Yes. They will have to be machined on a CNC Mill. I've done this many times. It's a relatively expensive feature to add, and tough to do on thin sheets. But it's possible.
It just occurred to me that you have a structural issue.
A standard bolt and washer provides a flat contact with significant area. A flat[ ]head screw in a slot is clamped to two line contacts with a theoretical contact area of zero. The screw will embed itself as you torque your nut. It will embed itself further as you stress the joint, possibly leading to a loose screw.
I've used chamfered slots before in specific situations where I needed the positioning provided by the flat head screws, which would not have been provided by a counterbore/cap screw combination.
@SwinnyGG: Fair enough about the positioning aspect of countersunk screws. Do you know how strong a countersunk screw in a slotted hole is in tension? I assume it's still significant, but less than normal listed values.
@glass99 - of the top of my head, no, I don't know. My guess would be 'not a lot'.
If your slot is short and the plate is 1d thick or better, you should get a value relatively close to the normal value I would think. As your slot grows in length, or plate reduces in thickness, I'd imagine it's going to drop down pretty quickly.
When I've used features like this in the past is has always been in applications where the fasteners were loaded in shear. In shear there has to be some reduction which scales with slot length, but my intuition says the rate is lower. I've never done the calcs to be sure; I've only ever used an arrangement like this where loads were light.
Thanks for all the replies. Are there particular low-profile fasteners? I'm after M12 size. I looked up low profile cap screws and the head is ~1mm deeper than a countersunk head, so would need to increase the plate thickness for the counterbore option.
The tension is nominal, just to stop the edge of the plate lifting when a slow vehicle goes over. It's not worth spending big dollars on if the countersunk slot is that expensive. I thought maybe fabricators have a quick trick.
I'm leaning towards partly counterbored button screws, or maybe just live with full protrusion.
If there's a rubber tire going over it, I'd just use button heads with no counterbore. A button head isn't going to puncture a bicycle tire, let alone a rubber tire on anything bigger than a bicycle.
