Countersink with M3 and bigger only?

jmsaltzman

I'm trying to make M2.5 countersunk holes, but I don't get the chamfer on them (or smaller), while I do get it on M3 size and bigger.

Here's a test document: https://cad.onshape.com/documents/ec5b355b32308a201247974a/w/22bc19797fcb30004dea4b95/e/e62ee5e5019dd294ac17a4c0

The dialog for M2.5 here shows the same value for "Diameter" and "Countersink diameter," but the latter is greater for M3 and larger hole sizes.

Is there a reason for this? I could enter a reasonable value myself but am curious why it's not setting it automatically.

M3: showing the countersink diameter of 6.72:

M2.5 showing the same countersink diameter as the normal diameter:

(edited for grammar)

philip_thomas

@jmsaltzman - Your curiosity shall be rewarded. I spoke to the keeper of the standards (Jason) - yes we really do buy the standards (they are ridiculously expensive if you were wondering), and the answer is that there is no specification for chamfer on fasteners less than 3mm (ISO). Because the value in the table (and subsequently shown in the dialog) has to be something, we set it to be same as the pilot diameter (why 2.9 for an M2.5 i do not know). Can you override it? Of course!
I hope this helps

philip_thomas

@jmsaltzman - Your curiosity shall be rewarded. I spoke to the keeper of the standards (Jason) - yes we really do buy the standards (they are ridiculously expensive if you were wondering), and the answer is that there is no specification for chamfer on fasteners less than 3mm (ISO). Because the value in the table (and subsequently shown in the dialog) has to be something, we set it to be same as the pilot diameter (why 2.9 for an M2.5 i do not know). Can you override it? Of course!
I hope this helps

jmsaltzman

Aaah... that makes sense. Interesting reason too: we learned something!

I overrode the diameter myself, works fine of course.

Thanks for digging and reporting back!