Screw Threads

[Dick]

This isn’t much different from writing books. Every paragraph I wrote was altered ten times before I got them right, or as right as they would ever be. I mention that because this is going to read like the first draft of a book – somewhat disjointed, unstructured and not necessarily in logical sequence. You will therefore, as intelligent people, have to extract the bits that matter to you as best you can.

There are many different types of screws, but put at its simplest there are two basic types, wood screws and machine screws. The difference is that wood screws are designed to cut their way into wood ie there is no prepared thread in the wood to accept the screw, while machine screws are a two piece thing in which a male threaded screw runs into a prepared female thread of the same size. The threaded hole can be a nut or a tapped hole in a jig, tool or machine. Although wood screws are a topic of their own I won’t say more about them because we are concerned here with machine screws.

Machine screws, what most people would call nuts and bolts, are found in cars, aircraft, ships and in almost every piece of mechanical equipment ever built. And of course firearms. Rifle barrel/receiver threads and guard screws are machine screws as are reloading dies. Nuts and bolts are of course widely used – perhaps the most obvious is steel bridges the components of which are, or used to be bolted together. I think, though I can’t say for certain, that more bolts are screwed into tapped holes than are used with nuts. But the key point is that, unlike wood screws, machine screws can’t cut their way into steel, they must enter a prepared threaded (tapped) hole or a nut. Someone is sure to tell me about self tapping screws and some very specialized screws that can be driven into steel without a tapped hole, but those are a special type and not relevant to this discussion.

Machine screws are made in four ways (1) cut with dies either by hand or machine (2) cut with machine chasers (3) screwcutting lathes and (4) rolling. Most hardware store screws and even engineering supply store screws are rolled, and are considered stronger than cut screws because the metal grain is preserved. But I mention that for completeness – it has no other relevance, except possibly for very small screws. Precision screws are die cut and may or may not be followed up with a chaser depending on the application. Very few screws are cut on lathes simply because the only need for that is screws that can’t be otherwise obtained, and for cutting threads on jigs and tools which must be shop made.

Female threads are tapped because there’s no other way, except for the very few that are machined on lathes for the same reasons of availability and shop made tools.

Hobbyists and small shop professionals like gunsmiths will do a lot more tapping than cutting male threads because machine screws can be bought cheaply for most applications but tapped holes can only be done by the user. In SA gunsmiths must make a lot of their own jigs and tools because expensive jigs and tools can’t be afforded for occasional jobs or might even be used only once. Even in my own short gunsmithing career, and as a hobbyist before and since, I have tapped hundreds of female threads but lathe cut only a few male threads. Earlier I said that the screws are lathe cut when they are otherwise not obtainable. Some old firearms have odd screws for which taps or dies are not obtainable at any price. Fortunately we seldom have to tap holes in firearms – it’s almost always the screw which must be made. But there are also standard threads that can’t be bought. For example, slitting saws are useful for various things including cutting screwdriver slots in screw heads. But you can’t buy lathe carriers for them; you have to make them, and because they have threads you have to cut them on the lathe.

All this is of course by way of general background. But what you guys want is the different types of machine screws and how they relate to each other.

There are several types – these are the ones I know about but there may be others :

British standard Whitworth (BSW) inch
British standard Fine Whitworth form (BSF) inch
Model Engineer threads, Whitworth form inch
British Association (BA) metric
ISO Metric metric
ISO Metric fine pitch series metric
Metric constant pitch for model engineers metric
American Unified Coarse (UNC) inch
American Unified Fine (UNF) inch
Small US National Coarse and Fine inch


It will be obvious that British and American inch threads won’t fit metric threads, but it might surprise you to know that British and American threads won’t fit each other eg an American ¼ inch x 20TPI bolt won’t fit a British ¼ x 20TPI nut. That’s because the British minor diameter is smaller than the US minor, because the British 55 degree flank angle makes the depth of thread deeper. The British bolt will enter an American nut but won’t fit properly although it may feel as if it does, and that’s because they will screw together but the different flank angles won’t engage as they are supposed to.

British and metric thread dimensions are expressed differently. British and American threads are described by the major diameter (OD) x the number of teeth per inch. Thus ½ x 12 means ½ inch diameter x 12 teeth per inch (TPI). Metric threads are expressed, more sensibly in my opinion, as diameter x pitch (pitch = distance between teeth), thus a 6 x 1 is 6mm diameter x 1mm between teeth.

