Why Decimal Inches Suck

[Jon Matcho]

I have so far refused to purchase that special Stanley tape measure with the "tenths of inches" gradients. My thinking is that any time I see something like 10.6" I could convert to "a little less than 10 5/8" (10.625"). So far this has worked without much problem.

 

Just as I was thinking to cave in and buy one of those stupid things, I noticed how much I use my other measuring tools: a bunch of squares and various rulers. I'm not about to replace all of them with some wacky measuring system.

 

So I got to thinking, "why are these measurements here at all?" I think it must be due to 10.6" being easier to write and understand on paper. For example, when the need is to measure 10.625" to locate where to swipe your big ol' black marker for where you're going to hack away with your chainsaw, it's obvious that 0.025" is not going to matter much.

 

The problem is that 0.1", 0.2", 0.3", 0.4", 0.6", 0.7", 0.8", and 0.9" cause everyone without one of those magic tape measures to pause. Why couldn't the original plans be done to the nearest 1/16"? Why not 1/8"? Keep in mind that I'm talking about parts that are quite sizeable -- bulkheads and fuselages for example. The tiny parts, particularly tubes and fittings, are a different story.

 

I firmly believe that this decimal inches muckery was chosen for one or more of the following reasons:

• 5/16" was too hard to type on 1970s typewriters.
• Someone thought 5 5/16" would be confused with "55 or 16 inches" (which is why 5-5/16" was invented).
• 28.15" somehow implies precision, and precision is good (after all, machinists use crazy decimal measurements, why shouldn't we when building an 8-foot fuselage?)
• The Stanley decimal tape measure was discovered, and all heck broke loose.

As for me, I am dead set on NOT purchasing one of those tape measures and am doing just fine without. I just find the nearest 1/16" and mark above or below the marker.

 

Thanks for letting me rant. What do you think?

[Jerry Schneider]

I think the decimal tape is like the secret decoder ring. You almost gotta have one to be part of the club.

 

I have 2. (I just wanna belong. )

 

On the practical side: With it, there's just one less thing to think about while you're measuring and cutting. (I need all the help I can get. )

[Remi Khu]

Well since you asked. I think you're a non-conformist, anti-establishment, anti-social, sociopathatic deviant that needs to be looked up in a cell measured in metric. <--- that's a SMILEY

 

You know I couldn't let that opportunity pass Jon. However, I'm in agreement with you on the tenths business, but I conformed and have the Stanley tape for fear that the plane will be out of spec and something horrible may ensue. The problem is the tenths become hundredths, and my marking tool isn't thin enough to differentiate.

 

There has been some discussion on dimensional tolerances and I've come to accept that the Cozy isn't going into space with the space shuttle, so close is good enough.

 

My latest struggle is the accuracy of the digital level. I have spent an entire afternoon leveling the canard and its incidence angle to near perfection (+/- 0.1deg) only to return the same evening to have the alignment differ!! I think I have a draft in the basement. So I begin to question the reliability of the digital level. Once again, some smart ass told me the early canards were built using the bubble levels and none have had any structural issues. DOH!!

 

I guess all this ranting exposed my idiosyncrises and insecurities, so I'll just fade back into the woodwork and lurk some more.

[Jon Matcho]

I think the decimal tape is like the secret decoder ring.

Well now that you put it that way... I'll at least look at it differently.

 

Once again, some smart ass told me the early canards were built using the bubble levels and none have had any structural issues. DOH!!

The plans do call for a 'carpenters level', which is no doubt important. I have one, fitted with a digital gizmo. I too labor over 0.1 degrees...

[Len Evansic]

Well, I'm still stunned as to why, but decimal inches is still used extensively at Scaled. Each fabricator in the shop is required to own a decimal inch steel tape. It's just the way it is.

 

That being said, in engineering, most thicknesses are expressed in thousandths of an inch, but most things are approximated to decimal fractions (i.e. 0.060" ~= 1/16"). If you are measuring thousandths, you are using calipers with a dial or digital readout.

