Cracked Screw Bosses
Cracked Screw Bosses
(OP)
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?



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?








RE: Cracked Screw Bosses
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.
RE: Cracked Screw Bosses
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.