Hi Hood
In retrospect I was a bit hasty in posting. It would be better if the references to a lathe were left out.
For the record I did use some Roslau piano wire .029 diameter. It appears to be available in most countries. Google search follows:
http://www.google.com.au/search?hl=en&q=Roslau+piano+wire+germany&btnG=Google+Search&meta=I strained the wire between two points and carefully aligned them to the Lathe bed using gauge blocks and used it as a straightness reference. To load it up a large gear maybe 25 pounds was hung just off the floor to tension it the wire did not break! Ideally it should be tensioned just short of breaking.
To measure the error, a measuring stage made from a 10 thou micrometer and a back lit slot only a couple of thou bigger than the wire. The stage was then mounted on a kinematicly correct frame two pairs of points on the v and one point of the flat of the lathe.
By moving the stage (carefully) until the slot was centered optically and then reading the micrometer it was possible to get repeatable measurements within about .0002. that cancels out any thickness error of the wire itself but The rosleau had no measurable error with a brand new micrometer. Obviously the slot and wire is not visible to the naked eye but no problem for a cheap student microscope. Set to about 50 power. Found one second hand for 10 bucks in a trash and treasure shop. The best way is to cut off the base and mount it on a steel support to fit your needs. (Keep the focus adjustment) The slot must not touch the wire. The bed is about 10 feet long with the headstock removed. Straight edges are out.
The horizontal plane is a different matter to check that a 10 second precision machine level (0.0005"/10 ") engineers level was used to level the machine. Buy working from both ends and averaging you can get a good result. For leveling machinery on a tight budget the only way to go. I have seen them on the internet for a couple of hundred bucks.
If you calculate the sag of the wire using the Catenary formula you can also use it to define a horizontal plane.
From reading some of the posts here there appears to be quite a lot of members building their own machines.
The wire method is not new as a browse of the internet will show, it produces outstanding accuracy.
An autocollimator would be nice but for 20 bucks a lifetime supply of wire is pretty cheap. I don’t reuse it. (When unwinding it off the coil make sure it is not kinked.)
The MIT paper Synchronous Proximity Detection for Stretched Wire Alignment Systems By J Paridiso, Link :
http://web.media.mit.edu/~joep/papers/WireReadout.pdf is very interesting as are many other posts on the internet using wire for the CERN linear accelerator for instance. It deals with reading the position of the wire electrically. That got me thinking? Could we sense the wire using a instrumentation amplifier bridge and use Mach 3 to read the error and position a tool? Even in the x plane if the Catenary formula was used?
Maybe this forum is more software oriented. And the practical side is of less interest to members. This post was made to possibly assist machine builder members on a budget. This method will allow sub thou accuracy over long distances. Not fast to set up but it works.
Cheers
John Mac