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Countersunk Holes and Implied 90
3

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!

RE: Countersunk Holes and Implied 90

The implied thing about implied 90° angle is that it is only implied when the right angle is formed by a horizontal and a vertical line.

RE: Countersunk Holes and Implied 90

Burunduk,

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

Quote (Burunduk)

The implied thing about implied 90° angle is that it is only implied when the right angle is formed by a horizontal and a vertical line.

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

engAlright,

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

It's a novel interpretation to use on a countersink. Normally it's considered for situations where there is an intersection between the related surfaces, not for elements of one to other elements on itself. I suppose it goes that a point on a sphere taken as a pole is at right angles to all the points on the equator relative to that point.

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

Quote (Belanger)

The verbiage of Fundamental Rule (j) doesn't say anything about the relationship of the two lines to the horizon

Yes, it doesn't say... that's why I said "the implied thing about implied 90° is that it is only implied when..." wink
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

Still disagree with you. I'm not really referring to countersinks; I am taking issue with your general statement: It's not even implied that implied 90º angles requires horizontal or vertical lines. You're introducing something that is not even remotely intended by Y14.5.

RE: Countersunk Holes and Implied 90

Quote (greenimi)

Would you consider a complete definition of the c'sink if only the hole is defined for its location (fig 1-39)?
Only if the dimensions and tolerances are given the way they are in fig. 7-24, only with the counterbore symbol replaced by the countersink symbol. A countersink is a cone, and it may be a gray area whether a countersink location can be controlled by position, but the only other option is profile, and we don't see that often for c'sinks.

RE: Countersunk Holes and Implied 90

Belanger,
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

engAlright,

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.

--
JHG

RE: Countersunk Holes and Implied 90

Do you want to get what you need? Or just what you end up with?

RE: Countersunk Holes and Implied 90

Quote (drawoh)

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°.

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

(OP)
Thanks for the responses everyone. I appreciate the recommendation to add the 90° angle for clarity, but that could be said about any 90° angle on a drawing. There are many instances where geometry lines or centrelines do not intersect but an implied 90 applies: see Fig. 7-21 of the 2018 standard as mentioned by Belanger.

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

How do you guys usually specify the tolerance for a countersink angle?
For me it is one of the exceptions in which I still use direct tolerances on angular dimensions.

RE: Countersunk Holes and Implied 90

engAlright,

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.

--
JHG

RE: Countersunk Holes and Implied 90

I'd want to be very sure to understand what the c'sink is used for. Sometimes the angle is important, and definitely not 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

not calling out the angle and symbol is just laziness

RE: Countersunk Holes and Implied 90

(OP)
Well seems like I'm in the minority...consensus seems to be that showing the angle is the way to go. It still seems weird to show a 90° angle dimension on a drawing but I understand that the tendency will be to assume the Engineer has forgotten to add a dimension, rather than assuming an implied 90 applies.

And for reference ANSI metric countersink angles are 90°.

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