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down scaling

5 inch to 3.5 inch formula

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Peter Townsend 205/05/2021 02:44:45
4 forum posts

hi I need to down scale drawings from 5 inch to 3.5 inch is there a formula or how can i do it thanks Peter

Jon Lawes05/05/2021 04:47:03
535 forum posts

For straight lines I guess multiply by 0.7, but obviously you wouldn't want to scale anything boiler related like that.

That's just a stab in the dark mind...

Paul Lousick05/05/2021 06:32:05
1724 forum posts
628 photos

Multiplying dimensions by 0.7 will scale down the drawing but you then have to round the answer to a size that will match available materials.

eg. 5" scale drawing uses 1/2" plate = 0.35" at 3.5" scale. Nearest imperial plate is 3/8" [0.375"] or 8mm [0.315"]

or 5" scale drawing uses 1/4" [0.25"] bolts = 0.175" at 3.5" scale. Bolts this size are not available and you have to select something different.

Everything on the drawing should be checked to compensate for these changes. Unless it is critical to achieve an exact scaling down of the drawing from 5" to 3.5" you may want to round lengths to whole numbers. Bearing in mind that your 5" drawings are probably an approximate scale of the original full size drawings.



Edited By Paul Lousick on 05/05/2021 06:36:54

J Hancock05/05/2021 06:32:39
585 forum posts

For quick comparisons , draw yourself a right -angled triangle.

Hypotenuse 5 " Bottom 3,5 " long , drop as many vertical lines as necessary., or 10" , 7" etc.

Nigel Graham 205/05/2021 08:32:05
1420 forum posts
20 photos

Fastenings are not so difficult. Most won't be far off the 3.5 / 5 ratio, though you'd probably want to use screws with size-smaller heads in many areas for appearance or accessibility - a spanner takes up a lot of room!

A rough guide to the right area is to assess the nut and bolt sizes in full size, and bear in mind they had to be manipulated by blokes with spanners. Looking at preserved machinery, a lot of the ordinary fastenings were no more than perhaps 3/4 diameter; much above an inch was usually for major, fitted bolts holding parts like cylinders to frames; or specials on marine-engine frames. The designers calculated joint strengths based on several moderately-sized rather a few huge, bolt or rivet shanks - and also considered how the thing was to be erected and serviced. .

The example Paul cites of a 1/4" bolt in 5" scale(?) creating a theoretical 0.175" bolt, equates practically to 2BA (0.180" dia measured from a sample screw), possibly M5 though M4 might be better for appearance.

I think we mean gauge here. The difference between scale and gauge?

A 1/4" diameter bolt on 5"-scale drawings is 0.6" dia in full size (5/8" in practice).

A 1/4" bolt on a 5" gauge loco (approx. 11:1 scale) would be nearly 3" diameter in full size!


The easiest way to re-scale the drawings is to use a spread-sheet, because you need only drag-copy the formula down the column.

Round plate thicknesses and rod diameters sizes to 1, maybe 2, decimal places; to cope with only metric materials being available create a further column converting them to mm ( X 25.4). For plate and sheet work it is probably better to go up a size but don't over-do it.

NB - Having to change the plate thickness from the original plans then compromise for stock sizes, can affect a lot of other dimensions, especially so in locomotive frames and traction-engine horn-plates with many components on both sides of the plates.

Paul Lousick05/05/2021 08:42:03
1724 forum posts
628 photos

It is very easy to downsize (or upsize) drawings/models if they are in CAD by applying an overall scale feature to produce a drawing at a different scale. Then a matter of modifying to use the available material sizes.


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