Three decimal places on a mm dimension, or cm? 1 µm tolerance is some pretty impressive machining. (Of course, four decimal places on an inch is 2.5 µm, which is impressive enough if you ask me.)
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Nytrunner
Pop lugs, not drugs
In Texas we skirted the problem by measuring distance in hours
Sorry Tex; we do that everywhere. (Some years ago I was relocating from the east coast to southern California, and the family decided to drive I-10 from end to end on the way. We found that in Texas, distance should be measured in days.)
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The Metrinch tools work good. They kind of ‘cam’ down on the flats instead of grabbing the corners. The only time they are tough to use is in tight spots where you don’t have much room to turn the handle...the designed slop in the sockets or spanners need an extra turn angle to grab the flats. My set is probably 20 yrs old.
I’ve owned a few vintage British bikes so I even have a set of Whitworth spanners. If that’s not weird enough, the battery is positive ground and it shifts gears on the right side. You have to recalibrate your brain before going for a ride. The first stop sign is the test....
I’ve owned a few vintage British bikes so I even have a set of Whitworth spanners. If that’s not weird enough, the battery is positive ground and it shifts gears on the right side. You have to recalibrate your brain before going for a ride. The first stop sign is the test....
On mm dimensions. Lots of medical, aerospace and Battlebots applications.
As the punchline goes, "I had a truck like that once."
We can blame Jimmy Carter for that. Dang it.
One of the biggest differences between the British and metric system is how mass and force are handled. The British system uses gc = 32 (lbm/lbf) ft/sec^2 ,where the metric system does not use gc at all, unless, you define kg(force) and kg(mass), but this most often is not done. If kgf and kgm are defined, then gc in the appropriate units must be introduced. Without gc, the metric system states that 1 nt = 1 kg-m/sec^2. Note this means that a kg mass will have a weight of 9.8 nt, because the acceleration of gravity is 9.8 m/sec^2. With "gc" in the British system a mass of 1 lbm will be numerically equivalent to 1 lbf at sea level. I treat "gc" with the collection of all the units as being equivalent to "one". Using "gc", force can be written as F= ma/gc. A lot of books will go around in circles on this topic, but this is essentially the basic facts. Other places where units can get confusing is in the perfect gas equation with the gas constant "R". This confusion can continue into finding the exhaust velocity. Sometimes a collection of units called "J" is used in the exhaust velocity equation, which can come in very handy.
I'm well acquainted with most of the stuff your saying here, but not with how you're saying it. 1 lbf is the weight of a 1 lbm object under 1 standard gravity, about 32.2 ft/s^2 (or exactly 9.80665 m/s^2). And "lb" is ambiguous, which is usually not a problem in daily life but can cause no end of confusion on occasion, and which gives rise to those lovely obscure units the poundal and the slug.
But what is this "gc" of which you speak? If, as you say, F=ma/gc then gc must be a dimensionless quantity equal to exactly one.
I've never had the pleasure of doing ideal gas calculations (or any others involving the ideal gas constant) in English units; I'm surprised to read that it makes a difference. The ideal gas law, PV=nRT, has quantities with units of pressure, volume, and temperature. R itself is typically expressed, at least in MKS, with units of energy and temperature. None of these are subjects of the mass-weight ambiguity.
I also have not experienced difficulty with exhaust velocity per sé, though my experience in fluid dynamic calculations is essentially nill. Are you referring to the disagreement over whether to express specific impulse as impulse per weight of propellent or per mass? Here the mass-weight ambiguity does indeed rear its ugly head once again. And tell me more about J; that one I've never heard of.
Also, you should be aware that the correct abbreviation for the Newton (MKS unit of force) is "N", not "nt".
But what is this "gc" of which you speak? If, as you say, F=ma/gc then gc must be a dimensionless quantity equal to exactly one.
I've never had the pleasure of doing ideal gas calculations (or any others involving the ideal gas constant) in English units; I'm surprised to read that it makes a difference. The ideal gas law, PV=nRT, has quantities with units of pressure, volume, and temperature. R itself is typically expressed, at least in MKS, with units of energy and temperature. None of these are subjects of the mass-weight ambiguity.
