Thread: converting imperial to metric visa versa

I think much depends on how precise your woodworking needs to be. Precision work is always going to be problematical if you are converting scales.

Most of the equivalents are rounded figures and incorrect. As others have said, although we think of 1" as 25mm it is 25.4mm. As your sizes go larger the error is magnified. Your 10" saw blade is really 254mm. A sheet of 3/4' plywood was 8' x 4' in imperial and is now sold in metric as 2400mm x 1200mm x 19mm. The exact conversion is 2440mm x 1220mm (occasionally sheets are still this size but not often).

I grew up with imperial measurements, but just the thought of it is enough to make me commit hari kari. Metric is so much more logical and simple. Only a bloody minded nation would persist with Imperial.

Having said that, I have pointed out before on these forums that with timber I still buy 3m of 4 x 2 .

Regards
Paul

And 4 x 2 is pre dressed size so it is more like 3 1/2 x 1 1/2 or sold as 90 x 35 or even 89 x 35 mm.

Confusing, is it not!

Originally Posted by Bushmiller

I think much depends on how precise your woodworking needs to be. Precision work is always going to be problematical if you are converting scales.

Most of the equivalents are rounded figures and incorrect. As others have said, although we think of 1" as 25mm it is 25.4mm. As your sizes go larger the error is magnified.

Regards
Paul

No it will not, think about it. If you convert a 12 inch measurement to metric you round the converted final metric, you do not use 25 x 12 as that will magnify the error. Secondly, you round to two decimal places, that means you calculate to 3 decimal places and then round to 2 decimal places ie an accuracy of thousandths of a mm, then rounded to hundredths of a mm. Most of us in the real world can't see that amount of accuracy and your marking gauge or pencil is not that fine, my pencil is 0.5 mm and it is the size for using my Incra rulers. Just imagine a mm cut into a thousand bits or a hundred if you can. I have never seen a regular workshop ruler with measurement markings that fine. A saw blade measurement is irrelevant if it is imperial or metric as it cuts where you make it cut to the mark on your timber.

SB

Originally Posted by Superbunny

No it will not, think about it. If you convert a 12 inch measurement to metric you round the converted final metric, you do not use 25 x 12 as that will magnify the error. Secondly, you round to two decimal places, that means you calculate to 3 decimal places and then round to 2 decimal places ie an accuracy of thousandths of a mm, then rounded to hundredths of a mm. Most of us in the real world can't see that amount of accuracy and your marking gauge or pencil is not that fine, my pencil is 0.5 mm and it is the size for using my Incra rulers. Just imagine a mm cut into a thousand bits or a hundred if you can. I have never seen a regular workshop ruler with measurement markings that fine. A saw blade measurement is irrelevant if it is imperial or metric as it cuts where you make it cut to the mark on your timber.

SB

SB

I think you missed my point.

The original post refered to converting plans from imperial to metric or just working in the original measurements. My contentionion was the commonly regarded equivalents between imperial and metric, which are rounded figures, are inexact. For example, if a plan calls for something to be 4" long and you make it 100mm your component will be the wrong length by 1.2mm. As I said, depending on the required level of precision, this may or may not be an issue.

You can convert the plans to metric using a calculator or conversion charts, but it is tedious and there is always the possibility of error. Round numbers are relatively easy. 1 11/16" maybe a little more challenging and introduces more potential for error.

On the subject of decimal places if you start with figures that have one place of decimals it is a meaningless accuracy to start working with three places of decimals and correcting to two. I would suggest that little if any woodworking is performed at that level. However, if like me, most of your work is marked out with a pencil I would agree that .5 of a millimetre is about the best that can be achieved. When I want to be exact I use a marking knife as I only regard a pencil as useful for rough (relatively ) work. I know that the forum members who achieve the best results use a marking knife as otherwise you work to one side of the pencil line and I can never remember which side I'm supposed to be on .

It's all a question of precision.

Regards
Paul

Originally Posted by Bushmiller

I think much depends on how precise your woodworking needs to be. Precision work is always going to be problematical if you are converting scales.

