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Or, can hot/cold humidity make them warp at all? Im talking a good one. Just wondering if you 'get what you pay for', as the measurements are in the thous...

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Here is what a Parlor Guitar is for!
http://www.youtube.com/watch?v=oEa8PkjO6_I

They hold up well. You would have to have some serious temperature fluctuations to warp one of these. If you have high humidity problems and the straight edge is not stainless it can rust though.

Just to be sure of terminology... We are talking about a Straight Edge -- a thick bar with ground flat edges used to check things are flat.... NOT a ruler.

If you are using a good one -- Starrett, Brown and Sharpe, Etalon, etc... They are really good... Regular ambient humidity and temperature swings found on a shop workbench shouldn't effect them....

They are precision measuring instruments and should be treated as such. The bigger problem with these is getting bent from abuse and from being dropped. They will bend if abused. They may warp if they are set on top of something hot (Side benders and glue pots, stove tops, etc....) They can also rust if left in wet, humid places... so they should be wiped down after use.

They occasionally need a bit of stoning to hone off burrs from clanking against things... but otherwise, they will last for years and hold their straightness for years and years and years. We have a Starrett 3' straight edge in our shop (In the day job..) and it is ~40 years old and still true.

Good luck.

John

-Mainly setup, and luthier stuff.

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Here is what a Parlor Guitar is for!
http://www.youtube.com/watch?v=oEa8PkjO6_I

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As others have said, as long as it doesn't get abused & is stored between uses (in a proper wooden box, which could be a seperate project if it doesn't come wiith one!) It will give you years of trouble free usage. A small bag or two of Silica Gel stored with it will keep the rust off it. A word about storage, if you store it in the 'flat' (widest side parallel with the floor) make sure it is supported in at least four points along it's length, If you store it on it's edge then it only needs supporting at two points along it's length. These points are called "Airy" points (if my memory from training years ago is still good) and are exactly 2/9ths in from each end, they could be considered 'nodes' and at these two points the straight edge supports itself keeping the edge perfectly straight.
Useless piece of info for you there but may come in handy if you intend keeping an expensive piece of equipment in good condition.

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Nick Oliver

http://www.oliver-guitars.com

As a machinist , most of the tools are hardened and are very durable. My tools , mostly hand made through apprenticeship were checked and rechecked and iso verified. It seems that you may have your nomenclature mixed. What you are describing is a parallel , while these are straight and square on all 6 sides they are different from a straight edge. A straight edge will be thinner and almost ruler like but the edge is more knife like and usually true on the edge only.
I have parallels that are 1/16 to 4 inches wide. They are great tools and come in handy in any shop. If the straight edge isn't hardened , it would have to be cared for . These things are designed to be used in a production climate and are relied on to set ups in large equipment. They had to perform . I hope you can or do have a set of machinist layout edges and parallels, they are worth it.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it