Scales!!!

[-steves-]

OK, so we all know the difference between a scale and a gauge.

So with that in mind, do we have a mathematical person that can tell me if I have a drawing in a scale of 22.5:1, what percentage do I need to multiply that to get it to 16mm (which is about 19:1 I think)? The formula for working this out would be appreciated as I am often given drawings in various scales. I was once told it's about 25% but I have no idea if that is anywhere close

Thanks in advance
Steve

[philipy]

100mm at 10:1 = 10.00 ie 100/10
100mm at 22.5:1 = 4.44 ie 100/22.5
100mm at 19.05: = 5.25 ie 100/19.05

22.5/19.05 = 1.18 ( conversion factor)
so 1.18 x 4.44 ( dimension from 22.5 drawing) = 5.24 which should be close enough to 5.25 for your purposes as long as you use the same conversion factor throughout.

[Jbs]

Hi Steve,
For enlargement I divide the scale I have the drawing of by the scale I want to change it to. 22.5/19 = 1.1842. It seems to work out OK. I usually check enlarged railway drawings on the track gauge and adjust slightly up or down to suit.
Hope this helps.
John

[-steves-]

So for the sake of a photocopier, would I use 118% to enlarge them?

[GTB]

OK, so we all know the difference between a scale and a gauge.

My personal experience is that a lot of railways modellers don't have the foggiest idea that there is a difference and the plethora of scales and gauges out there doesn't help......... The term dog's breakfast springs to mind.

So with that in mind, do we have a mathematical person that can tell me if I have a drawing in a scale of 22.5:1, what percentage do I need to multiply that to get it to 16mm (which is about 19:1 I think)?

In this instance If you divide 22.5 by 19 the result is 1.18 (rounded to two decimal places).

This factor can be used to multiply a dimension taken from the 1:22.5 drawing to give the correct dimension in 1:19 scale.

If using a photocopier to blow up a drawing then in this example you would enter 118% as the magnification factor.


The general formula for rescaling a drawing is F = O/C where F is the scaling factor, O is the original scale and C is the scale of the rescaled copy. It works in both directions and can be used to reduce drawings as well as magnify them.

In the english speaking countries scales are a mix of metric and english units, ie. Xmm/ft. The scale ratio is easily calculated by dividing the number of mm in a ft (actually 304.8, but for this purpose it can be rounded to 305) by the number of mm in the scale. So 305/16 = 19 when rounded off.........

The continentals are a bit smarter and just use scale ratios. 1:22.5 scale works out to about 13.5mm/ft and is roughly what you get when you use 45mm gauge for metre gauge prototypes.


It should be pointed out that paper is a lousy material when it comes to dimensional stability and photocopiers, scanners and printers aren't necessarily accurate when it comes to reproducing drawings.

We once had a photocopier at work that had a 10% difference in scale between the longitudinal and transverse directions. Didn't make a blind bit of difference when copying text, but drawings were a different matter. Since then I've always made a test print when I get a new scanner, or printer.


I could have just given you a number, but I think it's better to know how to go from the particular to the general. It's probably still as clear as mud in a beer bottle, but work through a few examples and you should get the hang of it. Just don't get hung up on how many decimal places the average calculator or computer can display and just round off the results.

Regards,
Graeme

[-steves-]

Thank you all for your replies. I knew this forum had some very knowledge people and that just goes to prove it, the in extensive knowledge is great