How straight is straight

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Forum topic by richgreer posted 03-16-2011 06:01 PM 2541 views 0 times favorited 33 replies Add to Favorites Watch
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4541 posts in 3246 days

03-16-2011 06:01 PM

I have a 12” el cheapo metal ruler that I bought Menards years ago. It is their Tool Shop brand. I don’t recall what I paid, but I am sure it was less than $5.00. It looks perfectly straight to me. I place it on edge on the surface of my TS and see no light coming through anywhere along the edge.

Yet, I often read about really good straight edges from manufactures like Lie-Nielsen and Bridge City. Their 12” rulers cost $20 or more. Longer ones can be much more expensive.

I usually prefer to buy the better quality tools but in this situation I cannot figure out what an expensive ruler does for me that my cheap ruler does not do. Can anyone explain?

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

33 replies so far

View Bertha's profile


13550 posts in 2865 days

#1 posted 03-16-2011 06:07 PM

I’ve got one 12” Starrett rule that I keep inside the house (a vestige of engineering in college). I buy cheap rulers & hold them up to the Starrett. If they’re not grossly wavy, I say they pass. I once read that a plastic CD case was the most square thing you have lying around. I think you buy an expensive ruler because you want an expensive ruler. A square is another matter.

-- My dad and I built a 65 chev pick up.I killed trannys in that thing for some reason-Hog

View Ripthorn's profile


1458 posts in 3157 days

#2 posted 03-16-2011 06:11 PM

I bought a precision straight edge from grizzly and only pull it out for things that require dead on accuracy. For regular, around-the-shop type tasks I use a cheap one whose limitations I’m well aware of. The nice one gets pulled out for jointer setup, checking guitar neck flatness after frets are in and making sure my table saw fence is parallel to the miter slots and blade.

I agree with Al, square is a whole different ball of wax. There is rarely a “close enough” for squares like there is for straight edges.

-- Brian T. - Exact science is not an exact science

View Bertha's profile


13550 posts in 2865 days

#3 posted 03-16-2011 06:20 PM

And how, Rip. I’m due for an expensive set of squares that I can keep protected somewhere.

-- My dad and I built a 65 chev pick up.I killed trannys in that thing for some reason-Hog

View Dan's profile


3630 posts in 3052 days

#4 posted 03-16-2011 06:21 PM

Rich, I think you got lucky and got a good cheap rule. I have several cheaper rulers and combo squares with the rulers and some of them to my naked eye are dead straight and others are off. The advantage to the expensive one would be you know your getting one thats right on.

However you can always fix a cheap combination square or ruler yourself. Using the same method you would use to flatten a blade or plane. You can just run the ruler alone some fine grit paper on a flat surface and sand down the high points. I have done this on all my cheap measuring and layout tools.

-- Dan - "Collector of Hand Planes"

View devann's profile


2246 posts in 2864 days

#5 posted 03-16-2011 06:23 PM

Straight is straight. If it’s not straight, well then it’s got a bit of a curve between the two points. I’ve purchased steel cork backed rulers over the years and if they’re straght the only difference was where I bought them. Artist pay more for steel rulers than carpenters. Some people want more $ for the same thing. I always thought it was the overhead cost that I was paying for. Square, plumb, level, or straight, for many is an absolute, ether it is or it’s not.

-- Darrell, making more sawdust than I know what to do with

View SteveMI's profile


1123 posts in 3466 days

#6 posted 03-16-2011 06:35 PM

I have bought the inexpensive steel rules and “calibrated” them with finest grit sandpaper on a sheet of glass, kind of like the scary sharp method. When I drop one on the concrete floor, I have a drawer it goes into until I can’t find a non-dropped one.

This works for me at 12”, but longer than that I think you need to pony up for the better brands.


View teejk's profile


1215 posts in 2856 days

#7 posted 03-16-2011 07:11 PM

I go cheap and test with automotive feeler gauges but for the most part subscribe to the “close enough” theory…it is wood! even if you get 100% accuracy on the mark, you are still at the mercy of your cutting machine and in reality does anybody notice a discrepancy of 1/64th or 1/32nd?

