Mitre Square Accuracy

[Charlie Woody]

This is a silly question ..... how do I check the accuracy of my Mitre Square?

I have been having difficulties cutting mitres so decided to make a shooting board and then I wondered if my Mitre Square is accurate!!! As if it is'nt then my shooting board will also be out!!

 

[woodbloke]

Not daft at all...I really don't know. Normal squares are easy, but a mitre square? :duno: - Rob

 

[marcros]

i have been pondering this prior to making a shooting board.

I would guess you do it the same, but twice from each direction, so lines/marking knife at 0, and assumed 45 degrees. move it to mark 90, then do the same backwards from 180 degrees marking assumed 135. If the 2 90 lines meet up, then all is well.

I am wondering how you do it in b and q to ensure you are buying a good one!

 

[Charlie Woody]

Not sure that I understand your methodology Marcros, could you please elaborate further?

 

[9fingers]

Make sure your table saw is properly set up. That is, mitre slot absolutely parallel to the blade and the fence absolutely parallel to the blade and slot.

Cut a piece of ply/mdf etc so you have a parallel strip using the blade and fence spacing of 200-300mm

Make a cut using the suspect mitre square with one of the parallel edges against the mitre square. flip that piece over face down on to the table and compare the cut edge against the square and blade.
If that looks reasonable, then make a more accurate comparison by starting with an approximate square labelling the edges in sequence A, B , C D

Place side A against the square and trim edge B
rotate 90 degrees (approx) and trim edge C with respect to B
rotate again and trim D with respect to C
compare edge A with the blade and edge D against the square.
This magnifies any error by 4 but if the mitre square is accurate, then there will be no gaps.
Check by measuring diagonals too.

hth

Bob

 

[James C]

Yes good point mitre square or mitre fence?

 

[marcros]

not without a picture, and that involves scanning which I cant easily do.

you know how you test a square, by flipping it over and checking that it reads the same from both sides- as described here http://www.newwoodworker.com/fxfrmsqr.html

well, I propose doing the same with the mitre, but you will have to do it twice to make 90 degrees.

Say for instance, it is dreadful, and is 40 degrees instead of 45.

You mark against a known straight and draw a line at 40 degrees. you move it to your line and draw another at 40 degrees, or 80 from your original straight. Moving back to your original straight, you flip the mitre gauge over and draw the same lines at 40 and 80. In this example, the middle lines will show a v, because there is a 20 degree error. If it was an accurate 45, they would overlap on 90 degrees.

At least in my mind that works!

 

[Charlie Woody]

Bob
My table saw does not have a mitre slot ... it has a sliding table but I find it difficult to hold small items against the sliding table fence. So I have been using a SCMS to cut the mitres and wanted to shoot them accurately, hence my post.

Marcros
I think I need to get my mitre square out and try what you say, so will attempt this tomorrow.

Thanks guys I'll let you know how I get on.

 

[9fingers]

Bob
My table saw does not have a mitre slot ... it has a sliding table but I find it difficult to hold small items against the sliding table fence. So I have been using a SCMS to cut the mitres and wanted to shoot them accurately, hence my post.

Ah! that approach breaks my first rule. it uses a hand tool! :mrgreen:

I'm sure it is possible the adjust the sliding table by the same method and to make some sort of jig to hold your work pieces firmly when sawing.

Bob

 

[Steve Maskery]

Simples!
Take a long parallel board.
Set your mitre fence (mitre gauge or sliding table, whatever) to 45 deg as defined by your mitre square.
Take a 45 deg cut, flip the workpiece and make a second one.
The result SHOULD be a triangle with a perfect 90 deg corner. If it isn't, your square (or technique) is out.

Or is that what the others have explained - it's late.
S

 

[wcndave]

Look on this video, at 14 minutes. http://logancabinetshoppe.com/blog/2009/06/episode-3/

Basically you do a square line on a board, then draw an arc through 0, 90, 180 degree positions (basically you could draw a semi circle). Then you put compass on points where circle overlaps your horizontal and vertical access (ie three point) and draw four more arcs, two from middle, one from each "side" and where they overlap is exactly 45 deg from the centre start point.

I tried the other method of drawing two lines and checking they add up to 90, however was never that sure, that it was 90 rather than 89.5 as thats hard to tell with a protractor.

The method in the video is supposed to be the traditional "correct", or perhaps, mathematical way of doing it, and is dead easy if you have a compass!

 

[CHJ]

Take A4 sheet of printer/copier paper.

Align mitre square along bottom edge and left side edge, Draw line at 45°

Turn Paper 180°, align mitre square along bottom edge
Repeat 45° Line, should be parallel with first line.