Threads are standardized for obvious reasons, thus a ¼ inch BSW or UNC diameter screw will have 20 teeth per inch and a one inch diameter will have 8 TPI. A 6mm screw will be 1mm pitch, a 10mm will be 1.50, and a 20mm will be 2.50. Thus, when you buy a 6mm machine screw in a hardware store it will be 6 x 1 and nothing else.

But you will have noticed in the list above, that British, American and Metric threads come in both standard and fine pitches. For example the standard metric 6mm is 6 x 1 ie 6mm diameter x 1.00mm pitch. The 6mm metric fine is 6.00 x 0.75. ½ inch BSW is 12TPI but ½ inch BSF is 16TPI. That’s because some applications need more turns per inch or per mm for either of two reasons (1) the female is too shallow to allow enough turns of a standard pitch or (2) particular applications more related to fine feed. An example of the former would be rifle receivers. The 6 x 48 normally used is 48TPI which is very close to 0.50mm. But here’s where you’ll see how complicated this business can be. 6 x 48 is used because “normal” UNF (US Unified Fine) doesn’t go small enough or fine enough. 6 x 48 is from the Small US National Fine series, but even then the standard No 6 is 6 x 40. 6 x 48 is therefore a special thread – presumably special for scope bases and rings as I’m not aware of what else it might be used for.

An example of fine feed would be micrometer spindles. Micrometer spindles are typically about 6mm diameter but the thread of metric mikes is 0.50mm because the spindle must move 0.50mm per turn. And of course when we get into watch making, clock making and instrument making the need for fine threads is obvious. One of my neighbours is an instrument maker, still working, but I have never asked him about this. I’ll make a point of doing so.

Reloading dies provide another example of coarse/fine threads. The standard 7/8 UNC is 9TPI. The 7/8 x 14 of reloading dies is UNF ie US standard fine.

Standard threads cover a wide spectrum of sizes. BSW Ranges from 1/16 inch x 60TPI up, UNC from ¼ x 20 up, and standard metric from 0.50 x 0.125 up. The smallest Small US Fine series is No 0 x 80TPI. That’s 0.060 inch diameter = as near as dammit 1.50mm. The biggest I ever saw was during the extension of the Iscor Steel Works at Vanderbijlpark – some holding down bolts for the steel structure were 3 inches (76mm) diameter and at least a metre long. The nuts were enormous and weighed something like 10kg.

There are some unusual threads. The BA (British Association) threads popular with model engineers have a similar proportional diameter/pitch relationship to ISO metric standard but are not the same. The diameters and pitches strike me as not logical but presumably there was some sort of logic when they were designed. No4 BA is 3.60mm diameter x 0.66mm pitch. I discovered that the hard way when I made a screw for the cocking piece of an SMLE. I measured one from another rifle and thought it was 40TPI, but my new screw wouldn’t enter the tapped hole – it turned twice and jammed. Another look at the factory screw showed it to be slightly more than 38TPI and my text book indicated that it was No4 BA. After some head scratching I managed to set up the gears on my lathe and the resulting screw fitted the rifle perfectly. The other peculiarity of the BA is its 47.5 degree flank angle, which of course requires the cutting tool to be so ground.

Then we have special threads. My text book lists Model Engineer and Special Threads Whitworth Form. 1/16 inch to 3/16 inch diameter are 60TPI. 1/8 inch to 3/8 are 40TPI. ¼ inch to ½ inch are 32TPI. They are constant pitch in which several diameters are all the same pitch. You will notice that there is some overlap between them. They have been superseded by the Metric Constant Pitch Threads for Model Engineers. 3mm–6mm diameter are 0.50mm pitch, 4.5–12.0 diameter = 0.75 pitch, and 10–20 diameter = 1.00 pitch. 20mm Dia x 1mm pitch is very fine. These are standard threads but I doubt that taps or dies are readily available – perhaps from model engineer supply shops in the UK. I do suggest that 4.0 x 0.50 Model Engineer or 4.0 x 0.50 ISO metric fine would be a better choice than 6-48 for scope mounts being only slightly bigger in diameter but significantly bigger in sectional area and similar (slightly finer) pitch. My Unimat 3 miniature lathe has 14 x 1 metric threads on the spindle nose and tailstock ram and 10 x 1 leadscrew. Neither is standard metric or metric fine; they are both metric constant pitch.

