cutting sheet with out a straight edge

[FireStorm]

this may be a silly questions here.

but whats the best way to get a straight edge and to cut from.

the reason I ask is i never seem to get a straight edge once i've cut something.

I tend to take the edge with the lowest point, measure across the sheet and make a point so i have one line.

then I measure down from the first spot and make it, then measure across to make my 4th corner, then join the dots up.

but what I seem to find is where I don't have a straight edge to go againest I don't get a straight line. so my points are all out and it's not straight.

does this make sense? if not i'll try to explain better or even take some pictures.

 

[bigbadbugga]

Do you mean straight edge or square corner?

Your description sounds more like you are trying to mark out a 90 degree corner.

 

[FireStorm]

i need a square plate say 15 by 15 inches, but i have no straight edges of the sheet metal

so whats the best way to cut a straight line?

or even if i was to cut the end of a sheet whats the best way to get a straight line.


E.G like this where i have no straight edges to base off.

http://i728.photobucket.com/albums/ww282/paulwar98/untitled.jpg

but now just thinking, I could use the L shape rulers "can't think of the name of them"

doesn't matter where on the sheet it is as long as A to B is the set amount and C to D is, i can use the ruler to get my 90 degree corners and then just joing the dots up.

but then on another note say I need to cut out a sqaure like in the picture and the edges were not straight , would i do the same? measure from A to B and c to D then join the lines up, but how would i get this sqaure to the sheet as i don't have a straight edge to measure from?

also whats the best way at straightening edges like this up, as I can't measure from side to side as they are not straight?

so how would I make a straight line if i don't have a straight edge in site?

 

[hotrodder]

L shaped rulers... framing square. You can use Pythagoras to square things, often referred to as the 3-4-5 rule in this context. Can swing an arc using a compass or string line to establish the corners, chalk line to mark a stright line in the absence of a stright edge

 

[invicta]

For any rectangle, the value on the diagonals is always equal. If you check this you will know if you have set out correctly. You will need a square to set out the rectangle. You can easily make one of these from flat plate riveted together. Screwfix do a framing square for £6.89.
http://www.screwfix.com/prods/17926...rs-Squares/Framing-Square-16-x-24-406-x-609mm
Hope this helps.