Comprehend the Countersink

When a tapered-bit countersink is adjusted for shorter screws shows, a small gap around the drill bit opens. That opening collects debris.

When a tapered-bit countersink is adjusted for shorter screws shows, a small gap around the drill bit opens. That opening collects debris.

The Countersink: The Real Cutter
The real workhorse of all-in-one countersinks is the countersink portion itself. This part of the tool cuts the opening for the screw head; and if you need a counterbore it does that as well.

Countersinks are generally made of high-carbon steel, but due to the amount of work done by this part of the tool, you can find countersinks  with carbide tips or high-speed steel, also. The high-speed steel and carbide countersinks hold an edge longer than high carbon and are often reserved for production work.

All-in-one countersinks with carbide wings are similar to the design of router bits. (See the photo at right.) Carbide-tipped countersinks are favored when working with melamine and/or particleboard. These countersinks are not inexpensive and could cost you more than twice the amount of a standard all-in-one countersink, but they will last several times longer.

Just as with the drill bits, there are many different designs in countersinks. One of the most important differences to me is that many of these countersinks sized for #8 screws (the screw size I find most used in my work) leave a counterbore that is smaller or larger than the standard 3⁄8″ for most plugs. This is also why many manufacturers have matching plug cutters available in sets.

The area between the cutters carries the waste from the countersink.

The area between the cutters carries the waste from the countersink.

Another area that garners attention is the number of flutes on the countersink. Some setups have two flutes while others have three or four flutes. And while an increased number of flutes makes for better cuts – think three knives versus two knives in your jointer or planer – it’s not the deciding factor for us.

The cutting edge of a countersink flute is no different from the cutting edge of a chisel. Sharper is better. So the amount of actual cutting surface and its angle is important. Some countersinks scrape the wood to form a recess; others have a lower pitch that cuts a shaving.

After waste material is cut, it has to be carried away. The area between the cutters helps accomplish that chore. If the waste jams in the recesses, it clogs the cutter and inhibits a clean cut. That also builds up heat and increases friction. The trailing edge of the opening should be angled or sloped so the waste material is swept away from the cut. (See the photo at left.)

In our testing, we found that the two-flute cutters tended to cut faster and with less effort. They also had the largest areas for carrying away waste. However, the four-cutter designs will stay sharp longer because you are spreading the wear across four cutters instead of two. So it’s a trade-off.

Finer threads on setscrews grip better.

Finer threads on setscrews grip better.

Another issue to watch is how the countersink affixes to the drill bit. Most countersinks are held  with setscrews. One setscrew, as it’s tightened, causes the countersink to push against one side of the drill bit, forming a small offset opening. Or if the setscrew falls into the flute of the bit, it can move the countersink position when tightened. With two setscrews you can secure the bit in a more centered position. One setscrew works, but it’s better to have two per countersink.

Some bits used with countersinks have flattened areas on the shank where the setscrew locks. Generally, these use a single setscrew. If you need to adjust the setup for shorter screws, it’s possible to move beyond the flat areas onto the rounded shaft. This results in surely a weaker hold.

A Word About Setscrews
Each all-in-one countersink is adjustable to match various screw lengths. To make an adjustment, loosen any setscrews holding the drill bit, slide the bit to increase or decrease the length, then tighten the screws to hold everything in position.

As you might expect, there is great force on the countersink as it makes contact with the wood. If the setscrews don’t hold, the countersink spins on the bit shaft, which damages the bit. Also, the countersink can creep and change the depth of cut. Neither result is acceptable.

These contemporary countersink designs all focus on the stop portion of the tools. Spinning attachments stop as the surface is reached.

These contemporary countersink designs all focus on the stop portion of the tools. Spinning attachments stop as the surface is reached.

When it comes to setscrews, its size doesn’t matter. Large setscrews don’t equate to more holding power. To be securely tightened, setscrews need to have fine threads. Finer threads hold best when tightened. And don’t hesitate to crank the screws into the shaft.

Stops: Use Them if You Must
Several companies that market all-in-one countersinks include depth stops. Most of the time, stops are used in production work. If you have the need to drill hundreds of countersinks, set the stop to reduce any potential for mistakes.

Most woodworkers seldom set the stops for countersinks. But if you’re a stop setter, here are a few things to keep in mind.

Make sure the stop fits snugly to the countersink. If the fit is sloppy (as some are), chances are you’ll set the stop at an angle. When the stop contacts the surface, a burnished ring can appear.

Also, make sure the setscrews used to hold the stop have fine threads. Plus, you should watch the position of the setscrews as you tighten them. I found one setup where the setscrews wouldn’t grab if they were positioned on the area of the countersink that cleared the chips.

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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