my new standard for accuracy needs help

[Dale R. Hamilton]

So I determined that I would tackle this new build with a degree of accuracy and precision that I've never used before. Instead of 9 foot and 3 iddy biddys, it would be 9' 3/32's. I went out and bought the incra system rulers, 5mm pencil, new glasses so I can see. But then it occurs to me that damn near everything you measure on a boat is going to be way longer than an incra ruler. Like how else do you measure 9 foot something except with a tape measure. And we all know how accurate a tape measure is. So how do you guys make measurements that are long accurate, and precise?

[Keith Wilson]

You don't. Fortunately, you don't have to. Remember the old joke; a house carpenter gets fits to the nearest 1/8", a cabinetmaker works to the nearest 1/32", and a boatbuilder makes fits to the nearest boat. Numbers don't matter; fits do. It doesn't matter if the part is 9' 3-3/32" or 9' 3-1/8", just that it fits well where it goes and the water stays outside the boat. Make parts to fit, not to dimensions.

[Ken Hutchins]

Fust, forget about the fractions go decimal [img]smile.gif[/img] , then get a good becimal foot tape measure that is certified and tracible to the national bureau of standards.
Actually the accuracy of the measurement is not as important as consistancy so always use the same tape measure and frequently examine the hooked end to be sure it is not bent. An occasional check against a flat steel rule is the only way to know if your steel tape is changing. Never check a steel tape against the readings on an aluminum rule which will be drastically affected by temperature, humidity and barametric pressure.

[ion barnes]

I avoid being that precise. Not much point with poor eye sight. Just cut oversize and trim to fit, as in 'good enough for me'.

[Keith Wilson]

If you find an aluminum ruler that's affected by humidity and barometric pressure, I'll eat it. (Well, maybe not, but I'll be very surprised ). Temperature I'll give you.

I agree 100% about decimal inches, particularly for small measurements. For $15 you can get a set of 6" dial calipers which will measure to an accuracy of .001" or thereabouts. Besides, calculators like decimals a whole lot more than they like fractions. Just remember that 13/16" is .8125 and you'll be fine.

[ssor]

Knife marks on a long stick are quite good and don't need numbers. For inside measuring use two sticks and mark the end of the overlap on one.

[dmede]

Well this is why you often don't measure things but mark them instead. Using a long batten, or the peice you will be cutting, you can mark off the leangths. Or use the tape measure and be a little generous, you can get the perfect fit when it comes time to install.

[Bruce Hooke]

Lots of good suggstions have been made. I will just add that I think the point at which actual numeric accuracy matters most is in the lofting and initial setup. From there on out you are working much more to the boat and the lofting than to a ruler.

So, before you start lofting check your tape measure carefully and try to stick with one tape measure. I find it very useful to have a 2' stainless steel ruler for measurements that are within that range, but if you are being careful and exacting you should check that ruler against your tape measure and make sure they agree before you start lofting.

[Jim H]

You might want to consider taking an old 25' tape apart. Drill out the rivet that holds the hook-end on. The tape is accurate, it's that adjustable end that gets sloppy. I remember this from an older thread that I think Dave weighed in on.

[Ken Hutchins]

If you find an aluminum ruler that's affected by humidity and barometric pressure, I'll eat it. (Well, maybe not, but I'll be very surprised ). Temperature I'll give you.

Keith, start eating, it is a fact barametric pressure will affect the dimensions of virtually
everything. I was involved in the investigation of why the OD of some jet engine rotors ground on a particular day were oversize and interfered with the casing they went into causing engine failures in test.

These parts are ground in a temperature controller environment. My theory was the atmospheric pressure because the particular day in question set a new record high in Boston 60 miles away. Nobody believed me of course, but after extensive tests I was proven right and the gage for measuring the parts which had an aluminum body had to be redesigned to change the material and change the method of locating it to decrease the length which could be affected by such things. The length of the gage part involved was only about 5 inches but the effect of the pressure on a unit of length times the total length was enough to cause the failures.

