Countersinks? 5

[biglaw22]

Do I need to call out the concentricity of a countersink? This would seem like overkill to me, but I am having an issue with parts coming in that have countersinks offset from the center of the hole. This causes the screw to not sit flush. (these are for #4 screws)

Any information you can provide would be helpful.

Thanks

 

[PeterStock]

It would control the offset you are seeing. You should call the main hole diam as a datum and relate the countersink (or counterbore if you have those) to that datum with a true position tolerance. If there is a hole pattern, you state the hole diam datum as "INDIVIDUALLY" and the same for the true position tolerance.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[dingy2]

I agree with PeterStock that positional should be used to control the countersink relative to each hole in the pattern.

Dave D.

http://www.qmsi.ca

 

[KENAT]

Are your countersunk holes located using position tolerance? If so the countersinks are already related to the hole diameters within the positional tolerance used. If you've toleranced the mating parts properly this level of control is typically adequate.

Are you sure the countersinks are off from the holes, have you confirmed this?

If the mating part has a pattern of threaded holes that the screws are going into then you may have another problem causing the screw heads to appear out of true with the countersinks.

Because of the cone shape of the countersunk they tend want to center the screws to the countersink. Threaded holes also tend to center screws in them due to the geometry of the thread. So you have 2 competing features trying to center the screws, this is called a fixed-fixed fastener situation. Unless you match drill the pieces it's very unlikely the countersinks and threaded holes will be very closely lined up, they will be off by a few thou' most likely. This in turn will cause the heads to not sit flush and/or create stresses in the heads which in some circumstances may cause them to fail.

So to avoid this:

1. Avoid fixed-fixed fastener situations where ever possible. When heads protruding are not acceptable consider counterbores with another type of screw. Button heads have a fairly low profile and can be counter bored flush into relatively thin material.


2. If you must have fixed-fixed fasteners then there are steps you can take to ensure the head is always flush or below flush however, IT DOES NOT PREVENT INCREASED STRESS IN THE SCREW HEADS/MATING PARTS. Essentially you can oversize the countersink so that even when the screw head is off center to it, it will still sit flush. When using positional Tolerancing, which is usually the most appropriate way for hole patterns, you can use the below method.

Use the tightest positional tolerance on both countersunk holes and threaded holes that you can get away with from a manufacturability/cost point of view. Threaded holes are difficult to hold very tight but countersinks can be held fairly tight with less cost increase. Add the total positional tolerance of the two holes to the minimum diameter of the countersink. This will ensure the screw head is always flush or under flush.

For example, say your threaded holes have position of dia .005 (which is about as tight as I'd want to go for a threaded hole and will probably increase cost). Say your countersunk holes have a position of dia .003 (again fairly tight with a possible cost impact). So the total position is .003 + .005 = .008.

For a #4 socket flat head screw I believe the max sharp head dia is .256.

So you would want your minimum countersink dia to be .256+.008 = .264.

However, as mentioned above, this may still cause stress in the screw head and the countersunk part.

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[ewh]

A star for common sense, KENAT.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[ModulusCT]

Yeah, no kidding... That was a great post.

 

[biglaw22]

Thanks Kent,
I will speak with my supplier, I'm going to guess that they will prefer to go with the larger countersink. They are not the most technical, but the price is right. The problem is only at final assembly visual/tactile.

 

[KENAT]

Glad it was of help, this isn't the first time it's come up but I tried to explain it a bit differently.

I've battled fixed-fixed at my current employer on a number of occasions, the above technique was shown to me by CheckerRon when I had the pleasure of working with him, so he deserves some of the credit.

If you don't have relatively tight pos control then your csk's get very over size and start to look poor, especially depending on color contrast between screws and cover. Also, the more holes in the pattern the more problematical it can be.

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[CoryPad]

biglaw22,

Are the countersinks offset from the hole in the countersunk part, or are they offset from the tapped hole in the other part? Your options will vary based on the answer to this question. KENAT's discussion of fixed:fixed holes is for the latter problem.

 

[KENAT]

Cory, I did address the first part with my first 2 paragraphs. I just found it unlikely that this was the issue, since the csk tool will typically follow the existing hole unless you make efforts to do otherwise.

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[Compositepro]

Making the countersink deeper will help keep the head from sticking up but will not help the head taper fit the countersink taper. Which problem do you have? You said the countersink is not concentric with the hole in the part. That is different than not lining-up with a mating threaded hole. The first issue can be handled with better tolerances or making the hole bigger. A countersink head should sit in the taper and the body of the screw should not touch the hole.

Getting matching parts to line-up is a more common problem, particularly where the countersink is in a thin part so there is little distance between the the countersink and the threaded hole. One solution is to counter bore the threaded hole to a small depth so there is some ability for the screw to flex between the countersink and the start of the thread. Otherwise, tolerances must be tight.

 

[CoryPad]

KENAT,

Yes, but I was thinking of corrective action for the eccentric condition. For example, using a single machining tool and operation to perform both drilling and countersinking, rather than two tools and two operations. There are benefits and disadvantages to both, but a single tool and operation should eliminate this condition.

 

[KENAT]

Cory, I took the view that the OP was asking how to specify the concentricity on the drawing, not the manufacturing methods.

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[CoryPad]

Yes, that was the right way to go. Concentricity requirements for fastener holes on engineering drawings are not very common, and one of the reasons is the use of different manufacturing methods. I am very interested in the subject of fastened joint drawing specifications, I even asked a question (thread1103-229636) here regarding spot faces. I think this is an area where industry best practices for making actual parts usually is so good that most drawings don't need/use notes.

 

[KENAT]

thread1103-197198 talks about position tol of coaxial features which would include countersinks. The relevant section in ASME Y14.5M-1994 is section 5.7. Unfortunately the ilustration is slightly confusing, that was the reason for this other thread. Sorry I didn't link to this when you asked before Cory, it wasn't one of the things you specifically asked about and I didn't think of it then.

It was section 5.7 I was alluding to in the first paragraph of my first post.

So I'd say that coaxiality (rather than concentricity which has a specific meaning) of fastener holes is commonly specified, though perhaps not widely understood.

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[CoryPad]

All good points

 

[MartinSr00]

If you need to control it, control it. If you leave it controlled, it's uncontrolled.

To control a countersink on a through hole, you would have a locational control on the hole, set the axis of that hole as a datum feature, and control the location or orientation of the countersink. Without it, you're relying on the part manufacturer to countersink the hole in a single setup with boring the hole.

Either make your overall design so the eccentricity doesn't matter, or, control it.

 

[KENAT]

Can I suggest all the people suggesting making the hole a datum and then adding an additional control to the CSK take a look at section 5.7 in ASME Y14.5M-1994.

For most simple fastener hole patterns I think the hole pattern tolerance quite adequately controls things.

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