I also have not experienced difficulty with exhaust velocity per sé, though my experience in fluid dynamic calculations is essentially nill. Are you referring to the disagreement over whether to express specific impulse as impulse per weight of propellent or per mass? Here the mass-weight ambiguity does indeed rear its ugly head once again. And tell me more about J; that one I've never heard of.
Also, you should be aware that the correct abbreviation for the Newton (MKS unit of force) is "N", not "nt".
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If you are not familiar with "gc" and get along fine without it, by all means don't use it. I am not trying to "convert" anyone here (pun intended). Actually, one can take the collection of units called "gc" and plug into an equation anywhere and not even call it "gc". I've done that for years and it works, because gc =1. jd2cylman would have done something like this way back in his first post of this thread, when he encountered "kgf" in someone's definition of torque.
I'm not in any danger of being converted, but I'd still like to understand what you were saying.
jlabrasca
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gc is dimensionless, but not unity.
https://www.risacher.org/rocket/
In which weird little book you will find R in ft•lb/lb/R° -- pound-force on the top, pound-mass on the bottom, and absolute temperature figured in Rankine .... just because.
edit: If you look in the "nozzle "section of that pamphlet, you will also see an occurrence of gc
I learnt to write it "Nt" (to avoid confusion with all the other instances of capital N that will crowd the page during the solution of a problem -- another academic affectation, to go with crossed 7, lower case q, and z.)
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I told you that this would go in circles. I would say that gc with it's numerical value and collection of units is equivalent to one. Let's say that I want 1 pound mass (lbm) to have a weight at sea level to be equal to 1 pound force (lbf) or go to the grocery get a 1 pound box of cereal and bring it home put it on the scale and it weighs 1 pound (force). I can plug these numbers into F=ma or weight equals mass times the acceleration of gravity or w=mg. I get the following:
1 lbf = 1 lbm x 32.2 ft/sec^2
Now I divide both sides of the above equation by lbf and I get:
1 = (lbm/lbf) x 32.2 ft/sec^2 = gc
I can treat this grouping as being numerically equivalent to one. Your mileage will vary. I can write these equations differently, say for example,
w = mg or w = mg/gc
as long as an analyst is careful, both of these equations are correct and in fact you will see both forms in print. If you put the gc just any place in an equation, you will get a hodge-podge of units that does not make much sense or is simply impractical.
If I have any doubt about units, I like to write out the equation in full with all the numbers and all the units, so that I can see all the units cancel out. Returning, for example,
w = mg/gc = 1 lbm x 32.2 ft/sec^2 x (1 / ( (lbm/lbf) x 32.2 ft/sec^2 ) ) = 1 bf
which is the desired result.
If I collect the numbers differently in the first equation of this post, I get:
1 lbf = 32.2 lbm x ft/sec^2 = (32.2 lbm) x 1 ft/sec^2 = 1 slug x 1 ft/sec^2
or the definition of a slug = 32.2 lbm
1 lbf = 1 lbm x 32.2 ft/sec^2
Now I divide both sides of the above equation by lbf and I get:
1 = (lbm/lbf) x 32.2 ft/sec^2 = gc
I can treat this grouping as being numerically equivalent to one. Your mileage will vary. I can write these equations differently, say for example,
w = mg or w = mg/gc
as long as an analyst is careful, both of these equations are correct and in fact you will see both forms in print. If you put the gc just any place in an equation, you will get a hodge-podge of units that does not make much sense or is simply impractical.
If I have any doubt about units, I like to write out the equation in full with all the numbers and all the units, so that I can see all the units cancel out. Returning, for example,
w = mg/gc = 1 lbm x 32.2 ft/sec^2 x (1 / ( (lbm/lbf) x 32.2 ft/sec^2 ) ) = 1 bf
which is the desired result.
If I collect the numbers differently in the first equation of this post, I get:
1 lbf = 32.2 lbm x ft/sec^2 = (32.2 lbm) x 1 ft/sec^2 = 1 slug x 1 ft/sec^2
or the definition of a slug = 32.2 lbm
I do both, but give me metric anytime. Much less prone to error with mental arithmetic IMHO.
