Are there 'Standard' thread sizes for model engines?

[terryd]

I am sure that this question gets asked repeatedly, but I am asking again. I am about to embark on my first small steam engine build from a set of castings i got on ebay. The 'Little Oscillator' found here: Steam Engine Price List | Precision Service I know that plans usually list out the fasteners needed for that engine, but I need to start purchasing taps and dies to make fasteners or start stocking up on fasteners to have on hand. Being in the US it can get tricky. Do I use UNC, Metric, or BA? The choices have large ranges of pricing for quality. Is there a list somewhere that shows the standard sizes for model engineering in each system? Is there any benefit of one over the other? Am i making this too complicated? Thanks for the input.

Hi Razzle,
I use M.E. (Model Engineers) UNF, Metric and BA. However my go to standard is Metric, both fine and coarse. Screws are generally available from around 1.2 mm up to as large as you need. The tooling and hardware are very easy to get over the internet even if you have to use eBay or similar postage from the far east is usually inexpensive (unlike the US and UK) and the quality is excellent generally despite what some diehards believe. Metric stock is available from teh normal suppliers in the US as much is used there in manufacturing, McMaster-Carr carry a wide range of metric materials.

The reason for teh lower costis simply volume production, all but three international countries use Imperial or Americam Customary Standard, they are Liberia, Myanmar (Burma) and of course the US. Of course if US manufacturers wish to sell their goods abroad for profit, vehicles being a good example, then Metric must be used if they are to be acceptable internationally, like it or not, and of course it is not economical to try to produce in both systems. So much manufacturing, if not all, use the metric system.

Also I cannot fathom out the use of Fahrenheit for temperature measurement, it is simply an illogical system. The German scientist who developed it, Fahrenheit, believed that Abs. Zero was 0°F and based his calculations on that. Of course we now know that Abs. zero is way way colder, around -273°C (approx -460°F). Celsius is based on measureable standards, i.e. the boiling and freezing points of pure water at sea level, 0°C and 100°C relatively 100 as the scale between the two for convenience - simple, repeatable and measureable. The metric system allows Engineers, Scientists and others to communicate around the world easily and with understanding and accuracy with no complex conversions.

Enough of that

TerryD

 

[terryd]

This has turned into quite the educational discussion. Lots of good advice and supporting information. Thank you. It looks like I will be investing in some taps and dies over the next few years. I will probably start with imperial first. The pick up the others as needed later.
Should I buy spiral flute or get 3 piece tap sets (straight, plug, bottoming)?

Hi again Razzle,

We are "two nations divided by a common language" so opined George Bernard Shaw. If you are buying 'Imperial' you will be buying tools with Whitworth thread forms on the whole. The common ones were BSW (British Standard Whitworth - coarse), BS Fine (Whitworth Thread form as are ME Model Engineers), BSB (British Standard Brass) and pipe threads to name a few, again all Whitworth Thread forms.

B.A. thread form were developed essentially for instrument makers and are widely used for that but are sometimes convenient in modelling, it is actually based on a metric definition but different to teh Metric System and are not generally interchangeable with it, for example 0BA is 6mm diameter with a 1mm pitchc but a 47.5° thread angle instead of 60°. There is a fixed calculation for determing each reducing size based on it's predecessor, they range down to 24BA using a numbering system but odd numbered threads are rarely used.

I doubt that you mean that! I expect that you mean 'American Customary' standards such as UTS (ASME/ANSI) e.g. UNF and UNC. These are not part of the 'Imperial System' which was defined by the British Government in 1824, with various modifications until metrication. I also note the use of the term 'mil'which I often read in US discussions and books to describe a thousandth of an inch. This can be confusing as in most of the worl. the term is short for 'millimeter' and a thousandth of an inch is a 'thou'. This is the sort of misuse of terms which leads to confusion. let's try to be more accurate.

TerryD

 

[Jasonb]

they range down to 24BA using a numbering system but odd numbered threads are rarely used.

