ANSI B18.6.3-1972, R1991 Appendix

[mackmoon]

I am trying to learn about why the Machinerys Handbook has a note at the foot of the Slotted Round Head Machine Screws table 12 stating "Not recommended, use Pan Head machine screws." So far I have not had access to the ANSI document itself. I am seeing this on page 1502 of edition #25 and on page 1577 of edition #26. Does this statement refer to all the round head screws in the table, or to part of them? Does anyone know why the std prefers pan head to round head? Thanx.

 

[arto]

Round heads get all chewed up at the slot edge & tend to "cam-out" the screwdriver blade. Pan heads are flat, so the edge contact it more a line than an arc, so the screwdriver works better.

 

[mackmoon]

Thank you very much for your response. It is interesting. The slot is deeper on the round head, even tho rounded, so I kinda expect the driver to stay seated into the slot better. Does the ANSI std point this out as why they do not recommend round head screws, do you know? Does your comment come from your own bench experience? Most of my recollected experience is with binder heads, and I find that it is very easy for the driver to slip out of the slot, which mars the screw. I have not seen myself do this to a round head screw yet, but I have much less bench experience with them. Thanx again.

 

[mackmoon]

I told my partner of your response, and he has seen the chewed edges on round head screws too. Cool. Does this happen to stainless screws too? I am guess that the stainless steel screws will be harder on the driver over time. I am working on CAD dwgs for a chassis assembly. The company this is for typically uses round head screws. When I saw the note in the Machinerys Handbook, I flagged the company of the note recommending pan head screws. I got back a this response from one of their mechanical engineers:

"Generally speaking, pan head screws have a larger head diameter and shorter head height as compared to the same thread size for round head screws. We do use some pan head screws here @ ___ but mostly round head. As I see it,
the "not recommended" refers to (ANSI B18.6.3-1972, R1991 Appendix) see pg. 1498 of edition #25, table 7. Very small diameter threads only from 0000,000 and 00 with diameters of .021 to .047" max. dia., NS designation. I would use round head."

I cannot see this in the Machinerys Handbook myself. I am guessing further info must be present in the ANSI specs, but I still need to learn this for myself, so that I can understand this issue properly. Please forgive me for seeming so stubborn and challenging. I am only trying to understand this better.

Is the guidance I was provided by the internal mechanical engineer quoted above true, or... do I need to persuade the company to change? No matter what, I would like a sound understanding of this issue, if only for myself and my future work. Does anyone have further thoughts on this to help me sort this out?

Hope my persistence doesn't drive anyone crazy! Thanx.

 

[Hush]

If you want to be sure buy ASME B18.6.3. Its only about $48.