Decimal or fractional

Originally Posted by Dana

It depends on what you're drawing. If the accuracy required is high (closer than 1/64") decimals are used. Decimal dimension are also generally used for machined parts. Fractions are often more convenient for larger parts cut and measured by hand.

-Dana

Place a half full glass of water before a pessimist, optimist and an engineer:

The pessimist says the glass is half empty.
The optimist says the glass if half full.
The engineer says the glass is too large.

Darn good answer ... and as for that "glass of water" item - outstanding! :-)

tyc

If you are going to draw the plans with a cad system, doesn't the system itself have a conversion factor built in? Couldn't you put a standard decimal dimension in and then put the metric conversion underneath? The bad thing about fractions is an individual builder having to convert them to metric, while converting them from a decimal is easy.

The thing to remember about the decimal system is that it actually is a fractional system...it just uses 10ths, hundredths, thousandths,etc.

And instead of writing 1/10 you write .1

Also, you should consider that the majority of kits are built in the USA and the bolts,pins,wires,etc will all be available in American standard sizes. Someone building your design in another country will obviously have to make allowances for metric equivalent hardware.

I believe the problem with listing both metric and the english system(fractional or decimal) is that the sizes don't match up. 1/2" = 12.7mm or 5/8 = 15.875mm. All my measurements are in english. If i were to list metric i would need to find the closest size that would work, which means if it was less i might have to calculate new stress limits to be sure they are ok, if it's more than you would be increasing weight. OH...what to do.

skeeter

Skeeter,
The Sequoia Falco is a plans built wood design with drawings made and sold in the USA - and if I'm not mistaken, aren't the drawings metric? Why don't you email and ask their opinion? They've been selling plans for a long time.

Isn't the falco originally from Italy, which would answer why or if the're metric? Maybe someone building one could elaborate. Falcobob are you listening? And how do you cross the dimensions to available size woods? Especially plywood?

skeeter

Skeeter: Googled for aircraft wood/spruce knowing that one of the big suppliers is in Western Canada and lo and behold, up came this link from our own J-man himself !

HomeBuiltAirplanes.com > Construction Junction > Wood Construction

We're metric up here but there is still a big holdover from "English" measurements - like our ply is still in 4X8 sheets, etc.
Check out Western Aircraft Supplies aircraft@telus.net
They supply a lot of the wood for Falco builders so if anybody has the answers you're looking for, they most likely do.

Personally, I got into metric a few years ago, and I'll never go back to fractions! Takes a while to fully convert but metric is soooooo easy - besides, the rest of the world uses it.
If you're going to market plans or a kit, you'll be amazed at the number of builders there are in Europe, Australia and NewZealand, not to mention South Africa. Why limit yourself to the USA?

In a never ending quest for homebuilding improvement, I once experimented with working with an inch ruler/tape in tenths and hundredths of an inch. My idea was actually to embed a tape in the table coinciding with wing station positions. It did not work out very well... The tenth was too big and the hundredth was too small to conveniently work with... My conclusion is to dimension in decimal but to fractional equivalents. As in, 0.250 which is a quarter of an inch.

FWIW

Which ever you use.....be consistent...just remember the JPL project that the programming was done in metric...and the data entered in english units...or vice versa...and a nearly $2,000,000,000 spacecraft went "poof" in the Martian atmosphere...oooooooops....

Don't ya know all JPL employees are required to use metric units on everything now...even when baking a cake at home...


Dave

Originally Posted by Birdmanzak

As another Australian, I get cranky enough using inches (grumble grumble Boeing grumble). If I had to work in fractions as well I'd have to change career in protest.

I'm suprised it's hard to find decimal tape measures. My tapes and rulers have one edge divided into decimals and the other into fractions.

I was building some shelves recently and used the traditional measure twice, cut once. Well, it didn't work out. All of my parts came out a little short or a little long. I re-measured and recut, and they still came out wrong. It wasn't until I realized that my tape measure had 10 inches to the foot and ten eights to the inch. Turns out that it was marked in feet, tenths of feet, and hundredths of feet. Boy, did I feel dumb!

Bruce

Wow, that truly is a weird tape. Where did you buy that?

One guy had an unfortunate experience... I do some metal casting... and I had a shrinkage ruler - which is a ruler where the measurements are X percent larger (I forget the exact number, but it was for aluminum which is one of the larger ones) so that when you make your pattern, you oversize it to allow for shrinkage.

Well, he borrowed the ruler (without me realizing just what he was taking...) and his hardwood cabinets did not come out perfect.

If you put the dual dimensions on the drawing, length and width dimensions should be fine in a metric conversion. As for hole sizes, a simple note stating that the builder should use the closest metric equivalent that is larger than
the stated conversion dimension should suffice.
I guess the thing I would wonder about is whether the tubing and sheets of material that are available to foreign buyers are also metric sizes. I also seem to remember that a long time ago, not all metric threads were the same in different countries. It could be a nightmare to try to convert it yourself.

For what it may be worth here, as is probably the case with most of you, I was not "raised" with the metric system and if you think switching to the metric system is difficult, it just might be, so just start with the basics; Inches is 2.5 centimeters (no not exactly but close enough), Yards is Meters (no, not exactly but close enough), A Kilo is a bit more than two Pounds but if you're looking for precision here - just ask your local drug dealer; sorry people, I just couldn't resist that one :-) A kilometer is 66% of a mile (no, not exactly but close enough.)

A simple and straight forward "trick" I started using years ago and continue to this date is to log in the weather every morning; Temperature is recorded in Centrigade, Wind Speed in Kilometers per hour. Do this for just a month or so and for want of a better term, "you're in" when it comes to using the metric system. The same applies for designs of any project; Picture Frames, Cold Boxes, Garden Fencing, Small Aircraft, etc.

The precision and associated simplicity offered by the metric system, which is decimal and not fractional, is difficult to argue against - at least that's been my own experience.

tyc

For weight, my high school physics teacher told us to remember that a raisin weighs about one gram.. easy to remember: "Raisin Gram!"

-Dana

+REAL+ programmers use EDLIN to create Windows apps

Hello All, A very meaningfull discussion indeed.

I think it is of great importance to use appropriate dimensions for parts. this includes of course "How to dimension". Dimensioning must also capture "What is important". This translates into how to describe units (Fraction or decimal) but it also should take into consideration the tools required to manufacture and inspect the part.

Simply put - If you plan to inspect the part with a tape measure; use the units that are on the tape measure on your drawing! If you will need to manufacture/inspect with great accuracy then use decimals!

Be careful to always redraw or design parts with the key features prioritised! and draw things so that inspection is possible! Many "professional" drawings have problems in this regard!

Safe flying and fun building!

Great discussion about unit confusion! you all probably know about it!
Math Mistakes in History: The Gimli Glider 360