Mouse ears and crescent moons: how to make sure parts always fit

Do you want to connect two metal parts? Then you can use a tab and slot joint: you make a slot in one part and a protruding tab in the other. By sliding the tab into the slot, the parts can be joined together.

However, tab and slot joints often don’t fit properly. In this blog post, we describe two causes and give some tips to solve them.

Residual metal in the corners of holes

The first cause is residual metal in the corners of holes. How does that metal get there? There are three reasons:

1. The radius we use to cut out the corner
2. The radius of the laser itself
   
3. The thickness of the metal 

The first reason is by far the most common.

Our flatbed lasers always create a small radius in corners. The main reason is that this is better for the cutting head of the machine. By following a gentle, smooth curve, they do not need to abruptly move to the left or right. The constant motion of the laser also reduces burr formation in the corners on the underside of the metal.

When cutting holes with corners, this means the laser never cuts a right angle. A small curve always remains in the corner. This need not be a problem, but it can be if another part must be inserted into the hole. A part with exactly the same dimensions will not fit, because the rounded corners will block it.

Should I make the tab smaller or make the slot larger? No, as that would reduce the stiffness of the joint.

The solution: mouse ears or dogbone fillets

What you can do is use ‘mouse ears’ or ‘dogbone fillets’. You draw small circles on the outside of the corners, which the laser then cuts out.

Thanks to these circles, there is no longer a radius on the inside of the corners, so the other part fits into the hole as it should. Because this only creates extra space in the corners, it has almost no effect on the stiffness of the joint.

Drawing mouse ears and dogbone fillets

You can read how large the mouse ears or dogbone fillets should be in our submission guidelines. Be aware that there must be sufficient space between the circles to maintain stiffness. The required distance varies for each part.

Please note: the image below – which does not have the correct dimensions – shows more clearly where the terms ‘mouse ears’ and ‘dogbone fillets’ come from.

Residual metal along the edges of holes

Protrusions that prevent a tight fit can often be found along the edges of holes. This can affect any shape – round, square, rectangular or other shapes – and is caused by one of the following:

1. Residual microjoints
2. The radius of the laser itself
3. The thickness of the metal

The first cause is the most common.

Microjoints – also known as bridges – are small temporary joints between the cut part and the rest of the sheet. They hold the small cut parts in place and ensure that the cut parts do not shift during laser cutting or fall out of the sheet when it is moved.

It isn’t always possible to avoid the use of microjoints. However, with flat parts, you can ensure that they don’t cause any problems when fitting parts into holes.

The solution: crescent moons

Crescent moons can be explained with an example, as shown in the image below.

On the left you can see a microjoint that connects a round cut part to the rest of the sheet. On the right, the cut part has been removed, but part of the microjoint remains. This small protrusion on the edge means a tube with the same diameter will not fit through this hole.

If you make a small cut at the location of the microjoint (in the shape of a crescent moon), the following situation will arise:

When the round cut part in the above example is removed, part of the microjoint will remain. However, this microjoint is recessed in the curve of the crescent moon, which means a round tube with the same diameter will fit through the hole.

Drawing crescent moons

How can you ensure that the microjoint is formed inside the crescent moon? 

By placing the starting point of the laser there. Place a green circle on the cut line in your drawing package or indicate the starting point in Sophia®. You can read how to do this here. Please be aware that this can only be done in DWG or DXF files or flat STEP files.

You can also add crescent moons to the straight edges of square or rectangular holes, for example, or holes with other shapes.

Finally, we have two tips.

Draw a quarter circle rather than a semicircle. With a semicircle, the hole will be unnecessarily large and the laser will have more difficulty cutting a neat circle. The ideal situation is shown on the left. The situation on the right should be avoided:

Place the starting point exactly in the middle of the crescent moon and not too far to the left or right. A microjoint in these positions will affect the stiffness and there will be a higher risk of protrusions that prevent parts from fitting once the microjoint is broken.

To be sure that the microjoint is placed correctly, you can also draw a crescent moon with a straight section (see the image below). If you do this, you must ensure that the protrusion is deep enough to reduce the risk of a protruding microjoint.

Do you have any questions?

Please contact our Customer Service. We will be happy to assist you!