Local machinists: I need a 10' straightedge.

[RJSakowski]

Channel is an unbalanced shape. I would choose a balanced shape such as I-beam or rectangular tube.

This is true but there is no load on the extrusion. I have used the 3" channel for more than twenty years and have found it to be flat in use. I use it as a "C" rather than a "U". If I were picking a form to make the tool, I beam would be my choice. I happened to have the channel so I used it. What the I beam form does is stiffen the beam in right angles to the web. It tends to make the beam resistant to twisting. Structural channel is just one side of the I beam.

 

[ezduzit]

...there is no load on the extrusion. I have used the 3" channel for more than twenty years and have found it to be flat in use. I use it as a "C" rather than a "U". If I ...

Not true. The "beam" will bend just from its own weight. So, if straightness is important, channel is a poor choice.
I understand that you used it because you already had it.

 

[Latinrascalrg1]

Ive never used this product but i can see this being very helpful to a noob tile installer. It wont resolve those problem high/ low spots but this product should do a good job with keeping each tile level to its neighbor. Add in the "+ spacers" separately to control spacing between and your good to go.

iRookie 100pc Reusable Tile Leveler Spacers with Special Wrench - Tile Leveling System for Floor and Wall Construction -T Shape Levelling Shim Flooring Alignment Leveller -Tile Positioning Tools Set - - Amazon.com

iRookie 100pc Reusable Tile Leveler Spacers with Special Wrench - Tile Leveling System for Floor and Wall Construction -T Shape Levelling Shim Flooring Alignment Leveller -Tile Positioning Tools Set - - Amazon.com

www.amazon.com

 

[RJSakowski]

Not true. The "beam" will bend just from its own weight. So, if straightness is important, channel is a poor choice.
I understand that you used it because you already had it.

Since the cross section of a piece of channel is one half a piece of I beam, the bending due to the weight of the beam will be the same. Yes there is bending due to gravity's effect but for construction purposes, it is negligible. By actual measurement around .005" over 8 ft. The contractors that I know work to +/- 1/8".

 

[ezduzit]

Since the cross section of a piece of channel is one half a piece of I beam, the bending due to the weight of the beam will be the same...

Simply untrue. The I-beam is a much more efficient shape for beam loading, which is still the case for supporting its own weight. This is why the I-beam and I-joist are chosen for supporting spans in construction.

The amount of bending in a 10-foot channel can be measured in 1/16'ths of an inch. It is very easy to check this for yourself by supporting one at its ends and sighting down it.

 

[Cadillac]

Having more info helps with a solution. For any tile job expecially a virgin concrete floor I would use a 16” wide scraper on a pole a scrape the entire surface. It will show high spots, find nails,screws, anything that might be a problem. Same style of scraper used to lift laminate tiles and glued carpet.
With a 12x24 tile your in the larger size notch probably 1/2x1/2 or larger. Back buttering the tile and a large notch gives you a ton of wiggle room to float the floor level. Using a 2’ level tile to tile keeping you flush and level. Don’t need much more than that other than some patience. Look into adding a latex bonding agent to mix in the mortar for helping adhesion to the concrete. Good luck.

 

[T Bredehoft]

It is very easy to check this for yourself by supporting one at its ends and sighting down it.

While the above is true, the use to which it will be put, determining variation in a floor, won't suspend it by the ends, but by many points across the floor. I'm sure an 8 foot 3" sq tube would work.

 

[ezduzit]

While the above is true, the use to which it will be put, determining variation in a floor, won't suspend it by the ends, but by many points across the floor. I'm sure an 8 foot 3" sq tube would work.

A square tube would be much better than a channel, as it is a balanced shape. A straight line would touch on only 2 points. A flat plane would touch on just 3 points. Because of sag, the channel would touch on possibly a few more.

 

[francist]

Off on a bit of a tangent here, but what else is new....

RJ mentioned in one of his posts about using a laser level on a grid to measure the relative flatness of a floor. And also that it would probably be a tiresome process. I actually did just that a few years ago, and yes, it was laborious!

I wanted to get a topographical map, as it were, of a fairly large section of floor (40' X 75' roughly) that had a questionable support member. We wanted to be able to document whether the member was continuing to deflect, and if so where. So I came up with this system using exactly what RJ mentioned: a laser level and means of measuring from level line to individual points on a half-metre grid. These points were plotted into an Excel spreadsheet and I had my topo map of the floor.

Well it kind of worked but kind of didn't. The laser line is actually pretty thick, so if you're going for 1/8" tolerance that's maybe doable, but finer than that and you're guessing. And of course my level, although it was a pretty decent one, was not a razu rotating kind. But it many ways as it often is, figuring the "how to" is often the most fun anyway.

I took the level and made a kinematic mount for it out of MDF. To let it rotate smoothly, I used part of a scrapped magnetic drive out of I'm not sure what anymore, but it was super smooth and very tight. Attachment to the rotating platform was accomplished with three rare earth magnets so I could easily assemble and disassemble the setup.

For the measuring part, a similar base of MDF held an aluminum scale at vertical and tensioned with a screw so that the scale could always be zeroed at a given point. Once the scale was zeroed to the laser line, I just moved the base around to the various points on my grid and any deviation would show as the line being higher or lower than my "zero" mark.

Then I painted them both banana yellow, yay!

I took a number of measurement sets this way and yes it was interesting but ultimately not one hundred percent viable. Our more reliable surveys were done using an optical level and stadia rod, land surveyor style. But it was a lot of fun in the making, that's for sure.

Now back to your regular scheduled programming.....

-frank

 

[RJSakowski]

Simply untrue. The I-beam is a much more efficient shape for beam loading, which is still the case for supporting its own weight. This is why the I-beam and I-joist are chosen for supporting spans in construction.

The amount of bending in a 10-foot channel can be measured in 1/16'ths of an inch. It is very easy to check this for yourself by supporting one at its ends and sighting down it.

I cantilevered my 8' beam from a single point which should cause almost 10x the deflection of a beam suspended from two points and could see no measurable deflection by eye, by laser, and with a straight edge.