3.5 to 7 1/4

[amahira1977]

Guys I need some help, my brain is getting seriously muddled in my 40s, I have bought some plans for my first build, the only one that took my fancy was for a 3.5 inch gauge track, so I'd like to convert all the measurements to build larger for 7 1/4 track, does anyone by chance have a decent conversion formula to do this. My head already hurts with this...

[borderer]

With respect, what you are suggesting is not very practical, because few parts can be scaled-up directly. It will be easier to design from scratch than to try and convert everything.

It is necessary to use stock size materials, and there are well-established standard dimensions in each gauge for parts such as wheels and axles. It is sensible to follow these, and if you are looking at a steam model, the boiler must be designed specifically for the size required.

For your first project, it is probably best to build an existing design, or at least to start with one which is similar to what you have in mind.

[flyingfox]

Greetings, whilst "Borderer" is quite correct in what he is saying, I have built a 3.5 and 71/4 gauge Bantam Cock, both locos performed well.

I would also draw your attention to the "Colchester" Design which was increased from 5 in to 71/4 gauge, Paul P in west Wales has built 4 of these, and also is making 2 off 71/4 Bantam Cocks.

I am also in the early stages of building a 71/4 gauge version of the late Martin Evans's "Jubliee" a 31/2 gauge Stanier 2-6-4, and hope to submit an article to ME about it.

Whilst data for the wheels, axles and valve gear are easily available, I would strongly advise caution in scaling up a boiler design yourself, I have mine made by Paul at Southern Boilers, who redesigns them to correct standards.

Finally, I use 2.054 as the multiplier to scale up dimensions from 3 1/2to 7 1/4. Also be prepared to juggle when using metric stock instead of imperial sizes.

Regards

Brian B

[oldnorton]

You need to say what 3 1/2 design you are looking at. I agree with all the above that you are taking on a challenge, while it is possible for someone with experience.

The boiler must be to a published design (not a rescaled design) or carry a commercial CE mark, otherwise you will not get a test certificate.

My reason for recommending not doing it is that a good 7 1/4 design, in theory, can carry more that is true to scale, that might not be seen in a 3 1/2 design. The 7 1/4 Black 5 Highlander is quite a different beast from the 3 1/2 Doris.

Norm

[SteveW]

amahira1977,

Consider going the other way towards gauge one or table top scale for your first loco. You'll be up and running in no time.

Popped round to my club yesterday (Sunday) and the table railway was busy and everyone was having loads of fun. Every thing was small and easily portable and running on either meths or gas. Its almost a no-brainer.

[kipford]

Norm

The boiler must be to a published design (not a rescaled design) or carry a commercial CE mark, otherwise you will not get a test certificate.

This is not quite true as you have written it or no one would be allowed to produce a new boiler! The Boiler test requires you to demonstrate prior to building it that a 'new design is sound see section 5.1 of the code.

5 Design Verification
5.1 The constructor of a boiler to other than a recognised design available through the model engineering trade and/or press shall produce design drawings and demonstrate to the satisfaction of the Inspector, either by calculation or by well-proven example, that the design and materials used have adequate strength.

A new boiler design can be be demonstrated to be technically structurally sound if you use the Aussie code as the basis. I have done this successfully with two boilers designs so far.

Regards
Dave

[kipford]

Steve
You are going a little off topic from the OP's original question and would it not be better being discussed in a separate thread?
Regards
Dave

[oldnorton]

Since the OP has not responded his question might not have been intended to produce a simple reply.

[terrier060]

Hi Amahira
I have a small program I wrote many years ago which I have used to reduce all my Terrier drawings from full size. It works in either direction in Imperial or Metric. I would be happy to send you a copy if you would find it useful. Details are on page 2 of General Chat under my 7.25 inch gauge Terriers, on page 6. You can message me for more details if you are intersted. It has saved me eons of time. It will not help a lot with thicknesses of copper hough in boiler design. You will need to look at similar designs for that.
Ed