Most rifle barrel/receiver threads are special, being finer pitch for diameter than either standard coarse or fine.

The difference between inch and metric screws is not just the diameters and pitches. BSW and BSF threads are 55 degree Whitworth. That means that the included angle of the teeth and the V between teeth is 55 degrees. Metric threads are 60 degrees. But the curved ball is US threads which have the same 60 degrees as metric. But it’s not only the different flank angles – many British and US threads have different TPI. British and US ¼ inch coarse threads have the same 20TPI, but British fine (BSF) is 26TPI while US fine (UNF) is 28. British ½ inch coarse and fine are 12 and 16TPI respectively but US are 13 and 20. My textbook lists BSW and BSF threads from 1/16 to 1 inch but UNC and UNF only from ¼ inch. From ¼ inch up to I inch the BSW and UNC are all the same TPI except ½ inch, but none of the BSF and UNF are the same - the UNF threads are substantially finer than the BSF.

This where I need to show a diagram of a screw thread to explain the nomenclature as well as other things. Unfortunately I haven’t figured out how to do that so I’ll have to do my best without it. But I recommend you should find it on the internet and I advise printing it for reference.

You will see that the Vee of the teeth is rounded at the root and crest. When machining in a lathe you can grind the tool to a sharp 60 degrees (US or metric) but that will have the effect of cutting deeper and reducing the core diameter. So you must round the point of the cutting tool. That’s impossible to get dead right but you can get close enough for practical purposes. I do it by first stoning a flat of suitable width then rounding the corners. Takes only a couple of minutes. However, if you make the radius too big the core diameter will be too big. The trick is to make the rounded point slightly too “sharp” by first stoning the flat slightly on the small side. It has to be done by visual estimation. I start by calculating the width of the flat from the thread diagram, and I use a watchmaker’s loupe to see clearly what I’m doing. It’s a lot easier than it sounds and fortunately there’s some room for error.

The crest is a bit different. There’s no way of machining the rounded crest – lathe cut screws have flat crests. Why would that be a problem ? If you machine the screw to nominal (full) diameter the two “corners” of the flat topped crest will be outside the required outline of the tooth and could be an obstruction to the screw entering the tapped hole. Therefore, it is standard practice to lathe cut screws slightly undersize, just enough to “cut off” the rounded crest. The required diameter can be calculated from the diagram. It’s not much, only a few thousandths.

As mentioned earlier, the male/female engagement is on the flanks not the root and crest ie the tooth and the V must be equal. As that is determined by how deep you cut, how to get it right? There are two ways :

(1) Calculate the depth of cut, start with the tool in contact with the work or use a feeler gauge to start a known distance from the work, and feed the required depth by reference to the graduations on the top slide leadscrew. For some very small threads I set up a micrometer head on the cross slide to control the feed.

(2) The best and easiest is to use the tapped hole as a “cut and try” gauge. But you can’t do that if the tapped holes are in a big and/or awkwardly shaped tool, in which case make a gauge by tapping a thread in a small piece of bar.

Of course this is a bit academic because most guys won’t be lathe cutting threads so I’ve included it for completeness and for passing interest.

Tapping is more important because it is much more likely to be done by most guys than lathe cutting and doesn’t need more tooling than a small drill press. Also because there’s an important trip wire that need to be understood ie the matter of thread engagement. What that means is the depth by which the male and female mate together. Surely, you may say, that’s the full theoretical depth of the teeth. Actually no, female threads are seldom tapped full depth in industry because :

1. There’s no need because pull out strength is very little affected by cutting the teeth less than full depth.

1. Cutting partial depth greatly reduces tap breakage and tap and machine wear and tear. I say machine wear and tear because most tapping in industry is by machine.

But what exactly does partial depth mean? It means using a bigger drill than the minimum, which means that the teeth of the tap will cut only part way into the sides of the oversize hole. To put it differently, the depth of the spiral V groove will be less than the depth of the tap teeth and less than the depth of the teeth of the male screw. It amounts to cutting off the top of the V and leaving it with a flat top the width of which will depend on just how big a drill you use. The trade jargon is “engagement depth.” 50% Engagement means that the female teeth will be only half depth and that the male/female engagement will also be 50% of tooth depth. It has the effect of dramatically reducing torque and tap breakage with no downside in the strength or utility of the screw and tapped hole. It is very easy to see in a diagram but difficult to explain in words.

