Is your straight edge straight??

[Bill Huber]

I have a straight edge, let me restate that, I had what I thought was a straight edge. I used it to set the wings on the table saw, the planer and my router table.

Things have been going good but I noticed that when I was cross cutting longer boards then like 10 inches they were not square to the face of the board.

I got the straight out and checked the wing and it was spot on with the straight edge. So what was going on, for some reason when I put the straight edge on again it was way off.... WTH is going on. I flipped the straight edge over and you got it, it was spot on.

So I ordered a straight edge from Lee Valley. the steel, 36 inch long 3/8 thick. What a difference, spent the time to redo both wings on the table saw, redid the planner. Things are much better now and square when I cut any length of board.

I don't really know how to test a straight edge unless you take it to a machine shop and have them check it. When you are only talking about 5 or 10 thousands of and inch its hard to check it in the your shop.

So I guess the moral to the story is. Is your straight edge straight?

[scott vroom]

I use a 1" square steel tube, 4' long. I believe it is straight but have not checked it against a "true" straight edge. For my TS extension I used my new 24" level which I believe to be true.

[glenn bradley]

I received the same LV straight edge as a gift a few years ago. I keep it in resistive paper and parked in its original tube. Its one of the few "shop" things besides my camera and my shellac that gets to live in house. First time I tried it I got a very different story than my builder's level and aluminum extrusions I had been using to get by.

He and his friends have solved many curiosities around the shop. If damaged, I am sure I could figure out that it was no longer straight by reversing it as Bill describes. But, I'd have no way to return it to 'straight'. If he ever gets dropped, I'll go right out after new one. I'd previously wasted too much time and material on 'almost'. I can mess up all on my own just fine. I don't need my machines to give me a head start on the process ;-)

[george wilson]

I wouldn't vouch for tubing to be straight,or any type of extrusions,either,speaking in terms of close tolerances. They are usually curved or twisted to some degree.

Being also a machinist,I have my own little 'Bureau of Standards." These include 3 cast iron straight edges that are hand scraped to within .0001". Then,I was lucky to get a black granite straight edge 30" long. It is laboratory grade.

You need these type standards to really see if anything is straight. The cast iron straight edges I have are the type used to rub onto worn lathe beds with Prussian blue. Then,you scrape the blued spots off,and repeat until the bed shows about 30 spots of blue per sq." all over. There are different standards of scraping,by the way.

The least expensive way to tell if your straight edge is true is to buy a black granite surface plate. The Grizzly ones are true enough for any of your practical purposes. The shipping is the worst part. These surface plates are also good for sticking down abrasives and trying up your plane soles. You need to get a decent size one as possible. A long straight edge hanging off both ends of a plate isn't the best situation.

Stewart Macdonald sells a straight edge made of rectangular steel tubing,but the working edge has been precision ground. I do not like the rounded edges of the tubing,though. Hard to see the small gap you may be looking for,unless you get down to eye level to look under it.

You could try tightly stretching a small music wire across the surface,but it needs to be pretty taut. Maybe make a stout wooden bow,and use the music wire for the string. You can see as little as .0001" of light through mating surfaces. It tends to look larger than it really is. Just be aware of this test. Before they had more sophisticated means,they used music wire to align critical parts,like the mountings for propeller shaft bearings on ships,which have to be very close.

I got VERY lucky,and also found 3 black granite squares of different sizes,for checking my squares.

If I hadn't found them used,and for much less than they were worth,I wouldn't have them.

However,I bought a granite square from CDCO for the toolmaker's shop when I retired. It was only about $65.00,and checked out very accurate against my old American granite squares. It was ABOUT 8" X 5",IIRC.

[harry strasil]

Perfect Straight edge that can not possibly get out of alignment = a piece of .035 solid mig welding wire.

1. Stretch it tight over what ever you want to check, go to hardware store and get some 3/8 or 1/2 inch keystock, cut into 2 inch lengths and use file to chamfer the cut edges.