View CharlieM1958's profile


16278 posts in 4390 days

#8 posted 03-16-2011 07:34 PM

teejk: Took the words right out of my mouth..

-- Charlie M. "Woodworking - patience = firewood"

View docholladay's profile


1287 posts in 3230 days

#9 posted 03-16-2011 07:42 PM

There is a very simple method to test if a straight edge is truly straight. Take two brads and drive them about 1/2 way into a board. Set them almost the length of the straight edge apart. Place the straight edge on the board so that it touches both brads. Draw a line along one edge with a very finely sharpened pencil. Flip the straight edge end for end and then draw the same line. Then flip the straight egde so that you are drawing along the opposite edge and the flip end for end again. After drawing all 4 lines, if they appear to be one line, then the straight edge has almost certainly got to be straight enough for woodworking. If the lines deviate at all, then you have some problems with that straight edge. Otherwise, if the tool works, who cares if the tool cost more money or not. Only if you are impressed by brand names.


-- Hey, woodworking ain't brain surgery. Just do something and keep trying till you get it. Doc

View dbhost's profile


5767 posts in 3404 days

#10 posted 03-16-2011 07:44 PM

To be straight I believe a deviation from the X, Y, or Z axis of +-0 is acceptable.

I have been around too many engineers!

I have an el cheapo Johnson metal rule, try square, etc… and they are as straight and square as I am willing to worry about. I have a set of blue point inside micrometers, as well as a set from HF. They both read the same thing, so no harm no foul there… I think for woodworking, worrying about anything smaller than about .003 is really being OCD. Through natural expansion / contraction of the wood, you will get larger variations than that in a workpiece over time…

-- Please like and subscribe to my YouTube Channel

View Woodwrecker's profile


4196 posts in 3747 days

#11 posted 03-16-2011 07:52 PM

I have a couple of combo squares I bought from Sears and they have tested straight & true as rules and squares when compared by a friend’s super expensive scientific set.
I’m with teejk & Charlie.
Believe me, with my work, it doesn’t really matter anyway…....LOL
Thanks Rich

View Loren's profile


10477 posts in 3820 days

#12 posted 03-16-2011 08:19 PM

Cheaper combo squares tend not to be square enough for fine work.

This is easy to fix in a sliding combo square. You just get in there with
a file and correct the error in the sliding head.

I’ve found where straightness matters most in woodworking is with
longer boards for door frames and face frames. I use a 78” level
a lot when milling up longer stock. Such a level is straight enough
for woodworking and comes in useful for hanging doors too.

I have some 2’ precision straight edges too, but only really need them
for setting up a jointer for the first time. I also use them when working
on guitars, but, really, it’s not that hard to make a pair of 2-3’ wood
straight edges that are quite straight enough for making furniture.

View DLCW's profile


530 posts in 2826 days

#13 posted 03-16-2011 08:24 PM

2 ways I approach it, I use a ruler/tape measure to measure. I use a dedicated straightedge to determine flat/straight, etc. For example, I use the straightedge on my jointer when aligning the knives, not a ruler.

-- Don, Diamond Lake Custom Woodworks - - "If you make something idiot proof, all they do is make a better idiot"

View TheDane's profile


5534 posts in 3835 days

#14 posted 03-16-2011 08:28 PM

I use Empire Level ‘True Blue’ straightedges and combination squares.

I realize they may not be in the same league as Starrett, but for my shop, they are good enough.


-- Gerry -- "I don't plan to ever really grow up ... I'm just going to learn how to act in public!"

View 8iowa's profile


1586 posts in 3933 days

#15 posted 03-16-2011 08:54 PM


I get great use out of a Veritas 50” aluminum straightedge. It is advertised to be straight within .003” over it’s entire length. This is certainly adequate for set-ups on woodworking machines.

In fact, I use this straightedge to level hevy duty roller stands both fore and aft of the infeed and outfeed tables of my 4” jointer. This set-up takes less time than one would think. I have often jointed and glued boards up to 104” length using this method. The Veritas straight edge is the key tool in this set-up.

-- "Heaven is North of the Bridge"

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