It all depends on how accurate you want to be. I'm sure a lot of people make a machine which worsk, without using this equipment, but they wont make a really accurate machine without it. For the machine I recently made I used a 24x12" surface plate to measure and correct the rail straightness. Fortunately I won it for about £20 at an auction!

Quote Originally Posted by silyavski View Post
Apart from parallel i would like the Y rails / the long ones/ to be straight, not just following each other curvature. Any other way to do this with simple tools like a Bosch profile?
Not cheaply. People have experimented on CNCzone with using a tensioned wire to make a straight(ish) line for reference, but it's not going to be as good as the proper device.