checking accuracy

[DEW]

How does everyone check the accuracy of their linear rails and such on big machines? I built a bridgesaw that has 12' rails and would like to check how straight they are and parallel.
Thanks,
Doug

[mxtras]

For the home-shop, you can use fine music wire stretched tight for the straightness. You can get fancy using small voltages, a buzzer and conductive (brass, steel, whatever) feeler gages to tell you the exact deviation, but your eye can pick up maybe .002" error.

Paralellism is a lot more difficult. Primarily because you need to look at the paralellism in two axis at once - in X down the length of the machine and in the vertical direction - Z. It is critical if you are using round rails because it is very possible to get a perfect indicator reading down the length only to find out later that the rails are way off.

I don't know where to start - it is an involved process to type up.

Let me see if I can find a web site for you real quick - the guy did a really nice job documenting his machine build.

---- I see you are new and offline now. If you come back, I'll dig into it....too many single posters have robbed me of my time in the past.

Scott

[trubleshtr]

Usually on production machines a "master rail" is set against precison rolled dowel pins, the dowels are precise to 0.0005" or greater. with the dowels set in the casting or frame base, the master linear rail is placed against the dowels and bolted down, The second rail is then located in reference to the master rail, usually using a dial indicator mounted to the truck on the second rail. Permanent Jacking screws are also installed beside the rails to get small movements to accurately dial the parallelism in. and check for any height differences in the rails (reverse dialing).
Once the saddle is mounted to the trucks, they ussually are doweled to the trucks on the rails, the rest of the build contiues using dowels or locating rails. Once the 3 axis unit, or whate ever you have built is done, a test part is cut, and checked on a CMM type piece of equipment to measure it for accuracy, if tappers are present or hole locations are off, adjustments are made mechanically first to tram the ways in parallel by using the jacking screws, from there offesets can be applied to the cut to better acuracy of hole locations.
That's how I do it at work in a nut-shell.
hope it helps