Speaknoevil
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My workshop, pieces/parts, tools, threads, scales, torque-measures, heck, even tiles on the floor are in feet/inches. When I see an Mx screw I throw it out! Too old to ever change (outside of motor nomenclature unfortunately); I even love Loki for keeping the 1/64th numbering system for nozzle throats! Down with decimals, long live fractions!
I love that you say that in praise of the god of chaos.
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I'm imagining nozzles denominated in 1/3 mm increments and I guess I don't entirely hate it? ... although it's a lot more etching.
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In normal life—well, depends on whether you think California is normal, I suppose—I use feet, inches, pounds and ounces. For instance, I have no idea what I weigh in kilograms or how tall I am in meters.
But from the very beginning, all of my products were designed solely in metric. Millimeters are a friendly scale for small products, and decimal math is just easier to work with in general.
I've gotten to where time-keeping and latitude/longitude have begun to bug me, too. I wish we used decimal math for those, too.
Internally, AltimeterThree stores time and GPS coordinates as simple integers, which are just as accurate and more compact than the normal minute/second or even floating point degree representation. I vote we put 32 bits each on latitude and longitude, and call it a day. That's 1 cm at worst precision at the equator, which should be fine.
But from the very beginning, all of my products were designed solely in metric. Millimeters are a friendly scale for small products, and decimal math is just easier to work with in general.
I've gotten to where time-keeping and latitude/longitude have begun to bug me, too. I wish we used decimal math for those, too.
Internally, AltimeterThree stores time and GPS coordinates as simple integers, which are just as accurate and more compact than the normal minute/second or even floating point degree representation. I vote we put 32 bits each on latitude and longitude, and call it a day. That's 1 cm at worst precision at the equator, which should be fine.
Anyone who thinks California is normal is probably from California. But all the rest above is normal enough for Mercans.
jlabrasca
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Nytrunner
Pop lugs, not drugs
Not from Mercury. From Merca! Merca, F-yeah!
Nytrunner
Pop lugs, not drugs
Hmm, never been there. Is it one of those central african countries folks keep forgetting about?
Now, son, I'm gonna 'sume yer just funnin' me and let it go. This time.
rharshberger
Well-Known Member
Actually I have always found the term Muricans/Mericans not neither funny nor appropriate, as someone who has seen friends die in the service of this country and have served my self the term is fairly repugnant.
Well, in that case I'm very sorry to have offered offense.
beeblebrox
8 C6-0, 12 D11-9, 20 D20-0, 20 E5-0, 3 Cinerocs
x = x right!
x² = x² Square both sides
x²-x² = x²-x² Subtract same from each side
x(x-x) = (x-x)(x+x) Left side, factor out an x, Right side Difference of two perfect squares = product of the sum and difference.
x = x+x reduce common terms (x-x)
Therefore if x = 1, 1 = 2, 2 = 3 ... all numbers are equal....bwaaaaaahahaha
x² = x² Square both sides
x²-x² = x²-x² Subtract same from each side
x(x-x) = (x-x)(x+x) Left side, factor out an x, Right side Difference of two perfect squares = product of the sum and difference.
x = x+x reduce common terms (x-x)
Therefore if x = 1, 1 = 2, 2 = 3 ... all numbers are equal....bwaaaaaahahaha
Nytrunner
Pop lugs, not drugs
You're just going to divide by zero like that in public?
beeblebrox
8 C6-0, 12 D11-9, 20 D20-0, 20 E5-0, 3 Cinerocs
It was a joke ... shows that following the rules is not always right....
NateB
Well-Known Member
For my job, we have to be familiar with imperial and metric standards. All of our medications at worked are dosed by metric measurements. We report milage for billing in statute miles, but milage is reported to us in nautical miles. Temperatures are reported in F or C depending on the source and that calculation is hard to do in your head.
Actually, it shows that seeming to follow rules while either not understanding them or applying them carelessly doesn't always get you the right result. Since it was a joke I assume that you did know what you were doing; I'm just saying if it's both a joke and a lesson then let's make it the right lesson.
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