Checking straightness over 7metres

[howardm]

Well,
I want to make my own screed vibrating beam, I have some left over sheeting rails which I can
weld together end to end with some sheet backing.
How does one check an item 7 metres long for straigntness.?
All I can think of is taut string, anybody.?

I don't have any flat floor or surface at all.!

thanks
Howard

 

[daleyd]

String or a laser level perhaps? When I used to level long industrial dryers (~17m)I used to use braided fishing line rather than string as it weighs less and therefore doesn't sag as much and you can get plenty of tension on it.

 

[Gareth0123]

Laser and a pin-point target that can be moved along the beam.

I've cobbled them up many times for horizontal and diagonal jobs. Somewhere about the workshop I have got some speaker magnets bored out to suit lasers that were bonded in place square with loctite.

 

[Rocketman]

Facing up the edge of a 1m steel rule against it along the length at intervals and looking for gaps under it.

 

[gordon stephenson]

For certain similar tasks i the turbine world we used a very taut piano wire,

 

[colnerov]

I take it Mk1 eyeball can't be used?

 

[RonA]

Well,
I want to make my own screed vibrating beam, I have some left over sheeting rails which I can
weld together end to end with some sheet backing.
How does one check an item 7 metres long for straigntness.?
All I can think of is taut string, anybody.?

I don't have any flat floor or surface at all.!

thanks
Howard

When I studied metrology as part of my mechanical engineering studies in the late 1960's the weapon of choice was a sighting microscope & target arrangement (very similar to an architects theodolite but working to much tighter tolerances). Doubtless the ubiqitous laser has now taken over the task.
If you didn't need such precise alignment what about the old water tube level method, perhaps with a sighting mark on the miniscous at both ends this would achieve the level of accuracy you need.
RonA

 

[piman]

Hello Gordon,

"taut piano wire,"


Or even MIG wire?

Alec

 

[malcolm]

A piece of taught string can sag a little, but if you turn the wood over and put the string beside it will be very accurate. There is fine builders string available.

What sort of tollerance are you going for? I wonder if the average builder would buy a long bit of wood especially, let alone check it is straight.

 

[Wendelspanswick]

I made a 7.2m beam by cutting a sheet of ply into three 400mm wide strips and glueing them into a groove I cut into lengths of 38mm x 63mm studwork timber, creating a timber RSJ for want of a better description. The edge of the ply is nice and straight so as long as you rip the sheet parallel to the edge it remains straight.

 

[ajlelectronics]

+1 You'd be surprised how little deviation from straight looks glaringly obvious with a bit of practice.

Unless like me, you have astigmatism! To me, a perfectly drilled hole looks eliptical.

 

[ajlelectronics]

A piece of taught string

What did you have to teach the string? ;-)

 

[stormin]

Laser line grazing down the face of a flat surface. Any slight deviation will show up as a variation in width of the lazer line. Greater deviation and the laser line will miss the surface altogether. The optical version Ron A speaks about is known as an auto colimator, bit over the top accurate for a screed rail. Any taught line will sag in the horizontal plane but if you measure to its vertical plane it will always be straight unless it's caught by the wind.

 

[tomsutton529]

If you are looking for a cheap option then don't even bother reading the rest of this post.

LEVALIGN Ultra is a flatness measurement system featuring a rotating laser in conjunction with the ultra-modern ROTALIGN Ultra computer. The system provides the ideal solution for quick and efficient measurement of surface flatness and levelness. The computer’s large colour backlit display shows all relevant information, including a context sensitive menu, in a self-explanatory manner. - See more at: http://alien.co.nz/products/industrial-alignment#sthash.MIfcMQFO.dpuf

have used that in the past. Not cheap but certainly does the job. I use pruftechnik everyday at work and its never let me down..

 

[brightspark]

just get a cheap set of water levels . theres nothing more accurate than that

 

[howardm]

Thanks for the input men.
The beam fabricated from wood has given me an idea, I have a few sheets of 1" structural ply, I will make a box section from that.
Howard

 

[Bladevane]

When I was building and installing my kitchen units I used a thrust bearing race and a ball to check horizontality (is that a proper word?) in both planes.

 

[brightspark]

i use one of these for work .there brill for leveling sockets and for marking and putting recessed lights in ceilings http://www.tool-net.co.uk/p-413371/...pls180g.html?gclid=CL-D0sWT7cYCFSgGwwodhdgGwg

 

[gordon stephenson]

Turbines and pumps, For long distance accuracy i used a Watts water gauge, Accurate to around half a thou over any distance you have to measure. Ideal for setting bed plates etc for multiple machine inline alignment.

 

[normspanners]

Most things unless, VERY light and strong will bow over 7mts maybe only slightly, but will bow, because of its own weight, try a piece of rsj or universal beam they sag, then bring into the equation pulling a mass of concrete along.I made mine out of 4x2 box with a strenghtening lattice but still bowed slightly. The secret is to knock the mix out fairly level to start with IMO. so that you are not dragging loads with you.