Converting fractions to decimals

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I haven’t got the will to argue about this, Duncan

Like I said in the first place: Each to his own

I just thought people might be interested

… and, as I have already stated: The example was trivial … the App gets increasingly useful when you are doing more complex calculations.

MichaelG.

I wonder how many people have machine dials or micrometers marked in fractions. That would mean boring that 11/64" hole to a +/- 0.0005" tolerance is always going to involve some futzing around with a calculator. Or a conversion chart - which gets harder to read the more complete it gets. Or log tables if you're some kind of luddite/weirdo/masochist.

My machinist grandfather offered this advice a long time ago: work in whatever units the drawing is dimensioned in - converting to your 'normal' increases your chance of cocking up. My machinist aunt repeated it a couple of weeks ago when we were talking about making stuff.

Edited By Nicholas Wheeler 1 on 17/08/2021 22:28:06

Well i have worn a shallow groove across my workshop with the new Hemingways knurling kit. All in Imperial. Looking up how many thou is this size. Worst thing is i have no imperial drills , so had to order a shiny new set. + borrow imperial reamers from a friend. Not so cheap a job. But more stuff for the workshop i guess.

Steve.

As a smart arse I have avoided all the angst by drawing everything that I make in metric, as I feel that inches, feet, fractions etc are best left in the realm of Ancient Oddities.

cheers

Bill

p.s.Basically though I agree with Tug.................

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[ says the man who never does the job ]

MichaelG.

[ says the man who never does the job ]

MichaelG.

Hmmm. Just as well that I'm not sensitive, have a look at my albums.............

cheers

Bill

Michael said,

[ says the man who never does the job ]

I can only assume you are refering to yourself there Michael - yes the topic has run on a bit but it's promoted some good comment from all points of view.

Nicholas said,

My machinist grandfather offered this advice a long time ago: work in whatever units the drawing is dimensioned in - converting to your 'normal' increases your chance of cocking up. My machinist aunt repeated it a couple of weeks ago when we were talking about making stuff.

With high respect to your grandfather and aunt's no doubted machining skills I can't agree that this is the right approach. It should be the machine being worked on that sets the parameter. Using imperial dimensioned drawings on a metric dialled machine without converting the dimensions is a fraught experience and vice versa. It compounds itself if the measuring kit doesn't match too but as said DRO eliminates it down to personal choice of the system preferred

That's based on a lot of working experience of this very issue - jobbing shops always had a myriad of machines in both and of course a constant supply of drawings varying daily including yes even the odd few in fractions! It certainly enabled me to decide that metric was far the better system (for me) to work in and apart from the Z movement on my old Linley mill all working is done in metric. - I get round the Z movement by using metric slips and a dead stop and of course reverse conversion using, dare I say it, a calculator

Regards - Tug

Feeling a little happier with my foolish self this morning because my mistake was trusting a calculator; ironic because my post was about distrusting calculators doing fractions.

I used Python because it supports Fractions and Decimals as well as floating point. Floating point arithmetic is super fast because the numbers are in binary, but this causes conversion errors. 25.4 is actually stored as 0.25399999999999998579x100, which can cause trouble in long complicated calculations.

Decimal numbers are exact - 2.4 really is 2.4 but they are slow.

In Python, I typed:

fd = 1 + Fraction(11,16)
fd = fd * Fraction(Decimal(25.4))
print( fd, fd.limit_denominator() )

which gives the slightly wrong answer 96517769514083937/2251799813685248, which was approximated by limit_denominator() to 3429/80

Doing the same sum in fractions throughout gives the right answer:

ff = 1 + Fraction(11,16)
ff = ff * ( 25 + Fraction(4,10))
print( ff, ff.limit_denominator() )

3429/80 3429/80

So my blunder was caused by whatever method Python uses to convert Decimal(25.4) into a fraction.

Although I dislike fractions intensely in engineering drawings, they are perfect for gear ratios: change gears and threads. In that context, Python's limit_denominator() function is good for finding approximations, for example:

Fraction(math.pi).limit_denominator(10) = 22/7
Fraction(math.pi).limit_denominator(100) = 311/99
Fraction(math.pi).limit_denominator(1000) = 355/113

The various methods discussed in this thread are just tools. Tug's calculator may be good choice for him but other tools are available, and could be exactly the right tool for others. Calculators aren't the ultimate calculating aid!

Spreadsheets are calculators on steroids, but harder to learn. Programming languages even tougher but can do almost any calculation. As I've demonstrated though, just like physical tools, the operator has to choose and use them correctly. Banging screws in with a hammer is rarely a good idea,

Dave

Measuring equipment is the key to this, not the machines! We're lucky that we tend to make small stuff, and that equipment is cheap. My machines are metric because I think in metric, when an imperial part comes up I press the button on the digital caliper and use an imperial micrometer.

Well, we'll have to agree to disagree on this Nicholas.

Size of workpieces is immaterial - like you my machines are all metric - now. I got so fed up of switching from lathe (imperial) to mill (DRO set to metric) I changed the cross and topslide leadscrews to metric. Though not quite so important if it had been possible I would have done the main leadscrew too.

Imperial drawing - metric machines - two sets of measuring kit? Yes, I have them both across a range of kit which hark back to those days (and early working ones too) but the imperial kit is virtually redundant and rarely sees the light of day.

