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Absolutely, Positively Square - And Cheap!

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Review by jonah posted 08-31-2015 01:04 PM 11369 views 0 times favorited 12 comments Add to Favorites Watch
Absolutely, Positively Square - And Cheap! No-picture-s No-picture-s Click the pictures to enlarge them

I have two of these yellow plastic speed squares. One was $3-4 at Home Depot years ago. One I purchased a couple of months ago for $4.

Summary/TL;DR version: these cheap plastic speed squares are absolutely 90°.

Long version:

After reading a thread in the forums a while ago extolling the virtues of “precision” layout and marking tools, I decided to resurrect my 9-10th grade geometry and trigonometry and figure out exactly how square these things are. I discovered first that the squares are absolutely identical in size. Both are exactly 7.25” x 7.375” – exactly as the HD website claims. I checked that with a measuring tape as well, my 12” Starrett combination square that I got off eBay some years ago, and a ruler. However, if you look at the picture, you can see that one corner is not a 45° angle but is clipped off at 90°. Using two rulers, I determined that if that side continued out at the angle it was at that the side would be about 1/8” longer.

Here are our options for discovering the angles and sides of this triangle:

1) Law of triangles (all angles add up to 180°)
2) Pythagorean Theorem (for right triangles)
3) Law of Cosines/Sines
4) Using trusted 90° angle, compare to the speed square to see if they differ
5) Using trusted flat/straight board, use the “two line” test to see if opposite sides of the square make parallel lines on the board

If the angle on the speed square is indeed a 90 degree angle (which it turns out it is, but lets not take that as a given for the moment), we could use the Pythagorean Theorem to find the measurement of the third side of the triangle. If that number differed from the actual measurement we’d know that the angle wasn’t 90.

I decided to not take a 90 degree angle as a given for the moment and to use a much more complicated Law of Cosines formula. Here's a good website that explains it.

3) Law of Cosines

c^2 = a^2 + b^2 – 2ab cos©

I ended up tracing the triangle onto a piece of paper using a really fine mechanical pencil, extending the line of the clipped off corner, and measuring that. The hypotenuse of the triangle ended up being just above 10 27/64”, so I took that to mean 55/128”. 10 55/128” is 10.4297.

After a bunch of simplification of the above formula we end up with with ~90.01° or ~89.99, depending on which angle you set to be C. We pass. That’s pretty square!

2) Pythagorean Theorem

I have sides of 7.375”, 10.4297”, and what I suspect is another 7.375”.

a^2 + b^2 = c^2

10.4297^2 – 7.375^2 = b^2

b = 7.3748 = 7.375

We pass the Pythagorean Theorem test.

4) Trusted 90° Angle

My most trusted 90° angle is the aforementioned Starrett combination square. Holding the square up to the speed square, I see no gap between them all the way down to the end of the blade. We pass this test.

5) Parallel lines test

Again using the Starrett square as well as my best straight edge, I found a piece of plywood that was absolutely flat and straight and square, with the face 90° to the edge. I drew a line with the speed square. I flipped it over and drew another. Both are parallel to my eye. I tried ones that were very close together as well as 1/4” apart. I could not see any obvious divergence. I tried the same test with my Starrett combo square. Same thing. We pass this test.

Conclusion: any divergence from 90° in this speed square is so slight that I cannot detect it with the measuring tools I own and cannot see it with my 20/15 vision. Five stars, especially considering the price.




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jonah

745 posts in 2934 days



12 comments so far

View CincyRW's profile

CincyRW

156 posts in 1286 days


#1 posted 08-31-2015 04:59 PM

Thanks for this. Its great to know I dont have to pay upwards of $25 (and more) for a square thats actually square.

View Tedstor's profile

Tedstor

1643 posts in 2268 days


#2 posted 09-01-2015 01:20 PM

I have one of these plastic squares and the aluminum model. They are indeed well-made. I use them all the time.

View JoeinGa's profile

JoeinGa

7672 posts in 1642 days


#3 posted 09-01-2015 10:10 PM

I also have a plastic one. Plus a 6” and a 12” aluminum one too. All three are dead on.

Oh, and by the way …
.

