Building from imperial plans in a metric world

[timothyb]

I've been looking over various plans and images of plans on the 'Net (Google Images has all sorts of interesting tidbits, but a lot from model aircraft). Obviously any older plans or US-originating plans are going to be in inches & feet rather than mm/cm/m. If you need to source spruce (or say Qld hoop), or metal tubing and the plans all call for 1", 1.5", 2", 2.25:" etc. etc, do you buy and cut wood down to plan specified size or just oversize when it comes to metal? Even the EAA table is all specced in inches and feet - http://www.eaa1000.av.org/technicl/worktabl/tablefig.htm

 

For those interested in looking at plans, I've also discovered the Aircorps Aviation Library which has some free images of WW2-era plans.

 

Tim

 

 

[timothyb]

Aircorps Aviation Library at https://aircorpslibrary.com/

 

 

[old man emu]

It doesn't really matter when using materials whether you use stuff sold by metric or imperial size.

 

Just remember that 1/16" = 0.0625" = 1.5875 mm, or 1/16" = 1.5 mm. Then:

 

1/16" = 1.5 mm

 

1/8"  = 3 mm

 

1/4"  = 6 mm

 

1/2"  = 12 mm

 

3/4"  = 19 mm

 

1"     =  25 mm

 

These approximations are just a poofteenth out, but over a 10 ft length using 1" ~ 25 mm will have you 2% (60 mm) shorter than using 1 " ~ 25.4 mm (60 mm)

 

Since quality hardware used in aircraft of all types is mainly made in the USA, then get yourself some rulers marked in imperial units and learn the duodecimal system, and its  common lengths in divisions of 1/32"

 

 

[timothyb]

Great info. Thanks and marked as informative. 

 

 

[onetrack]

That's a first world problem I never even thought about before. However, it's been one that has been with us for a long time, and it'll be with us for a long time yet.

 

Interestingly, I was reliably informed about 3 decades ago, that all the major American (read, global) manufacturers factories were set up to go metric, at the drop of a hat - but the U.S. Govt has refused point blank to change, citing the major cost to smaller businesses.

 

I think sticking to inch measures is more to do with American stubborness and a desire to "set the standards". They hate metrics with a passion, probably believing the system to be a French and German plot, designed to snatch technical leadership from the U.S.

 

However, we have all worked with dual systems since the mid-1970's. Every now and then, there's a push to make Imperial (inch) measurements totally obsolete - but it always fails.

 

I can remember one time, there was push to ensure all tapes sold in Australia were metric only. The push failed.

 

The bottom line is, it is only when you start to deal with metric sheeting and tubing dimensions, that problems with inch plans start to materialise. 

 

One of the major problems is that many metric-built items are the "near-equivalent" of the original inch dimensions - but 9 times out of 10, the metric item is smaller in diameter or thickness.

 

This "smaller metric size" could easily lead to disaster if the dimension is tubing or sheeting related, and over a sizeable area or length.

 

It's easy enough to keep threads and fasteners in inch sizes. Finding A/F tools is getting harder.

 

I repair/restore moderate numbers of older plant and equipment that is usually American in origin. Some of it is also much newer, but it's still largely inch measure.

 

I spent all my working life owning, working with, and repairing, American-built equipment. My toolbox is still three-quarters A/F tools, and my workshop fastener reserves are largely inch sizes.

 

I was educated in the 1960's and fortunately learnt both systems. I can swing between metric and inch sizes readily and easily - your brain remembers all the equivalents. 

 

Not surprisingly, one often finds metric fasteners and metric measures in American-made equipment today, due to global manufacturing sourcing components from multiple countries and sources.

 

It's the mix, is where the problems start. Looking at something you've picked up - is this item, metric or inch? You need to measure carefully. Mixing metric and inch threads is a nightmare, and this has caused problems in the past.

 

If the build plans specify a certain inch dimension in tubing or sheeting that is a critical component, you really need to source that material in the specific dimension stated, not use a metric equivalent.

 

When it comes to engineering supplies, there is usually a readily-available choice of inch or metric dimension materials.

 

 

[red750]

The only metrics the Americans adopted readily was $ and ¢.  

 

Last weekend we were looking at a journal written by my late father in law, with amounts written in £/s/d. My brother in law, who is 60, had no idea how to add up the column of figures.