Calipers, digital or standard are not what are used for accurate measurement in a machine shop. If you choose to do so that's fine but it's not for me. Measuring across a part in a lathe or mill with a pair usually leads to differing readings to take your pick from.

Sorry, can't agree on this one

Tug

Dave wrote

The various methods discussed in this thread are just tools. Tug's calculator may be good choice for him but other tools are available, and could be exactly the right tool for others. Calculators aren't the ultimate calculating aid!

Well I wasn't saying they are Dave but, WOW, looking at your impressive layout and explanation I'm glad I do not share such questioning mathematical skills. That's not being dismissive but recognising them for what they are and the fact that such would have an effect on what I want to do - which is, of course, - machining

As from the start if I'm faced with an imperial drawing then it's a matter of converting the dims to metric beforehand. The drawings can then be worked from the off and one can settle into the job equally so. The usual readily available calculator is, and has been, more than capable to convert the dims to the accuracy that will be worked on in any home workshop on the basic home kit likely to be found in one.

My interest is to produce a piece of 'model engineering' - I've always sought to do so in the most accurate terms using the most simplest approach. So far I'd like to think that has worked well for me.

To someone however, who was a dismal failure at maths, that's an impressive ability you have there

Tug

PS Agree about the Hammer though 

Edited By Ramon Wilson on 18/08/2021 11:12:53

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I was referring to the job of converting dimensions, Bill … and your own explicit statement.

I am in no doubt of your [determinedly metric] engineering ability

MichaelG.

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P.S. __ I did look at your albums, last night

Edited By Michael Gilligan on 18/08/2021 11:57:13

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For the avoidance of any further misunderstanding : Please see my response to Bill

MichaelG.

This looks be getting a little bitter and twisted, when we are taking of UNITS.

The same parameter can be dimensioned in whatever unit you like, Angstrom units, Light years, yards; rods, poles or perches; spans, or bricks, if that is what takes your fancy. Just depends on how many decimal points you are prepared to tolerate, even if you prefer duodecimal, or whatever..

Film speeds were measured in Hurter and Driffield, Scheiner, Weston, Gost, and ASA, all for the same material,

Before anyone says it, not all at the same time!

They are only units, you choose the one that suits best. Best to try to use the same units as other people, which is where Whitworth, Sayers, BSI and ANSI, etc came in.

Because my kit is all Imperial, I am an "Ancient Oddity" in all senses, but occasionally use Metric,WHEN IT SUITS.

My shop made Workshop Comparator incorporates 8-32 UNC, 5/16 BSW and half round 4 mm pitch 0.0625"deep threads, because

a ) the UNC and BSF fixings were available, and

b ) I wanted a coarse thread, and the lathe could be set to 4 mm pitch, into which I could match a 1/8" ball bearing..

They happened to be the horses that suited my courses, did the job, and do not need to be interchangeable with anything else....

Howard

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Thank you for trying to inject the Wisdom of Solomon, Howard … but the thread is [or at least was] really about converting between factions fractions and decimals.

MichaelG.

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Edit: __ corrected my Freudian slip of a typo 

Edited By Michael Gilligan on 19/08/2021 06:45:44

"Or log tables if you're some kind of luddite/weirdo/masochist."

I resemble that remark and only work to the "rough" sizes using four figure logs and not super accurate six figure versions, but I can convert ingrish to meterick without having to use t'nternet or Collosus and can still work to a "Proud" sixteenth or a "Shy" thirtysecond and it might not to be to NASA standards of accuracy but it works.

Regards, Wot tiller.

Edited By Circlip on 19/08/2021 09:05:08

Ah, but the subject has hidden depths. The discussion leads to the need for change, which is mankind's greatest enemy. Hard to teach old dogs new tricks because old dogs see no reason to change. This is why I'm handling my mother's hospitalisation by landline telephone while the rest of the family are in a WhatsApp group. I ring family to tell them the latest news and everyone in the group is already better informed than me. I resent it slightly, because they've been using new technology for years while I still hanker after dials, telephone directories and pressing Button B to get my 4d back in a red box smelling of wee.

Very upsetting - I have trusty old friends like Landline telephony, Imperial Measure and Fractions, and some young whipper-snapper barely out of nappies points out they're all actually a bit sh1t, giving reasons I don't understand or give two hoots about.

Our hobby sits on a fault line; is Model Engineering about retro-technology like steam engines, Whitworth and HSS on a 1947 lathe, or is it about Quadcopters, microcontrollers, CAD, laser-cutters and 3D-printing? I think it should be about both. I admire craftsmanship and enjoy my manual lathe and milling machine, but I'm also interested in state of the art methods and what's in the pipeline.

The history of engineering shows new techniques always win and it's a bad mistake to cling to the past. Discuss!

Dave

OK Dave, I'll bite!

Good, provocative thoughts. I thought the subject deserved its own thread, so I've started one, in the Tea Room.

Silly old duffer says:

The discussion leads to the need for change, which is mankind's greatest enemy.

I remind you all that without change (enemy or not) we would still be living in the sea along with lots of identical precambrian jellies.

Further - I had not anticipated the nooks and crannies into which the discussion would flow. I will try to think of something else as inspiring, in case this thread runs out.

Regards, Tim

This Moore & Wright booklet includes an extensive and very clearly tabulated set of conversion tables: **LINK**

https://ia802901.us.archive.org/26/items/moore-and-wright-micrometers/Moore%20and%20Wright%20Micrometers.pdf

MichaelG.