of the oval


C = 0.66 times A, then subtract 0.002 from the result, this is the height of the side


D = 0.173 times A, then add 0.015 to the result, this is the width of the slot


E = ½ times F, this is the depth of the slot


F = 0.134 times B, then add C to the result, this is the height of the head



For Flat Head Screws with a 82 degree included head taper for counter sunk holes:


A = Diameter of the body


B = 2 times A, then subtract 0.008 from the result, this is the diameter of the head


C = A-0.008, then divide the result by 1.739, this is the depth of the head


D = 0.173 times A, then add 0.015 to the result, this is the width of the slot


E= ½ times C, this is the depth of the slot


For Round Head Screws:


A = Diameter of the body


B = 1.85 times A, then subtract 0.005 from the result , this is the diameter of the head


C = 0.7 times A, this is the height of the head


D = 0.173 times A, then add 0.015 to the result , this is the width of the slot


E = ½ times C, then to the result add 0.01, this is the depth of the slot

View attachment Screw Proportions 1.jpeg Screw Proportions 2.jpeg
 

Attachments

NYECOGunsmith

Obsessed Member
Staff member
Moderator
#3
Threads are common sizes, like 3-56, 4-40, 4-48, 5-36, 5-40, 6-48, 8-40, so that part is easy to make, but not knowing how to duplicate the head and have it match other factory screws on the gun is the difference between an armorer and a real gunsmith.

Making a screw that doesn't follow these proportions for the head will have it sticking out like a sore thumb.

Brownell's sells screw blanks in lots of regular machine screw shank sizes, ( 3, 4, 5, 6, 8) with those four types of heads already made to those proportions, so all you have to do is thread the shank and cut it to length, and then slot the head.

But if you and Brownell's are both out of the blanks, or you have an odd size for which no one makes a blank, then with these formulss, you can make one in short order.

The screw slots are best cut on the vertical mill with a slotting saw on an arbor, then finished up with a gunsmith's screw slot file.
 

NYECOGunsmith

Obsessed Member
Staff member
Moderator
#7
I suppose if you had one of the 3D printers that prints by using powdered steel, stainless steel, or titanium you could make a screw that way, but you still need to know the proportions in order to program the printer so that the head comes out looking like the factory screws.
Since those printers are very, very expensive I doubt any gunsmith has one in the shop, yet, but give it time, and they will come down in price and then it will be a real boon to gunsmiths, easier to print a part than machine it.

There was a company at the Shot Show 3 years ago demonstrating their 3D metal printer, printing 1911 frames that needed little to no finish work prior to installing the internals, but they were expensive and a CNC machine center could turn out a dozen or more 1911 frames in far less time than the 3D printer could print just one.

Once they are faster, and the price comes down, big advantage to having one in the shop. Now, not much advantage at all.