 

 

[derekliston]

Used to be fun in the UK years ago when you needed three sets of spanners , imperial, the sizes of which made no logical sense, AF and Metric. Not quite so bad nowadays but the plans for my Zenith CH701 did suggest I would need ‘an 8ft metric tape rule!!!’

 

 

[Yenn]

When I built my RV4 I ordered a yard long imperial ruler, just to make life easier. that was 2010 and it still hasn't arrived at "Tool Supplies" our local tool experts. I made do with inch / metric tapes and uase a calculator to work out all the measurements as I went. No real problems. If the design is good most of the sizes will be standard for the materials, such as 4130 steel tube or 2024 aluminium. anyway and no doubt most of it will come from USA as we make very little in Australia.

 

 

[spacesailor]

The old inch size, "Whitworth" did make More sense than the others in that you measured the Head of the nut or bolt-head for the size of spanner to use,

 

All the bolt heads & nuts were made smaller for saving metal in the war era, And that's the reason it's got no sense at all!.

 

Iv'e come across metric threads that are not normal in their thread pitch size, and have to get specialist taps or dies to rethread the job in hand, & it's still called SI.

 

It's no longer Imperial as it's AF, american -fine, sized threads.

 

spacesailor

 

 

[Yenn]

Surely when you are referring to spanner sizes AF means across the flats.

 

 

[Old Koreelah]

What annoys me the most is how the USA is holding the rest of the world back. It has used its dominance of the market and world aviation to force its medieval measurements on the rest of us. A few years back I had words with an American who was trying to sell an interesting engine to us. When I pointed out to him that we in Australia adopted the metric system decades ago, and suggested that he adapt his marketing to our needs, he arrogantly claimed the whole aviation world uses imperial measurements and that pounds, ounces, feet and inches had got Americans to the Moon.

 

i wasn't quick enough to to reply that if the USA had been using metric measurements they might have made it to Mars by now.

 

 

[danny_galaga]

The USA use metric extensively in the military and in science. Since a lot of Americans start off in the military I think over time there will be an influence in changing. After all they are world leaders in science and military 

 

 

[old man emu]

Just remember that 1/16" = 0.0625" = 1.5875 mm, or 1/16" = 1.5 mm. Then:

1/16" = 1.5 mm

 

1/8"  = 3 mm

 

1/4"  = 6 mm

 

1/2"  = 12 mm

 

3/4"  = 19 mm

 

1"     =  25 mm

 

These approximations are just a poofteenth out, but over a 10 ft length using 1" ~ 25 mm will have you 2% (60 mm) shorter than using 1 " ~ 25.4 mm (60 mm)

 

Since quality hardware used in aircraft of all types is mainly made in the USA, then get yourself some rulers marked in imperial units and learn the duodecimal system, and its  common lengths in divisions of 1/32"

I was using an approximation in the wrong place here. The inch approximations are a poofteenth short of the metric conversion. These small distances relate to lengths and diameters of bolts, rivets, screws, washers and other hardware. When I said that a 10 ft length would be 2% shorter if the approximation 1" = 25 mm was used instead of 1" = 2.54 mm, this error would not occur when cutting a length of tubing for an airframe. Obviously, if the plan said to use a 10' length, then you would cut it at 3048 mm, which can be measured easily.

 

If you can't find a ruler marked in the appropriate imperial divisions of an inch, have a look here https://www.printablerulers.net/ where you can download a PDF file that is accurate in scale and you can print the PDF and stick the paper to a rigid strip of material. I needed a ruler marked at 1/10th-inch so I printed a PDF of what I needed and stuck the paper to on old plastic ruler.

 

Most electronic vernier caliper readouts can be switched from metric to Imperial.

 

 

[Marty_d]

When I built my RV4 I ordered a yard long imperial ruler, just to make life easier. that was 2010 and it still hasn't arrived at "Tool Supplies" our local tool experts. I made do with inch / metric tapes and uase a calculator to work out all the measurements as I went. No real problems. If the design is good most of the sizes will be standard for the materials, such as 4130 steel tube or 2024 aluminium. anyway and no doubt most of it will come from USA as we make very little in Australia.

I've got 2x 600mm steel rulers, they have metric on one side (mm one edge, half mm on the other) and when you flip 'em they have inches on the other side - including a section that has 16ths, 20ths, and 32nds.

 

Bunnings has heaps of them - no need to send away!