Help needed with understanding accuracy in measuring tools. I want to go straight :-)

[Boyan Silyavski]

Hi, i need some help there.

I am on my way to purchase some precision instruments needed for my build and hopefully for next builds. On the cheap

Basically i need instruments for precision squaring of the CNC.

1. I just bought from ebay Super accurate pair 3" and 3.5" machinist square accurate to +/- 20 sec of arc

Sellers description:

This is a pair of solid machinist squares. One square is 3" x 3" with a 90 and dual 45 degree angles and the other is 3 1/5" x 2" with 90, 60 and 30 degrees angles. Angular accuracy is insanely accurate to 20 seconds of arc. So just what is a "second of arc?" A full circle consists of 360 degrees. One degree can be divided into 60 minutes of arc. Each minute of arc contains 60 seconds of arc, so a second of arc is an angle that is 1/3,600 of a degree. 20 seconds of arc would be 1/180 of a degree, or a deviation of 0.000055" per inch. These squares are made from hardened carbon tool steel and are ground and lapped for straightness and parallism. These are the most accurate squares you will find for this price anywhere.

2. Now i am contemplating of buying from aliexpress 2 pieces of Universal Bevel Protractor Angular Dial Length 150mm 320 Degree

The first buy will be used to adjust the 2 universal protractors squarish.

3. Now i need a straight edge. It will be used to check rails mounting surfaces and so on. Aslo i intend to use it for connecting it to the 2 squarish protractors and check and adjust rails parallelism. Here comes the question.

I will like this to be as large as possible without breaking the bank. I saw on ebay 2 options:
a/Igaging 36" straight edge beveled precision ruler hardened steel

Sellers description:
These straight edges have hardened, satin chrome finished blades that are ground and lapped for straightness and pallalelism and have true right angle on all edges with one long beveled edge. Beveled edge is graduated in 64ths and is precision photo etched and then black filled for easy readability. Rules are 36 3/16" long, 1 9/16" wide and 3/16th thick and weigh in at almost 4 pounds. These are substantial rules. Accuracy is guaranteed to be +/- 0.0015" (one and a half thousandths of and inch) over the length of the 36" rule. Rules come in an imitation leather pouch.These straight edges are perfect for setting up machines, checking work for straightness and scribing straight lines. These rules are US engineered and manufactured to strict tolerances in factories overseas. These are well made and precise and I am convinced they offer a reasonable alternative for anybody unwilling to spend the money required for the more premium line of US made precision tools.

b/ Maun Carbon Steel Straight Edge 48in

sellers description:
Accuracy is 0.01% of the length of the straight edge.

Size.48 inch.
Width 2in.


Hence the questions:

- am i on the right way?
- obviously the maun straight edge is longer 42 inch versus 36 inch, but what does the stated accuracy mean? I can not understand it, percentages, etc? can somebody translate?
-any other options, better or cheaper? And preferably from Europe.
-other ideas?

Thanks!