Except for model engineering where some of the odd numbers are in very common use, typically 5BA and 7BA where they are used on 1/8" and 3/32" and to a lesser extent 3BA on 5/32" stock. Also 1BA has been and is still a standard for gas jets

 

[terryd]

The 'quality' message keeps cropping up in more than few areas of engineering, not just 'model' engineering. What's the 'quality' really worth and isn't a lot of it to do with something akin to 'national pride' or maintaining the status quo? I have mentioned my threading tackle as including metric, and metric can be extremely expensive. Nor is metric free of confusion. Take 12mm., which is generally available in five 'standard' pitches: the situation is not unlike that with the plethora of so-called 'imperial' systems. Five commonly used 'thread types' for the one size, so there's no loss or gain, is there? Like it or not - and I wasn't particularly keen about buying lots of new tackle - metric is here to stay. So for me, the issue is one of dollars and/or pounds. Quality? Just how is that assessed? By the manufacturer's say so? By 'standards? Or perhaps the price is a guide to quality? Tell me another one!
I regularly and routinely use taps and dies down to 1.4mm and while there are breakages, the Asians vs 'reputables' show no differences in durablity or finish, but if a 38mm tap is priced at $A800-odd and the Asian at $A100, then one is well advised to ask about the differences. Sure we can bat on about supporting the local industries, and that's laudable, but what does a non-sale at $800 support? I simply won't buy it! And if I had to, I'd certainly be factoring those sorts of prices in: it's a bit of a circular argument, isn't it?
I don't rush to Hong Kong to buy my stuff, but more than a few visits there have shown Australian (and perhaps others' countries) retail margins as being pretty well obscene. Four to five hundred percent is not unusual. And we, as model engineers with our very small quantities, are very much one of the retailers' targets. Then there's the 'scale' appearances' considerations and for this I do keep some very old BA hex heads, but only as decoration. They are in brass and a bit of torquing might shear 'em off: I'm sure I'm not alone in this! Enter the Asians, where I can have fasteners in brass, mild steel (zinc plated) or stainless steel and there was one occasion, in HK., where I had asked about stainless 2mm hex head screws (not bolts) and was surprised when asked how many thousand would you like, sir? Now the heads aren't as small as 'scale' might dictate, but if the whole project is 'metrified', what's the loss? And the SS can be better torqued up for steam tightness.
Last, but not least, is the 'net and there are firms who'll do almost any nut or bolt you might think of and there are others who'll supply taps and dies by the bushel and those bushels will be very comparable, in price and quality, to the single purchase at the 'shop'. You might counter with that perhaps you don't need 1,000 of xyz screws, but if the price is the same as ten offered at the 'shop', does the shop appeal for anything other than simple convenience? Then, they might have to 'get it in' for you. Oh, yeah.....

Hi Wazrus,

I'm with you all the way on your comments, National Pride often gets in teh way when discussing 'quality'. I buy tools from Aliexpress and Bangood as well as some British suppliers (here in the UK Tracy tools are a very competitive suppliers of just about any top or die imaginable) and I have no complaintr, any breakage is usually my fault through hasty or sloppy working practices. I access screws etc from various suppliers via eBay where almost any screw, bolt or nut in any grade or material can be obtained relatively inexpensively. I rarely need anyother than metric course and fine up to 12 mm thread. I usually use carbon steel as they a eeless brittle than HSS or carbide and if broken are easier to remove, as I'm a model maker now, I don't need production quality tools.

Interesting point about BA is that teh hex heads are over scale, if you need the strength but want scale you can buy specials which have a hex which is a sized below giving a more scale appearance. A good friend of mine who built a beautiful of a Vee 8 aero engine and manually reduced the size of the BA hex heads with a dividing head and milling attachment for a true scale appearance.

TerryD

 

[terryd]

Except for model engineering where some of the odd numbers are in very common use, typically 5BA and 7BA where they are used on 1/8" and 3/32" and to a lesser extent 3BA on 5/32" stock. Also 1BA has been and is still a standard for gas jets

Hi jason,

I quite agree and use these on my Stuart models which I'm refurbishing and building, i just didn't want to confuse the matters more as these are quite specialised applications, but yes, I do use them.

Regards

TerryD

 

[delalio]

One other thing which has not been mentioned here is the use of a thread gauge, and a micrometer.

I have several and find them invaluable when looking at an existing thread.

If you're doing models, then make sure you get the thread gauge which caters for the sizes you will be using.

If you want any tips feel free to PM me.

Del

 

[Bentwings]

Hi Razzle,
It's a moot point but the'Imperial ' system is not that used in the US. Over there it is, I believe classified as 'American Standard'. There are some differences between the two, so best not to get confused. There are measures, especially in volumetric measurement that are radically diferent and you use a few measures that are archaic as far as the Imperial system is concerned.

