Metric or Imperial ?
Nuts, bolts, screws and fasteners. The subject is worth its own section to simplify the choices facing the newbie model engineer. In my own case I started out with BA fasteners but I found these expensive with a limited choice of heads and somehow they did not seem to offer the holding power of their metric counterpart – though this could have been my imagination.
So, I quickly opted for the metric system. Metric coarse or fine ? I can say that I have never used metric fine in my model engineering so it’s metric coarse all the way with me. My short period of experience has also narrowed down the choice of fasteners to just two main sizes, M2.5 and M3. I did occasionaly use M2 in my early days for very small engines and I sometimes use M4 for holding engine frame to its base. Build a small stock of the main sizes and buy in ‘specials’ as and when you need them.I also prefer stainless steel as I don’t think brass looks authentic on our small engines and steel is prone to rust.
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- Gaskets cut from PTFE sheet
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- M3 stainless cap screws
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- Tapping tools
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- Two washers and nut fitted to stud
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- Stud and face of nut filed down
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- One washer removed and nut turned over
Threaded Rod
Now whilst I think it is a good idea to have a small selection of M2.5 and M3 screws and nuts at your disposal I would also suggest a small stash of threaded rod is a good investment. In situations such as holding down bolts for cylinder end plates and valve chest covers studs look far more authentic and scale than screw heads. In addition you are less likely to run into problems of stripped threads during assembly and final tuning where you may have to remove covers many times before you get your engine timing spot on. It is much easier to strip a thread screwing into relatively soft brass than is likely with stainless threaded rod. Once you have your studs in position with a spot of Loctite you are much less likely to suffer the agonies of a stripped thread. Threaded rod is also much cheaper than buying equivalent screws so it’s a ‘win win’ situation.
To achieve a really neat appearance to the stud and nut place two washers over the stud followed by the nut. File down the stud and continue filing to remove approximately a third of the nut depth. Remove nut and one of the washers. Turn over the nut and tighten down.
Taps and Dies
You will also need the appropriate M2.5 and M3 taps. There are quite a few specialist suppliers of fasteners and associated tooling out there on the internet but my preferred choice is http://www.modelfixings.co.uk/ In addition to fasteners and threaded rod Model Fixings can also supply tools such as spin socket spanners and a good selection of bearings useful for our models.
Tapping is often the last stage in producing say a cylinder or steam chest which may have taken several hours to machine and could so easily end in disaster. Nothing is more frustrating than snapping off a tap at this stage leaving the broken end buried in the hole. Proceed with the utmost caution ! Refer to your tapping chart to ensure you are using the correct size tapping drill. Keep backing off the tap and remove to clean out the flutes. A spot of WD40 may help to reduce friction binding.
In addition to the small tap sizes I bought a cheapo set of metric taps and dies, running from M3 to M10, from Machine Mart in my very early days and these have proved to be useful on many occasions. I also bought a dieholder set for use in the tailstock of my lathe. The big advantage of forming threads on the lathe is that everything is nicely squared up and you are less likely to get a threaded shaft looking like the leaning tower of Pisa. When threading in the vice I use a homemade guide which keeps the thread running vertical.
Drill Bits
If, like me, you choose to go down the metric route don’t completely ignore the merits of imperial drill bits. For example, a 3/32″ imperial drill bit equates to 2.38mm and is ideal for tapping M3 and probably a lot cheaper ! I buy boxed sets of metric drill bits from 1mm to 6mm in 0.10mm steps and replace the most frequently used smaller sizes in packs of ten.
When building from USA originated plans (e.g. Elmer Verburg’s plans) you will be faced with the conversion of dimensions from imperial to metric. Initially I was constantly referring to the inch/millimetre conversion chart in The Model Enginners Handbook. Eventually I photocopied the chart and had my local stationers encapsulate it in plastic. I refer to this handy chart several times a day. And remember your digital vernier scale will switch from imperial to metric, or visa versa, at the touch of a button.
Plan conversion
When starting a new project from plans presented in inches I usually enlarge the A4 plan to A3 then Tippex out all dimensions. Photocopy again and re-enter all dimensions in metric. A certain amount of creativity and adjustment is often required. For example the plan might specify a 3/16″ dia shaft. This translates into 4.76mm – I would adjust that up to 5.00mm so I could use available stock. At the same time I would ensure that I made the appropriate allowance to any affected items such as a flywheel or crank that may be fitted to the shaft. Equally, with care, you can mix metric and imperial on the same project. My beam engine for example is built in metric but features a 1/4″ diameter crankshaft rather than the 6mm specified on the plan.
A calculator is a pretty useful device to have on hand in the workshop and if you get one that converts metric to imperial (or visa versa) then all well and good. Take a look at the Aurora. I have used one for the last couple of years and would be lost without it.
Gaskets
One of the reasons why I suffered from stripped threads in my early days was in tightening down cylinder and valve covers too tight in an attempt to contain air (or steam) under pressure. Recently I have taken to producing gaskets in 0.25mm P.T.F.E available from http://www.directplasticsonline.co.uk/ Not only is this material perfect for forming air tight and heat resistant seals it is a joy to cut with scalpel or safety razor blade and bolt holes can be punched neatly using a set of cheapo hollow punches. I have even taken to making small circular gasket/washers to insert into pipe unions for more efficient sealing without the risk of stripping a thread.
All this assumes that you will be building model engines similar to mine. For larger engines or projects other than model engines you will of course need to adjust your requirements accordingly.
