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

[Clive S]

The way I see it is that the straight edge will not vary by more than about .12mm over its length. ..Clive

[EddyCurrent]

There's nothing wrong with getting more gear but do you need it ? what's wrong with measuring diagonals with a strip of wood having a chisel point at each end to get into the corners ? I'm thinking those squares are quite small so that over a distance of say 1000mm any error would be magnified. There again I've just rechecked the prices and you might as well get them.
There's a 'law' that goes something like this, in your workshop you need 3 things, tools, materials, yourself, but there's always just enough room for 2 of those.

[Boyan Silyavski]

I was interested in the straight edge, for checking the gantry profile after welding. And as i have nowhere to machine it and havent found around where to do it, i was contemplating checking and filing until is flat. i believe its called " elbow grease " in English.

My idea also was to do the same with Bosch profile i have, as its quite straight. The small triangles i bought as i needed sth really straight as a starting point for all projects.
maybe i am wrong, but how to make a precision machine if i don't have a straight ruler to draw straight line?

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?


So you say the price is ok?

[EddyCurrent]

I think they look very nice for the price and I agree a straight edge is essential but I'm not sure you really need the other items though I'm sure they will be useful for some jobs.

As Clive said 0.01% of length = (0.01/100)x1200mm = 0.12mm

[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.