TerryD

I looked up some of these tap and die sets. They are over $100 for 3 taps alone .
there was a discussion about threads that noted thes may be whitworth. I can’t even spell it. I’d have to get the machinery manual out for the. Even my master car has a steep price tag on these hopefully anything I get will already have tapped holes or threaded parts

byron

 

[Richard Hed]

I looked up some of these tap and die sets. They are over $100 for 3 taps alone .
there was a discussion about threads that noted thes may be whitworth. I can’t even spell it. I’d have to get the machinery manual out for the. Even my master car has a steep price tag on these hopefully anything I get will already have tapped holes or threaded parts

byron

Look up the same taps on Shars--they will be a fifth that price and the quality is pretty good too. I have been buying taps from Shars for a long time and haven't had one break yet. However, I am very careful, as the taps I am buying are very small. I always get a spiral tap as they are easiest to start, then the 3 part set if possible. Too often I need that blind hole bottom tap and they are not always available.

There are other shops besides Shars also that have good stuff at much lower prices. Here's one that is not well known but I consider one of my go-tos when they have what I needs.:

https://www.victornet.com/ord/basket.html

 

[Mike Ginn]

I’ve been following the “best thread” discussion with interest. Here is my take on the decision process.

It doesn’t matter which thread you use be it metric, BA, ME American etc. What does matter is the size of the nut/screw head.

In Europe models specify metric – see Jan Riders marvelous designs. In the UK older models specify BA with ME for larger threads – Wyvern is a good example. More recent designs from Anthony Moult have moved from BA to metric. US designs such as Little Brother use American 4-40 etc. All of these threads, at the appropriate size, have sufficient strength for the application.

I have taps and dies for all the common thread flavo(u)rs and I use metric for all general engineering work – usually cap head. Metric screws are cheap and strong and a few Allen keys are the only tools needed.

When I come to needing screws and nuts for models, I try different sizes to ensure that they have the “right look” for the scale. This usually means that I need to make my own nuts and hex screws. My goto thread is BA since it is well document and well proportioned between the sizes. I usually find that the customary “one size down” works well and these can’t easily be purchased economically. Sometimes 2 sizes down are needed.

To hold down the base of a recent engine I used a 2BA thread over 25mm long with 7mm of 4BA at the end and then 4BA reduce to 5BA full and half nuts. Looked good!

There has been mention of the difficulty of BA single point cutting. The angle is 47.5 with a very rounded crest and a very odd pitch. In 40 years of modelling, I have never found the need to screw cut a BA thread – I have always used dies.

FYI I usually mill the hex heads using either an ER25 collet on a hex base and then carefully round the corners in the lathe. The best method is to use my small CNC mill since I can program in the corner radii but there is more setup time.

BA spec for screws, nuts and washers may be found here – www.royalsignals.org.uk/articles/ba screws

The image below shows the carb and head of my Wyvern. Note that the carb extension is oval to give it strength and to allow a 1/4inch bore. The head had already been tapped 6BA and to give the right look the screw head was made 8BA with a 8BA washer.

Hope that helps. Mike

 

[methuselah1]

Hi Razzle,
I use M.E. (Model Engineers) UNF, Metric and BA. However my go to standard is Metric, both fine and coarse. Screws are generally available from around 1.2 mm up to as large as you need. The tooling and hardware are very easy to get over the internet even if you have to use eBay or similar postage from the far east is usually inexpensive (unlike the US and UK) and the quality is excellent generally despite what some diehards believe. Metric stock is available from teh normal suppliers in the US as much is used there in manufacturing, McMaster-Carr carry a wide range of metric materials.

The reason for teh lower costis simply volume production, all but three international countries use Imperial or Americam Customary Standard, they are Liberia, Myanmar (Burma) and of course the US. Of course if US manufacturers wish to sell their goods abroad for profit, vehicles being a good example, then Metric must be used if they are to be acceptable internationally, like it or not, and of course it is not economical to try to produce in both systems. So much manufacturing, if not all, use the metric system.

Also I cannot fathom out the use of Fahrenheit for temperature measurement, it is simply an illogical system. The German scientist who developed it, Fahrenheit, believed that Abs. Zero was 0°F and based his calculations on that. Of course we now know that Abs. zero is way way colder, around -273°C (approx -460°F). Celsius is based on measureable standards, i.e. the boiling and freezing points of pure water at sea level, 0°C and 100°C relatively 100 as the scale between the two for convenience - simple, repeatable and measureable. The metric system allows Engineers, Scientists and others to communicate around the world easily and with understanding and accuracy with no complex conversions.

Enough of that

TerryD

Any temperature measurement requires a base point and a cross point. Danny Fahrenheit found a self-stabilising brine/ammonium chloride (eutectic) mixture, which was the base point, and used human body temperature for his cross point- it may be apocryphal, but I've heard said that it was the temperature of his wife's armpit! Either way, the cross point has been revised at least twice after his scale was proposed. Only the U.S. and three former British colonies still use it.