Metric Conversion for Technical Drawings

Recently acquired many drawings from Germany and other European countries. If course, all the parts, etc are dimensioned using metrics.

I remember reading, many, many, years ago, that if you use the metric dimensions and put them over 32, changing it to a fraction, you will construct the design about 20% larger than originally drawn. For example, if a part is 5mm, then 5/32 = 0.156 is the Imperial conversion, plus 20% enlarged, IF the finished size of the project is not critical. All dimensions are converted into fractions, and you don't have to figure out the direct metric/Imperial conversions.

The above is TOTALLY by memory and may not be valid. Disregard it all, if it's a crock. I did Not try this out before posting this question.

Are there any rules-of-thumb for QUICK conversions, assuming final size of object is not critical. I wouldn't mind if my next engine is 20% larger than drawn if it saves me time converting everything to inch equivalents.

Just asking.



Frank

Frank,
It will end up 20% smaller.

Hi Frank, in actual fact 5mm is 0.196". I was brought up on the imperial system at school and used it at work for a number of years. Australia changed to the metric system over 30 years ago and I find it a lot easier to use than imperial.

When I convert from imperial to metric I just multiply by 25.4 to get the result, when going from metric to imperial I just divide by 25.4.

This is all based on 1" equalling 25.40mm.

Paul.

You still need to convert the threads. Conversion is bit of a pain.

Jim

Not a crock but you have it backwards. making converting to sae from metric if you make 1 mm +1/32 inch the model will be smaller. 1 mm is aprox 1/24 of an inch. if you Want a bigger model make 1mm= 1/16 of an inch.
Tin

Yes -

/32 will go smaller (25.4/32 < 1)
/16 will go larger (25.4/16 > 1)

I'm doing a /16 treatment of a metric design at the moment. It makes things easier in terms of buying finished round stock, drilling holes, etc when you are dealing with the imperial units found typically in the USA.

Don't want to start a religious war, but I grew up in Australia (metric) and moved to the USA. In general metric is more systematic and rational than imperial, but for machining I've grown to like inches- using fractions has a nice, old-school analog-y feel to it....

Jason H.

Jasonh, et al.

In my rush to jot down what I thought I remembered, as usual I got everything ass-backwards. Thanks for the obvious corrections. Maybe I should think before I post.

You bring out the points that I tried to make. My memory ain't what it used to be, and I knew there was some advantage to using /16ths. That was what the original article tried to convey.

Any tool that I will use will have dials, etc. calibrated in thous. of an inch. I don't even like to read drawings with some dimensions shown in fractions, and some in inches. Metric is probably easier for some, but, in my world, not so much.


Gentlemen....Thanks for helping me, and correcting my errors. Much appreciated.


Frank

In México the legal system is metric but our proximity to The USA make us live un a mixed system where we have to use both systems. I certainly prefer the metric system but I have had to equip my shop with both metric and imperial tools. Metric drill bits for instance are more expensive than the imperials and harder to find. Both my lathe and mill have imperial dials ... it was a bit frustrating for me not being able to purchase a metric machine in a country were the metric system is the legal one.
Therefore I have to convert units all the time. A trick I oftenly use is to make a rough conversion: 0.040" is about 1 mm which works well for rough cuts, when approaching to the final dimension I switch to a more exact conversion: 0.039" = 1 mm. With this little trick I've been able to be happy with my machines' imperial dials.
Kozo Hiraoka mentions something about this matter in his wonderful (metric) Book "Building the Shay". Making an inch equal to 32mm makes a model 20% bigger than the original but the conversions become a piece of cake.

Wow. I just click on document properties, then click on units, then click on "mm" or "inch"

On the shop floor though... we use 25.4

albertorc19 said:

Making an inch equal to 32mm makes a model 20% bigger than the original but the conversions become a piece of cake.

Click to expand...

Albert, how does that work in practice? Assuming a dimension on the metric plans of 5mm, what would you convert it to?

If you are going to go to the trouble of changing from metric to imperial by doing a calculation you may as well just do the direct conversion and get the results in thousandths of an inch, then use the handwheel dials that you already have. Its no harder to make a part 1.968" long than it is 3.125" long

The only things to change for ease of construction would be stock diameters and fixings for example a 5mm shaft would be best changed to 0.1875" or 3/16" but the length kept as the direct conversion. Same with threads the M5 would convert to #10x32 on the 3/16" shaft.

The fraction conversion does not work in all cases to give convenient sizes. Say you are using the metric/16or32 method and the drawings specify a standard metric 15mm rod, where are you going to easily get 15/16" or 15/32"stock?

J

Not sure where 20% came from for 32mm instead of 1":
32mm / 25.4mm (1") = 1.2598 = ~26% increase.

