Converting Imperial to Metric

[DaveRC]

Hi All,

I have in front of me my first set of plans, an Upshur Vertical Single, arrived yesterday in the post, all very exciting....!

Good looking set of plans and looking forward to starting this in the new year.

So first question, of I would imagine quite a few, my first job, I am going to go through all the plans and convert everything to Metric.

Is it just a case of multiplying everything by 25.4, and how accurate do you need to be, what decimal place to you go to, for example there is a distance of 0.406 on the plan, X 25.4 = 10.3124 would you round this off to 10.3, I am not sure my skills could get much closer then that.

Also, converting threads to metric, so far I have found 4-40, 10-32, 2-56, 5-40, I actually have no idea what these mean, is there any "nearest equivalent" table kicking about?

Thanks in advance, I know these are quite basic question, I am sure they will get more technical as I go on.

Dave

 

[ruzzie]

Hi dave
As long as the mating holes all line up it wont matter how many decimal places you use.
Here is a link to a thread chart

#4 x 40 = OD 0.1120" & OD 2.8448mm

http://en.wikipedia.org/wiki/Unified_Thread_Standard

Hope this helps

Merry Christmas All

 

[DaveRC]

Spot on, good chart, that will help me.....

Thanks


Dave

 

[arnoldb]

Hi Dave

Yes, just multiply all measurements by 25.4
As to rounding, it would be best to round to 0.01mm even if you think you cannot machine to that level yet, and strive to hit those 0.01s while machining; it will make your work much more accurate, and you'll soon find that hitting it is not that difficult as you build up experience. For a start, at least try to hit 0.02mm accuracy, as that is about a thou.

I've found that storing 25.4 in my calculator's memory works a charm for converting the values - for example 1 3/16" on the calculator: "3 / 16 + 1 = x RM = "

For converting threads to metric, you can use the sheet Bogs has posted - It is here - I normally look up the closest metric equivalent that will suit me; depending on the piece either larger or smaller. For example for a #4-40 replacement I usually use M3, but if things look tight I'll drop down to M2 (I don't have M2.5 taps & dies). Watch out for screw/bolt head sizes to make sure you'll have adequate room for fitting them and for jobs like counterboring and so on.

I regularly use metric equivalents for shafting sizes etc as well - for example whenever I see a shaft size of 1/4" I'll use 6mm rod. Just be careful to check through all the parts that will be fitted to a shaft if you do this and take care to size the holes appropriately - for the 6mm example crankshaft, one has to make the bearing holes, the flywheel hub hole, and if needed eccentric hole all 6mm.

There's quite a bit of leeway on some sizes - but always try and identify the critical dimensions of an engine and get those close. If you do divert from sizes just check carefully how other parts of the whole engine will be affected.

The first couple of times you sit and convert a plan to metric can be a bit daunting - but once you've done a couple it becomes easier and is just a minor chore that is far overshadowed by the satisfaction of getting a running engine in the end

Good luck with your build Thm:

Kind regards, Arnold

 

[Ken I]

I call that metricising (simply multiplying by 25.4).

Personally I would redesign it - staying close to the original but then converting to metric threads and metric sizes.

Example a 1/4" shaft is 6.35 - not much use in a country that seldom stocks material or reamers in those sizes.

So change it to 6.0mm etc. etc.

Sorry to be a wet blanket but I find metircised drawings a PITB - when I see "funny" metric numbers - I divide by 25.4 and check out the fractional equivalent - Ahhhh ! now I know the original was in inches so I end up having to convert back to inches - so I know what it was.

Consequenly I just don't see the point - work in inches or metric - I don't care, I'm happy with either - but consfusing the two is pointless.


I'll go back in my cave now.

Regards,
Ken

 

[jonesie]

what i do is just change the print i am working on or am going to work on, so i do not feel like all i am doing is changing number for a long time, or i do it while i am watching the news. jonesie

 

[Tin Falcon]

The guys are right just a case of multiplying. but. you will inevitably end up with odd numbers to deal with.

another method to convert plans involves a little bit of scaling up or down. You simply make 1 mm = 1/16th or 1/32 of an inch . (yes I know it is actually (1/25.4) but you get even numbers and stock sizes come out to standard sizes. an option to consider.
Tin

 

[Ken I]

Make 1/16" = 1mm and scale down (whatever) - damn - that's a good idea.

