Is your straight edge straight?? [Archive] - Sawmill Creek Woodworking Community

Bill Huber

10-25-2009, 4:20 PM

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

10-25-2009, 4:29 PM

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

10-25-2009, 4:49 PM

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

10-25-2009, 5:45 PM

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

10-25-2009, 5:53 PM

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

10-25-2009, 6:00 PM

We're thinking the same way,Harry!

Tom Adger

10-25-2009, 6:18 PM

harry strasil

10-25-2009, 6:36 PM

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

10-25-2009, 8:20 PM

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

10-25-2009, 8:24 PM

Peter Quinn

10-25-2009, 8:29 PM

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.:D 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

10-25-2009, 8:30 PM

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

10-25-2009, 8:41 PM

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

10-25-2009, 8:41 PM

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.

:confused: 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

10-25-2009, 9:14 PM

scott vroom

10-25-2009, 9:29 PM

You guys are kidding, right? How would a straight edge that is "out of straight" by 1/10,000 of an inch matter unless you're planning to ride your cabinet to the moon?:D

Bill Huber

10-25-2009, 9:29 PM

:confused: 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.

You would not have to have it parallel to anything, you set it on the end an zero it, that angle is the same all the way the length of the straight edge.
I just don't think that when you measure over a 2 inch span you will get a very good reading.

Cliff Rohrabacher

10-25-2009, 9:30 PM

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

Yup. You can also simply stretch that over a good strong piece of wood to make a bow. The wire will describe a very straight line.

Yes the weight of the wire will pull it out of true but, not by any amount you can detect - - unless your bow is not done using a stout enough piece of lumber. It's gotta be bloody tight. And you can hang it in your shop with the humidity and mistreat it all day and it'll still be a straight edge.

The down side to using a wire straight edge is that it is very hard to sight with it.
When a good steel edge you can hold it up toi any natural or incandescent light and sight through the edge and the gap in the work and very accurately tell exactly how big the gaps are from 0.010" all the way down to 0.0005" (yah, half a thou) by looking at the color of the light that passes through. Can't do that with a wire, ya gotta break out the ohm meter.

If you want a steel straight edge and don't want the price tag, here is a way to make your own.
http://home.comcast.net/~jaswensen/machines/straight_edge/straight_edge.html (http://home.comcast.net/%7Ejaswensen/machines/straight_edge/straight_edge.html)

george wilson

10-25-2009, 9:43 PM

I'd suggest using a much thinner wire,say .010" diameter,like a thin guitar 1st. string. The thinner wire will weigh less,and gravity will affect it less. You could saw a slit in 1 end of your wooden bow,and just use the ball on one end of the string as an anchor. The other end could be wrapped around something stout,like a cut off nail,and secured through a slit at the other end. This will give about a 30" straight edge,if you don't waste too much of the string securing it around the nail.

When I mentioned my precision straight edges,they are intended,as I mentioned,for precision machine tool work. Since I happen to have them,though,they also serve as master straight edges for checking anything else against them,as do the granite squares. .0001" is of course not needed for woodwork!!!

michael osadchuk

10-25-2009, 9:47 PM

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.

Harry,

Great method, but either I'm seriously misunderstanding the method or there should be a reversal of the direction "no go/center high;a go under center, wire low".

?

thanks

michael

Bill Huber

10-25-2009, 10:00 PM

You guys are kidding, right? How would a straight edge that is "out of straight" by 1/10,000 of an inch matter unless you're planning to ride your cabinet to the moon?:D

It takes very little to have a 3/4 inch board to be off of square to the face.

Here was my problem, the straight edge that I used to start was off a good 1/10,000. So when I set the wing it was high on the out side end. The straight edge showed it flat.

131066

James White

10-25-2009, 11:00 PM

It takes very little to have a 3/4 inch board to be off of square to the face.

Here was my problem, the straight edge that I used to start was off a good 1/10,000. So when I set the wing it was high on the out side end. The straight edge showed it flat.

131066

Bill,

Perhaps you meant to say your strait edge was off by .010" not 1/10,000. That would be .0001".

James

Joe Jensen

10-25-2009, 11:04 PM

It takes very little to have a 3/4 inch board to be off of square to the face.

Here was my problem, the straight edge that I used to start was off a good 1/10,000. So when I set the wing it was high on the out side end. The straight edge showed it flat.

131066

I just did the math and assuming an 8" wing, and 8" from the wing to top joint. If there is an .010" gap under the straight edge at the joint, that would translate to about .001" of error on the cut, which makes the angle about 89.924 degrees. Not 90, but REALLY close.

How much off was the straight edge? .010" in 8" would be really bad.

Bill Huber

10-26-2009, 1:13 PM

I just did the math and assuming an 8" wing, and 8" from the wing to top joint. If there is an .010" gap under the straight edge at the joint, that would translate to about .001" of error on the cut, which makes the angle about 89.924 degrees. Not 90, but REALLY close.

How much off was the straight edge? .010" in 8" would be really bad.

Yes Joe I am all screwed up.

Just for the heck of it I took the old straight edge and put it on the new one. I could put a .016 feeler gauge under the center of it. I then turned it over and it would rock on the new gauge and I could get a .038 feeler gauge under the end of it.

