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What is a Countersink Drill Bit?

By C.L. Rease
Updated May 16, 2024
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A countersink drill bit can be secured in any chucked drill motor and will create a recess to allow a countersink bolt to sit flush with the surface of a material. The angle of the countersink screw head determines the angle of the countersink needed to allow the bolt head to sit flush while maintaining full contact with the base material. Using the wrong degree countersink drill bit to countersink a screw leads to poor contact between the two components. This leads to a weak connection between the screw and the base material.

Measuring the degree of the countersink screw head with a bevel degree finder allows the selection of the proper tapered countersink bit. When the inside edges of the bevel finder contact the top of the countersink screw and the length of the countersink screw head taper, the proper angle is shown. Utilizing the bevel degree finder ensures that the connection remains at full strength after the connection has been made. Selecting the type of countersink drill bit for the drilling operation ensures that the bit creates the proper type of hole.

Cross-hole and fluted countersink drill bits both drill through a variety of materials. A cross-hole counter sink drill bit is the most common type of countersink drill bit. The tapered cone of the drill bit reams a correctly sized hole set at a chosen bevel for seating countersink screws at or slightly below the surface of a material. Fluted drill bits use a steeper entry angle for tightly beveled screws or as a starting point for machining of metal base materials.

Tapping and burr removal are two machining process that benefit from using a fluted countersink drill bit. Tapping threads in metal requires an evenly surfaced entry to a hole. Uneven hole entries cause the cutting threads of a starter tap to wander, leading to out-of-level threads, which cause a screw to thread into a metal part at an angle, decreasing the surface contact between a fastener and the base material. Burr removal removes sharp edges from the outside rim of a drilled hole. This increases safety and avoids damaging the screw threads when the fastener is inserted into a drilled hole.

Lubricating both types of countersink drill bits while in use decreases friction at the contact point of the bit taper and the base material. Friction creates heat at the drilling point. Heat causes the cutting edges to dull rapidly during the beveling process.

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Discussion Comments
By Izzy78 — On Jan 17, 2012

@JimmyT - I think I know what the article is talking about to use to find the angle, but I use something different. What I have is just a piece of plastic with various angles of common screw bevels drawn on it. It is clear, so you can either put the screw on top and match up the angle, or you can hold the screw up to the bottom of it and look through to match up the lines.

Unfortunately, I don't know exactly where you would find one, since mine came with a set of countersink bits that I bought online. I would guess that in a store, they would most likely be with the drill bits.

What I would suggest is just searching for a bevel finding online, and I'm sure you would be able to find both kinds pretty easily. I can't imagine they would be all that expensive. All in all, though, it's a pretty handy too to have, and it doesn't take up any space.

By JimmyT — On Jan 16, 2012

Does anyone know if you can buy countersinks for a masonry drill bit? I have a project where I would really like to have the screws recessed below the brick. I have looked around a few stores, and I haven't seen anything like this, though.

Also, what is this bevel finder that the article mentions? I don't think I have ever seen one of these before. I have had the problem on a few occasions where I really need to make sure I have the right size bevel, and I have just had to drill some test holes and see how the screw fit in them. It would be really nice if there was a guide I could use that would tell me without having to spend time testing different bits.

What exactly do the bevel testers look like, and where would you find them in a hardware store? Would they most likely be with the screws or with the drill bits?

By jcraig — On Jan 16, 2012

@stl156 - Good questions. As far as the various sizes, there aren't really as many as you would initially think. Even if you don't have exactly the same size, getting a countersink that is roughly the same will usually work. A lot of times if you don't have the right angle, you can use a little flatter bevel, and the force of the screw going in will create the depth you need. Getting the right bevel when you are drilling metal might be important for the reasons the article mentions, but I've never had a problem with wood.

A lot of the countersink sets actually have an adjustable collar that lets you pick the depth you want the bit to go into the wood. That way, if you have long drill bits, you can set the depth to whatever you want, and the collar will just let the bit go in to the depth that you set at the beginning.

By stl156 — On Jan 15, 2012

So, I have been looking to buying a set of countersink drill bits, but I don't really know how to start. First off, are there just a set number of screw heads, in general? At least from my experience, it seems like there are a lot of different bevels of screw heads. How do you buy a countersink drill bit set that covers all of those different sizes? You have to add onto that the fact that different diameter screws would need their own specific size.

The other question I had was, how do you know when to adjust the drill bit to the countersink part? For example, if you only needed a hole to be drilled about an inch deep, but you had a long drill bit that was 3 or 4 inches long. How do you get the bit to stop at only 1 inch and still get the countersink hole made? Any help would be appreciated.

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