I've read a lot of the threads about building routers and have seen some questions and suggestions about how to get the table/frame flat so the rails/guides are parallel and true to each other. When I built my machine, this was one of my most basic concerns, since the first set of rails (X axis in my case) determine the trueness and "twist" of the entire machine. I used plumb bobs, 2 lb monofilamnet fishing line (sewing thread proved to be too fuzzy) and a micrometer to measure the plane-trueness/twist of the frame. With the frame standing on end, I hung the plumb bobs off the top two corners and VERY CAREFULLY measured the gap between the line and the bottom two corners. Even after "jigging' the frame to weld it, it was twisted .033. I mixed up a 50/50 mix of JB Weld and Brownell's Atomized Steel and made mount pads for the rail supports. The X rails are well within .001 of each other in twist, now, over a 44" distance.

.... Even after "jigging' the frame to weld it, it was twisted .033. ....

....The X rails are well within .001 of each other in twist, now, over a 44" distance.

44 inches with a .033 twist in a welded structure!!!!!

.001 over 44" for final rail alignment!!!!!

That is very impressive.

Thank you for pointing out that method of taking the measurement - I had wondered how to approach such a measurement problem and that is an interesting approach.

I am always amazed at how well people can measure things compared to my own crude work.

Plumb bogs are probably the most accurate method of measuring plumb over any appreciable distance. Use something accurately sized as a cross member at both the top and the bottom of the table/frame, such as drill rod or cast jig plate. You'll also have to have a room that is absolutely free of drafts. When you stand the table/frame up, adjust it's vertical so you have a 1/8 or so gap between the plumb bob line and the temporary cross members. Use a magnifying glass to accurately determine the measurement from the plumb bob line and the cross member. With care, you can easily (although its tricky holding a magnifying glass and using a mic) measure to the .001. Once you have the gaps measured at the lower end, use feeler guages to copy the measurement at the two bottom corners. The feeler guages will be the "pedestals" for flattening the JB Weld/Atomized Steel "pads" across the length of the table/frame rails so you'll have accurate mounting pads for the rails/rods.

If you want to up the accuracy, buy some 0.005" Stainless wire and hang your plumb w/ it. Then you can put a 5vdc supply into the wire which is series connected to a buzzer or a LED. When you use your feeler gauge you'll get a MUCH more accurate indication ranther than by just looking [even w/ a mag and a mic] It's what I used to align my mill which has rails that are 20feet long.. and 10feet appart..

FWIW

J