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do not assume

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carlossa52(at)gmail.com
Guest

Posted: Tue Jul 15, 2008 6:51 pm Post subject: do not assume

So, you already know the bottom line to this note.

Now please bear with me while I vent a little - and share a lesson.

Eons ago, when I started building, I obsessed about getting a true straight edge (check archives, around late XXth century).

 I eventually settled on (1) accepting that the edge of a sheet 4x8 or 4x12 is straight and (2) an 8' melamine shelf as a "ruler".

I also (3) assumed that an aluminum sheet, as delivered by the mill/dealer, is rectangular. 

 

Weeks ago I cut the (outboard wing) skins from 4x12 sheets, using the assumptions above.

I have drilled the (HD) outboard nose skin and the top skin. All nice and clecoed.

Today, when I positioned the bottom skin in place, I noticed something was off. About 5 mm off.

 As if the rear z was offset to one side by 5 mm. Pardon y French, but (at)#%!

So, I disassembled the whole thing, and, to verify if the top and bottom skins were (are) square, I placed them one of top of the other.

Well, they disagree. By about 5 mm.

 I either made a mistake when cutting one of them or assumption (3) above isn't true. 

Of course, it's now impossible to verify which one is true.

(In a few days I might admit it was my mistake, but for now I have to blame somebody/thing else.)

 So there goes a hundred bucks (or so, we'll see when I buy another sheet) out the window and, more important, umpteen hours (technical term for several evenings) polishing the darn thing.



Does anybody know where I can get a precision, real large, square?

 



Carlos

CH601-HD, plans (as in "plans to finish someday")

One step for man, two steps backwards.



  [quote][b]

ch701builder

Joined: 07 Feb 2006
Posts: 119
Location: N38.9947,W105.1305,ALT. 9,100'

Posted: Tue Jul 15, 2008 7:42 pm Post subject: do not assume

Hey Carlos,

One way to see it your corner is square is to do the 3-4-5 triangle measurement on the corner. That should get you a square corner to start with and go from there.

(3'side -- 4'side -- 5'diagonal) or any multiples of this,

such as 300mm--400mm--500mm, or double the distances 600mm--800mm--1000mm

Using these numbers should get you back to a square corner.



Good luck,

Keith

ch701 -- scratch

N 38.9947

W 105.1305

Alt. 9,100

*************************************************************************

--

jaybannist(at)cs.com
Guest

Posted: Wed Jul 16, 2008 4:04 am Post subject: do not assume

Carlos,

 

 Try the hardware store.  Look for a "drywall square".  

 

 Jay in Dallas

 

   

 

   

 

 --

Al Hays

Joined: 01 Oct 2007
Posts: 42
Location: Gore, VA

Posted: Wed Jul 16, 2008 4:04 am Post subject: do not assume

Carlos,



As an alternative to the excellent 3-4-5 triangle measurement Keith  

suggests, I use a drywall square.  It's like a 4 foot version of the  

thing I've seen used at drafting tables before CAD came along.  I  

also use a carpenter's square for smaller items.  I'd be lost at a  

drafting table and don't know squat about CAD, but I trust the  

drywall and framing squares.  If you find a used drywall square, just  

be sure it hasn't been bent or gotten loose from rough treatment.   

Home Depot or Lowe's are convenient places to get new ones at  

reasonable cost.



Al

CH701 & 601XL, N701GH & N5892H reserved



On Jul 15, 2008, at 10:46 PM, Carlos Sa wrote:



 	  Quote: 	
		   Does anybody know where I can get a precision, real large, square?

 Carlos

 CH601-HD, plans (as in "plans to finish someday")

 One step for man, two steps backwards.

rsteele(at)rjsit.com
Guest

Posted: Wed Jul 16, 2008 7:52 am Post subject: do not assume

And don't assume any square you buy will be square, most aren't.  In

particular framing squares usually need to be adjusted.  You do this by

peening the corner to 

open or close the square a bit.  Peen the outside to close, the inside

to open.

I've found drywall squares to be better, but check them regularly.



If it makes you feel any better, I'm now fitting my first rear wing

skins.  The pre-punched holes, done by a cnc machine, are off by 4mm

from rr1 to rr9.  This, I

can tell you is a real pain in the neck.



