7 1/4" Gemma ?

[jeremie88]

Hi there,

Just received the drawings from Reeves for the little 5" Gemma. I'd like to scale it up to 7 1/4" and after that study the possibility of making my own castings. If the project turns to be too complicated or too expensive for what it is, I will instead switch to Jessie. 

As I need to built it in metric, my first question will be: should I first scale all measures up in inches to 7 1/4" and from that transcribe them in millimeters, or should I do the opposite ? First transcribe everything in millimeters THEN scale it up in 7 1/4" ? 

Is there any specific way of calculating 5" measures to 7 1/4 ? Is the ratio simply 1 = 1.45 ? 

Thanks

[uuu]

Why not do the metric conversion and scale-up in one go, by multiplying by 36.83?

Wilf

[jeremie88]

That sounds like a good idea, didn't even think of it! 5" x 36.83 gives 184,15mm, shouldn't it be 36.80 ? 7 1/4" gauge is 184mm in metric

[uuu]

25.4 * 7.25 = 184.15

What you will find is that you'll have to tweak quite a few dimensions to make things sensible e.g. if the frame thickness comes out at a size you can't get. That will have a knock-on effect on all sorts of other things. It's an issue even with a published design, if it's in inches - you just can't necessarily get material of the specified size.

Wilf

[dscott]

YES She is a bit small in 5 inch gauge but on the other hand liftable and still pulls 3 Adults.
Ours lives in the Living room on 2 very rusty rails.

7 1/4" Would be nice. I had problems with the valve as drawn and had to make new to worked out marks on stuck on card.
1988 I finished her!!!

David and Lily.

[jeremie88]

Hi David and Lily,

Good to know that someone here built one Gemma, as I certainly will have plenty of questions about it ! Next to this valve problem, did you remember of any other problem from drawings or from building the locomotive?

Thanks, Jérémie

[dscott]

A quite DUSTY model of Gemma with a younger pair behind.
The 2 stones by the Beans are from Tibet and Lily has decorated her with other bits. The 700 Class also seems to live in the area.

The brass safety valve cover is from a firehoze turned and polished. Stuff for me seems just to arrive and needs a home until the day it is needed. I built her in the Mid 1980's and copied the ones on The East Cornwall Mineral Railway that ran from Calstock round to Kit Hill near Callington. I then lived in the area so she was a Local Model.
Later moving to Enfield and visiting North Woolwich old Station where they had a preserved example.

GER Class 209 Currently under Overhaul at the Flower mill Lydney Gloucestershire. They may have a drawing or let you measure up.
Go and take too many Photos and go for a more scale horn blocks and springing.

I could not resist using one of the sand boxes for covering the Lubricator!

Do keep us informed of progress.

David and Lily.

[millman]

I built a Gemma many years ago when the castings were supplied by a company up in St Helens, think it was called MESAS. I had trouble with the valve gear, hardly any port opening and by this time the design had been sold on to Hugh Mothersole, the chap that bought up a lot of Don Young’s designs, I contacted him and eventually I got a drawing of the corrected valve gear, I don’t know if the drawings were ever updated by Hugh Mothersole before he sold out to Reeves.

[jeremie88]

Hi, I will check in Reeves's drawings about the valve gear. But how could I notice if the drawings are now corrected from the old version ? Do you eventually have the corrected version to compare ? Thanks

[Shawki Shlemon]

Many years ago I build one in 5" G , it is a great engine and gutsy .

www.youtube.com/watch?v=kzZd_Vag9lw

[ejparrott]

I've just done the drawings for laser cutting for a customer, scaling up is easy once you have digital files. Multiply by 1.4118 (1.5 scale) or 1.4494 (1.54 scale)