Imperial or Metric?

[Keith Weber]

Actually, there is a cute trick I saw by Mr. This Old House Tom Silva on dividing up boards or just about anything and I now use it all the time. Take your tape measure and put it on one corner of your board and stretch it to the other side. You'll probably need to hold it with your finger as you then move the tape measure at an angle (creating the hypotenuse of a triangle) until you reach a NICE number to divide by and then tick off the mark on along the tape measure (the hypotenuse of the triangle).

Now that's a neat trick. I hope I can remember that one!

[Phillip Gregory]

Phillip,

Without getting into too much detail, I was wondering where you are located, and what your line of work is.

The reason I ask, is that I am curious as to what machine shops are using for measurements these days. I recently fabricated a new, re-sized overarm blade guard support for my Altendorf F-45 because the saw was too wide for my narrow shop. When I went to buy 3 pieces of metric tubing for the job, I was shocked at the price quote of around $1100. I was told by two big metal suppliers that they don't make metric tubing in the US, and they had to get them from Europe. The closest imperial equivalent was only $130, so I went that way and machined the ends where I needed to mate to the existing metric pieces.

If machine shops were working almost entirely in metric, I would think that it would be a pain to machine every bit of imperial stock down to get metric sizes. It wouldn't be that big of deal if the piece was machined out of a larger block, but if the end product included a bunch of metric flat bar and plates, it would entail a significant amount of extra work (or extra expense to order the metric stock). I guess they would just pass these expenses on to the customer.

It's easy enough to get measuring tools in metric and anything digital can be switched at the push of a button, but choices in US-made cutting tools are limited in metric. Also, a lot of the metric hardware that you see in the hardware stores is in those metal "special hardware" bins that sell individually-packed at a significant markup to the bulk-bin prices on the SAE hardware. I guess that it wouldn't make much difference to a manufacturing company which would order hardware by the pallet, but for a passionate hobbyist like by myself, metric is an expensive pain in the butt.

All of my metalworking stuff is pretty much imperial (collets, end mills, drill bits, indicators, edge-finders, etc.) I do have a metric tap and die set and a full compliment of metric hand tools (wrenches, sockets, allen keys, etc.), though, and I like my temperatures in °C, so I'm not completely in the dark ages.

Keith

I work in healthcare in Missouri. Nope, not a machinist but we do a lot of work with various measurements and it's all metric, at least internally. The goofy part is that there isn't one metric set of units in healthcare. The most notorious example is that Europeans typically use molar concentration such as mmol/L where we use weight per volume such as mg/dL. Both are metric but the units are very difficult to convert between as you have to know the molar weight of the substance in question. Just like the imperial vs. metric conversation here there is a bit of debate over which one of those is "correct."

I mis-typed and really meant "mechanics' tools" rather than metal fabrication or milling tools. The only real significant fabrication and milling I do is with wood, which is why I am here instead of on the Practical Machinistforum. I do some shade tree mechanicing at home on various equipment and nearly all of it uses metric fasteners and metric measurements for parts (e.g. hub bolt circles). You can metric fasteners in most hardware stores and yes, it stinks to have to buy them individually or in packs of three where you can buy 100 similarly-sized SAE fasteners for about only twice as much. Bar, flat, angle, and sheet stock however is in imperial and I measure and cut in imperial units since that's what my measuring equipment (the same rules and tapes and squares I use for woodworking for the most part) are.

[Duane Meadows]

And what is the plywood thickness of the week?! Ugh, drives me crazy. I have 'nice' cabinet grade A1 maple plywood and found a sheet or two that DID vary thickness within the sheet. When my Domino arrived, I threw out (not really, but it's gathering dust) my dado blade.

Actually, there is a cute trick I saw by Mr. This Old House Tom Silva on dividing up boards or just about anything and I now use it all the time. Take your tape measure and put it on one corner of your board and stretch it to the other side. You'll probably need to hold it with your finger as you then move the tape measure at an angle (creating the hypotenuse of a triangle) until you reach a NICE number to divide by and then tick off the mark on along the tape measure (the hypotenuse of the triangle).

So for your 20 mm board (rather narrow!) into 7 equal parts, simply move your tape measure up the side of the board until it reads 21 mm and then tick off every 3 mm.

I agree, Chris. That is the easy way to do it. Doesn,t change the math issue though! A fraction is a fraction, whether it is a fraction of a meter(better?), or a fraction of a foot.

Yikes! 20 mm? Shows you my mind wasn't thinking metric! Who says you can't make error with metric?

[Terry Therneau]

I use metric as much as possible - I just find it easier. When ripping boards or planing I can make them what I want. For the router I bought an adjustable (dial) slot cutter, and smile every time I use it -- plywood variation was driving me bonkers. But mostly I'm practical, and switch from one to the other as needed.

Terry T

[Curt Harms]

Just a get a story stick without any units at all, and you won't have to deal with either! Just make everything "about that big", and all the matching parts the same size.

