# Accuracy of a square



## cocheseuga (Dec 15, 2010)

Don't have one, besides assembly squares from Rockler. So while I was at HD I picked up their cheapest steel square to see how accurate it was, and if it wasn't to practice bringing one into square. 

I measured just now, and got about the following: as best I can measure, I had between a .7 to 1mm delta over a length of about 196mm. That's a .0035-.0051 variance. I give a range, as I measured the graphite lines as best I could. 

Would you consider that square enough? I know wood is going to move, but the more dead on I am in the beginning, the better off I'll be I reckon.


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## GeorgeC (Jul 30, 2008)

That is absolutely sufficient accuracy for all normal work. If you were going to use that square to square a 10' sheet then I would be concerned.

George


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## WillemJM (Aug 18, 2011)

cocheseuga said:


> Don't have one, besides assembly squares from Rockler. So while I was at HD I picked up their cheapest steel square to see how accurate it was, and if it wasn't to practice bringing one into square.
> 
> I measured just now, and got about the following: as best I can measure, I had between a .7 to 1mm delta over a length of about 196mm. That's a .0035-.0051 variance. I give a range, as I measured the graphite lines as best I could.
> 
> Would you consider that square enough? I know wood is going to move, but the more dead on I am in the beginning, the better off I'll be I reckon.


No, this has been discussed here before with various opinions, but I cannot live with that. I'm actually doing work on a project tonight where that will be major issue, with spindles glued together and going off center. The other problem is, that variance will worsen rapidly, as you start using the square. Ask me how I know.:blink:


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## Dave Paine (May 30, 2012)

Only you can say if the present square is accurate enough for your needs. 

The old Beth Steel mantra comes to mind. "Beat to fit - paint to finish". :laughing:

I use a square for not only laying out the wood, but also for checking the table saw blade tilt and today I needed to re-calibrate my mitre gauge after making an adjustment.

All these matter to me. I recently did a segmented bowl, and found that the 12 segments I cut did not end up being exactly 360 deg when glued. 

Some projects require more precision than others.

I purchased 2 or 3 squares before I finally purchased "DIN reference" 90 and 45 deg squares from Grizzly. These are dead on and so are the ones I normally use.

Shame that my rosewood handle square with the nice brass rivets is off. Nice looking piece, just gathers dust. 

I also have a 2 ft carpenter square. I only use this for cutting sheet goods.


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## WillemJM (Aug 18, 2011)

Dave Paine said:


> I also have a 2 ft carpenter square. I only use this for cutting sheet goods.


I have one of those too, mine is older than me. For rough carpentry work, or case work I actually squared mine up by measuring a perfect square on ply and then using a metal punch along the corner metal to stretch the steel and get the square, square, with another check by flipping it 180 degrees.

Nothing Pi&^% me off more than working carefully and ending up with a cabinet door that does not hang pefectly, or two pieces with a joint that has to be forced closed, or a miter which is off ever so slightly. So even with my Starret, I still build in double checks in my methods, to guarrantee accuracy. The el-cheapo triangle aluminum squares I find are pretty accurate.


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## railaw (Nov 15, 2011)

I have a Stanley 12ts 12" square that appears to have a very straight edge when compare to my starrett (albeit the starrett is 6"). I don't have an accurate way to check the Stanley's squareness but I'm ok with that for now. I am a little concerned though because the Stanley has a slight bow to it when looking down the thin edge. Should I be concerned about this when using it to check flatness by holding the edge to a board (or a plane I want to flatten)? Should I bother trying to take out the bow by stretching the metal, or would that make more problems?


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## JBSmall (Jul 6, 2012)

My Starrett 12" combination-square is god in my shop. (If you've never held one, you have a treat in store...they have that "Yes, I can do good work with this tool"-feel.)
You need one square which you decide is perfect, and trust it. 
Just don't drop it. Ever.
I decided my table-saw table is perfect, too. 
This obviously helps preserve sanity.


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## JBSmall (Jul 6, 2012)

railaw said:


> I have a Stanley 12ts 12" square that appears to have a very straight edge when compare to my starrett (albeit the starrett is 6"). I don't have an accurate way to check the Stanley's squareness but I'm ok with that for now. I am a little concerned though because the Stanley has a slight bow to it when looking down the thin edge. Should I be concerned about this when using it to check flatness by holding the edge to a board (or a plane I want to flatten)? Should I bother trying to take out the bow by stretching the metal, or would that make more problems?


I was taught to true-up an aluminum carpenter-square by peening the corner, outside to close it, inside to open it. You could "straighten" it this way too. 

Then file the edges to remove a little upset metal. 

Or buy a new square.

You can check things by scribing a line, flipping the square and scribing another, thus doubling any error. 
I am quite concerned about tools I can't trust. I'm capable of screwing up without their help.


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## WillemJM (Aug 18, 2011)

JBSmall said:


> My Starrett 12" combination-square is god in my shop. (If you've never held one, you have a treat in store...they have that "Yes, I can do good work with this tool"-feel.)
> You need one square which you decide is perfect, and trust it.
> Just don't drop it. Ever.
> I decided my table-saw table is perfect, too.
> This obviously helps preserve sanity.


+1:thumbsup:


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## BigJoe16 (Feb 20, 2012)

That square is probably more accurate than you will ever be able to cut. Saw aren't going to cut 100% perfectly every time. So as far as i am concerned squares are plenty accurate


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## GoIrish (Jan 29, 2012)

Probably good enough but it should be easy to find one better than that.


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## Wrangler02 (Apr 15, 2011)

BigJoe16 said:


> That square is probably more accurate than you will ever be able to cut. Saw aren't going to cut 100% perfectly every time. So as far as i am concerned squares are plenty accurate


No doubt, you are correct. The problem arises due to the compounding of error. It the square is off and the saw is off, you risk doubling the error.


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## railaw (Nov 15, 2011)

JBSmall said:


> I was taught to true-up an aluminum carpenter-square by peening the corner, outside to close it, inside to open it. You could "straighten" it this way too.
> 
> Then file the edges to remove a little upset metal.
> 
> ...


I remembered the two line trick after posting the above. I suppose that would help reveal any lack of straightness too. Thx.


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## cocheseuga (Dec 15, 2010)

I did a little work on it, and adjusted my measuring methods. Used a knife to mark the line. Measured a 3/32" gap at the bottom of the long side, a 2/32" gap at the top. 1/32" difference, and then I think you're supposed to divide that in half to get the true error. 1/64", and the distance was 12". I'm not sure I need to be more accurate than that.


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## GeorgeC (Jul 30, 2008)

cocheseuga said:


> I did a little work on it, and adjusted my measuring methods. Used a knife to mark the line. Measured a 3/32" gap at the bottom of the long side, a 2/32" gap at the top. 1/32" difference, and then I think you're supposed to divide that in half to get the true error. 1/64", and the distance was 12". I'm not sure I need to be more accurate than that.


You would be hard pressed to mark and saw even that accurately.

George


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## woodbutcher360 (Jul 1, 2012)

railaw said:


> I have a Stanley 12ts 12" square that appears to have a very straight edge when compare to my starrett (albeit the starrett is 6"). I don't have an accurate way to check the Stanley's squareness but I'm ok with that for now. I am a little concerned though because the Stanley has a slight bow to it when looking down the thin edge. Should I be concerned about this when using it to check flatness by holding the edge to a board (or a plane I want to flatten)? Should I bother trying to take out the bow by stretching the metal, or would that make more problems?


Lay that square on a piece of hardboard's machined edge and mark a fine line. Flip that sucker over and mark the same line. If the lines are the same, that square is a true 90 degrees. Pretty simple test.


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