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Converting fractions to decimals

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Tim Stevens16/08/2021 11:53:16
1623 forum posts

I have just downloaded and printed a page from the web which might be of help to others. It would be an ideal tool for anyone faced with converting an old drawing to something a modern young engineer might get to grips with :

It is a chart of fractional inch sizes (down to 64ths) to millimetres (and four decimal places) for all dimensions from 0 to 12", and then in 32nds to 24".

Find the Koken UK site, and their PDF catalogue - page 314 gets all this detail on one side of A4. (NB the page is numbered 314 but my printer counts it as 316)

And the catalogue includes lots of useful tools, too.

Cheers, Tim

AlanW16/08/2021 12:05:52
92 forum posts
12 photos

A useful find Tim. My calculator is usually buried 'somewhere' under a pile of useful stuff (rubbish).

The .pdf reader reader recognizes the un-numbered cover and inside cover, hence page 316 rather than 314 as printed.

JA16/08/2021 12:41:38
1405 forum posts
81 photos

I have just started on this task in the last few days. Using a calculator it was becoming a pain. Then I found my old slide rule which is far quicker and gives good enough accuracy.

I will take a look at the Koken UK site.


not done it yet16/08/2021 13:25:39
6889 forum posts
20 photos

JA, you are spot on. 1/64” is a measurement to only plus or minus 1/128” (unless a tolerance is provided). So the possible error could be as much as 8 thou (~0.2mm). Not many of those conversions would be needed to four decimal places, in mm - and not many on here would be machining to that precision, methinks.

Henry Brown16/08/2021 13:27:15
559 forum posts
119 photos

Zeus book is my go to for that if I'm in the workshop, if I was marking a drawing up the large text would be handy...

Paul Lousick16/08/2021 13:44:11
2078 forum posts
728 photos

If I am working on a drawing which is on my computer (eg. converting and old imperial drawing to metric), I just use a calculator program on the computer called Calculator Plus, free to download from Microsoft .

It sits in the taskbar and always ready to convert vertually everything. Area, Lengths, Currencies, Power, Pressure, Volume, Velocity, Weights to almost every other form of that unit.

Calculator types are Standard, Scientific, Conversion

For Imperial fractions to mm, use the Conversion calculator, Set convert from inches and Convert to mm. Enter the fraction and hit the convert button and get an answer to about 25 decimal places.

If you are making a CAD drawing, (and you are like me with a bit ol oldtimers memory loss) you do not even have to remember the calculated answer, use the calculator copy function and paste the answer into the CAD dimension.


duncan webster16/08/2021 13:48:22
4127 forum posts
66 photos

If using cad you can draw it in fractions but make it do the dimensions in decimal inches. You can also make it put the metric equivalent alongside

JasonB16/08/2021 13:52:38
23076 forum posts
2771 photos
1 articles

If I'm doing it CAD I just enter the dimension in imperial and the program does the rest giving me the dimension in metric to whatever decimal places I'm working to. just need to remember to put " at the end of the entered dimension which can be entered as fractional or decimal imperial as you don't need to enter mm if the default is metric

As I often change the sizes of an original model I just make a simple spread sheet that gives me both the conversion and scale alteration as a single metric dimension. Then I decide whether to use that or alter to nearest nominal metric size.

Ramon Wilson16/08/2021 14:15:01
1401 forum posts
448 photos



Edited By Ramon Wilson on 16/08/2021 14:20:03

Ramon Wilson16/08/2021 14:19:35
1401 forum posts
448 photos

Sorry, but I'm afraid I can't quite understand why using a simple basic hand held calulator is so challenging - I really have been doing this for years

For example

'one' divide by 'sixty four' tapped in = .0156 and mutiplied by 25.4 = 0.396mm if you prefer to work in metric

First three digits are usually good enough but if 15 thou isn't you can go the extra 6 tenths or, call it 16 thou and make it 4 tenths oversize!

I always find going through a drawing that is fractionally dimensioned to convert them an opportunity to see how the drawings fair dimensionally and potentially spot any errors.