[Keith Wilson]

Mmmmm, crunchy!

How much did a 5" part change due to barometric pressure?!? Not boatbuilding tolerances, I'd bet, at least not MY boats.

[Billy Bones]

Some good points here. Some lunacy too! There's a reason why offsets are in feet inches and eighths. Accuracy is much less important than consistancy.

To sumarize, the error in tapes is in the first inch (that is to say where the hook and tape fit together.) There SHOULD be some slop in the attachment, however. This is so that when you measure INSIDE something (banging the end of the tape up to an inside edge) and OUTSIDE something (say, to cut a piece to fit inside something else) they end up the same measurement and are not off by the thickness of the hook.

KEY: Use your fancy new rules to ensure that the the 1" mark is at one inch both pushing (inside measure) and pulling (outside measure). Then, use the same tape throughout your job (rechecking it when you drop it on a hard surface to ensure the hook is still square and floats properly on the tape.

When several carpenters show up on a job together, the good ones will synchronize their tape measures this way periodically.

[George Roberts]

Ken Hutchins ---

Modulus of elasticity of metals is over 10^6. You can figure the change in size for 14psi.

I expect that what you say is more myth than fact.

[Gary E]

Ken,
" certified and tracible to the national bureau of standards."

Yeah, datz da keeper of da accurate stuffs, they changed the name to NIST a long time ago to cunfuddle us, or maybe just to justify the raise in the calibration charges, but you probably knew that. [img]smile.gif[/img]

[Steve Lansdowne]

Let's see, didn't somebody build an ark long ago using the length of his arm and of his thumb to the first knuckle as standard measures ...

[ssor]

Chop down a tree and chop away everything that doesn't look like a boat. One tool, no tape measure. All we have done since is refinement. Some of them look pretty fine.
As long as we set a standard for a project it doesn't matter what the values are unless you are trying to convey them to someone else. "Make it as long as your arm" won't be the same size as "make it as long as my arm". But we can depend on 4-3-4, feet, inches, eighths

[Del Lansing]

These parts are ground in a temperature controller environment. My theory was the atmospheric pressure because the particular day in question set a new record high in Boston 60 miles away.

The skeptic in me would guess the atmospheric pressure change affected the test areas' thermostat by compressing the diaphram. The temperature that day jumped up. A wopping change of 3 inches in AP would represent 1/24 psi, enough to deform aluminum?

[Ken Hutchins]

Change the aluminum it did, you all can imagine the skeptics at work. But there had been occasional problems with these particular parts for several years despite many theories and usually ending up blaming the machine operators the most recent 'improvement' to the process was the conversion to electronic gages in place of the original dial indicators. The records from the electronic gages proved the operators were not at fault. So after a lot of meetings and investigations in an attempt to disprove or approve my theory we made a pressure test chamber to do a side by side comparison between a piece of hardened tool steel and a piece of aluminum. The test results showed exactly what I had predicted.

[Paulyboy]

Ken- u da man! Reminds me of a stry from my early days in the car parts bidness. 20 years ago I worked for my father-in-law at a parts store near Cincinnati Milacron- VERITABLE NEST OF SLIDE RULE FANATICS. One day an engineer came in and asked to see front wheel bearings for his 69 Chevy Impala. He looked at them for 10 minutes turning them over and around and looking at every nook and cranny. He said he'd be back. The next day, he brought a buddy along and they both looked at them for some time, discussing measurements, etc, and still didn't buy them. The third day, they brought in a red shop rag with a dirty, worn out, blued from heat nasty piece of metal. They took out their calipers and started measuring. They measured the new one, then the old one, then the new one. One of them looked at me and said "These aren't the right bearings. I need some that measure the same as this one." It took me 5 minutes to explain to frick and frack that if they actually did find some that measured the same as their old ones, they wouldn't be fixing anything because they were measuring WORN OUT PARTS!

