Model Rocket Math: Converting Decimal Inches to Fractional Inches

[dr wogz]

I dont *beleive* imperial units, which while still are used in other parts of the world, are considered part of the International Standard...could be wrong though..

3 actually:
the US, Myanmar, and Liberia

many are "forced to" because of the US; me / Canada namely.. GM, Ford, etc.. as well force it on others..

https://www.statista.com/chart/1830...ries – the U.S.,body parts or everyday items.

 

[dr wogz]

I don't know about WA but for a time CA had those signs.

Personally I will never convert to the metric system. It is under 400 miles between San Francisco and Los Angeles but it is over 600 kilometers and that's just too danged far to drive!

But you can drive it at 130kph!!

(sure beat a slow 70 knots..)

 

[kuririn]

As to high speed computers, I had not heard that reason given before. I do know that back in the 80's (could have been late 70's as well) the US Govt tried to get everyone over to the Metric system. I vaugelly remember seeing speed linit signs in WA or CA that were in both KPH and MPH.

I remember gas pumps showing liters instead of gallons.
That didn't last long.

 

[brockrwood]

You only need worry about the 0.784. Multiply by 64, gives 50.176 or just slightly over 50/64. Both numerator and denominator are even, so divide both by two (do you really need a calculator for that?). Gives 25/32, so 6 and 25/32.

FWIW 25 is 2 x 12 1/2. 32 is 2x16. So it's a bit more than 12/16. Which is 3/4. So...slightly more than 6 3/4.

Let's go back to millimeters, it's a heckuvalot simpler....

Thanks for pointing out that one can just forget about the part of the decimal inches figure to the left of the decimal point. That part is already in whole number inches.

So, 6.784 inches is, well, 6 whole inches plus .784 inches - the only part that needs converting to the closest fraction. That makes it a lot easier. As another commenter said, just multiply .784 by the denominator of smallest fractional unit shown on your ruler. Then round that number to closest whole number and put it over the desired denominator.

Now I know why my ruler only shows 1/32’s of an inch for the first inch. These small units are only needed when you get to the END of the length you are measuring.

 

[brockrwood]

Hmm. 1/32 of an inch is noticeably smaller than 1 mm. I do, occasionally, have to split a millimeter in half when working on LPR rockets. Maybe the metric system is not perfect? Maybe millimeters need to be a bit smaller?

 

[Hyak]

I don't know about WA but for a time CA had those signs.

Personally I will never convert to the metric system. It is under 400 miles between San Francisco and Los Angeles but it is over 600 kilometers and that's just too danged far to drive!

No, No, No.... You are wayyyy overthinking this. The SFO>LAX trip is easy, I can explain it in two words: "FLY DELTA!"

 

[Hyak]

Hmm. 1/32 of an inch is noticeably smaller than 1 mm. I do, occasionally, have to split a millimeter in half when working on LPR rockets. Maybe the metric system is not perfect? Maybe millimeters need to be a bit smaller?

View attachment 518041

There is actually (I believe its the Micrometer) but you're probably building rockets for NASA if you need those tolerances!

 

[Steve Shannon]

There is actually (I believe its the Micrometer) but you're probably building rockets for NASA if you need those tolerances!

The micrometer or μm (1 x 10^-6 meter) is also commonly referred to as the micron. Next smaller is the nanometer, then the picometer, etc.

 

[brockrwood]

There is actually (I believe its the Micrometer) but you're probably building rockets for NASA if you need those tolerances!

Yep! I thought of that. There is no way to mark an ordinary ruler in micrometers.

I was thinking more of day to day use of measuring sticks by hobbyists, technicians, and tradespeople. My idea is that the millimeter, to be perfectly suited to use as the smallest hashmarks on a measuring stick, should be a unit of measurement small enough that, using any ordinary ruler, and any ordinary set of human eyeballs, you can't distinguish any unit of measurement smaller clearly.

I am old. I have bad vision. But I can see clearly enough when a point falls directly between two millimeter hashmarks on a ruler. Then I have to measure the half millimeter, like so: 1.5 mm, or 3.5 mm, or 16.5 mm. I wish the millimeter were small enough that I could just use whole numbers and dispense with "half a millimeter".

Case in point: There are metric sized drill bits. Those drill bits comes with 1.5 mm, 2.5 mm, 3.5 mm, 4.5 mm, et cetera, bits, as well as whole number sized bits, 1, 2, 3, 4, and 5. The millimeter is small, but not quite small enough.

But it is far too late now. The meter was established as the basic unit of measurement of length, for all of humanity, for all practical, day to day purposes, and we are stuck with it. It just is a bummer. If the meter were smaller, in absolute length, then the millimeter would be the perfect size to use as the hashmark unit on a ruler.

Oh well. With my imperfect ruler, I will return to the task of making an LPR parachute out of a dry cleaners bag...

 

[Steve Shannon]

Yep! I thought of that. There is no way to mark an ordinary ruler in micrometers.