Never heard of that one before. So obvious and simple.

Nice one Tin.

Ken

 

[Tin Falcon]

that is cool isn't it not an original Idea it is from an old home shop machinist magazine.
but making a 6mm shaft into a 3/8 shaft or a 3/16 shaft the numbers come out even and you do not have to custom size everything.
Tin
Sorry metric to imperial instead of imperial to metric but this will still work .

 

[DaveRC]

Thanks guys, the is some good tips in there, I like the idea of the scaling, seems so simple.

I have worked my way through a couple of the sheets and in doing this I am getting a good idea of what fits to what and what are the critical conversions and what I can change to make it simple.

Thanks again,


Merry Christmas

Dave

 

[Tin Falcon]

the late great Elmer Versburg encouraged folks to redraw each part . this help to understand the part. also if drawn on an index card easy to have the print next to the machine without damaging the book (if you are luck enough to have one) or print.
Tin

 

[prof65]

Hi Dave

I always convert plans with a ratio of 1" = 24 mm.

1/32" = 0.75 mm, 1/16" = 1.5 mm, 1/8" = 3 mm, 1/4" = 6 mm... the size of most parts matches exactly the standard size for bar stock and drill bits; the engine is just 5.5% smaller than the original one.

I built 5 engines from Elmer's plans this way, works well for me.

Merry Christmas

Roberto

 

[Ken I]

1" = 24mm

Oh man - two great ideas in one day.

It must be Christmas.

Ken

 

[chbeyer]

Hi,

if you are lazy and don't mind things getting a bit larger you can also assume 1" to be 32 mm the advantage is obvious I think

cheers
christoph

 

[Ken I]

I'm an experienced engineer - not an experienced modeler.

So it never crossed my mind to deviate appreciably from the imperial sizes.

But WTH we're making models - pick a scale - any scale.

An elegant solution - thanks for the comments.

Re my earlier comments - if I find a drawing with (say) a dimension of 23.81mm diameter ??? what on earth is that ?? divide by 25.4 = 0.9375" - Hmmm maybe I'm getting warmer - what's that as a fraction ?? some trial and error produces 15/16" AH-HAA now that reamer I have.

Now would someone explain to me how that "metrication" made my life easier.

Metricate it properly or leave the drawing in inches.

Ken

 

[Loose nut]

It would probably be easier to sell all your metric tools and equipment then rebuild your shop with Imperial stuff. It will save you the aggravation of doing the conversions :big:.

 

[doubletop]

I'm with Arnold B on this and it is modeling so go with what you've got. Most of my tooling is imperial except my 1930's lathe is German, so metric. My new mill is metric. I've got digital calipers which do both and a calculator is always handy for the conversions. If you have a copy of the Zeus manual all the drill sizes even for the imperial threads are given in metric

As for thread sizes I tend to use what I have or is available BA or metric, but not being from the US never Unified. You have the equivalents table so off you go.

Pete

 

[mzetati]

If I had access to Imperially sized tool and especially stock material, I'd do Imperial works without any conversion.

As it goes, I'm usually converting measures from imperial to metric, now with the aid of a cheap 'euro-converter' I bought on a street market a few months ago: I stored in it the new 'rate' of 25.4 and it now automatically converts any GB Pound I key in into metrical Euros
That made, I usually select the nearest metric stock I have at hand, and change the sizes accordingly: I mean, a 6mm shaft wil do in place of 1/4" one, providing the bearing hole..., or a 12mm plate instead of a 1/2".

Threads, expecially on the smaller sizes, show an additional problem: there's quite a good choice of metric threads among 1mm and 3mm...
No need to tell what happens when I trust my memory about the thread chosen for the already made matching part, instead of checking again.

Marcello

 

[hopeless]

all this angst over a simple problem. ??? I grew up with imperial measurments and coinage but then Australia Government) decided we should be metric and in 1966 we got decimal coins and over time the rest of the metric system. I can use either so plans can be whatever the designer decides and I then make it to that or adapt easily using bolts or make them using taps & dies I have on hand such as BA or ME etc. As for measuring I have both available for both metal & brown stuff so no real problems in the modelling world. Just use the materials you have or can easily purchase/borrow/beg/steal ;Dand go for it.
Cheers
Pete