The (currently in abeyance) marine engine drawings were all fractions - After deciding to undertake it at 5/8ths scale of the original size I spent a few pleasurable hours converting the fractions to metric dimensions - dim x .625 x 25.4 - all the red ink annotation. The drawing in the forground is a secodary copy with just penciled dims added as machining took place. Don't suppose the major parts drawings took more than 3-4 hours in total broken down into several short sessions. The final, actual, dimension was worked to and not rounded up or down - that way the design is not changed.

marine compound  (5).jpg

CAD is no different just the need to draw or re-draw it adds more time.


JasonB16/08/2021 14:28:06
23076 forum posts
2771 photos
1 articles
Posted by Ramon Wilson on 16/08/2021 14:19:35:

Sorry, but I'm afraid I can't quite understand why using a simple basic hand held calulator is so challenging - I really have been doing this for years

The chart or a spread sheet simply saves having to keep punching the numbers into your calculator, simply look at chart and read corresponding decimal

Ramon Wilson16/08/2021 14:36:50
1401 forum posts
448 photos

With a calculator Jason, the answer is right in front of you - you don't have to move all over the chart to find the answer - have tried it but it's a pain in comparison - for me. '1' divide '64' = is just five taps - an onerous task for some perhaps but if one doesn't have time to do this what patience is left for the job


Edited By Ramon Wilson on 16/08/2021 14:37:36

JasonB16/08/2021 14:43:24
23076 forum posts
2771 photos
1 articles

But how many times do you have to tap in those 5 digits or 9 if adding in the scaling factor?

Do it once on a spread sheet and also include the 5/8th scaling factor and you get a sheet like this so simply look at it to see what say 17/32" comes out as in metric at 5/8ths original size.


Though for that particular job I would actually use 16th = 1mm  or 1/32 = 0.5mm and then you can do it all in your head and get round metric sizes 

16 to1.jpg


Edited By JasonB on 16/08/2021 14:52:39

Tim Stevens16/08/2021 14:43:24
1623 forum posts

And of course, calculators never need batteries ...


SillyOldDuffer16/08/2021 14:51:04
8906 forum posts
1999 photos
Posted by Ramon Wilson on 16/08/2021 14:15:01:

Sorry, but I'm afraid I can't quite understand why using a simple basic hand held calulator is so challenging ...

Bad memory, finger trouble, distractions and error checking.

Spreadsheet is my preferred method. The numbers are typed in, remembered, and can be printed if required. The calculation is applied once to the whole column, and the result posted next to it. If necessary a third column can convert to millimetres or inches, and a fourth used to back convert to the original as a cross-check. Best of all, three weeks later, when a mistake is suspected, it's all there ready to be diagnosed. Good for up to about a thousand entries, beyond that consider a database, but I rarely tackle big projects.

Many CAD packages support spreadsheets too, which saves a lot of time when a drawing error affects several dimensions.

End of the day though, whatever technique best solves the individual problem. No point wheeling out a computer for a few conversions. Pre-tabulated answers save typing, but look-up errors are more likely, and it's down to the operator to manage them. Calculators suffer finger trouble and have no memory. Redrawing is most time-consuming except I find it helps me understand the design and the conversions are almost a side benefit. Slide rules and Nomographs are remarkably effective - faster than pre-tabulated, but less accurate.

Proper record keeping is an engineering skill in itself. In theory, I take careful notes. In practice, I don't. Shameful.



Ramon Wilson16/08/2021 15:31:47
1401 forum posts
448 photos

Well each to his own of course, but having done it for so long it simply doesn't seem an issue to me. And, as said. it gives you the opportunity to 'read' the drawings for mistakes beforehand - and we've all heard about drawing 'mistakes' before haven't we. Charts and spread sheets are of little help there I'm afraid especially after the part has been machined.

Jason - you are showing off your computer skills now - whilst I do know what a spread sheet is I have absolutely no knowledge of how to create one - and no I'm far too past caring to want to know

Bad memory Dave ? is that the user or the calculator laugh. Where best placed I tremor to ask is the dimension to be - on the drawing of course - one calc, one annotation in red there for the rest of the build - simples.

