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

[EddyCurrent]

You almost got me there :-) . But isn't it that a square straight edge has 4 edges that could be used for that against the ones that have only 1 sharp edge?
I believe their purpose is a bit different and definitely i believe i need the square- to compare between rails in all directions / the twist i mean/ .

If I'm reading you correctly you mean this;



But if you angle the straight edge on the left it will bend and not be straight any more.

[Boyan Silyavski]

If I'm reading you correctly you mean this;



But if you angle the straight edge on the left it will bend and not be straight any more.

Hi,
maybe you misunderstood me because of my English. I meant there are 2 types, beveled/like a knife/ and ones that look like bar. So as far i understood, you said that the beveled is better, cause you can see easier the light, when touching some surface to be measured.
So what i believe, the square could be more useful for me, constructing a cnc, because it has 4 edges versus one of the beveled.



First it can serve as a master guide to straighten the first square shaft supported rail when mounting on the red axis, fig 1 , so its true straight

Then check the straightness on blue axis, fig2 /hence proper tightening on hopefully straight surface or if not-if properly shimmed.

Then check twist/ green axis/ fig 3 and 4.

With a beveled edge i don't see how you can perform and alight twist, as many of the beveled edges i saw are straight only at the knife beveled blade. fig 2 is where the beveled edge shines, but if i simply flip and use one corner, its the same.



PS: edited, axis not showing

[JAZZCNC]

I've got both and the bevelled edge is far more versatile and the one used most.

You cannot angle the straight edge like you show because your straight reference edge is lost because it's bending and will just follow the curvature of the surface. The reference edge is only meant to be used in one direction and must be 90deg to surface your checking any tilt/bend and error is introduced.

[Boyan Silyavski]

I've got both and the bevelled edge is far more versatile and the one used most.

You cannot angle the straight edge like you show because your straight reference edge is lost because it's bending and will just follow the curvature of the surface. The reference edge is only meant to be used in one direction and must be 90deg to surface your checking any tilt/bend and error is introduced.

Thanks, i get it now. Ok, i will look for a beveled one.

Now recapitulating everything it seems that the Maun even if not ultra accurate is made from carbon steel opposed to all other mentioned. And at theit web site i saw it was beveled.

So i send a couple of emails.

-To Maun, to ask them if its hardened carbon steel, cause at their web they don't say so, say steel. No carbon at any place. All sellers say carbon. We will see.
-To Measuresop which Greeny suggested, cause here they have some beveled straight edges which look quite cheap , so i asked about the accuracy.
-To Precison tools to see if they will ship to Spain this and if this can serve as a straight edge, cause it states tolerance 40micrometers
- to ebay member who is selling the Igaging 36 inch to ask if he could check straightness before sending. yes, back here, as the brand ones a very expensive.

Will see what happens.

[mekanik]

Hi M8
This is only my opinion and i am sure others may disagree but i would forget about the Maun straight edge, i couldn't see any thickness quoted so it's more than likely a bendy one,the other bevel edge rule is quoted as steel and galvanised and i didn't see any tolerance/flatness quoted, i would stick with the magic numbers DIN 874/2 a parallel straight edge of reasonable thickness should be fine you can see the smallest of gap by placing a light behind the straight edge.
Take no notice of e-bay sellers the DIN 874/2 rules will be checked under controlled temperature conditions in a tool room using some rather expensive gear. a box for your precision straight edge is a must buy.
Hope that helps, i am now awaiting the flack.
Mike

[JAZZCNC]

Hope that helps, i am now awaiting the flack.
Mike

No flack from me I'm 100% in agreement. My bevelled edge is 5/8" thick came from Vickers tank factory and would cost more than a small car today and weighs nearly has much. It also has stamped on it working temperature range it's accurate at and it's quite narrow, it will also be fair way from calibrated by now so I treat it with kid gloves and it's away in it's wooden case ASAP.

You get what you pay for.!!

[Boyan Silyavski]

Thanks a lot for the info!

The Maun edge if happens to be from hardened steel/ still no info/ , could be good for drawing straight lines
Here is the drawing from their web:



I will stay with the magic number.
Will fiddle until the end of the week and if nothing more interesting jumps, will buy from Germany. They answered me and told me that the scale also is a straight edge. Yes, i wanted beveled, scale and hardened and inox but ...$$$.