How straight is your straightedge?

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Forum topic by ChuckM posted 12-05-2011 01:30 AM 4960 views 0 times favorited 14 replies Add to Favorites Watch
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625 posts in 4143 days

12-05-2011 01:30 AM

Topic tags/keywords: straight edge

To ensure that when I use a straightedge, the straightedge in my hand is truly straight and doesn’t just look straight, I checked my straight edge tools (squares, steel rulers, the combination ruler blade, etc.) against my Bridge City Toolworks (24”) straight edge. The results show that several edges aren’t that straight and next time, when I need a short straightedge, I know what to grab.

Here are two sample tests:

A Steel Center Finding Ruler (,43513,43529)

A LV Double Square (,42936,42945,44279)

-- The time I enjoy wasting is not time wasted

14 replies so far

View jmos's profile


916 posts in 2846 days

#1 posted 12-05-2011 02:01 AM

It is interesting to check these things. I’ve got a Pinnacle 48” precision straight edge, and I’ve also found other edges off more than I would like.

Another good thing to check are your right angles. I bought the same LV double square you show above, and found it’s not 90deg. The spec says 0.001”/inch, over three inches of the extension, the error is enough to see a good wedge of light. I sent one back and to replacement was the same. It doesn’t sound like much, but it makes it unsuitable for checking for square off the jointer. I bought a Starrett used of Amazon for cheaper than the LV and it’s dead on.

-- John

View ChuckM's profile


625 posts in 4143 days

#2 posted 12-05-2011 02:23 AM

Hi John,

Good reminder.

Yes, we should check our squares, too. Last time when I checked, two of my three engineer squares (the longer ones) were off a tad. But I have had better luck with my LV 4” double square (yours 6”?) as well as my combination square (,42936).

This is the test photo I took when I checked my double square a while ago – that kind of squareness is good for both machine set-up and other woodworking purposes:

-- The time I enjoy wasting is not time wasted

View ChuckM's profile


625 posts in 4143 days

#3 posted 12-05-2011 02:25 AM


-- The time I enjoy wasting is not time wasted

View jmos's profile


916 posts in 2846 days

#4 posted 12-05-2011 03:00 AM

No, mine is 4” also. Compared it to a number of other squares (Starrett 12” combo square base, 4” and 6” Pinnacle reference triangles, and Rockler engineers square) and no matter how I fiddled with it I could not get rid of a small but noticeable wedge of light. Disappointing, I’ve come to expect better from LV; usually excellent, at least with the Veritas line.

If first found it when checking a edge for square off the jointer. Started cussing the fence and resetting it before I realized it was fine and the square was off. Then there was more cussing…

-- John

View PurpLev's profile


8551 posts in 4125 days

#5 posted 12-05-2011 03:25 AM

and this is exactly why you should really use a straight edge when checking for flat and not whatever alternative you have laying around…

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View Dark_Lightning's profile


3509 posts in 3586 days

#6 posted 12-05-2011 03:52 AM

It is a shame when tools have to be played off against each other until a true straight, flat or square is found. I have the same problems in my shop. Also, the boards nowadays aren’t straight enough to depend on, either. I bought a bunch of pine (I know, I asked for it) at the blue borg to make some drawers to fit an old computer hutch I bought on the cheap. The boards weren’t even all the same width, nor were their edges straight- they were pretty wany. I want that kind of stuff, I can buy it cheaper where these conditions are acknowledged. At least they weren’t cupped, warped or twisted (much). I managed to force them into shape. But I don’t expect to have to rip store-bought S4S lumber to width. Guess I’ll have to, from now on.

oh- my little Starrett 4” machinist’s square like above is dead-on, as is my framing square and my little 16” Stanley is square, too, surprisingly.

-- Steven.......Random Orbital Nailer

View pintodeluxe's profile


5975 posts in 3290 days

#7 posted 12-05-2011 04:03 AM

How do you know which one is straight?
How do you know when you only have one coffee filter?

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View ChuckM's profile


625 posts in 4143 days

#8 posted 12-05-2011 05:21 AM

Hi Willie,

“How do you know which one is straight?” It’s easy, verify it with a 100% true straightedge… just kidding.

There’re several suggestions that have been discussed:

Among them, this seems a straightforward one:
“It is really prett easy to test for straightness. Much the same as testing a framing square. First, lay it flat on a table. Draw a ine down one edge. Then roll it over to the other side of the line you just drew, but with the other flat surface on down. Check it against the line that you just drew. If it matches perfectly, then it has to be straight. Any amount of out-of-true-ness will be clearly visible.”

-- The time I enjoy wasting is not time wasted

View Tootles's profile


808 posts in 2978 days

#9 posted 12-06-2011 01:05 PM

Remember also that a string stretched between two points is the true definition of straight – the shortest distance between two points. A chalk line might not produce a line fine enough to do your checking, but Mafe's Japanese ink pot might be better.

-- I may have lost my marbles, but I still have my love of woodworking

View nobuckle's profile


1120 posts in 3238 days

#10 posted 12-06-2011 06:03 PM

I often find these topics intriguing. I too am obsessed with wanting every tool I use to be “perfectly” straight, flat, level, parallel, etc… I have come to the conclusion that there must be levels of acceptance. In my experience as a tool maker I compare everything to inspection grade equipment. The problem is that as a woodworker this simply is not needed. If I find that my combination square or my steel scale is off by .002” I have to accept this. The material I’m working with will move way more that the discrepancy in my scale. So, at what point does one say “it’s no good” and thus seek to replace the tool? I hate to say this, but there is a point when it’s close enough.

-- Doug - Make an effort to live by the slogan "We try harder"

View ChuckM's profile


625 posts in 4143 days

#11 posted 12-06-2011 06:27 PM

Hi Doug,

I agree with you that as woodworkers (and as not space rocket builders), we can be more tolerant of the less-than-perfect tools and outcomes. I usually go by the stated tolerance levels that go with my tools: straightedge (+ or – 0.001” (or whatever the stated spec is)), handplanes (a slightly concave sole is acceptable but not a convex one), etc. I true my own squares and how true are they? As true as I as a woodworker need.

Using my straightedge and square, I’ve found almost all the things in my house that’re supposed to flat, straight or square not so. But my eyes can’t tell it nor anyone else’s unless we measure it. One thing that’s pretty straight, flat, and square, though, may be found in our wallets: a mint gift card. And I use credit cards in good condition as my squares a lot.

-- The time I enjoy wasting is not time wasted

View Grandpa's profile


3263 posts in 3152 days

#12 posted 12-06-2011 06:36 PM

besides not being true or accurate, they change with the temperature! My son took his good $5 framing square to his work place and had the quality assurance people to check it on the coordinates machine. It was nearly perfect. Far more accurate than we needed in a woodworking shop. Never know where you will find the accurate tools.

View FlairWoodworks's profile


71 posts in 3016 days

#13 posted 12-06-2011 06:49 PM

If you demand accuracy for squares and angles, you need one of these:

I don’t need one.

-- Chris Wong -- --

View DS's profile


3260 posts in 2897 days

#14 posted 12-06-2011 06:52 PM

cr1: Inside a gravity well, such as Earth’s gravity, the shortest distance between two points happens to be a curved line. Hmmmmm…. makes you think, right?

At some point, the practical application, renders the phrase, “Close enough for all practical purposes.” apropo.

There are times I think rocket science is easier than cabinetmaking.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

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