 

-- Len

[Lifessamsara]

Well guys, I think we all agree that working to base 10 or 100 is far easier, intuitive........... perhaps common sense?

Of course I respect that people, communities, and countries can do as they wish, but......... why not just join the rest of the world (and other professions) and use SI units? It would seem that using decimal inches is just acknowledging how outdated and awkward imperial measure really is.

Just from the record, I am old enough to know how to use both systems of measure, and am technically training to use both

 

Just my 2 cents worth (another example of base 100 use).

 

Cheers,

 

Bruce.

[Jon Matcho]

A major PLUS for the English system is that units are readily divided into halves, which is a common need. For example:

 

• half of 1' is 6"
• half of 1" is 1/2"
• half of 1/2" is 1/4"
• half of 1/4" is 1/8"
• half of 1/8" is 1/16"

That's very natural for me, compared to a similar use case with the decimal system:

• what's a third of a meter called? or 10cm?
• half of 1cm is 5mm
• half of 5mm is 2.5mm
• half of 2.5mm is... zzzt
• ...

To each their own. The English system is never going away IMHO.

[Jon Matcho]

It's just the way it is.

Hope you're doing well Len. I suppose if you don't need your shiny new decimal tape measure, I'll take it.

 

For the record, I'm not against the metric system at all. I just don't have any aircraft plans that are written 100% in the metric system.

[Phil Kriley]

My latest struggle is the accuracy of the digital level. I have spent an entire afternoon leveling the canard and its incidence angle to near perfection (+/- 0.1deg) only to return the same evening to have the alignment differ!! I think I have a draft in the basement.

I had the same problem, and found that I had not calibrated it correctly. I was trying to follow the instructions on the level itself, and was getting nowhere, and getting very frustrated.

 

So I took a look at the instruction booklet and VOILA! - now when I get a 0.0 reading it STAYS a 0.0 reading! And 90 degrees is 90 degrees no matter which end of the level is up!

 

I'm just glad I figured it out before I messed up the plane! And yes, I worry over 0.1 degree...it's just the way I'm wired, I guess!

[argoldman]

A major PLUS for the English system is that units are readily divided into halves, which is a common need. half of 2.5mm is...To each their own. The English system is never going away IMHO.

John, John,

 

 

You are so lengthocentric. You look at measurements using 1/2 etc etc, because that is the way you were brought up in the measurement world.

 

Change measurement to global politics and we have wars!

 

I like the inch method because it is that upon which I was also brought. (how's that for you english majors.)

 

The inch system is based on an arbitrary measuremet of a foot which is devided into 12 parts. The foot was the measurement of some king or other. Then for some reason they decided to think in base 12, except for yards which are base 12 thought of as base 3 and then miles which are the king's foot thought of as being in base 5280. Seems simple to me.

 

With metric, you have a basic, arbitrary measurement (such as the foot originally was). Everything is in base 10 after the original assumption. Infinitely easier--- If you have done it all your life.

 

All of science uses base 10. (except for the clowns with the original hubble iteration)

 

You even use the base 10 (metric-type system daily) What is 1/16th of a dollar???? It is all bas 10.

 

The odometer on your car, the hobbs meter, your navigation for your aircraft, your MP and RPM, Oil pressure, temp, OAT, interest rates, are all decimel (metric).

 

But then we make it harder. Our clocks are based on 60, except for the days which are based on 60*60*24 then there is the week, 60*60*24*7 then there is the month which is 60*60*24*30,31,28 or even 29 add to this the yearly computation-- multiply this by 12. BUT then they go back to metric in terms of decade and century.

 

Yes I, agree with you, that our system is much less complex than the UGH -- metric system.

 

But then , of course, there is always Whitworth (I believe another english creation)!!!

 

This bit of baffoonary took 325.236 seconds (I'll let you convert that to minutes if you wish:p

[Jon Matcho]

John, John,

Rick, Rick,

 

You look at measurements using 1/2 etc etc, because that is the way you were brought up in the measurement world.