Both straight edges are 36 inches long. My wings are 10 inches and from the blade to the edge of the table is 17 inches total.

Dan Friedrichs

10-26-2009, 1:36 PM

You guys are kidding, right? How would a straight edge that is "out of straight" by 1/10,000 of an inch matter unless you're planning to ride your cabinet to the moon?:D

Nope. Especially with jointer tables and knives, getting to within 0.001" during setup is necessary for good results. Having a table saw blade that isn't parallel to the fence within ~0.003" will give you burning.

You don't need to measure that precisely when building cabinets, but it's hard to do good equipment setup without that kind of precision.

tyler mckenzie

10-26-2009, 1:37 PM

Here's a vote for the LV 36" steel straight edge, one of the first tools i bought.

tyler
tealandgold.blogspot.com/

Cliff Rohrabacher

10-26-2009, 4:19 PM

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

Not a problem.
Grab a slave.
Grab his family.
Tell him he's either cutting the stone right your he gets to watch you sacrifice the family to the Nile crocs
Works every time.

Billy Trinh

10-26-2009, 7:31 PM

Any pictorial for this process? I'm lost from #1 but I'm interested to learn :).

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.

Kent A Bathurst

10-26-2009, 7:54 PM

Here's a vote for the LV 36" steel straight edge, one of the first tools i bought./

Soon after getting planer and jointer, and trying to figure out what I was doing wrong on setup, I got the Starrett 48" straight edge. Realize now it was prolly overkill, but the problems went away, and I have never had any snipe since. .0002 per foot means under one thousandth over 4 feet - as I said - prolly overkill.

Soon after that, I made a nice hardwood case for that sucker. Only sees daylight a few times per year, but it'll never be up for sale - by me, that is.

mickey cassiba

10-26-2009, 10:09 PM

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 .
Thanks Harry... I saw this done once when a couple of techs were setting up our first CNC lathe, and I could not figure out what they were doing. As I was apprenticing at the time, I could not get out of the job I was doing(trying to do) long enough to ask questions. Good way to get kicked out of the shop, not bein' where you were tol;d to be.

Glen Butler

10-28-2009, 2:29 AM

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.

Agreed, those wixey's are pretty good. I have tested them time and again. Mathematically they are within .003. Certainly not machinist tolerances, but when I check a used machine for purchasing I put the wixey on it first to see if it stays zero the whole time. If it changes I don't bother to go much further. Just recently I was looking a a delta dj-20. The wixey measured .2 out from one end of the fence to the other. My precision square confirmed that there was .004" of twist in the fence.

Nissim Avrahami

10-28-2009, 12:40 PM

This is my Aluminum straight edge - 98.5" long so it can "cover" also the 8' length...

It's an 1/2" thick and 4" wide Aluminum profile that is used by the concrete guys to straighten concrete floors or the "wall-to-wall carpet" guys to cut straight edge.

I checked it in the shop by attaching another straight edge, turning one of them to any possible direction and checking against the day light...I was surprised how straight they are and they cost only $20 (in Poland - should be less in the States)...

I use it to cut long sheets or to make a glue line with the router.

http://i158.photobucket.com/albums/t104/nanikami/Sanding%20Paper/S05.jpg

http://i158.photobucket.com/albums/t104/nanikami/Sanding%20Paper/S07.jpg

http://i158.photobucket.com/albums/t104/nanikami/Sanding%20Paper/S08.jpg

I have another one that is 1/2" thick, 2" wide and 67" long solid Aluminum that cost (if memory serves) something like $30 (in Japan - should be less in the US)...also very, very straight (it's my master straight edge).

131227

Regards
niki

Brian Penning

11-20-2009, 8:05 PM

Matter that much if it's an aluminum or steel straight edge?

James White

11-20-2009, 10:08 PM

The only difference is that the steel is more durable.

James

Paul Simmel

11-23-2009, 2:25 AM

I prefer aluminum cheap ones because they can be easily jointed on your jointer if screwed to a piece of relatively straight piece of ply.

-=-=-=-=-

And BTW, the Egyptians got their PERFECT level bases for their structures by digging trenches around the foundations and filling them with water and using the water level as an indisputable gage.

Stretching a guitar string seems like a very good idea, but for me that is good conversation in interesting conversation, but not practical in the least. A jointed straight edge is going to be about the straightest… and most very practical straight gage one is going to get (within the woodworking world short of $300)

That’s not to take away from stretching/pulling string to get a straight line (ages old and very workable) in use all the time in many trades.

JUST JOINT the bloody cheap (soft) aluminum and be done with it. If you think it is off after time… joint it again… just like sharpening a planer iron or chisel. It’s easy, accurate, and will NOT mess with your knives if you shave a few thousandths.

Take a SHARPY and mark the straight side. REALLY no big deal. I’ve done this several times. Glad I did. This was MY idea, and it works great.

-=-=-=-=-

I realize there a lots of newbies here. Just do it. Nothing will blow up in your face. You’ll have the straightest section of aluminum you’ll ever have. With HSS knives you’ll not notice a thing, and with carbide… you can do it all day long. Just screw the thing to a piece of ply and run it across. Go to the back side of your jointer if you want.

-=-=-=-

I posted on this last year along with pictures.