Checking a mitre square
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19 November 2017 at 10:02 am #374483
Hi All,
I know how to check a square for square, but I was wondering if anyone knows of a similar trick or technique for checking a mitre square or the 45 degree part on a combination square?
Thanks,
Rob.19 November 2017 at 12:56 pm #374580Hi –
one possible way, that just requires a board or sheet or whatever with 1 straight edge:
Mark any point along the edge, and draw a line square from the edge (after you’ve checked the square is square!) Measure out two equidistant points along the edge, and use the mitre to draw a 45deg line from both points towards centre. If the two lines meet at the square line then you’re good. If they diverge, you’re off!
I may be explaining that terribly, but I quickly sketched it out in cad- attached a pic of what I mean. Obviously this requires accurate measuring out of the distance from the centre point.
Another option would be if you have a board that you know 100% to have two parallel edges would be to do on 45 deg line from one side, and see if you can match the angle with the mitre from the other edge. But I think it is easier to be 100% confident on having a straight edge and measuring accurately than having a perfectly parallel surface.
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You must be logged in to view attached files.19 November 2017 at 1:06 pm #374590although funnily enough if i zoom in sufficiently on that supposed accurate cad drawing, I have a slight slight divergence as well!
attached shows a 0.2 mm divergence. Interesting.I’ll have to try this out again with the real thing tomorrow when I am back in the shop.
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You must be logged in to view attached files.Another method would be to check the 90 degree first. If that is okay, scribe a line 90 degrees to a straight edge. Next, go an arbitrary distance to the left and scribe a line at 45 degrees that meets the 90 degree line. You now have a triangle. (Just one triangle. Not two, as in the diagram in the earlier post.) Get a pair of dividers and check that the base and height of the triangle are equal. If you do not have dividers, just mark off the distance on a piece of paper.
This only checks one side of the square, so now go to the right of the 90 degree line and scribe another 45 degrees. Note that this requires flipping the square over onto its other face. I would not try to make the two 45 degree lines meet because then they smudge each other. Just do the two triangles separately.
I have no idea how the old timers would have done it. I don’t see anything wrong with the method in the earlier post, but it does require you to get the left and right distances away from the center line exactly the same. In the single triangle method (here), you just draw anything. It doesn’t matter, as long as the square is tight. Then you check for equal lengths of rise and run, so it seems less chance for making a mistake. Of course, drawing a bigger triangle gives higher sensitivity in both methods.
19 November 2017 at 2:55 pm #374670using a marking knife would increase accuracy and avoid smudging also, depending on what surface it is you are checking the mitre with.
some straight scrap plywood big enough that you don’t mind marking, for example.The nice thing about a pencil in this case is that you can make it super ridiculously sharp and then pull a thin, gentle line with close to zero pressure on the square, but a knife leaves the possibility of nudging the square, especially for a 45 degree line. It’s true that the pencil line may be a bit thicker, but with a decent sized test triangle, super sharp pencil, the angle error for half a pencil line at the end of, say, 5 inches is small…. 1/64″ at the end of 5″ is 0.2 degrees, and you might even do better than that. Both approaches could work, so it depends upon personal comfort.
20 November 2017 at 10:40 pm #376076Thanks for the suggestions guys, I haven’t had a chance to try them yet, but I’ll do both and report back.
21 November 2017 at 5:30 am #376377One other method, which is only a variant of the others, is
First test that the square is good for square.
Then actually cut a square corner on a board and check it. Use your shooting board if you have to. You may already have a board with a true square corner.
Then knife a 45 line from your 90 degree square corner off both edges of your square corner. Start both knifeed lines at the same point.
If the combination square is true for 45, you will only have one knife line If it’s not true, you will have two knife lines diverging in a V.
And if you can’t tell which, it’s close enough.
- This reply was modified 6 years, 5 months ago by Larry Geib.
24 November 2017 at 2:21 pm #379512Isn’t this what the 3-4-5 method is for?
Go down 3″ on a straight side and make a knife nick.
Use the 45 to scribe a knife line 5″.
The distance between the ends of the scribed lines should be 4″
This works with any line lengths, but the point is that the lines should end up as 3, 4 and 5 times the unit of measure.
This has been the method for centuries.24 November 2017 at 11:26 pm #3812053-4-5 sounds cool. Never heard of the trick before (a reflection on me, not the trick!).
24 November 2017 at 11:34 pm #381220Reporting back from actually trying juggling my triangles around earlier-
Used an off cut of MDF, using the machined edge as my straight edge.it just happened that the Starret combination square I bought online arrived this week so I was equally confident I had bought a good tool that should pass the test, and apprehensive that I would find I had bought a dud!
But, all good. photo (not great quality) attached of the result. I guess if I did zoom in 100x, having used a tiny tiny tiny pencil point, I would maybe find a slight divergence, but for me the result is more than accurate enough for woodworking needs.
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You must be logged in to view attached files.28 November 2017 at 1:10 pm #385322Forgive me, I neglected to say that you would check both halves of the resultant 90 degree to see that they match.
29 November 2017 at 1:49 pm #386226I had a sheet metal apprenticeship in the early 70’s and was taught the 3, 4, 5 method for squaring at that time. Used it over and over through the years. A right triangle (one that has a 90 degree corner in it) in which one leg is 3 units long and the other leg is 4 units long will have a hypotenuse (the longest side) of 5 units in length. What unit you use is not important, just that they have the relationship to each other. Said another way, “a” squared plus “b” squared is equal to the square of “c”, or in other words 3 squared (9) plus 4 squared (16) is equal to 5 squared (25).
- This reply was modified 6 years, 4 months ago by SmokyRick Crawford.
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