Using 5/32" as a substitute for 5mm dimension:
5/32 = 0.15625
* 25.4 = 3.96875 mm ...
3.96875mm(5/32") / 5mm = 0.79375 = ~80% of original size.

Same as above but with 5/16":
5/16 = 0.3125
* 25.4 = 7.9375 mm ...
7.96875mm(6/16") / 5mm = 1.5875 = ~59% increase of original size.

Making an inch equal to 32 mm makes conversions easy when the final dimensions are not important and you are fine with a bigger finished model. This is: 1/32 = 1mm, therefore 5/32 (0.156 in) in the plans means a piece of 5mm (0.197 in) in the real world

Working the other way around you'd have to make 1mm = 1/32 and you'll end up with a smaller model, so 5mm (0.197 in) in the plans translates to a piece of 5/32 (0.156 in).

Of course, there will be issues that are not as easy to solve such as screw cutting, tapping, etc. I think that there are other potential issues with this method: a 0.5mm dimension means that you have to work in 1/64th and if you find a dimension such as 1.7mm then the conversion is not longer straighforward.

Sometimes I make copies of the imperial drawings, erase the fractions with liquid paper and write all dimensions in mm or in thousandths of an inch, this means that I do all the conversion job before I start to cut metal. Of course I have to check twice to be sure that I did everything right. With this method you can have a model with the original dimensions.

My experience with all these different systems can be a bit maddening sometimes: I plan to purchase a Stuart model but first things first I bought the construction manual from Camdem, after a few enjoyable afternoons reading the little book I realized that I'll have to buy BA taps and dies if I want to use the screws that come with the kit and I'll have to import them since there's no chance I can get them here in Mexico.

Yep, 1/32 = 1mm, not 1mm = 1/32.

Haha, I've edited this too many times, it's midnight and I'm not firing on all cylinders

Hello members,

I don't think there's a really easy way to do this conversion.

Living in the metric world I have to do this conversion always from imperial to metric, therefore my experience with doing it in the reverse direction is nil.

There are proposals to convert 1 inch to 16, 24 or 32 mm. While each has some benefits, each also has large disadvantages and problems. Besides this, the only system to keep the size close to the original is 1" -> 24mm, as everything else will significantly change the size You are getting in Your build.
This may be less important in model building, but in any other application it will matter.

Lets see some examples:
1" to 16 mm system

1" gives 16mm. Fine for bolts, M16. If You need a ball bearing, no luck, nearest size 15mm. Square bar stock in 16mm? Difficult al least to buy.

2" give 32mm. Bad for bolts, M30 and M33 being the standard. No ball bearing available either. No bar stock in 32mm available too.

1" to 24 mm system

1" gives 24mm. No problem with threads, M24 being a standard. ball bearings - no. 25mm is the size. Any bar stock in 24mm? No, except in hex bar, standard size is 25mm.

3/8" give 9 mm. You are completely off. No threads, no bolts, no raw materials this size.

You can choose any system, at some point You get stuck. Exchanging bolts and threads usually isn't too difficult. What I find more challenging is raw material acquisition. No chance to get a 3/8" round silver steelbar. No chance to buy 3" square tubes.
How do You overcome this? Every raw material turning and milling to correct size before really starting the build? :fan:

I don't believe there is an easy way to do these conversions. :wall:

Mike

I don't know what plans you bought, but everything related to volume will grow (or shrink) with different mathematical power. Such as mass, inertia, flow, and so on. eg when simply scaling a model engine, the result won't be as similar as expected. This influences directly compression ratio, balancing, valve timing, carburetor size, high voltage insulation, and much more.

Wow! So much discussion... I've never put this much thought into it... it's terribly simple.

0.001inch = 0.0254mm
1inch = 25.4mm

Just look at a gage block set: the big long 4 inch block is basically the same as the 100mm block.

Multiply or divide to get the unit you need.

Let's not overcomplicate!

But that does over complicate actually making the engine.

If say a shaft on the drawing is specified as 25mm dia, does the builder really want to turn down 1" stock to 0.984" or if that were the bore where to source 0.984" O rings.

Once you have done a few conversions it becomes second nature to know what sizes to do an exact conversion of and what is the nearest substitute for easily available materials.

J

jwcnc1911 said:

Just look at a gage block set: the big long 4 inch block is basically the same as the 100mm block.

Click to expand...

Basically, JW

Just imagine, You supply the 4 inch sliding bearing, and I supply the 100mm shaft. Do You think these two parts will form a winning team ?

Mike

If you do the conversions, yes. They are part of separate sets. I didn't say they were the same, but comparable. No matter the unit, you still have to preserve the dimensions given.

Either I was unclear or you misunderstood.