Author Topic: Blue print re-scaling  (Read 933 times)

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Offline wolfstone

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Blue print re-scaling
« on: March 02, 2016, 10:08:04 AM »
Hi All,
I have just discovered some blue prints in my shed originally purchased from John Skinley Southend in 1960 they are for 4mm to ft.
Does anyone know how these could be copied and re-scaled to  G3
They are white on blue background.
LMSR 12Ton refrigerator van  DWG No G.16H
SR 10Ton Banana Van  DWG No G165H
LMSR 57 ft 3rd class flush sided brake compo (coronation scot)
   "     "   "    "     "       "      "     Vestible Car         "          "
BR 64ft 6" Standard 3rd class corridor coach
Anyone interested
Tim

Offline IanT

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Re: Blue print re-scaling
« Reply #1 on: March 02, 2016, 11:27:33 AM »
Hi Tim,

There are software products that claim to do this but some years ago I tried a "30-day" version of one and found it to be almost useless in getting an accurate CAD image. I'd be pretty sure there are probably other 'open' solutions around these days but I haven't investigated this area recently.

The other approach is to import a scanned (.Jpeg) image as a background layer to your CAD drawing - and effectively 'trace' the outline using the CAD facilities. I've also tried this but again found it a somewhat frustrating process (problems with getting the image 'square' and also 'out-of-parallel' errors)

So what I (still) do is work from a hard copy of the drawing using a scaling factor(s) [the X & Y axis may be slightly different scales when checked] and effectively re-draw the image full-size. The reason I do this is because (in practice) many dimensions would have been to whole numbers or (large) fractional values and it's much easier to guess how to round up/down to a 'real' number in full size rather than scale sizes. Another advantage is that once you have the drawing 'full-size' it can be scaled to anything you (or a friend) might desire. As another comment (now I use CAD extensively) I just scale (in G3) directly to 1:22.6. This is easy to do and exact.

So in summary, in my experience it's as quick (if not quicker) to simply redraw your image, rather than try to resolve the various issues that scanning or tracing bring with them (such as misaligned edges and dimension variations caused by enlarging a smaller image).

Not the answer you'd hoped for (I'm sure) but the only one I can offer.

Regards,

IanT
Nothing's ever Easy - At least the first time around.

Offline Andy B

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Re: Blue print re-scaling
« Reply #2 on: March 02, 2016, 01:02:46 PM »
Tim,

I'll echo Ian's comments to a large extent.
50 years on, I think the Skinley drawings should be considered to be excellent examples 'of their time' - but these days there are so many original works drawings available which provide a much more accurate starting point - especially if you are going to produce a CAD file.

But to directly answer you question, here is how I would do a simple conversion:
- scan the Skinley drawings at a high resolution (minimum 300dpi) in black/white or greyscale (not colour)
- import into GIMP (https://www.gimp.org/)
- Convert to negative
- clean up as much as needed / you can be bothered with
- work out the resolution (pixels per cm) you need to print out at 13.5mm/ft
- Print

Andy

Offline wolfstone

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Re: Blue print re-scaling
« Reply #3 on: March 02, 2016, 03:56:20 PM »
Thanks Ian and Andy. Not quite as simple as I thought it might be.

tim

Offline MikeWilliams

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Re: Blue print re-scaling
« Reply #4 on: March 02, 2016, 08:52:32 PM »
Perhaps I'm missing something here but Tim didn't mention CAD. He did ask "how these could be copied and re-scaled to G3".  Is the answer just a photocopy machine with an enlargement scale which goes up to 337%?  If can also produce a negative that's even better but not essential.

I also happen to dislike Skinley drawings because those of older vehicles can we wildly inaccurate, but I believe those of more modern (i.e. post-Grouping!) stock are a lot better.

Mike

Offline wolfstone

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Re: Blue print re-scaling
« Reply #5 on: March 02, 2016, 09:15:40 PM »
Well spotted Mike  your right it's almost a case of "what's CAD" the copier is more my degree of but ability.
Tim