Comprehend the Countersink

CountersinkThis seemingly simple tool has lots of details that vary by brand. We investigate.

By Glen D. Huey
From the Spring 2009 issue of Woodworking Magazine

When the idea to write about all-in-one countersinks was suggested, I was somewhat amused – as I’m sure you might be. What’s to learn? You drill a pilot hole, then drill a recess so the screw head sits flush to your surface (or you drill a little deeper – called a counterbore – so you can tap in a wood plug). Then you drive your screw and insert a plug to cover it. And most times you won’t even use a plug.

Well take a look at the next few pages. You might be surprised how many different tools are available – and we’re not even including traditional countersinks.

An all-in-one countersink has a pilot bit, a countersink/counterbore and a depth stop combined into one adjustable tool. Take a close look at different brands and you’ll find some countersinks have a single cutting edge while others have two or four cutting edges. Some setups use hex-shaped shanks for quick changes while others rely on the round shaft of the drill bit only. Some have a straight drill bit for the pilot hole and others have tapered bits. And do you even need a depth stop?

An all-in-one countersink consists of these three parts: a drill bit, countersink and stop.

An all-in-one countersink consists of these three parts: a drill bit, countersink and stop.

How do these parts work in unison? How do you know you have the right tool for the task?

Countersink Basics
Screw installation begins with a pilot hole. The pilot hole has to be accurately sized based upon the screw’s gauge (#2 to #18) and length. The gauge part is simple. Match the pilot hole to the screw size. As for depth control, it’s critical when using a tapered bit, while a straight bit is more forgiving. Additionally, differing materials such as softwood, hardwood or man-made material have to be considered. A simple rule of thumb is to drop down one bit diameter (1⁄64″) when moving from hardwood to softwood.

To properly create a hole for a screw, you need a clearance hole, too. This hole extends from the hole’s countersink and through the upper piece of stock and is sized to allow the screw’s shank to pass without its threads engaging the wood. As a result, the head of the screw pulls the upper piece tight to the lower piece as the threaded portion of the screw grabs the pilot hole. In today’s hurry-up world, woodworkers seldom take the time to make a proper clearance hole.

A properly drilled screw hole should have the recess at the top, followed by a clearance hole that extends through the upper stock and a pilot hole drilled into the lower stock.

A properly drilled screw hole should have the recess at the top, followed by a clearance hole that extends through the upper stock and a pilot hole drilled into the lower stock.

Also keep in mind that some countersinks are designed for screws with different angles on their heads. The standard wood screw angle is 82º, but it’s always best to confirm you have a match.

After making the pilot, countersink/counterbore and clearance hole, you’re ready to drive your screws. It’s a good idea to add lubricant to the screws to aid as you drive. I use paste wax as a lubricant, but I must caution that if you smear the wax around the hole, some stain and finishes might not bond there.

There are some woodworkers who lubricate  screws with bar soap or liquid soap.

However, there are also woodworkers who say the soap is caustic or attracts moisture that will corrode screws. We have yet to investigate this claim.

Even if you properly countersink your screws there’s a possibility that the top piece won’t draw tight to the second piece. Here’s a quick fix for that: After using a countersink on the show face of the top piece, flip the board and countersink the inside face slightly. Then, as the screw pushes through the stock, any potential blowout is reduced.

All-in-one countersinks have replaceable drill bits. If a bit breaks, simply replace it (as long as the countersink is in workable order). And you can swap different bits – tapered for straight and vice versa.

The difference between tapered and straight countersink drill bits is obvious in this photo. Both pilot bits work, but we think the tapered bits offer some advantages.

The difference between tapered and straight countersink drill bits is obvious in this photo. Both pilot bits work, but we think the tapered bits offer some advantages.

Drill Bits: Don’t Get the Shaft
Most drill bits used in countersinks are high-speed steel and most have two flutes. With a quick glance at the different countersink drill bits, it’s the shape of the drill bit that likely catches your attention. Some are straight and others are tapered. Is a tapered drill bit better than a straight bit? Why should you select one design over the other?

According to Leonard Lee, the founder of Lee Valley Tools, tapered countersink drill bits were designed for use with cut-thread brass screws. At one time, cut-thread brass screws were considered the best screw for quality furniture. Threads that are cut into brass screw blanks add strength to the screws. The result is a screw of a specific shape – cut-thread screws taper from shank to the tip.

Today, most screws have their threads rolled into the screw’s shank and the screw maintains a uniform diameter until the tip. There isn’t a continuous taper. (For additional information about screws see Issue 10.) When asked if he would use a straight drill bit if he were not using cut-thread brass screws, Lee’s answer was a resounding “yes.”

Manufacturers of tapered-bit countersinks disagree. They contend that a tapered pilot hole accomplishes two things. First, the design of the bit creates a true clearance hole at the top of the hole. (We have found that this depends on the relationship between the screw length and the bit length.) A straight bit pilot hole is engaged by the screw threads over the entire length.

And second, a tapered hole allows additional thread contact as the screw travels toward the bottom of the tapered pilot hole. As the hole narrows, the threads cut deeper into the wood.

A third advantage that the staff found in side-by-side testing that the tapered bits have deeper flutes and cut faster and with less effort.

We have found one disadvantage to using tapered drill bits. As you set the countersink for shorter screws, the opening of the countersink around the bit increases because of the taper. This allows additional waste to gather in the opening. As the material builds, it gains heat. Excessive heat leads to a build-up of residue on the bit and that increases friction.
Given our experience and testing, we recommend countersinks with tapered bits because they cut faster and result in stronger screw connections.

CATEGORIES
Power Tools, Tools

About Glen D. Huey

Glen Huey is editor of American Woodworker Magazine, and former managing editor of Popular Woodworking Magazine. He's an accomplished period furniture maker and author of numerous woodworking books and videos (as well as magazine articles).

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