# How to check that an engineer’s square is accurate

Shop for Engineer’s Squares

## Other equipment you will need:

### Marking knife

You will need a marking knifeto mark vertical lines in order to check that your engineer’s square is accurately square.

### Engineer’s straight edge

You will need anengineer’s straight edgeto check that you have a straight edge on the wooden board and the outside edges of your engineer’s square.

### Wooden board

You will need a flat wooden board to test your engineer’s square on.

### Wooden block

A known square wooden block is needed to test the squareness of the outside edges of your engineer’s square.

## Begin work

### Step 1 – Check wooden board

The first thing to do in order to check that your engineer’s square is accurately square is to get a flat wooden board with a straight edge.

To do this, place your engineer’s straight edge against the edge of your wooden board and check to see if there is a gap between the two of them that would indicate the edge of the wooden board was not straight.
This is best done with the board sat upright, with a light shone behind it, as any gaps of light between the two will be more visible like this. If the edge of your board is not straight, use a plane to straighten it.

### Step 2 – Place square against wooden board

Once you have a wooden board with a known straight edge on it, take your engineer’s square and place the stock up against the straight edge of the wooden board.

### Step 3 – Mark outside edge of blade

Then, using the marking knife, place a small line along the outside edge of the engineer’s square’s blade at both the base and tip.

### Step 4 – Flip engineer’s square over

Next, flip the engineer’s square 180 degrees so that the outside of the blade is on the other side of the marks you have made.

### Step 5 – Bring edge to meet mark

Place the marking knife in the line you made at the base of the blade, and slide the engineer’s square along so that the blade meets the knife.

### Step 6 – Make mark at tip of blade

Hold the engineer’s square in this position with the stock against the straight edge of the wooden board and the blade against the marking knife. Then, take the marking knife and make another mark at the tip end of the outside edge of the engineer’s square blade.

If this is in the same position as the previous mark you made at the tip of the blade, then you will know your engineer’s square is square between the outside edge of the blade and inside edge of the stock.

### Step 7 – Check marks for alignment

If the two marks do not line up then your engineer’s square is out of square between the outside of the blade and inside of the stock by half the distance between the two marks over the length of the blade.

### Step 8 – Repeat with inside edge of blade

Repeat the procedure in steps 2-7, but use the marking knife to place the marks on the inside of the blade in order to check the accuracy of the square between the inside of the blade and inside of the stock.

You will now know if your engineer’s square is square between the inside of the blade and inside of the stock (angle shown in red), as well as the outside of the blade and inside of the stock (angle shown in green). If your square is square between both these positions then you will also know that the inside and outside of the blade are parallel to one another.
To check the outside of the blade to the outside of the stock is square, you will need a block of wood you know to be square.
As you know the inside of the blade to the inside of the stock, and outside of the blade to the inside of the stock on your engineer’s square are square. You can use these known square sides to make a wooden block with known square sides. That can then be used to check the outside edges of your engineer’s square.

### Step 9 – Place square against block

Once you have your known square block, place it on a flat surface, then place your engineer’s square on the same flat surface and move it up against the known square side of your wooden block.

If no light is visible between the edges of your engineers square and the edges of the wooden block or flat surface, then you know the outside edges of your engineer’s square are square, and also that the inner and outer edges of the stock must be parallel to one another.