-- Perform A Random Act Of Kindness Today ... Pay It Forward

View jonah's profile

jonah

745 posts in 2934 days


#4 posted 09-02-2015 01:56 AM

Well, I did dramatically simplify the math part…

View Richard's profile

Richard

1912 posts in 2326 days


#5 posted 09-02-2015 08:05 PM

2) Pythagorean Theorem

I have sides of 7.375”, 10.4297”, and what I suspect is another 7.375”.

a^2 + b^2 = c^2

10.4297^2 – 7.375^2 = b^2

b = 7.3748 = 7.375

This Does Not Pass the KISS Principle in my book. :)

View jonah's profile

jonah

745 posts in 2934 days


#6 posted 09-03-2015 12:27 AM

It doesn’t get much simpler than the Pythagorean Theorem!

Even landscapers who claim to be terrible at math use the 3-4-5 method for squaring up a corner.

View Dark_Lightning's profile

Dark_Lightning

2743 posts in 2744 days


#7 posted 09-03-2015 01:00 AM

I saw that other thread- the one with the rant? Is that the one you are referring to? There are several, anyway. The 3-4-5 method has been around for millenia, and is covered in my Millwright’s Handbook, as well. I was talking to my wife about this the other day, and she uses plastic squares to make pieces for quilts. She’d be pretty mad (OK, incendiary) if the pieces didn’t line up. She’s making a quilt for our California King bed, out of pieces as small as 2.5” on the short side of the 45°-45°-90° triangles. There are hundreds of these triangular pieces, which include a small section that gets folded over and sewn. This quilt is going to hang over the edges of the mattress…and her quilts come out square.

-- Random Orbital Nailer

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jonah

745 posts in 2934 days


#8 posted 09-04-2015 10:00 PM

Not to pick on Woodpeckers, because they are far from the biggest culprits (I’m looking at you, Lee Valley), here are two of my favorites:

$110 for what amounts to a try square.

$35 for a speed square that isn't really suitable for use as a speed square.

I’m all for well engineered stuff, but sometimes it’s overkill, guys.

View Richard's profile

Richard

1912 posts in 2326 days


#9 posted 09-04-2015 10:54 PM


Not to pick on Woodpeckers, because they are far from the biggest culprits (I m looking at you, Lee Valley), here are two of my favorites:

$110 for what amounts to a try square.

$35 for a speed square that isn t really suitable for use as a speed square.

I m all for well engineered stuff, but sometimes it s overkill, guys.

- jonah


$110 for that square is Over Kill on my Wallet.
Think I would buy 10 of the plastic ones first , and I don’t like plastic tools.

View TObenhuber's profile

TObenhuber

152 posts in 1228 days


#10 posted 09-05-2015 12:05 AM

LOL!!! WOW, those are insane. Far from my low budget style. Of course if someone wanted to give it to me as a hand me down I would cherish it. I promise I would make a space beside the others from Harbor Freight. Of course I think I would still lean for my trusty Harbor Freight Speed Square. I just know the cheap ones and trust them.

-- Travis, Virginia, www.facebook.com/CreativeWoodworksHybla

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TObenhuber

152 posts in 1228 days


#11 posted 09-05-2015 12:06 AM



Not to pick on Woodpeckers, because they are far from the biggest culprits (I m looking at you, Lee Valley), here are two of my favorites:

$110 for what amounts to a try square.

$35 for a speed square that isn t really suitable for use as a speed square.

LOL!!! WOW, those are insane. Far from my low budget style. Of course if someone wanted to give it to me as a hand me down I would cherish it. I promise I would make a space beside the others from Harbor Freight. Of course I think I would still lean for my trusty Harbor Freight Speed Square. I just know the cheap ones and trust them.

-- Travis, Virginia, www.facebook.com/CreativeWoodworksHybla

View woodDuffer's profile

woodDuffer

4 posts in 425 days


#12 posted 03-28-2016 01:50 AM

I like Empire, I think they have very good products for exceptionally inexpensive prices.

I bought a couple of their 36 inch aluminum straight edge rulers, one for upstairs and one for downstairs, at $2.97 each at Home Depot. Less than a dollar a foot!

When I came back home after working out of state for a few months, they were both gone. This despite the fact I also bought one for both of my wive’s two sons.

-- Life support failure. Check oxygen levels at once.

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