Most of the equivalents are rounded figures and incorrect. As others have said, although we think of 1" as 25mm it is 25.4mm. As your sizes go larger the error is magnified. Your 10" saw blade is really 254mm. A sheet of 3/4' plywood was 8' x 4' in imperial and is now sold in metric as 2400mm x 1200mm x 19mm. The exact conversion is 2440mm x 1220mm (occasionally sheets are still this size but not often).

I grew up with imperial measurements, but just the thought of it is enough to make me commit hari kari. Metric is so much more logical and simple. Only a bloody minded nation would persist with Imperial.

Having said that, I have pointed out before on these forums that with timber I still buy 3m of 4 x 2 .

Regards
Paul

I agree that 25.4 is the magic number, I've hand to make the switch from inch to metric over the past 5 years and it's served me well.

1 inch is also 25,400 microns.

And even: 1 inch to yoctometre - Google Search

Inches	Off by (mm)
1	0.4
12	4.8
60	24
96	38.4

I wouldn't want my 8 foot long canoe to have a 3.84cm hole in it. (

If you're going to convert, might as well do it right.

Originally Posted by jnjh

And even: 1 inch to yoctometre - Google Search

I have just checked my digital vernier gauge and it doesn't seem to have those Yoctometre thingys .

I suppose you only need that if you can trace your ancestry to a primordial, protoplasmal, atomic globule .

Regards
Paul

just thought i would share an article i found on slashdot. related to getting the US to officially convert to the metric system.

"Without any prompting from the US Metric Association, a We The People petition to standardize the US on the metric system has received 13,000 signatures in six days. That's half the number needed for an official response from the White House. It looks like ending the US's anti-metric alliance with Liberia and Burma (the only other countries NOT on the metric system) might rank up there with building a death star."

Heres the URL of the We the people article where you can cast your vote if you wish to try and get the white house to listen. https://petitions.whitehouse.gov/pet...ystem/FndsKXLh

I should cast my vote then i guess

Originally Posted by jnjh

I agree that 25.4 is the magic number, I've hand to make the switch from inch to metric over the past 5 years and it's served me well.

.......I wouldn't want my 8 foot long canoe to have a 3.84cm hole in it. (

If you're going to convert, might as well do it right.

Steve, I agree that you should convert to precise figures if you are going to do conversions, and a calculator makes it easy.

However, harking back to the point of the original post: Unless you are making an exact reproduction of something, most furniture-making demands accuracy, not precision, meaning parts have to fit together, but it doesn't matter a hoot if they are a mm shorter or longer than drawn on a plan. if I am making a one-off, using a plan drawn in one system, but wishing to work in the other, I think it's unnecessary to convert most of the numbers on the plan. Not only is it a waste of time, it is too easy to make mistakes (well,, easy for me!). There are a couple of other reasons why I think this. The stock I wish to use may not be the exact thickness called-for by the plan (it rarely is!) which will throw out lots of dependant dimensions. And/or I may choose to vary the size of components like drawers or door stile/rail widths for various reasons, e.g to to use a particularly nice bit of book-matched crotch figure for the panels, which is 15mm too short if I were to stick with the plan dimensions. So it is far safer to do a sketch converting the major dimensions only, then derive all dependant measurements as you go. The plan is your road-map, and shows the order in which bits need to be fitted, for example, but the job itself dictates the sizes of all critical parts, which are set off their respective matching parts. I can work half a day marking directly off a job and not even touch a tape measure (which often means I can't find the damn thing when I do need it ) I find it simpler, & far less likely to result in errors, this way.

Pre-cutting a whole bunch of parts from a plan is a perfectly good & efficient way to work in a production shop, when you are making multiples from a tried & tested plan, & particularly if you are using jigs and machines. To set up, conversions to two decimal places from an original plan are warranted, but you would have made a prototype & checked each part carefully before launching into full production, I hope!

Cheers,

What Ian said, for one off work a cutting list is only a guide to see if you have enough timber. I don't cut anything to size till it's time.