Maths Question

[Gary Morris]

When I get my scroll saw I plan to make a toy truck / crane from a free plan I downloaded on the internet, in pdf format. When I printed a sheet (a simple circular disc) the printed diameter of the circle shows it shold be 100mm, yet the actual measurement is 109mm - 9mm oversize.

I know there's a percentage calculation that will give the % to reduce it to 100mm; my maths fail me, can anyone show me how to do it please? I can then reduce the % in the printer screen.

thanks

Gary

 

[marcros]

right...

109 minus 100 divided by the original (109) times 100 = 8.2569%

Then 100 - the above is 91.74%

That is your scaling factor.

There may be better way of doing it on your printer though.

 

[Gary Morris]

Thanks Marcros, thats what I wanted

Gary

 

[marcros]

oh, and always test it by doing 109*91.74% = 99.9966mm. probably near enough for wood!

 

[Jacob]

Or 100 divided by 109 = 0.9174

 

[Gary Morris]

Will do, thanks both. Commiting it to memory and paper - I get my scroll saw on Friday, I can't wait!!

Gary

 

[mseries]

Or 100 divided by 109 = 0.9174

times 100 to give the percentage

 

[Fromey]

Or should you be scaling according to the area? A circle of 109 mm in diameter has an area of 9331.31558 mm2 and a circle of 100 mm in diameter has an area of 7853.98163 mm2. Thus, the 100 mm diameter circle is 84.167999% of the 109 mm diameter circle.

 

[JakeS]

Or should you be scaling according to the area?

This is a good question - printers often do scale by area. Check by printing out a document at 50% - if it comes out covering half the page, it's scaled by area; if if comes out covering quarter of the page, it's scaled linearly.

(To work out the scaling factor you don't have to work out the literal area of a circle - a square bounding that circle will be scaled in exactly the same way, so you can simply do:
(100x100) / (109x109)
- that is, the area of the square you want divided by the area of the square you have (and again, multiply by 100 to get a percentage) to get the same answer Fromey gave you. ;-)

 

[Jacob]

The OP wants to scale linearly, to reduce 109 to 100mm.

 

[Fromey]

Think about it more Jacob. The object being scaled is not linear but two dimensional. Anyway, should be an easy thing to test by simply printing it!

 

[Gary Morris]

I measured the print out useing the 91.74% figure. The circle has a uniform dia. of 100mm The end view dimension (original 109mm H x 5.5mm W) came out at 100mm H x 5mm W (correct size, as per drawing)
For the purpose of this project I think 91.74% will be close enough.

thanks
Gary

 

[baldpate]

It may however be that the Print function of your PDF display software is automatically applying a scaling that you don't want - for example it is scaling the actual image up so it best fits your paper size. Before trying any 'manual' resizing, check the "Size Options" in the printer dialogue and ensure you have the "Actual Size" option (or whatever is your equivalent) selected - this may save you some effort.

 

[graduate_owner]

I'm not sure about the effects of printing, but basically to increase a quantity by say 10% you multiply it by 1.1 i.e.( 100 + 10) / 100, similarly multiplying by say 1.25 increases your quantity by 25%. To reduce a quantity by 10% multiply it by 0.9 i.e. (100 - 10) /100

This is the same as working out how much VAT to add - if something costs say £60, multiply it by 1.2 giving £72

K

 

[Fromey]

Well then I stand corrected (although I suspect the software is doing some interpolation so that it performs in this intuitive manner).

 

[Gary Morris]

Tried it at 'actual size' and it comes out at the larger size, (109mm) it's odd that they have drawn it slightly larger than 'actual size', thanks for all the comments

Gary

 

[JakeS]

Well then I stand corrected (although I suspect the software is doing some interpolation so that it performs in this intuitive manner).

I've seen printers behave in either manner - the linear scaling is obviously the easiest to program, while the area scaling is more like what most people want. As I understand it, it's entirely down to the printer drivers.

 

[thething84]

very few home printers will printer perfect size. You can get specific printers designed for drawings which print exactly correct. but these are expensive.

 

[DonJohnson]

Just for background info', Matthias Wandel sells a program called BIGPRINT - http://woodgears.ca/bigprint/index.html (about £14 sterling)

This enables the matching of particular printers to computers exactly for very high accuracy when dealing with problems such as described by the OP.

It also enables big prints to be made using several sheets of paper.

You can trial it for free, but it overprints 'Eval' on each page.