Cheap straightedge please

[justCNCit]

Yeah I've been looking for a straight edge thats at least 58 inches for cheap.

I know they are expensive and I should expect to pay alot. The ones I looked at were $400.00.

So I didn't jump on it, but still really feel the need to get one.

I have built a big dialing setup with arms and chrome shafting sitting in bearings to tune in the x,y and z planes for my 3 axis CNC wood router.

The machine is about 60x42 inches. I have put alot of work into this dialer as I am determined to get as much as it dialed in as possible, because I want everything to be straight and rolling smoothly.

I already tied a bit of .050" twine down the span of the machine, but the deflection of the metal twine makes it impossible and I used screws to support the twine at various points but can't tell what the deflection's like there either.

Please let me know of places I can look to get the straight edge or tell me of a accurate way to make a solid straight line over that span.

Thanks guys!

[DSL PWR]

Use high tensile wire, it's commonly used in pianos. You can tighten the crap out of this wire and for those short distances it won't deflect much. You could also try a cheap laser, the problem would be getting a jig to hold it straight.

[jmkasunich]

I second the suggestion to use wire. Music wire can be pulled very tight. That will give you a straight line in X and/or Y. Z will be trickier, because no matter how tight you make it there will be some sag. However, you can calculate the sag and take it into account - the sag will depend only on the wire size, the tension, and the distance between supports - all of which you can measure.

[jstiltner]

How about a drywall square? i think most are 48" and by its nature is supposed to be straight, if it were a mill i doubt it would be good enough, but for a wood router probably. Or a 5' level, or just a 6' length of drill rod ?

[vlmarshall]

How about a piece of flat-ground steel bar, from a tool supplier like McMaster-Carr? I've always ran to the materials racks at work when I needed a not-TOO-precision straightedge.

[Astonlee]

Why don't buy a bench ruler, as they make good straight edges.

[BobWarfield]

Look for an SPI straightedge on sale. Enco has them, for example.

Cheers,

BW

[jmkasunich]

We've got folks suggesting everything from metrology techinques (tight music wire, calculate the sag to compensage) to drywall "squares" (which are neither square nor straight by precision standards).

The original poster never specified how straight he wanted/needed to be. If he is aiming for 1/32" per foot, then the drywall square will work fine. But he might be aiming for 0.010 per foot, or 0.001 per foot, or 0.0001 per foot. Each step along that scale gets trickier and more expensive. Steel rules probably beat 0.010 per foot, but I wouldn't trust them for 0.001 per foot. And 0.0001" per foot will require something thick enough that you can run a sensitive dial indicator along it, strong enough that it won't sag from its own weight, and handled very carefully so expansion from the heat of your fingers doesn't bend it.

[justCNCit]

The straightness/flatness would preferably be within a few thousandths over a span of 72". I've been pricing out straightedges of this price range and am reluctant to pay $600 for a straightedge for this router as that accuracy isn't worth that much to me.

I've been having a tough time trying to find a ruler or bench ruler that's 72 inches long to begin with. At this point, I would consider just about any type of ruler to get this project out of it's rut.

I'm looking into the string idea right now, and still am open to suggestions. It would be nice to find some kind of ruler 50-72 inches long that is relatively straight... I want to name a tolerance, but my straightness tolerance is pretty tight, since I've gone through all the silly trouble of making a rotating dialer out of bearings that can be dialed in very accurately but didn't realize I was looking at a $600 (exchange rate / shipping & local price) ruler.

I just don't understand how you can dial in a 6 foot long machine without a precise machinists' straight edge, and expect it to run properly. I have all the parts sitting on my bench ready to bolt together but really the $600 is not what I had in mind, just doesn't seem right.

[HuFlungDung]

A piece of ground and polished shafting could be fairly straight and is easy to check with a dial indicator and any sort of Vees. Even a couple of chunks of angle iron will constrain a shaft while it is rotated for a quick checkup, prescision vee blocks are not required at all.

Now of course a perfectly straight shaft will also sag due to gravity, but only in the vertical axis. At 90° to the vertical, you'll have a good straight line. Now using that as a 'master' you can try a few pieces of aluminum flat bar until you find a piece that checks out. I wouldn't be surprised if a 1/4 x 3" extruded aluminum flat bar could be had off the shelf that was quite straight, but you might need to sort through a pile to find a piece. However, it should also not be too onerous to hand sand the edge of the piece until it checks out against the side of the long straight rod.