A set of taps often has a label that says drill size. I have a set of M5 that says 4.1mm drill. That’s wrong because 4.1 = 100% engagement = very high torque and risk of breakage especially with taps smaller than 4mm. I use a 4.5mm drill for 5mm threads – that’s less than 50% engagement and I’ve never had a problem. A 5mm drill for 6mm tap = 100% engagement. I use a 5.3mm drill = 65% engagement with complete satisfaction. The difference in torque between 100% and 65% has to be experienced to be believed. NEVER tap to 100% engagement – it is pointless.

That’s easy for threads bigger than, say, 4mm, because it doesn’t much matter whether you use a 5.3, 5.4 or 5.5mm drill for a 6mm tap because the differences of thread engagement won’t affect the utility of the thread, and the torque required to tap the hole won’t vary too much. But very small threads are a bit more tricky. To start with you have a much bigger risk of breakage with these small taps, so thread engagement and therefore torque are a bigger issue, and 0.1mm difference of drill diameter has a proportionately bigger effect than with bigger diameters.

Let’s take an M3 Standard metric as an example. A 2.70mm drill = 60% engagement. 2.60mm = 75%. So 0.10mm difference of drill diameter = 15% difference in thread engagement and probably three or four times the torque for no practical benefit. It’s worse with the M3 metric fine, for which 2.78 = 60% and 2.71 = 75%. The recommended drill is 2.75mm. Best of luck finding a 2.75mm drill. You might find 2.70mm at an engineering supply shop like North Engineering Supply in Bellville, but then be prepared for the high torque of 75% engagement. You can’t use a 2.60mm drill because you’ll almost certainly break the tap. Not many of us will need to tap M2 but it takes only 0.06mm difference of drill diameter to increase engagement from 60% to 80%.

Drill diameter is therefore much more crucial with small diameters. Fortunately few of us will want to tap below 3mm and not often at that.

Taps were traditionally supplied in sets of three. British nomenclature is taper, second and plug. US nomenclature is taper, plug and bottoming. The taper tap is what the name says, it is quite heavily tapered for easy entry into the hole. The second tap is much less tapered and also finishes the slightly undersize thread left by the taper tap. The plug tap has a 45 degree taper on only its leading tooth and is intended to ensure that the completed thread is to the bottom of the hole ie so that the bottom of the hole is not tapered thus preventing full entry of the screw. That’s not always important, and many indeed most of my tapped holes can be drilled a good bit deeper than required, so some taper at the bottom doesn’t matter.

These days some taps are supplied in sets of two. Presumably it has been found that two can do as well as three. I have one such set which works perfectly well. Machine taps are a bit different. I have a 6mm machine tap – it has a fairly sharp point unlike hand taps. It obviously won’t work for tapping to the bottom of blind holes, but it was all that was locally available at time, and I needed it only for “all the way through” holes for which it works perfectly. Machine taps are sold singly and work perfectly for through holes and blind holes that don’t need to be tapped all the way to the bottom.

There’s one more important item – lubricant. You can get away with tapping in aluminium or brass without lubricant but it’s more difficult in steel, and the greatly increased torque invites breakage. Light machine oil helps but it’s not that good. What you need is a special fluid – the two I can think of are Tapmatic and Tap Magic. They are not lubricants they are coolants. Don’t ask me why coolants work so much better than lubricants but they do – it’s like chalk and cheese – a drop down the tap flutes every second turn or so and it’s like cutting butter with a hot knife.

I haven’t gone into how to tap, how to keep them straight etc because I figure y’all know that, and this is more about threads than how to cut them. But if anyone needs to know I’ll explain separately.

My little textbook is “Drills Taps & Dies” by Tubal Cain. It is No 12 in the Workshop Practice Series originally published by Argus Books but now published by Tee Publishing as far as I know. I don’t believe it is possible to find a book with more detail – it is incredibly complete. I believe there are nearly 50 books in the series – I have one which isn’t great but I have half a dozen others that are as good as Drills Taps & Dies. I strongly recommend them to hobbyists and even professionals.