I have a model.Hudson loco with the plans from Little Engines
The plans call for 2-56. And 1-72 screwsplus other sizes.
As I live in the UK I am not familiar with this description having always worked in BA for model engines.
This is a very small loco live steam 0 gauge about 10″ long so the screws must be quite small
As the loco is part finished I need to get materials to complete the job can you please tell me where these can be obtained
Kind regards
Martin Murray
Hi Martin
Seems to me you have two options. The first is to source fasteners from a supplier in the States though I haven’t any experience of this method so am unable to advise you. I have built a number of steam engines (stationary) from American plans, notably engines by Elmer Verburg and I have opted for metric fasteners with M2, M2.5 and M3 covering 90% of my requirements. I personally prefer metric over B.A. fasteners as there is a larger range of materials and heads. In addition they are much less expensive and more widely available than B.A. I find Model Fixings have an excellent range. In terms of sizes I believe that if it looks right it will be right (if you know what I mean). For a more precise conversion from American standards to metric there is a useful table in The Model Engineers Handbook by Tubal Cain.
John
I am looking for 1mm cheese head screws/bolts with nuts i have looked everwhere?
Sites on line that advertise 1mm cheese head screws, when asked for them all I get is no items found. Please can you help me? I don’t care what the thread is just so long as they are 1mm about 1cm long.
Yours frustrated Derek Allen
Hi Derek
Are these anywhere near ? Rarer than hens teeth at the size you require. Maybe someone will come up with the name of a supplier. Can’t help wondering what the project might be – Rolex watch maybe ???
John
Thanks again,John.I already feel better!It’s a pity,though,that Boxford”translating gears” are so expensive-I’m certain that one day cutting a hefty Imperial thread will be unavoidable.Now to photocopy that conversion chart.
Thanks for a great site.I last worked in engineering(Fitter/turner in ship-repair)some 35 years ago,and am now an oap with a yen to build model steam boats,but recognise that small-scale demands a very different set of skills and attitudes.I recently bought a Boxford AUD.Only prob lem is that it’s metric whereas,overwhelmingly,I worked in Imperial.Having read a few books on beginner engines by such heroes as Stan Bray,Tubal Cain and Harold Hall I’m relieved at your comments that relatively few metric threads appear to suffice for a host of BA,Whit and BSF,(requiring number,letter and Imperial drills).Do you find that”40tpi” can safely be replaced by a metric-pitch near-equivalents also?
Hello Pete
Like most of us brought up in our generation Imperial standards were the order of the day along with (to me at least) a confusing array of thread types. When I first took to model engineering four or five years ago I tried to work in BA but soon recognised the limitations and expense of this standard. I found it to be like a breath of fresh air to discover metric coarse and for my model engines I worked mainly in M2.5 and M3. As i am now tending to build larger engines i also use M4, M5 and sometimes M6. As for 40tpi threads these are still retained by the producers of steam fittings for which I am sure there is good reason. For such situations most of us keep one or two ME 40tpi taps and dies on hand.
John
John,
I’m considering buying a lathe, and came across your site while searching for truely basic information. THANK YOU for investing your time to create an interesting and useful site.
Going back 40 years to when I played on cars, I seem to recall that we essentially glued pressure gaskets down to ensure a tight and permanent seal. So gaskets made of zero friction PTFE seem counter intuitive: Do the markedly lower pressures let you “get away with it”?
Oh, I have a woeful amount to learn.
Very respectfully,
Larry
Hi Larry
The PTFE material is resistant to oil and steam, it cuts cleanly and easily and has just sufficient compression to absorb minor machining irregularities. The low friction properties of Teflon does not appear to reduce its ability to form an air and steam tight seal. In practice I have found it to be the most effective method of sealing cylinder heads and steam chest covers whilst avoiding stripped threads through over tightening.
As for the lathe go for it – I am sure you won’t regret it !
John
Thanks John for your words of wisdom,I also like the threaded bar tip which I will put into practice,and thanks for a great web site 10 /10. I’ll be back!. regards Harry.
Hi Harry
Congratulations on the completion of Elmer’s Wobbler – its surprising how Elmer’s engines perform despite our best efforts ! I would suggest that you forget imperial fasteners as they are very limited in range of heads, sizes and material. When I started I soon discovered that there was much more choice in metric and for far less money. I also found that most fasteners for Elmer’s engines were either M2.5 or M3 if it looks right it will be right ! I could never get my head round american thread sizes though there is a conversion chart in Model Engineers Handbook. Get yourself a copy as there are some useful conversion charts to get you from imperial to metric – essential if you are building Elmer’s engines. For my more recent build of Elmer’s masterpieces I have upped the plans by 50% – very little extra work but the results are more impressive (in my opinion !). Anyway good luck with Elmer’s #43, it’s a nice little runner with forward and reverse.
John
Hi,came across your site looking for info on thread cutting on my myford ml2 (the old cheep one). anyway your site drew me into the world of steam, I’ve made and built Elmer’s *25 wobbler, only one mistake, I drilled the 7/32 recess for the spring the wrong side
of the frame dooh!,but it RUNS yarhoo!!!.
So I’ve started my next project, Elmer’s *43 Horizontal Mill Engine only to realise that the nut and bolt sizes are very small? and I don’t understand the terminology used ie *3-48,my smallest size tap /dies are 6ba or m4.I would be greatful if you could enlighted me on this problem. regards Harry