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

[Jonathan]

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!

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.

[Boyan Silyavski]

Thanks guys,
so it seems clear to me that i will pull the trigger at the straight edge. A sort of investment.

The 2 angle finders from aliexpress will wait for the moment. My idea was to use them like this:

Look at the gantry picture bellow. Using the straight edge or epoxy , like for the long rails, i could flatten the upper side/1/
But then how to flatten the opposite side? I don't have a fixed gravity epoxy formed horizontal surface at home. If it was my proper house, i would have done it long ago. So just flipping sides and pouring epoxy would be simple. So i thought of connecting the 2 angle finders un U like shape so i can measure distance and parallelism bots sides. So using the epoxy leveled top or bottom as a reference i could file , sand and so on, the other side. if they were on one plane , it would be easier, but damned overhang...i don't like it.

Anyways, i bought 2 packs of 1kg west system epoxy kit 105/209. After pouring some where necessary, the rest i will use to form a flat table surface and fix some short adjusting legs. As i have a very precise inclinometer / ~0.5mm per 3m, or so i calculated it when i bought it, don't remember but was the most precise on ebay/ i can fix me a portable level base precision surface for similar projects. I need a bigger garage:-)

Another thing is that just now i found how to make precision straight edge at home. 3 edges in fact. very interesting read. Now i am contemplating how to make it in reality. will need some polishing paste. maybe vibrate the edge for quicker removal. or slowly, like making a katana, while watching movies at night at home...

PS. i want to be as most as precise as possible, cause i know that even if you try, you are not. So if you don't try, its even worse

[mekanik]

G & M Tools - Moore & Wright 24 inch Hardened Steel Straight Edge

Grizzly.com
grizzly is US but this is the sort of kit you need, a basic rule will be to flexible sideways(that's what i have)

Just found this on e-bay looks like the grizzly
http://www.ebay.co.uk/itm/New-36-iGa...item1e7bed8717

[EddyCurrent]

That Moore & Wright seems cheap for the brand. Also silyavski you do want a straight edge with a beveled edge because you can see the light shining through gaps under it very accurately.

[Boyan Silyavski]

Thanks for the help, its highly appreciated!

At the end i bought the two triangles / deviation of 0.000055" per inch/ + 2x 7" Electronic Digital Protractor Goniometer Angle Finder Miter Gauge/ Resolution: 0.05 degree /

I bought last month the protractor as a present and i liked it very much, very substantial feel, precise enough, quite handy, unbeatable for the money, so it was on my purchase list.

Now for the straightedge. Let me share. It will be great if somebody finds it helpful. After 2 days and tons of research, here is what i have found:

Which straight edge?

1. the Igaging straight edges even if advertised as top precision are shot in the dark. Many people received a bad ones, so as i don't like Russian roulette when spending hard earned money i dismissed them. Source:Amazon reviews and forums

2. Axminster straight edge- same as above. Could be perfect or a total crap. Source: Axminster reviews and forums.

3. Starret straightedges: good but expensive. People complain about spending insane cash for a long straight edges and receiving them without a box , wrapped in oiled paper only. No hook hole. Heavy.

4. Moore & Wright- dismissed- brand new, insanely expensive.

5. Second hand straight edges- dismissed. Jump in the dark if you dont buy from an respected machinist, even better fellow forum member

6. Maun straight edges- not precise enough, at least for me, i found more precision rulers for the price. If it was a ruler, i would buy it for the thickness

7. On the cheap: people use Stanley and Stabila ground beam levels with great result +- 0.5mm/per meter, which is quite good for the price and

8. Other rebranded mostly black in color sold " for cylinder rectification" , no specs or whatsoever.

9. Other brands in USA mostly, normally priced but expensive to ship from abroad + eventual tax.


Now come the good ones/ for the buck/

9. Veritas : people love them. At Axminster they sell aluminium ones with 0.075mm precision over the entire length and steel ones. Citation: " These Canadian-made precision steel straight edges are ground flat over the entire length on both edges, the 610(24") length to within 0.025mm and the 915mm(36") length to within 0.038mm. All have been stress relieved to remain true under temperature variation. Useful for checking whether surfaces are flat or straight, they are 38mm wide and easily stand on edge unsupported, leaving both hands free for precise measurement or tool adjustment. All have hang holes, allowing storage on a nail or hook"


Good choice for the money, either steel or aluminum. Couldn't find a bad word for them.



10. It seems the magic word that i needed to find is din 874

But what DIN 874 /00 874/1 874/2 mean?





So now knowing the magic word./ for EU i mean/ it seems DIN 874/00 on ebay made in Germany can be found at short lengths quite cheap. Longer than 500 become prohibitively expensive.

But hey, i am happy with Din 874/2 . 33 micrometers = 0.033 millimeters / on the entire length.

So now i am contemplating which length exactly, 1200mm or 1500mm, which would be easier to handle. Or maybe even 1 meter. Made in Germany-I like that. Cheap, i like that also.
Example/ from very nice page with all kinds of stuff :
Straight Edges Steel DIN 874/2 Z032021000
lenght 1000 mm, height 40 mm, width 8 mm, Accuracy in accordance with DIN 874/2 lenght 1000 mm, hei...
Price without Tax: 63,65 EUR Price incl. Tax: 75,74 EUR

That Moore & Wright seems cheap for the brand. Also silyavski you do want a straight edge with a beveled edge because you can see the light shining through gaps under it very accurately.

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/ .


That is all folks. If you have some info i missed just say it for the future generations who read this:-)

Conclusion: Maybe i am crazy, but i don't see how i can make a precise machine without precise instrument, so i need one/ at least, i am a kind of instrument junkie/

[mekanik]

Top marks M8 i have bookmarked that supplier just in case i need one @ some stage, love a guy that does the research PPPPPP
Mike

[Boyan Silyavski]

PS.
Just discovered on the page of the DIN 874 straight edges, that there is another page. Its called: Straight Edges made from Steel, no DIN, Testing Accuracy in Item Description, Faces grounded, Length from 500 mm to 6000 mm. Here are the even cheaper " scrap like" :-) edges. Scrap for them, treasure to me.
Example:

enght 2000 mm, height 40 mm, width 8 mm


Straight Edges Steel
finely ground,
Tolerance flatness: 112 µm,
Net- weigth: 4,8 kg

Made in Germany

Price without Tax: 114,50 EUR
Price incl. Tax: 136,26 EUR

So 112 micrometers =0.112 millimeters, and thats for 2 meters. Not bad for 136 euro. If i make a big build i will definitely buy one similar

[Greeny]

This one looks ok.
MeasureShop.biz: Flat straightedge, steel

Part no: 1689 104
2000 x 60 x 12mm
din 874/2. 58 micro meters
£111.80 + vat

[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