If you say so. I just find it easier to divide 12 by 2, 3, or 4 which happens to be a common need. Maybe you find 33.333 centimeters an easy thing to measure.

 

Yes I, agree with you, that our system is much less complex than the UGH -- metric system.

Not sure if you're serious, or cynical.

 

I have no issues with the metric system. I've used it plenty, have a bundle of metric tools, and know that a bottle of wine is 750ml. The original point of this diversion was that decimal inches that do not fit into a 1/32nd of an inch multiple have no business beyond 1.0 inches! There, that's it, end of story.

 

This bit of baffoonary took 325.236 seconds...

You've actually made my point -- 325.236 seconds is as silly as 325.236 inches. They're both better described as "about 5 and a half minutes" and "just over 9 yards (or 8.25 meters)", respectively.

[Len Evansic]

Hope you're doing well Len. I suppose if you don't need your shiny new decimal tape measure, I'll take it.

Ah, if I only had bought one. I'm a die-hard REAL inches guy, and for that my decimals are only equivalent to inch fractions. I'm not a fabricator, so I'm not required to have one of the decimal inch tapes .... yet.

 

-- Len

[argoldman]

Rick, Rick,

 

If you say so. ...............

 

 

You've actually made my point -- 325.236 seconds is as silly as 325.236 inches. They're both better described as "about 5 and a half minutes" and "just over 9 yards (or 8.25 meters)", respectively.

Yes, John, I was just pulling your 1 meter leg (or is it 1 yard and some fraction) My point was that you are comfortable with what you are comfortable with. The end result will be the same.

 

That's my $0.02 7/16 worth or is it $0.06375?

[Spodman]

33.333

To somebody not used to it the US preoccupation with fractions & fear of decimals is a bit bewildering, why anybody would have a decimal currency and still refer to 102 33/64 cents defies me, particularly seeing the smallest coin we have in circulation here is 5 cents! What would be so wrong or bewildering about $1.025? I note Qantas hit a low of AUD4.930 this morning, If I buy 10 shares at 4.935 I can write a cheque for it!

 

Why decimal inches? Buggared if I know but using whole inches leaves a big gap between possible measurements, and using one decimal place leaves only 2.5 mm between graduations, this gives me an indication of how accurate to be with my cuts. If Burt wanted me to be more accurate he'd have said 33.33 inches giving .25 mm between graduations. A third of 100 inches? 33.3 inches, further decimals are not significant to the task.

 

Why did Burt use decimal inches? Coz he's a smart man, that's why.

[Jon Matcho]

I need to clarify a few things, such as the title of this thread. It should really have been one of the following:

• Why decimal inches suck for larger cosmetic parts
• Why decimal inches suck when they're larger than my micrometer
• Why decimal inches that don't work out to halves, quarters, eights, or sixteenths suck
• Why decimal tape measures suck

I was half on the fence with thinking that measurements should be written in fractions, such as 5-1/4", but now I completely agree that this is best written as 5.25 inches -- maybe even throw in an extra zero (5.250) for added precision .

 

Why did Burt use decimal inches? Coz he's a smart man, that's why.

Not arguing that Burt isn't smart. It's just that whether the seatback is 28.8 or 28.75 inches doesn't matter. The Long-EZ and Cozy's seatback angles are driven by (or supported by) a 1" wide surface offset by 0.7" from forward to aft surfaces. The AeroCanard designer thought it was better to use 0.65". Give me a break.

 

I'll bet that most who have actually assembled a fuselage would agree that 0.05" one way or another doesn't matter one iota. So... why not make decisions for these things to come out to measurements that are readable on the 5 tape measures I have lying around my shop in strategic easy-to-grab locations?

 

That was all in good fun. I'm done now. Think I'll get this Christmas thing behind me, do my year-end stuff, and set some real goals for making progress on this big model airplane of mine.