2. Stick a couple of pieces as near the ends of the stretched wire as you can to raise the ends and add extra tension.

3. Use another piece or pieces to try and slip under the center sections of the taunt wire. if its a no go the center is high, if it go's under without moving the wire, its low in the middle. Use mechanics feeler gauges between the keystock and the wire to determine how much you need to get it true.

A more sensitive method is to insulate the wire when you clamp off the ends with some clear plastic or other non conductive material and fasten the leads of an ohm meter to the wire and the keystock or metal table and using the feeler gauges contact will be made when you have the proper feeler gauge between the two .

FWIW, and its foolproof, it can never get outa wack. Before wire feeds came out we used to unwind starter or generator windings to get fine wire for this method.

[george wilson]

We're thinking the same way,Harry!

[Tom Adger]

Are you familiar with the Wixey digital angle guage? It is very accurate. It is what I use to set up my table saw blades, etc. If you think your staight edge is not straight, put it in a vise, put the Wixey on one end, zero it, and put it on the other end. If it still isn't zero, you have a problem.

[harry strasil]

Oops, forgot that a pen light or other source of light on the back side of the wire will show even a half thousands of light betweem the wire and keystock . The same technique will work with straight edges laid on top of something.

[Jamie Buxton]

A different straight-edge test is this.. On a piece of fine-grain wood -- maple, poplar, or the like -- use the straight edge to scribe a line with a marking knife or razor knife. Don't make a deep scribe -- just enough so you can see it. Flip the straight edge end-for-end and check to see if it fits the scribed line. Then flip it over, face-down, and again see if it fits the scribed line. If it fits in both flips, the straightedge is straight.

Of course, this depends on how fine your scribed line is, and how good your eyes are. (I use a binocular loupe for things like this.) I'm pretty sure I'd be able to see 5 thousandths of error in the straight edge -- that would be 10 thousandths in the flipped position. 5 thou should be good enough for most woodworking situations.

[Rod Sheridan]

Are you familiar with the Wixey digital angle guage? It is very accurate. It is what I use to set up my table saw blades, etc. If you think your staight edge is not straight, put it in a vise, put the Wixey on one end, zero it, and put it on the other end. If it still isn't zero, you have a problem.

You have to be kidding if you think a Wixey can determine if a machinist grade straight edge is straight.

Regards, Rod.

[Peter Quinn]

I bought a 3'X 3/8" steel straight edge from some machinst supply company a while back, it was supposed to be ground or scraped or maybe licked straight to within .003" over its length. It seems to be the straightest thing I have, myself included. I have no way to check it and mostly couldn't care less at this point, but it has been used to set up every major machine in my shop and I have no complaints. Now if I could use it to keep the wood straight, that would be a thing to see.

[Gary Herrmann]

A friend told me about a guy on the bay that sells Starrett straightedges pretty cheap. I really like Starrett tools, but I've rarely seen them and cheap in the same sentence.

I've also got the steel LV straightedge. Comes in very handy.

[Dan Karachio]

There is a guy with a podcast, Modern Woodshop, that covers getting this same straight edge as a present. Sounds like it helped him realize his entire shop was not straight!

Now we have to ask ourselves how the ancient Egyptians and Greeks built the pyramids and Parthenon (respectively of course) to such amazing tolerances.

[Dan Friedrichs]

Are you familiar with the Wixey digital angle guage? It is very accurate. It is what I use to set up my table saw blades, etc. If you think your staight edge is not straight, put it in a vise, put the Wixey on one end, zero it, and put it on the other end. If it still isn't zero, you have a problem.

What?

That would only work if the gauge was VERY precise, and if you clamped the straight edge precisely parallel to the ground (I believe the gauge you speak of uses some gravity-pulled means of determining "down").

Also, that doesn't tell you the edge is straight along the length - just at two points.

[Bill Huber]

Are you familiar with the Wixey digital angle guage? It is very accurate. It is what I use to set up my table saw blades, etc. If you think your staight edge is not straight, put it in a vise, put the Wixey on one end, zero it, and put it on the other end. If it still isn't zero, you have a problem.

I have a Wixey but it is only good for 16 inches and I don't thing it would be accurate enough, to see much of a difference in 16 inches it would really have to be off a lot.

I was thinking of the Digital Protector...