Cracked Screw Bosses

[ryandias]

We are having trouble with some screw bosses I have designed...

I followed the standard design guides available online.

The screw is a #4-20 plastite 45 screw. It secures the connector into a metal housing. The 2 of the 3 screws use a half slot, rather than a hole. These are both exhibiting cracks.

The screw boss details are;

ID 2.40mm
OD 6.00mm
Depth 6.5mm

The 4-20 screw depth past the metal is 5.95mm but realistically when measured around 5.5mm.

I have contacted the supplier and they have not had anything to say about the plastic specifically. They suggested that it might be the half slot "pushs" the screw toward the open, causing a crack in the boss.

I do not have a counterbore on top of the boss. Is this important to strength, or just starting the thread easily?

Would love to hear some theories or suggestions?

 

[hendersdc]

Looks like your pilot hole diameter is too small. Not sure what you mean by a half slot vs a hole as your pictures aren't clear about it but anything less than a full boss around a hole for a thread forming screw is asking for trouble. It could be done but you would need to open the pilot hole diameter up even further to reduce the radial force from the screw.

A counter bore is only needed if you want to mount something flush to the boss surface. Thread forming screws will deform the plastic around the hole outward a little, a counter bore gives this plastic somewhere to go where it won't effect the mounting surface.

 

[ornerynorsk]

I don't think you mentioned what the material is.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.