Thread: converting imperial to metric visa versa

It is better to work with the given measurements so you don't end up with weird and wonderful fractions. Having said that some times the scales on your machines aren't the ones you want and you still have to do the maths.
Newer British plans come with both measurements but warn you to stick to one or the other as they are not exact.
Hope that's some help
Regards

Thanks A duke, So even if i do convert them correctly it can still be out a little? So i guess i will just attempt to get used to the imperial measuring system then

The best in my opinion, just do enough to get your basic dimensions and then make every thing else to fit. Some of the old timers (even older than me) work with story sticks with out any measurements, or size one piece ageist the the next.
You also have to watch translations especially English to English like Australian Woodsmith they translate from the American article just changing the measurements and suggested timber, and even they don't always get it right.
Regards

There is a fair bit of crossover between the systems, not to mention blurring of the lines and the imprecise nature of wood.

Many of the 'metric' measurements are a simple metrification of an existing imperial standard, and in some cases you'll find the metric size on something as nominal, and the actual size translates back to the imperial measurement. Other times, it'll be a strict conversion to metric and the imperial measurement is approximate.

The approximations that I find useful are:

3mm = 1/8 inch
6mm = 1/4 inch
9mm = 3/8 inch
12mm = 1/2 inch
16mm = 5/8 inch
18mm = 3/4 inch
25mm = 1 inch

You'll note that materials like MDF are commonly available in sheet thicknesses of 3, 6, 9, 12, 16, 18 and 25mm...so most of the conversion is already done for you!

Many 150mm & 300mm rulers have conversion tables on the back for all common fractions up to 1" and whole inches up to 39". You won't need the accuracy of 4 decimal places, but they're handy to have as a reference.

It depends on what you are making, but for one off pieces dimensions on plans are generally suggestions only. Laying out individual elements should be done in such a way that all pieces that need to be the same length are marked together or off one another.

Even when using the same ruler I find if I measure things individually I'll mark at least one piece the wrong length. It has nothing to do with the unit system being use.

Originally Posted by A Duke

The best in my opinion, just do enough to get your basic dimensions and then make every thing else to fit. Some of the old timers (even older than me) work with story sticks with out any measurements, or size one piece ageist the the next.
You also have to watch translations especially English to English like Australian Woodsmith they translate from the American article just changing the measurements and suggested timber, and even they don't always get it right.
Regards

What A Duke said. I convert the 3 outside dimentions and just fill in my own as needed. I'm 65 Duke and use a storystick. All the important sticks hang on a loop of wire for future reference. At trade school we had to set out the whole job on the stick down to draw and panel size before cutting a job. NowI just fill it in as I go. It's a full size drawing of every component.

Delbs

There are plenty of metric conversion programs available to you when you "google" them.


Regards Mike

I'll add my vote to working in one system or the other. I grew up with Imperial, & had to make a conscious conversion to Metric. During my 'conversion' I found myself using both, and got into a few right muddles! I'm comfortable enough in either now, but work most often in Metric these days. Sometimes, I find myself digging out my few remaining Imperial rules when working on an old piece because they are usually built to multiples & simple fractions of inches, so I am not trying to remember multiple odd lots of mm. You can, of course, use any units, but my ageing noggin is just happier with the simplest posible numbers...

While it's easier to stick with the prevailing system for a number of reasons as the others have said, it's not all that important what the units of measure are. What is much more important is to develop habits of work that lead to accuracy, as suggested. Dimensions on plans are all too often wrong, & particularly those converted from one system to another. Murphy's law ensures the mistake is always in the most critical one! Always check before cutting if it's at all critical - it's too easy to mis-measure by 100mm or something equally foolish (DAMHIK ).

It's also important that you transfer measurements in a way that doesn't create errors. Use a marking knife where you can, for at least three good reasons: it makes a very fine line, you can use the cut at the corners to register the knife when extending lines all round a piece, which avoids the 'creep' you often get by following the line by eye, and the cut allows you to register a chisel exactly when chopping shoulders, etc. If you're marking something that will show, & you wouldn't be able to remove knife marks, use pencil, but make sure you use a sharp one. Wherever possible, harness the repeatability & accuracy of 'set' tools like marking gauges, & of course, always use these off the face edge or face of your work (which you clearly marked in your initial stock preparation!).

It's habits like these that lead to well-fitted parts & nice, square openings for drawers etc. In my earlier & more impatient days, I would often blunder on with bits that were not quite right, in the vain hope that small errorss would cancel each other out. Not a chance! Good old Murphy has decreed that all errors are additive - they never cancel out!

Cheers,

I have found the best way for me when using imperial plans is convert the measurements as I go, but when going to mm round the dimension to the nearest .5mm (trying to measure 125.4125mm with a rule is a pain! the bloody lines are 0.2mm wide!) So 4-15/16" would read at 125.5mm. Unless your engineering a watch the 0.2-0.3mm here and there dont matter so much. Another advantage with this system is it builds the familiarity with imperial, which, sadly still comes in handy!

TigerAce

If a plan is in feet and inches then try to stick to that as there is less chance of error when calculating the conversions. Most rules and tapes still have both systems on them. This works well when working in solid timber as you dimension the wood as required for the plan.
Now here is the bit that gets harder. If you use sheets of ply, mdf, or whatever and it is in mm thickness then joints need to match that thickness. Same goes with using router bits or chisels to cut mortices. They can also be got in both measurements but be sure you use like for like. So if your tooling is metric and you use sheet material then perhaps best to convert the plan to suit.
Regards
John

If you are doing sheet work and using Euro hardware, metric is a must. " System 32 " has been around for yonks. All the boring centres are multiples of 32mm which is also the thickness of 2 16mm gables butted together ( modular cabs ). The big multi spindle borers are set this way. Measure the centres of any hole bored by Ikea. We all love Ikea.

Think metric, it's a dozen times easier.

Originally Posted by AlexS

Think metric, it's a dozen times easier.

That's an oxymoron metric does not work in dozens.
Regards