Seriously, though... While metric has its benefits, I find it to be easier just dealing with imperial in the US. Most people here wouldn't know a centimeter from a centipede, and most of the hardware and tooling that you'll find are in imperial measurements. It's easy to get a 1/2" router bit, a 1/8" drill bit, 3/8" bolt, or an imperial tape measure without going further than your local hardware store. But try to find a 10mm router bit, a 7mm drill bit, a 8mm bolt, or a metric tape measure, and finding things you need just got a lot harder -- especially if you need them right now.

In woodworking, I tend to work in fractions as long as it's easy (say, midpoint of 26-1/2" is 13-1/4".) When it does get more difficult, (say 17-5/16" divided by 3), I might just switch to decimals, just to make the math easier. Using decimals in imperial measurements is no different than doing it in metric, so long as you have a method of conveying those measurements to your workpiece. I have both imperial scale and digital readouts on my slider, so I can quite easily work in either. The thing is, though, that the number of times that you need to resort to digital imperial measurements in woodworking is almost never. I also find it humorous when some woodworkers start making measurements to the 1/1000th of an inch (for purposes of accuracy vs. ease of deriving a measurement), and claim that their tablesaw cuts are able to cut to that accuracy.

Well said. I think the two biggest problems with using metric in the U.S. right now is the relative scarcity of hardware and unfamiliarity. Most people can tell with a glance whether a piece of plywood is 3/8" or 3/4". Thickness in millimeters? Not so much - even though that sheet of plywood might actually be metric.

[Warren Wilson]

John -- I hesitate ever to offer an opinion in such august company, but the notion of basing a system of measurement on "human-sized" units would forego many of the benefits of the SI system. One of the aspects of the metric system I find most appealing is that it is NOT based on arbitrary "human" measurements such as the length of a dead king's foot or the twelfth part of that.

And once we begin measuring things other than length, it seems the metric system really shines. Relating length to volume to mass (i.e. 1 cubic cm of water weighs 1 gram) makes all calculations simpler than relating inches to gallons to pounds -- even after we decide whether we mean US gallons or Imperial gallons.

And as to temperature, assigning 0 degrees to the freezing point of water then calibrating 100 of those degrees to the boiling point makes sense. I can't remember why Fahrenheit chose 32 degrees as the freezing point of water then sliced the difference to boiling into 180 degrees, but I do know the old system no longer makes sense to me.

And in spite of my respect for the SI system, like Roy I am "bi-metric," and think of body weight in pounds, distance in kilometers, temperature in celcius, and usually think of plywood as 3/4" and buy 8' lengths of 2x4s. Just don't ask me to quickly calculate half of 13' 7 7/8"!

[John Coloccia]

They defined the meter, rather arbitrarily, as 1/10,000,000 the distance from the equator to the north pole. It seems odd to me as there was already a common measurement in many different countries that, more or less, equaled an inch...even in France. They had the pouce. There was no compelling reason, other than it it was philosophically pleasing, to introduce an entirely new unit when they could have simply used the distance between the equator and the north pole to precisely define the unit they already had. It would have caused a heck of a lot less trouble to have a meter defined as 10 pouce, for example, and then continue on from there defining whatever other quantities they wished.

For example, when we wanted more experimental precision in defining our unit of length, we didn't introduce a new unit. We used the speed of light to more precisely define the unit we already had...the meter. I guess with the revolution and all going on, there was change in the air.

[Kent A Bathurst]

And what is the plywood thickness of the week?! Ugh, drives me crazy. I have 'nice' cabinet grade A1 maple plywood and found a sheet or two that DID vary thickness within the sheet. When my Domino arrived, I threw out (not really, but it's gathering dust) my dado blade.

Yeah - that pretty much sucks. No getting around it, AFAIK. In addition, any teensy out-of-dead-flat conditions can exaccerbate the problem when the carcass and shelves are run. All I can say is that my approach is this [assume nominal 3/4" ply]:

1. 3/8" x 3/8" stopped dado in ply carcass, using TS and dado stack.
2. With same dado set, double-rabbet the insert boards [shelves?] to leave 3/8" wide x 23/64" deep tongue. Sneak up on a scrap test piece for depth of cut [which means width of the tongue].
3. Carcass/tenon saw to remove leading edge of tongue to matched depth of stop.
4. Dry fit, with face float and / or wood block and 80g adhesive-backed sandpaper on block's 2 faces only [not on the edges]. A few quick swipes as/where required for tight fit.
5. the actual joint is hidden by the shoulders, so any imperfections there cannot be seen.

[Ernie Miller]

The thing is, though, that the number of times that you need to resort to digital imperial measurements in woodworking is almost never. I also find it humorous when some woodworkers start making measurements to the 1/1000th of an inch (for purposes of accuracy vs. ease of deriving a measurement), and claim that their tablesaw cuts are able to cut to that accuracy.

