Countersink vs. Counterbore

[BCRider]

Like the others above I tend to pick flat head screws and countersinking where the material is thin and I don't want cap or button head screws sticking up.

A slight downside to flat head screws is the thickness of the rim around the edge of the head. Unlike classic wood screws this is not a sharp edge. So some degree of wider countersinking is needed. In an ideal world we'd have sized countersinks for each screw so we could go deep enough with a countersink that is just a whisker bigger than the diameter of the screw heads and bury that rim without a big surrounding "halo" of countersinking.

Another source of confusion for me is the total lack of standardization on optimal countersink angles. Most of the black finish with allen socket flat head machine screws I have seem to be 90*. But most of the generally available countersinks seem to be 82*..... Or at least the countersinks I've bought seem to be. The mismatch of angles makes for either a non optimal job with the outer rim of the flat head taking all the pressure or the screw head having a gap all around as it sits in a flatter countersink.

Mind you it doesn't help that all but a few of my countersinks were bought at the old Boeing Surplus center..... I really should break out and do something about getting some proper 90* CS's.

[MattiJ]

Elf,

I misunderstood your question.

I think it's a question of strength - cylindrical (or hexagonal) headed fasteners can usually be tightened more than equivalent thread diameter countersunk fasteners. The hex key that fits a countersunk M12 fastener is a lot smaller than the one that fits an M12 cap screw for instance. It may also be that there are more friction losses in a tapered seat than a flat one, making the problem worse.

Ian

I don’t know about US sizes but at least the metric sizes are such that you can torque even 10.9(grade 8) countrrsink bolts to recommended max torque.
M10 countersink bolt has 6mm hex socket. Torque for 10.9 strenght bolt is 75Nm, good 6mm hex key can handle about 100Nm.
problems usually start when loosening even slightly stuck fastener and the hex key doesnt have sufficient margin to break the bolt loose.

[J Tiers]

Yeah for thin material. That's why you can buy them 1 1/2" dia x 6" long.

The flat head is BEST for thin materials. You do not get much good result using a small standard head with thin materials.

That does not mean you cannot get large ones, nor does it mean that using one for thicker material will cause your toenails to get fungus and your hair to fall out. Nobody says you have to use them for thin material, or that they can only be used with thin materials.

[Dave C]

Duh!

[larry_g]

Not all flat head fasteners have the head concentric with the shank of the screw. When this happens you have line contact when tightening up the fastener till the fastener bends a bit or something gives to allow full contact between the fastener and the work piece. Secondly if fastening two ridged pieces together the cumulative position error between the threaded holes and the clearance holes will induce the same bending force in the screw as things get tightened. I prefer not to use a FHCS if something else will work.

Don't ignore low head SHCS's or button heads. They have their places

lg
no neat sig line

[rickyb]

Elf,

I misunderstood your question.

I think it's a question of strength - cylindrical (or hexagonal) headed fasteners can usually be tightened more than equivalent thread diameter countersunk fasteners. The hex key that fits a countersunk M12 fastener is a lot smaller than the one that fits an M12 cap screw for instance. It may also be that there are more friction losses in a tapered seat than a flat one, making the problem worse.

Ian

Right on the money. Torque tension relationship cannot be maintained with tapered screw heads. Angles never match up, too much surface area and the head will distort upon installation. If clamp load is important, never ever use counter sinks.

[754]

I disagree that taper head cannot hold torque , we used them a lot. And flat head cap screws should be 82 degrees NOT 90 degrees.
and yes the angles should match.

[J Tiers]

...........And flat head cap screws should be 82 degrees NOT 90 degrees.
........

Among the savages, yes.

[JoeLee]

Wood screws are countersunk and SHCS holes are counter bored.

JL..............

[larry_g]

https://www.fastenersuperstore.com/f...0head%20angle.

As for counter sink angle standard angle on SAE is 82* and metric is 90*. Others are variable but not standard.

lg
no neat sig line

[Paul Alciatore]

A countersink will tend to locate the part being attached due to the cone of the countersink and the matching cone on the screw. This can be good or it can be bad, depending on other factors. If an only moderately good alignment is needed than countersunk screws may be a good choice.

