Building imperial locos in a metric world – confused!

[Nick TParticipant @nickt]

Evening All. I am exploring beginners locos and have today received Jack Bucklers book about the Sweet Pea, which seems to be very useful for the likes of me. The measurements are all in imperial but he does mention metric equivalents for items such as the frames.

Could someone please tell me if I have to equip the workshop with items such as imperial drills or does one convert to metric? Equally would I have to convert all the drawings to metric if I wanted to stay metric?

I am a plumber so I am used to the world of metric copper tube and fittings and imperial for iron pipe, but this one has me stumped.

Many thanks, Nick.

[Nick TParticipant @nickt]

[JasonBModerator @jasonb]

It really depends how far you want to go. For example some people are happy to convert say 1/4" to 6.35mm and work to that but you can run into problems if you need to put a thread on the end as M6 is a bit tight if you only have metric taps & dies

On the other hand you can do a full conversion and the part that is 1/4" can become 6.0mm, easy if you want to buy say 6mm dia rod and easy to put a M6 thread on the end. But if you round all the imperial fractions to the nearest convinient metric size then by the time you have worked your way along the engine you could be quite a bit out.

In the end its best to decide on some form of compromise, keep  lengths to a direct conversion but diameters and stock sizes to nominal metric. There will be instances where this does not work as you may not be able to get a metric tube for the boiler barrel and if buying steam fittings will not find them with metric threads in the UK. Bit like copper plumbiong pipe being metric but threaded fittings in BSP.

J

Edited By JasonB on 05/03/2015 18:31:58

[Paul LousickParticipant @paullousick59116]

Hi Nick,

I had a similar problem with the traction engine which I am building. The drawings are imperial but I decided to work in metric because I already had a lot of metric tooling and I normally work in metres in my job. (easier than using fractions). You should also consider what size of material is available for making your model. Many have been changed to metric. The biggest problem I had was finding pipe to make my boiler barrel. The drawings specified 10" (254mm) outside diameter but the closest pipe that I could buy was 273mm OD. Consequently the size of many components had to be modified.

I converted the drawings to metric and re-drew everything to verify that modified parts would fit together. Other model engineers stay with imperial dimensions. Your choice.

Paul.

[Rex HanmanParticipant @rexhanman57403]

Work in what you are most comfortable in…..less chance of making mistakes. Avoid mixing metric and imperial threads as you will soon forget which is which!

[Another JohnSParticipant @anotherjohns]

Nick;

I'm almost 100% metricated, but somethings (e.g., SS balls, some materials, etc) are still in inches, at least on my side of the Atlantic.

I'm working on TWO locomotives; (don't ask) one has plans in inches, the other in MM. The one in inches has pencil marks on drawings indicating what the dimensions in MM are.

For screws, I just look and decide what is the most appropriate (BA to MM, thread pitches, etc)

The plans are, in general, just a "rough idea" anyways, so once you start, and think about things that MUST be standard (such as track gauge) you'll figure it out.

(if you ask; once the current locomotive projects are complete, anything following will have plans fully in metric, at least that's what I'm saying now. There are other issues other than inches/mm; all the metric plans I have are much better than ANY of the inch-based ones, with one exception – Kozo Hiraoka designed the Heisler in inches, to appease criticism, I'd guess)

[mickParticipant @mick65121]

Converting between the two systems is very straight forward. To convert from imperial to metric just multiply the decimal equivalent of the fractional dimension by 25.4 Metric to imperial, divided by 25.4 As for threads, the smaller metric sizes from 5mm downward are as near as dam it to BA with the added advantage of it being easy to source fixings such as socket head cap and counter sunk screws in metric sizes. Who would ever know that instead of 7BA you used M2.5.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by mick on 08/03/2015 16:41:16:

… with the added advantage of it being easy to source fixings such as socket head cap [screws]

.

Just a quick note of caution: The proportions of metric cap head screws are different to the Imperial, BA, and unified 'equivalents'. … In many applications this will not matter, but; if you are using counterbores beware!

MichaelG.

[mickParticipant @mick65121]

Wouldn't be the first time I've turned the knurling off a cap screw to fit a counter bore!

[VicParticipant @vic]

I noticed years ago that cap head socket screws have much smaller heads in comparison to metric ones. The imperial always looked nicer to me. I always use non standard size counterbored holes as the reccomended sizes look far to big and untidy to my eye!

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