Countersinks?
Countersinks?
(OP)
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
Any information you can provide would be helpful.
Thanks





RE: Countersinks?
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: Countersinks?
Dave D.
www.qmsi.ca
RE: Countersinks?
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.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Countersinks?
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: Countersinks?
RE: Countersinks?
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.
RE: Countersinks?
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.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Countersinks?
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.
RE: Countersinks?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Countersinks?
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.
RE: Countersinks?
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.
RE: Countersinks?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Countersinks?
RE: Countersinks?
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.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Countersinks?
RE: Countersinks?
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.
RE: Countersinks?
For most simple fastener hole patterns I think the hole pattern tolerance quite adequately controls things.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?