Countersunk Holes and Implied 90
Countersunk Holes and Implied 90
(OP)
Section 1.8.12 of Y14.5-2009 indicates that for countersunk holes "...the diameter and included angle of the countersink are specified.".
In the corresponding figure 1-39 the countersink feature is shown dimensioned with a 90° angle.
Why is an angle dimension required when the included angle is 90°? Would an implied 90° angle (section 1.4i) not apply? This is the only instance I can think of where a 90° angle needs to be shown on the drawing.
Thanks!
In the corresponding figure 1-39 the countersink feature is shown dimensioned with a 90° angle.
Why is an angle dimension required when the included angle is 90°? Would an implied 90° angle (section 1.4i) not apply? This is the only instance I can think of where a 90° angle needs to be shown on the drawing.
Thanks!
RE: Countersunk Holes and Implied 90
RE: Countersunk Holes and Implied 90
Would you consider a complete definition of the c'sink if only the hole is defined for its location (fig 1-39)?
Example: Ø6.8 ± size tolerance and position|Ø.xxx|A|B|C|
RE: Countersunk Holes and Implied 90
The verbiage of Fundamental Rule (j) doesn't say anything about the relationship of the two lines to the horizon. Only to each other.
See Fig. 7-21 of the 2018 standard for an example of implied 90º where the lines are neither vertical nor horizontal. It's rule (k) in that case, but the same as Rule (j).
RE: Countersunk Holes and Implied 90
Ask yourself this - whats the chances you think someone will call you up to confirm if two surfaces at 90deg on the drawing (but aren't labeled) are at 90deg? My guess is pretty much nil. Okay, what about if a 90deg countersink is also not labeled? I would say much higher, considering 82deg is a very common countersink angle, and trying to tell which is which by eye would be tough without a side by side comparison or 3D model to measure from. Its best to be as explicit as possible on your drawings, and try to avoid ambiguity wherever possible.
RE: Countersunk Holes and Implied 90
I don't recall often seeing a countersink depicted as having intersecting sides, at least not by direct contact (an inverted cone cut) or via extension lines, so it doesn't exactly fit, but neither would it fit if there was a chamfer shown on the edge of a rectangular part that cause a similar depiction.
More concerning is that there's no control that the axis of the countersink is parallel to that of the hole, a case I have seen in junky parts and which neither the diameter or position tolerances (when applied) do a great job of restricting - fortunately metal and plastic can bend.
RE: Countersunk Holes and Implied 90
RE: Countersunk Holes and Implied 90
Yes, it doesn't say... that's why I said "the implied thing about implied 90° is that it is only implied when..."
But apparently less so according to the 2018 standard. If one line of the countersink section is at 32° to the horizon of the view and the other one is at 122°, can you tell at a glance from looking at it that the included angle is 90°? I would personally not be sure. Anyway, I also agree with chez311.
RE: Countersunk Holes and Implied 90
RE: Countersunk Holes and Implied 90
onlyother option is profile, and we don't see that often for c'sinks.RE: Countersunk Holes and Implied 90
I would agree that where there is a physical corner between surfaces (not c'sinks) the implied 90° is clearer regardless of whether the lines are horizontal/vertical or not, so maybe I shouldn't have generalized.
RE: Countersunk Holes and Implied 90
I walked into a hardware store here in Canada and I asked for a 90° countersink. They stared at me like I was a space alien from the planet Zorklon, and they reminded me that countersinks are 82°. They can also be 60°, 100°, or 120°.
You need to call up the angle.
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JHG
RE: Countersunk Holes and Implied 90
https://www.globalfastener.com/standards/detail_15...
RE: Countersunk Holes and Implied 90
RE: Countersunk Holes and Implied 90
Yeah, that hardware store needs to get with the program. A 90 degree countersink is as common, if not more common these days, as an 82 degree. 82 degrees is the included angle for imperial unit flathead screws and 90 degrees is the angle on metric flathead screws. Things here in the US are moving more and more towards metric so those things are in every machine shop I've worked in for the past 30 years.
John Acosta, GDTP Senior Level
RE: Countersunk Holes and Implied 90
In this instance I think it's fair to assume an implied 90. If the countersink was 82° an angle dimension would be required. Since no angle is shown, we assume an implied 90 applies.
RE: Countersunk Holes and Implied 90
For me it is one of the exceptions in which I still use direct tolerances on angular dimensions.
RE: Countersunk Holes and Implied 90
In Ontario, if you don't specify your countersink angle, it will be assumed to be 82°. Another possibility is that a machinist will pick up the phone and call you to ask for the correct countersink angle, wasting both of your times. He has to pull the correct tool off the shelf.
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JHG
RE: Countersunk Holes and Implied 90
Like powerhound said, the c'sink angle needs to properly match imperial vs metric flat head screws.
A less frequent C'sink use for us is the chamfer on the clearance hole under a SHCS etc to miss the screw underhead fillet.
Machine center c'sink 60°.
Lugnut seats 45° or 60°. Better get that one right.
RE: Countersunk Holes and Implied 90
RE: Countersunk Holes and Implied 90
And for reference ANSI metric countersink angles are 90°.