A Counterbore a Tenth Too Shallow Holds the Plate Off the Seat
What the counterbore actually controls
A counterbore does two things: it provides a seating face for the screw head, and it sets how much thread engagement the screw achieves in the plate below. Too shallow and the screw head stands proud of the counterbore floor, so the plate it is clamping is held off its seat by the screw head rather than gripped by it. Too deep and the screw runs out of thread before it seats.
In a plate stack, this is not a local defect. A plate held off its seat by a proud screw head loses contact over its whole face, changes the stack height, and can move the alignment of anything dowelled through that stack.
Tolerance and drawing practice
Depth should be toleranced on the drawing, not left to the default block tolerance on a dimension that matters functionally. The screw head height for the fastener being used is the starting figure; the counterbore floor flatness matters as much as the depth, because a floor that is square to the hole axis wins over a depth that is exactly nominal.
Practically, counterbore depth is worth checking on the first plate of a batch rather than at final inspection. It is a feature that is easy to cut to the wrong reference — measuring from a rough face instead of the finished face is the most common cause of a stack-up error, and it reproduces across the whole batch once the setup is wrong.
FAQ
Why does counterbore depth matter on a mold base?
It sets both screw head seating and thread engagement. Too shallow holds the clamped plate off its seat; too deep runs the screw out of thread.
Should counterbore depth be toleranced?
Yes, explicitly. Default block tolerance on a functionally critical depth is how stack-up errors get through.
What is the most common cause of depth error?
Cutting to the wrong reference face, typically a rough face instead of the finished face.
When should counterbores be checked?
On the first plate of a batch. A wrong setup reproduces across every plate, so catching it early avoids a whole batch of rework.
All figures above refer to standard product data sheets; final confirmation is subject to written quotation.