Countersink vs. Counterbore

[elf]

I ran across this question in another forum: How do you choose between a countersink or a counterbore for a machine screw? Is there more to it than looks?

[Ian B]

Use a countersink if the screw has a countersunk (conical) head.
Use a counterbore where the screw's mating face is flat, and you wan to sink the screw into the material - commonly done with socket screws.
There's a 3rd one, a spot face - use this on rough castings to provide a flat face for a bolt or nut to bear on.

Ian

[754]

Countersink makes a bevel on edge of hole.
Counter bore has flat bottom and parallel sides.

[elf]

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.

[darryl]

The countersink is likely to be a larger 'feature' on the part, larger than the head of the screw or bolt. The counterbore leaves a smaller footprint.

If the material is thin, you're probably stuck with using a countersink and suitable bolt heads. But you couldn't counterbore in thin material- you would not be able to sink the bolt head, and you'd mostly be weakening the joint.

Another option, which I use often enough, is to combine the methods. A conventional countersink makes a tapered hole that will show larger than the bolt head if the head is to be sunk flush. What I've done is make the OD of the countersink the same diameter as the bolt head, then push it in until it has bored in a short ways. That way it has the same look as a bolt in a counterbored hole, and the head can be flush at the same time. It's the smallest blemish you can make on the material while still allowing a thinner material to be securely held.

I ended up making myself a set of these special countersinks. Some of them bore a tad smaller than the diameter of the countersink bolt that's going into them. That lets me grind the od of the bolt head to be a precise fit. It also allows the bolt to be closer to an edge without cutting into the edge. I made these tools to suit the exact angle, which I think is 82 degrees- not 90 anyway.

I've also used the spot face idea- a very shallow counterbore. Normally I use it to hide the thickness of a cap screw, or button head screw. That leaves a kind of nice look- you still have the bump of the cap screw, but the thickness of the edge is submersed into the material. There's nothing for things to catch on.

Sometimes I pick based on simply the look. Any of the methods allow you to paint the recesses before the bolt goes in. A socket head bolt looks neat in a painted bore. I usually use black, but red looks pretty sharp.

[David Powell]

Counter bores can be used when you want a flat surface for bolt heads to sit securely on. The clearances around them are such that the location of the parts cannot be guaranteed to repeat each time they are dismantled and rebuilt, hence if EXACT relocation is needed another means of location( Usually Dowel Pins) is added.
Countersunk screws do ensure quite a good relocation of parts. Not as good as dowel pins thanks to some screw heads not always being quite concentric with the threads. It is easy to make up a threaded bushing and spin the screws and discard any malformed ones. I know use in this way is not really " Good Practice" but it is a quick and easy method of getting realignment of parts within a couple of thous on each reassembly, Regards David Powell.

[MattiJ]

The countersink is likely to be a larger 'feature' on the part, larger than the head of the screw or bolt. The counterbore leaves a smaller footprint.

If the material is thin, you're probably stuck with using a countersink and suitable bolt heads. But you couldn't counterbore in thin material- you would not be able to sink the bolt head, and you'd mostly be weakening the joint.

Another option, which I use often enough, is to combine the methods. A conventional countersink makes a tapered hole that will show larger than the bolt head if the head is to be sunk flush. What I've done is make the OD of the countersink the same diameter as the bolt head, then push it in until it has bored in a short ways. That way it has the same look as a bolt in a counterbored hole, and the head can be flush at the same time. It's the smallest blemish you can make on the material while still allowing a thinner material to be securely held.

I ended up making myself a set of these special countersinks. Some of them bore a tad smaller than the diameter of the countersink bolt that's going into them. That lets me grind the od of the bolt head to be a precise fit. It also allows the bolt to be closer to an edge without cutting into the edge. I made these tools to suit the exact angle, which I think is 82 degrees- not 90 anyway.

I've also used the spot face idea- a very shallow counterbore. Normally I use it to hide the thickness of a cap screw, or button head screw. That leaves a kind of nice look- you still have the bump of the cap screw, but the thickness of the edge is submersed into the material. There's nothing for things to catch on.

Sometimes I pick based on simply the look. Any of the methods allow you to paint the recesses before the bolt goes in. A socket head bolt looks neat in a painted bore. I usually use black, but red looks pretty sharp.

At least the countersinks that I have are each sized according to screw head. So as long as I select the correct size and not three steps larger I can countersink to sufficient depth without making too large crater around the bolt head.

countersinks done with oversize tool look kinda ugly hack done in a hurry.

[Ian B]

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

[MattiJ]

And I select between countersink and counterbore mostly based on material thickness. In home shop it is also based pretty often what screws and tools are available

[cpole]

"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"

Ian answered this perfectly. I'm not so sure you know what you are asking.

[old mart]

Counterbores in thick material can be deeper if your fixings are not long enough.

[Mcgyver]

Assuming you want a flush finished surface (which you almost always do), for a given size fastener, counter sinks will work with thinner material. However when the material is thick enough for a cap screw, it has a smaller head than the large dia of the countersunk so fits in more places. Countersunk screws also positively locate which is less desirable for a fastener, where cap heads with a flat bottom hole allow for some adjustment

[john b]

Counter sinks can be used to keep parts in alignment better than cap screws. They also have a self locking feature. You don't need lock washers with them. John b.

[J Tiers]

You pick the type of tool to fit the type recess you want, which is based on the type of screw required. Yes the angled recess of the countersink works best for thin materials, the counterbore for regular head screws is better for strong attachment and high torqued fasteners

At least the countersinks that I have are each sized according to screw head. So as long as I select the correct size and not three steps larger I can countersink to sufficient depth without making too large crater around the bolt head.

countersinks done with oversize tool look kinda ugly hack done in a hurry.

All countersinks of a given angle work the same, so it is just a matter of how deep you go. If you want the close-sized countersink that Darryl mentioned, then you must use a close sized countersink tool.

"Aircraft" countersink tools have an adjustable stop and can be set up to make any size countersunk recess needed. And for thin materials, you use an "undercut" screw, so that the recess does not have to go into the part you are attaching the sheet metal to.

[john b]

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