Or divide by 3/2 – ie divide by 1.5. All the same thing, mathematically.

Anyone got any suggestions as to how best do the conversion work? I think it depends on how many dimensions there are to do and how complicated the plan is.

For a small number of dimensions and a simple plan, I'd incline to a paper pad and pocket calculator × 0.66666, double checking before cutting metal. (Error prone, but not as bad as pencil and paper.) If one is available, slide rules are excellent for this sort of conversion. After setting the slide to × 0.666, all the answers can be picked off by moving the cursor. (Disadvantage perhaps only good to 2 places of decimals, which I think is plenty good enough.)

A lot more dimensions on a simple plan would have me tabulating them in a spreadsheet and applying ×12 ÷ 8 to the whole lot. (Less error prone, but have to know how to drive a spreadsheet. Another advantage is the figures are stored, can be checked by someone else, and corrected later if mistakes are found.)

A complicated drawing would have me redrawing the plan in the original scale in a 2D CAD package. (I like QCAD, others available.) Then I'd rescale the whole drawing × 0.666666 in one go. This method highlights errors very effectively, including mistakes in the original drawing. Disadvantages are the time taken to re-draught the plan and needing to know a CAD package. But it is fun making an entire drawing change scale at the push of a button!

As an ex-computer programmer I'd love to write code to do the job, but that's hard work for very little return in this case. Spreadsheets are a far better tool unless the number of dimensions to be converted is enormous (millions…)

Dave

Hi Brian, Did all my calculations from 1.1/2" (1/8) to 1/12th using the option you suggested in your question.

x8 to full size and down to 1"- ft. Much of it long hand (keeps the brain active), otherwise calculator into thou.

Do I presume you are downsizing Alchin Dims for your Minnie as well ?. .

Another way is not to convert it as such but make yourself a new measuring stick with marks made at 1.5 in intervals. Then subdivisions of fifths made by marking off every three tenths in real inches. Then put in the tenths by eye. Or do the same for fractions whatever you prefer.

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Yes I assumed it was Michael

Even then a fractional calculator won't always work as you get those sizes that don't suit such as 5/16" which would not easily convert down to a fraction. In cases like this decimal is better and depending on what th dimension refered to may need to stay as that or be adjusted to suit nearest available stock diameter as no body really wants to turn long shafts down to 0.208" as it's often better to opt for 7/32" stock.

I do also know of people who like to express all the imperial measurements on an old imperial drawing in metric so in brian's example that 0.375" on a smaller version of the model would be written as 6.35mm and that is the size they would work to, still 1/12th scale.

Well that's embarrassing! Good job I said 'Another advantage is the figures are stored, can be checked by someone else, and corrected later if mistakes are found'.

Thank god I don't do this stuff for a living!!!

Maybe I should have said practical fractions. Not many rules or drill bits made in those fractional sizes.

Edited By JasonB on 20/01/2020 20:26:11