In my work as a harpsichord builder, I use digital Imperial exclusively for measurements under 6". In some assemblies, errors of a couple of thousandths of an inch can mean the difference between success and failure. It's not that I can cut wood to an accuracy of 1/1000 of an inch, but instead it's laying out what I need to cut accurately. Here's an example. Let's say the width of an octave on a piano is 6 1/2". There are seven white keys to an octave and they must be equal in width. So, divide 6 1/2" by seven and you get .9285". How do you draw a line .9285" away from another line onto a piece of wood? Using a ruler, we can take the closest available measurement which is 59/64" - if your eyesight is good enough to see those divisions. But 59/64" equals .9218" per key. Times that by 7 (for the seven white keys), and you get an octave that is only 6.4531" wide instead of 6.5". So what, you may ask. Well, if you multiply that error 7 more times (for the seven octaves in a piano keyboard) you end up with a width for the entire keyboard that is 45.1718". However, 6 1/2" (the desired width of a single octave) times 7 equals a keyboard width of 45 1/2". So using 59/64" as a measurement results in a keyboard that is more than 5/16" too narrow. That would be a disaster in a keyboard as the width of a key must coincide with many other assemblies in the instrument in order for it to function normally.

In order to avoid this cumulative error, I've abandoned the use of rulers altogether in certain areas - the keyboard being one of them. Instead, I use paper story sticks printed by a CAD program on my computer. I draw a series of lines 6 1/2" wide and use the story stick to divide those 6 1/2" divisions into 7 equal parts. This avoids the cumulative error problem - as long as I can accurately measure 6 1/2", I will at least know that I will end up with a keyboard of the proper width.

Unfortunately, this stuff keeps me awake at night.

Ernie

[John Coloccia]

In my work as a harpsichord builder, I use digital Imperial exclusively for measurements under 6". In some assemblies, errors of a couple of thousandths of an inch can mean the difference between success and failure. It's not that I can cut wood to an accuracy of 1/1000 of an inch, but instead it's laying out what I need to cut accurately. Here's an example. Let's say the width of an octave on a piano is 6 1/2". There are seven white keys to an octave and they must be equal in width. So, divide 6 1/2" by seven and you get .9285". How do you draw a line .9285" away from another line onto a piece of wood? Using a ruler, we can take the closest available measurement which is 59/64" - if your eyesight is good enough to see those divisions. But 59/64" equals .9218" per key. Times that by 7 (for the seven white keys), and you get an octave that is only 6.4531" wide instead of 6.5". So what, you may ask. Well, if you multiply that error 7 more times (for the seven octaves in a piano keyboard) you end up with a width for the entire keyboard that is 45.1718". However, 6 1/2" (the desired width of a single octave) times 7 equals a keyboard width of 45 1/2". So using 59/64" as a measurement results in a keyboard that is more than 5/16" too narrow. That would be a disaster in a keyboard as the width of a key must coincide with many other assemblies in the instrument in order for it to function normally.

In order to avoid this cumulative error, I've abandoned the use of rulers altogether in certain areas - the keyboard being one of them. Instead, I use paper story sticks printed by a CAD program on my computer. I draw a series of lines 6 1/2" wide and use the story stick to divide those 6 1/2" divisions into 7 equal parts. This avoids the cumulative error problem - as long as I can accurately measure 6 1/2", I will at least know that I will end up with a keyboard of the proper width.

Unfortunately, this stuff keeps me awake at night.

Ernie

When I print a pattern, I'm also sure to print a witness square, usually 4x4 (so I can accurately measure with my 6" rule). It's surprising how often it's off significantly. I thought using a printing house would be better. Nope. I haven't found a solution yet other than keep printing until it works.

[Fred Heenie]

I build in metric and sell in imperial. precise and no client math lessons required.
;^)

[Yonak Hawkins]

Try ... dividing 8 7/16 into thirds.
It would be wonderful if the US could go metric.

It probably would be good if the US went to metric but try dividing 1cm into thirds !

Really, we should go to base 12. More of our numbers would be shorter, and fractions easier.

[Ernie Miller]

When I print a pattern, I'm also sure to print a witness square, usually 4x4 (so I can accurately measure with my 6" rule). It's surprising how often it's off significantly. I thought using a printing house would be better. Nope. I haven't found a solution yet other than keep printing until it works.

It's worse if you try to glue the pattern onto wood (or anything else). The paper will swell from the moisture in the glue and throw your pattern off. A long time ago, I thought it would be great to print out a pattern for an entire keyboard. The printing came out quite accurately, but gluing the pattern down was a disaster.

I have my full sized plans printed by a digital print shop that deals mainly with architects. I have to check every plan to make sure the dimensions have been printed correctly. In fairness to the print shop, they have very little experience in printing anything full size. Almost all drawing for architects are scaled down to a workable, readable size. It seems there is quite a learning curve involved in using these large scale printers. Nothing comes easy.

[Steve Milito]

It probably would be good if the US went to metric but try dividing 1cm into thirds !

Really, we should go to base 12. More of our numbers would be shorter, and fractions easier.

Then I'd need two more fingers!

[Chris Parks]

Nah. Like I said earlier, it's pretty trivial for us to work is both units so there is absolutely no reason in the world to change.

Your international industry has changed though, automotive being a prime example, is that correct? It might be trivial at your level but if a company is in an international metric world they have no choice.