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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.

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.