And while we are on the subject, don't trust tape measures either.  If

you use more

than one they should be checked against each other over whatever the

maximum length

you are using.  I've found most I've used to be in agreement, but I've

seen at least

one that was off an inch in 25ft.



Ron



------

carlossa52(at)gmail.com
Guest

Posted: Wed Jul 16, 2008 9:07 am Post subject: do not assume

Thanks to all that replied.



I too found that tape measures aren't to be blindly trusted. 

Actually, this goes for any measuring tool bought at a hardware store, whether they have an aerospace department or not.

 

Carlos

do not archive



2008/7/16  <rsteele(at)rjsit.com (rsteele(at)rjsit.com)>:

[quote] --> Zenith-List message posted by: <rsteele(at)rjsit.com (rsteele(at)rjsit.com)>

 

 And don't assume any square you buy will be square, most aren't.  In

 particular framing squares usually need to be adjusted.  You do this by

 peening the corner to

 open or close the square a bit.  Peen the outside to close, the inside

 to open.

 I've found drywall squares to be better, but check them regularly.

 

 If it makes you feel any better, I'm now fitting my first rear wing

 skins.  The pre-punched holes, done by a cnc machine, are off by 4mm

 from rr1 to rr9.  This, I

 can tell you is a real pain in the neck.

 

 And while we are on the subject, don't trust tape measures either.  If

 you use more

 than one they should be checked against each other over whatever the

 maximum length

 you are using.  I've found most I've used to be in agreement, but I've

 seen at least

 one that was off an inch in 25ft.

 

 Ron

 [b]

bryanmmartin

Joined: 10 Jan 2006
Posts: 1018

Posted: Wed Jul 16, 2008 2:34 pm Post subject: do not assume

If you want to know if a sheet of metal really is rectangular, measure  

the distance from diagonally opposite corners. If both diagonal  

measurements are exactly the same, you have a rectangle. The same  

method can be used to verify that the rectangle you have marked up for  

cutting actually is rectangular before you cut it.

On Jul 15, 2008, at 10:46 PM, Carlos Sa wrote:



 	  Quote: 	
		   So, you already know the bottom line to this note.

 Now please bear with me while I vent a little - and share a lesson.

 Eons ago, when I started building, I obsessed about getting a true  

 straight edge (check archives, around late XXth century).

 I eventually settled on (1) accepting that the edge of a sheet 4x8  

 or 4x12 is straight and (2) an 8' melamine shelf as a "ruler".

 I also (3) assumed that an aluminum sheet, as delivered by the mill/ 

 dealer, is rectangular.




	




-- 

Bryan Martin

N61BM, CH 601 XL,

RAM Subaru, Stratus redrive.

do not archive.

_________________
--
Bryan Martin
N61BM, CH 601 XL, Stratus Subaru.
do not archive.

raymondj(at)frontiernet.n
Guest

Posted: Wed Jul 16, 2008 3:25 pm Post subject: do not assume

This method will work IF you verify that parallel sided are the same length.



Raymond Julian

Kettle River, MN



"Hope for the best,

but prepare for the worst."



---

Ron Lendon

Joined: 10 Jan 2006
Posts: 685
Location: Clinton Twp., MI

Posted: Wed Jul 16, 2008 8:12 pm Post subject: Re: do not assume

Carlos,



The diagonal measure is the quickest and most accurate to find out if it's square.



I have been using a full sheet of metal as a straight edge for making cuts, using the factory edge.  Just set two sheets up, one is the part the top piece is the cut guide.

_________________
Ron Lendon
WW Corvair with Roy's Garage 5th bearing
CH 601 XLB
N601LT - Flying
http://www.mykitlog.com/rlendon
Corvair Engine Prints:
https://sites.google.com/site/corvairenginedata/

Darryl Legg

Joined: 08 Nov 2007
Posts: 18
Location: Darwin, Australia

Posted: Thu Jul 17, 2008 5:13 am Post subject: Re: do not assume

Hi Carlos,

I use a large ceconite square generally used by glaziers. It is 6ft long by 3ft on the 90 deg angle. Very handy. Your local glaziers should be able to point you in the right direction to purchase one.

Darryl.

do not archive.

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