A counterbore will create a flat surface for the flat bottom of the screw head to seat against. SO, it will allow the part being attached to move around just a bit due to the clearance between the screw's OD and the hole's ID. This can allow manual alignment OR it can provide for a somewhat sloppy fit so that tight tolerances are not needed OR it can allow the use of pins for a more precise alignment than a screw of any kind would provide. On that last point, I would not use countersunk screws if a really precise alignment was needed. In that case alignment pins plus screws or bolts with flat bottoms on their heads are the best solution. And if those bolts must be flush, then a counterbore is used.

A countersink allows the screw head to be at or below the surface with a minimum amount of thickness being required. One frequently seen application of this is in fastening a sheet metal panel where the thickness of the panel is rather small but the screws must be flush. They even make small screws with countersink style heads that are less than the full height of standard countersinks for that size screw. This style screw is common in electronic equipment. This would be impossible with counterbores.

A counterbore would need not only the thickness of the screw head, but also the additional thickness of the part being fastened to allow for the screw head to be at or below the surface.

A counter sink can tend to expand a ductile metal like aluminum, making the hole larger and distorting the part.

A counter bore, which requires more material in the first place, is less likely to distort that same ductile aluminum when the fastener is tightened.

I am sure there are other reasons for using one over the other. Each application is different and must be considered on it's own requirements.

I'm not asking what the physical difference between the two styles of machine screws. I'm asking why you would choose one style over the other in a design.

[elf]

A countersink will tend to locate the part being attached due to the cone of the countersink and the matching cone on the screw. This can be good or it can be bad, depending on other factors. If an only moderately good alignment is needed than countersunk screws may be a good choice.

A counterbore will create a flat surface for the flat bottom of the screw head to seat against. SO, it will allow the part being attached to move around just a bit due to the clearance between the screw's OD and the hole's ID. This can allow manual alignment OR it can provide for a somewhat sloppy fit so that tight tolerances are not needed OR it can allow the use of pins for a more precise alignment than a screw of any kind would provide. On that last point, I would not use countersunk screws if a really precise alignment was needed. In that case alignment pins plus screws or bolts with flat bottoms on their heads are the best solution. And if those bolts must be flush, then a counterbore is used.

A countersink allows the screw head to be at or below the surface with a minimum amount of thickness being required. One frequently seen application of this is in fastening a sheet metal panel where the thickness of the panel is rather small but the screws must be flush. They even make small screws with countersink style heads that are less than the full height of standard countersinks for that size screw. This style screw is common in electronic equipment. This would be impossible with counterbores.

A counterbore would need not only the thickness of the screw head, but also the additional thickness of the part being fastened to allow for the screw head to be at or below the surface.

A counter sink can tend to expand a ductile metal like aluminum, making the hole larger and distorting the part.

A counter bore, which requires more material in the first place, is less likely to distort that same ductile aluminum when the fastener is tightened.

I am sure there are other reasons for using one over the other. Each application is different and must be considered on it's own requirements.

I think that is a good summary of the thread. Thanks.

[754]

Gentleman and bike builders use 6061 and its not that ductile..
on bikes if we countersink, we try to go to bolt head diameter.. which leaves the thin shoulder proud of the work..but looks best.
in the pic it was already drilled, some are low. . 9 and 11 o'clock are too deep..

[754]

One thing bears mentioning here if you are counterboring SHCS for motorcycles , boats or hot rods... beware standard counterbores, way to big a hole.. never looks right .. we use custom ground counterbores or regrind endmills to get a tighter hole.
be careful if chroming..but remember that chrome fasteners available from places like GardnerWescott maybe a bit under size if needed. Like knurl ground off types in chrome.

[Paul Alciatore]

6061 not that ductile? Really? What are you comparing it to, modeling clay?

Gentleman and bike builders use 6061 and its not that ductile..
on bikes if we countersink, we try to go to bolt head diameter.. which leaves the thin shoulder proud of the work..but looks best.
in the pic it was already drilled, some are low. . 9 and 11 o'clock are too deep..