Finger trouble - well agreed but that can occur when putting the dimensions in a spread sheet

Distractions - so what's different twixt calc and anything else - like shaving that last tenth of a crank shaft - distractions are a way of life

Error checking - well if you don't put it in the calc right it's going to be wrong but I'd question if you weren't sure about making a correect calculation to that extent then perhaps stamp collecting might be a better alternative

Ah Tim, what can I say, flippancy at it's best but you are right of cousre - my simple Tesco bought calculator doesn't need batteries at all - it's 'solar' powered and charges off the lighting. Well if doesn't I don't know where it gets its energy from - the batteries have lasted well past the externals of said calc and it never - ever - sees daylight!

We will all have our favourite way - I'm just stating that a calc is fine for me and can be for others - if they choose.


JasonB16/08/2021 16:16:46
23076 forum posts
2771 photos
1 articles
Posted by Ramon Wilson on 16/08/2021 15:31:47:

Well each to his own of course, but having done it for so long it simply doesn't seem an issue to me. And, as said. it gives you the opportunity to 'read' the drawings for mistakes beforehand - and we've all heard about drawing 'mistakes' before haven't we. Charts and spread sheets are of little help there I'm afraid especially after the part has been machined.

The chart can be used to get the numbers to write in red, I see no difference in using a chart or calculator to arrive at the figures to put on a print of the drawing so no difference in the ability to check for errors.

I would usually use one like that example of mine so that I can enter the correctly converted and scaled metric size straight into my CAD model as I redraw the whole thing from the old imperial drawing. I can then do far better checks on the CAD as the assembled engine can be turned over and I can see if things like the conrod or piston hits, that the valve moves correctly and so on. Also a function that will highlight any clashes eg where a shaft is assembled going through too tight a hole.

Another variation is that if working from say an old general arrangement that has been copied a number of times you can find that they get distorted so in a case like that I may have too columns one for what is scaled off the image horizontally and another for what is scaled off vertically.

That's why I personally would choose to possibly use a spreadsheet or chart over repeated punching in of numbers on the calculator. I am not trying to change anybodys ways but simply answer the "why would anyone"

I'm still likely to reach for the calculator in the workshop if I need to convert to decimal or from metric to decimal inches to suit the lathe handwheelssmiley

Edited By JasonB on 16/08/2021 16:30:24

noel shelley16/08/2021 17:45:54
1448 forum posts
23 photos

Calculators, Batteries ? Mines solar ! Doing my best to save the planet ! If it's so dark It doesn't work it's time to go to bed ! Noel

not done it yet16/08/2021 20:06:09
6889 forum posts
20 photos


You might note that 1/64*25.4 is the same as 25.4/64. A few keys saved and, if 25.4 happens to have been saved in the memory, 25.4 is only one press of the memory return key.🙂. If your calculator has a ‘constant’ key, things can be even easier/simpler/shorter/quicker.

I still like slide rule. No batteries, so planet saving. If it is dark same thing, go to bed!

Solar power can be stored in suitable power-packs, so one can continue in bed. Not quite doing without batteries, but still using solar energy 100%. Might also need a little more in the way of solar cells, mind.

I charge our power-packs (rechargeable batteries) from solar, for later use of the stored energy.

Ramon Wilson16/08/2021 20:10:33
1401 forum posts
448 photos

Well the colour really is acacdemic of coursewink it's just an indication that the drawing dimension has been changed/scaled decimalised to suit ones preference.

Theres also the fact that a spread sheet/conversion chart does not add a series of dimensions together so if it's out with a calculator for that then why not use for all.

For me this all stems from my first experience in a jobbing shop where one rapidly learns two things - mistakes are extremely unwelcome and time is money. With varying metric/imperial machines to be tasked to work on moving from one to the other on a daily/weekly basis meant a constant change of measurement too. The quickest way was to redimension the drawing to suit the machine dials and use the appropiate measuring kit to suit. Punching a dimension into a calculator was the simplest and quickest way to do so - scanning charts for the conversion - even on the side of the micrometer was a much slower process.

It's also easy to forget that such things as CAD, spread sheets and even DRO are only relatively new - on the scale of things.

I retrained as a machinist in 1980 - not one of the ten mills had a DRO and the first firm I worked for only had them on one of two Beaver mills. The second only had it on the Bridgeport.

Like the carbide thread - I'm not saying one shouldn't use such modern aids but I am saying one doesn't need to.

Just a point Noel but as said it was Tesco's cheapest - no way it was bought to save the planet

Sleep tight


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