[Keith Wilson]

Ken, exactly how much dimensional change are you talking about? I've worked with small tolerances off and on, and I know that very odd things can happen and factors that any reasonable person would swear would have no effect, do. I've never run into this one, though.

[Ken Hutchins]

Keith the change was about .0005. the gage measured the radius of compressor rotor blade tips. That change is more than enough to really cause expensive problems with something that runs at 45,000 RPM and relies on virtually zero clearance between the rotating and stationary parts.

[Keith Wilson]

Jesus H. Christ, HALF A THOU due to barometric pressure!?!?! That's like a mile or two in close tolerance machining. You said a radius, though, not a linear dimension; was it due to bending of the part or the guage? That seems like an enormous change due to less than 1 PSI variation.

[Del Lansing]

Keith the change was about .0005.

That seems like a lot, the change for aluminum is 12 ppm per degree F. I picture this guage as checking the gap between the rotor and the stationary shroud? I see the need for proper tolerances but in operation the unit would go through more extreme changes in temperature and pressures.

[Ken Hutchins]

The aluminum gage was linear, the part measured is a radius. I forget the actual number but it was near the half. This sure affected the thoughts on gage designs. Ironically the reason I thought about that being the cause was I had to take a trip to another plant on the day in question and I heard on the radio about the record high while I was driving.

[Ken Hutchins]

Del, all the operating conditions are taken into consideration in the design and the proper clearance is calculated, in fact the casing is not even round when cold to allow for expansion differences. If the clearance is too great the engine won't meet perforance specs, not enough clearance causes a major problem to occur.

[David Tabor (sailordave)]

Let's see, didn't somebody build an ark long ago using the length of his arm and of his thumb to the first knuckle as standard measures ...

I can't believe one of our resident WISEGUYS hasn't chimed in with an " ALLEDGEDLY"

Then again this sure ain't the Bilge [img]smile.gif[/img] (thank God!)

[ 03-24-2005, 09:31 AM: Message edited by: David Tabor (sailordave) ]

[essaunders]

"measure to the nearest boat" was the phrase that recently kicked my off my latest slump. I was reading through Scott Landis' Workbench book and read that. That made me realize that I won't be able to pre-build my canoe parts - I have to put them on the canoe and shape them to fit.

[Alan D. Hyde]

Dave Fleming (for one ) will tell me if I'm wrong, but aren't traditional boats historically built to 1/8" tolerances???

Alan

[John B]

If it were a racing yacht I think it might depend on the sort handicap its looking for.
A recent restoration here discovered that the 50 footer which is 50' on every register/race form from 1930 odd when it was launched is actually 51'6"
on a dry day.
in a dry month.

[ 03-24-2005, 03:31 PM: Message edited by: John B ]

[pipefitter]

All tape measures are accurate to within 5200ths of an inch [img]smile.gif[/img]

[Ocean Spray]

Recently while measuring a room for a wrap around carved sign, the old time finish carpenter I was working with refused to take my dimensions in feet and inches.
He would only take the dimensions in inches, he said converting the feet to inches was just another mistake waiting to happen.
Makes sense!

[ion barnes]

That right Ocean, as in 6.0' or 60". Another is never measure using the end of the tape or rule incase the end has been damaged. I have heard of someone with a wooden rule, broke the end, trimmed it to the 1st inch, everything came out a little short!

[ssor]

One shop where I worked had a folding rule that had duplicate sections following the thirty six inch hinge so that you could read 39 inches at two separate points. Finally one of the guys put it on the table saw and cut the whole thing in two.

[RonW]

SIMPLY AMAZING - Are we talking about a wooden boat, or some kind of part for the space shuttle?

[paladin]

I remember reading a biography of John Moses Browning.he walked aroud the shop with a ruler in his pocket that didn't measure less than 1/32nd of an inch.....and his designs always worked....

and a few years ago a jet fighter was being manufactured at several plants across the u.s....and when all the parts wuz shipped to california, nothing fit due to ambient temps at different parts locations.....but boats izzint built to those tolerances..