I was thinking more of day to day use of measuring sticks by hobbyists, technicians, and tradespeople. My idea is that the millimeter, to be perfectly suited to use as the smallest hashmarks on a measuring stick, should be a unit of measurement small enough that, using any ordinary ruler, and any ordinary set of human eyeballs, you can't distinguish any unit of measurement smaller clearly.

I am old. I have bad vision. But I can see clearly enough when a point falls directly between two millimeter hashmarks on a ruler. Then I have to measure the half millimeter, like so: 1.5 mm, or 3.5 mm, or 16.5 mm. I wish the millimeter were small enough that I could just use whole numbers and dispense with "half a millimeter".

Case in point: There are metric sized drill bits. Those drill bits comes with 1.5 mm, 2.5 mm, 3.5 mm, 4.5 mm, et cetera, bits, as well as whole number sized bits, 1, 2, 3, 4, and 5. The millimeter is small, but not quite small enough.

But it is far too late now. The meter was established as the basic unit of measurement of length, for all of humanity, for all practical, day to day purposes, and we are stuck with it. It just is a bummer. If the meter were smaller, in absolute length, then the millimeter would be the perfect size to use as the hashmark unit on a ruler.

Oh well. With my imperfect ruler, I will return to the task of making an LPR parachute out of a dry cleaners bag...

Common metric scales are graduated (hashmarks) in 0.5mm increments, which allows estimating 0.25mm measurements.

 

[modeltrains]

... which is pretty smack-dab in the middle of . . .

Sandy.

Tuesday Tangent Time ...

http://verbmall.blogspot.com/2007/11/smack-dab-in-middle.html
"What does smack dab in the middle actually mean, and where did it come from?"

 

[brockrwood]

Common metric scales are graduated (hashmarks) in 0.5mm increments, which allows estimating 0.25mm measurements.

I need to get one of those rulers with .5 mm hashmarks! That would be really handy!

 

[brockrwood]

Common metric scales are graduated (hashmarks) in 0.5mm increments, which allows estimating 0.25mm measurements.

Such rulers do exist! I found a set on Amazon for $7.99! Woo hoo! Amazon, here is my credit card number...


Thanks for turning me on to this useful tool!

 

[bjphoenix]

Hmm. 1/32 of an inch is noticeably smaller than 1 mm. I do, occasionally, have to split a millimeter in half when working on LPR rockets.

For creating things like fin marking guides or paint patterns I will mark between 2 millimeter marks. I've found that with a sharp pencil the best I can mark is to about half a millimeter precision, maybe just a tiny bit better.

 

[brockrwood]

Please, I am genuinely curious: why did high-speed computers defeat the move to the metric system? I haven't' heard this.. curious to the reason(s)..

The problem of converting units from Imperial to Metric and back again has to be done manually in a paper and pencil society. We are now a computerized society. All calculations, including conversion of units calculations are done instantly, in the background, without any human intervention, by the computerized devices that surround us. Who care what units something is measured in when the computer just calculates it to whatever decimal place you wish in any measurement system you care to use?

The Metric Conversion Act was passed in 1975. We were basically a pencil and paper society then. All we had were pocket calculators and slide rules and paper files. What units you measured things in was more critical because you did it all manually. Converting was a pain. Trading with metric-based countries was a real problem if they didn’t wsmt Imperial measured stuff.

After the Apple II and the IBM PC, everything changed. Other countries, metric countires, sell us stuff messured im Imperial units. No biggie. The computers convert everything and then control the robots that make the stuff. Think about CAD/CAM - click a button on the screen and use any units you want to.

 

[anbhtblr]

I do my best to work in metric. Whether using QCAD or Fusion 360 for design work it's mm. Of course I have to do conversions as manufacturers/vendors haven't convert to metric (shame on them [thanks motor manufacturers]) and the RSO expects CP/CG in imperial units (I'm thinking of starting to use statute miles). Also, shame on NAR/TRA for not ACTIVELY pushing metric.

 

[Funkworks]

... Maybe millimeters need to be a bit smaller?

I think a millimeter should be ten times smaller exactly. If the entire metric system was scaled down by a factor of ten, everything would be perfect.

 

[Steve Shannon]

I do my best to work in metric. Whether using QCAD or Fusion 360 for design work it's mm. Of course I have to do conversions as manufacturers/vendors haven't convert to metric (shame on them [thanks motor manufacturers]) and the RSO expects CP/CG in imperial units (I'm thinking of starting to use statute miles). Also, shame on NAR/TRA for not ACTIVELY pushing metric.

The RSO expects the Cp/Cg in imperial units? Really? Then you didn’t do as told.
Cp should be permanently marked on the rocket and the flyer must be able to demonstrate where Cg is. Neither of those require the use of units. They should be at least one caliber apart with the Cg ahead of the Cp. A caliber is also without units.
Our motors are measured in SI units for thrust and impulse.
Our Safety Code provides both systems because the one place you don’t want people fumbling around trying to convert is in safety related things.

 

[Blast it Tom!]

I do my best to work in metric. Whether using QCAD or Fusion 360 for design work it's mm. Of course I have to do conversions as manufacturers/vendors haven't convert to metric (shame on them [thanks motor manufacturers]) and the RSO expects CP/CG in imperial units (I'm thinking of starting to use statute miles). Also, shame on NAR/TRA for not ACTIVELY pushing metric.