[oafpatroll]

Thanks for this Dick!

[Sean KZN]

Intriguing, I have found great difficulty in matching up screws for electrical light switch covers, now I think I have an explanation.
Also, looking at my old German verniers which have a drill and tap chart on the back, the threads listed are Metric and Whitworth, no UNF or UNC which must hark back to days gone by.

[oafpatroll]

Intriguing, I have found great difficulty in matching up screws for electrical light switch covers, now I think I have an explanation.
Also, looking at my old German verniers which have a drill and tap chart on the back, the threads listed are Metric and Whitworth, no UNF or UNC which must hark back to days gone by.

I have inherited tools and workshop bric a brac from people all the way back to my great, great grandfather. They covered trades including cabinet maker, tool & die maker, mechanic, typewriter technician and others I forget. The range of unidentified threads in my miscellaneous fastener bucket boggles the mind. Once when my daughter was still keen on getting her hands dirty I paid her to gauge the threads and bag them by type. She wasn't a half hour in when she called to show me the pile that she couldn't identify with the three gauge sets to hand. It was bigger than the ones that she did identify.

[atunguyd]

I would like to add, when buying taps don't waste your time with carbon taps.HSS is not that much more expensive and cuts a lot better.

Sent from my SM-S908E using Tapatalk

[oafpatroll]

My little textbook is “Drills Taps & Dies” by Tubal Cain. It is No 12 in the Workshop Practice Series originally published by Argus Books but now published by Tee Publishing as far as I know. I don’t believe it is possible to find a book with more detail – it is incredibly complete. I believe there are nearly 50 books in the series – I have one which isn’t great but I have half a dozen others that are as good as Drills Taps & Dies. I strongly recommend them to hobbyists and even professionals.

I have his Building Simple Model Steam Engines which my great grandad gave to me when I was a kid. I got to spend some time with him building one of the stationary engines it described in the old man's largely self equipped workshop. The book and the memories it evokes are priceless to me.

[A-R]

It may be worthwhile also listing and discussing barrel and suppressor threads?

[Dick]

It may be worthwhile also listing and discussing barrel and suppressor threads?

By all means, although I'm not sure what we can do with them in view of the extreme difficulties of doing our own gunsmithing. I like rifle shooting but ranges long enough for centre fire rifles are scarce and I don't want to shoot rimfire because I want to load my own. I'd like to build a medium weight target rifle on a small action like mini mauser for a pistol cartridge to use on my local range at low cost. Would be a nice project. We need to be rid of the FCA. Maybe after 2024. Somewhere I have Frank De Haas's book which lists many barrel/receiver threads - just don't where it is right now. Remember also that for threading a barrel blank you need a lathe with at least one of two specifications (1) long enough for a barrel which means a lathe too big and expensive for most of us or (2) a smaller lathe with a spindle bore at least 20mm and preferably bigger. In the past small lathes didn't have spindle bores that big. But I did see that Warco in the UK sells nice relatively affordable small lathes with 20mm spindle bores.

[Dick]

I have inherited tools and workshop bric a brac from people all the way back to my great, great grandfather. They covered trades including cabinet maker, tool & die maker, mechanic, typewriter technician and others I forget. The range of unidentified threads in my miscellaneous fastener bucket boggles the mind. Once when my daughter was still keen on getting her hands dirty I paid her to gauge the threads and bag them by type. She wasn't a half hour in when she called to show me the pile that she couldn't identify with the three gauge sets to hand. It was bigger than the ones that she did identify.

What a rare privilege to actually works with one's great grandfather. Mine died long before I was born.

[oafpatroll]

What a rare privilege to actually works with one's great grandfather. Mine died long before I was born.

Indeed it was. He remains, amongst the people I have known personally, the one I most admire. He lied about his age and enlisted in the navy at 14 and survived two sinkings before he was 17. When the Great War ended he came to South Africa and worked on the railways for many years while slowly building his workshop and most of his tools from scratch. After retiring from the railways he worked as a fine cabinet maker full time for more than 30 years. He passed away when I was in my very early twenties and I have a photo of me, my dad, grandad and the oubaas getting pissed at the Guild Hall in JHB. A rare thing to have four generations of men alive and drinking together at the same time.