Use barleycorns - 1/3 of an inch.

I had a whole bunch of archaic units defined in my Mathcad template so that I could prank the boss every so often with volume in firkens or hogshead, lengths in chains, fathoms, hands, hanks, and so on. As the thread has noted, units-aware programs have worked around the whole imperial-metric thing quite nicely!

 

[StreuB1]

People who argue about metric and imperial make me giggle.

 

[brockrwood]

Use barleycorns - 1/3 of an inch.

I had a whole bunch of archaic units defined in my Mathcad template so that I could prank the boss every so often with volume in firkens or hogshead, lengths in chains, fathoms, hands, hanks, and so on. As the thread has noted, units-aware programs have worked around the whole imperial-metric thing quite nicely!

Cubits. Like Noah. What is a cubit, anyway?

 

[bjphoenix]

Please, I am genuinely curious: why did high-speed computers defeat the move to the metric system? I haven't' heard this.. curious to the reason(s)..

I think stubborn people defeated the move, but some things did change for instance all the fasteners in my American manufactured automobiles are metric and I had to buy all new tools some years ago.
I just realized that my cheap digital caliper will instantly switch between different units with the push of a button and it's very far from a high speed computer. A person could actually use it for conversions- open it up until it reads the dimension then push the button and it will convert the dimension to different units.

 

[cerving]

I remember gas pumps showing liters instead of gallons.
That didn't last long.

Yeah, some of the gas stations started showing price per litre... instant 70% discount. Once people started asking "What the Hell is a litre????" then that stopped real quick.

 

[Blast it Tom!]

I think stubborn people defeated the move, but some things did change for instance all the fasteners in my American manufactured automobiles are metric and I had to buy all new tools some years ago.
I just realized that my cheap digital caliper will instantly switch between different units with the push of a button and it's very far from a high speed computer. A person could actually use it for conversions- open it up until it reads the dimension then push the button and it will convert the dimension to different units.

I thnk it's more than stubborn people - it's no small thing to re-tool factories, etc. Now since we don't have many of those any more, if the idjits in DC ever fulfill their outright lies, ahem, solemn promises to bring back manufacturing, well, we could start fresh with metric, I suppose... I like your handy-dandy caliper converter. I just bought an old Starret 122 12" vernier caliper at a flea market for $20, but the slide is frozen, been soaking it in PB Blaster for 3 days now. (tangential remark). To cheap to send it back to them.

 

[Sandy H.]

A sales guy years and years ago said his company went 'soft metric' in the 1990's. That meant, in their case, they kept imperial fasteners, but switched to metric dimensions on castings, hole patterns etc.

So, basically the thing that never changes (the tapped hole) they kept imperial, which seems logical unless every tool kit you bought in the past 20 years had both metric and imperial wrenches. The things that a guy would need to measure and do math on switched to metric and therefore the imperial tape measure was always introducing bad error.

I say scrap both systems and come up with a new one we can argue about. Or accept that people should be smart enough to figure out what they are dealing with and select the right system.

Sandy.

 

[dr wogz]

Ever wonder why your big bottle of Coke is 69oz? that's 2 liters..

Or in16.9oz bottles? that's a 1/2 liter

packaging machines..

Drugs are usually in 'mg'

1 cup = 250ml (237ml actually)


metric is quietly creeping in..


snarky mode: /off

 

[anbhtblr]

The RSO expects the Cp/Cg in imperial units? Really? Then you didn’t do as told.
Cp should be permanently marked on the rocket and the flyer must be able to demonstrate where Cg is. Neither of those require the use of units. They should be at least one caliber apart with the Cg ahead of the Cp. A caliber is also without units.
Our motors are measured in SI units for thrust and impulse.
Our Safety Code provides both systems because the one place you don’t want people fumbling around trying to convert is in safety related things.

I stand corrected about TRA - thanks. When writing I was thinking about artifacts like the safe distance tables and my memories were/are of launches where those are only spoken of in feet. The Unified Safety Code definitely has both metric/imperial measurements. But my question is why not metric only? I do mark CP/CG on the airframes, but if a discussion ensues its almost always in imperial measurements.

 

[Steve Shannon]

The Unified Safety Code definitely has both metric/imperial measurements. But my question is why not metric only? I do mark CP/CG on the airframes, but if a discussion ensues its almost always in imperial measurements.

I already answered that. My last sentence read:

Our Safety Code provides both systems because the one place you don’t want people fumbling around trying to convert is in safety related things.

Put more simply, it’s not our mission to convert people to a different system, but it is our mission to make safety as easy to achieve as possible. History has shown that issues arise when people are required to convert from one to another.
The majority of our members still use English units. Until the USA has fully converted, it would actually increase risk to publish our rules in metric units only.

 

[StreuB1]

When my coworkers in the UK bust my chops in their own dry fashion about inches....I kindly remind them that I have been to the Uk many times and I know their secrets.

If you know, you know.

 

[dr wogz]

A "Royale with cheese'?