Author: Lost Art Press

One of the most common questions I get is why I don’t use Forstner bits much in chairmaking. The bits are readily available, make flat-bottomed holes and cut cleanly.

The answer is basically this: The Forstner’s lead point is too short*. That means if I want to drill anything other than a shallow angle I need to start the bit nearly vertical then tilt my drill to get to the desired angle. It’s do-able, but it’s easy to over- or under-shoot things.

So most of the bits I use in chairmaking have a long lead point. This long lead point allows me to tilt the bit to the right angle, lock my elbows then drill. Here are the three bits I use the most, with their advantages and disadvantages.

Star-M F-Type Bit, 16mm, by WoodOwl

This is my favorite bit, but good luck finding it. Most reliable sources are regularly sold out. This bit can handle just about any common chairmaking angle. I can tilt up to 30° off vertical if I make a small starter divot with an awl for the bit’s point. 

Other advantages: It cuts clean holes without any splintering on the exit side. This makes drilling through the arm and seat a quick and painless operation. 

Disadvantages: The side flutes are sharp. So if you move your drill sideways while boring, you will end up with an overly oval hole. The solution is to practice (sanding down the flutes doesn’t seem to help much).

The bit is metric and drills a 0.629”-diameter hole. So you’ll need to adjust your tenon-cutter to get a snug fit.

Finally, the bit seems to dull faster than my other WoodOwl bits. This is a problem with the other two bits discussed below. I get about five chairs out of one of these bits, and I haven’t found a way to sharpen them (yet).

WoodOwl OverDrive Bit, 5/8”

This bit is much easier to find than the Star-M. So keep searching. Lots of little suppliers have them in stock. The bit makes a true 5/8” hole (0.625”). And it also leaves a clean exit hole.

So what’s the catch? The lead tip isn’t long, so you are limited in the chair angles you can bore. I can easily bore 11° off vertical. And 14° when I am pushing things (and if I make a small starter divot with an awl for the bit’s point). That range of angles will get you through most dining chairs without too much trouble.

Like the Star-M’s, the side flutes are sharp – so practice makes round. And the bit doesn’t last as long as its big auger cousins from WoodOwl, which seem to last forever.

WoodOwl 5/8” Spade Bit

Really, any spade bit will do. The WoodOwl just happens to come sharper than most cheap bits. Another good option is to look for vintage (meaning ye olde 2020) Irwin bits that have the rim cutters. Other people have had luck with Milwaukee and Makita bits. Basically, look for spades that look like the WoodOwls. The bit needs two rim cutters (the little cat ears). The bit’s faces need to be surface ground (otherwise the bit will fail to bore gouda). And the lead point should *not* be a screw. These lead-screw spades are a sin against the Chair Gods.

Spades can handle almost any angle – up to 34° off vertical with ease. They can be resharpened. You can adjust their diameter on a grinder in seconds. They are cheap and plentiful. And they don’t have the side-cutting problems that the two above bits do.

But they blow out the backside like an American tourist after 10 currywursts. So you need to clamp backing blocks below the arm and seat when you make through-mortises.

— Christopher Schwarz

*There are Forstners out there that have a long lead point – usually a replaceable brad-point bit. But they are hard to find. And expensive when you do.

When you drill a hole with a 5/8” bit, then use a 5/8” tenon cutter to make the tenon, you should be golden…right?

Nope – unless you get lucky.

If you dare enter the fascinating world of boring you will quickly realize this truth: It is up to you to get your drill bits and tenon cutters to work together. You cannot trust the measurements stamped on the tool.

A quick example: a 5/8” WoodOwl spade bit and a 5/8” plug/tenon cutter from Lee Valley are not compatible. The tenon will drop into the mortise like throwing a hot dog down a hallway. It’s not a good joint.

There is no way I can cover all the bits and tenon cutters out there. But here is the methodology I use to get my mortises and tenons nice and tight.

Suggestion No. 1: Ignore the Official Measurements & Make Samples

A WoodOwl 5/8” spade bit and a WoodOwl 5/8” OverDrive bit make different-size holes. Because of its cutting geometry, the 5/8” spade bit cuts a hole that is .010” oversized. And that difference is enough to ruin a joint.

So when I get a new bit, I drill holes in oak, cherry and whatever species I have sitting around. Then I measure the holes and test them against the tenons I am making.

Suggestion No. 2: Ensure One Tool of the Pair is Adjustable

I can pair my Veritas 5/8” Power Tenon cutter with almost any 5/8” or 16mm bit because it’s adjustable. So usually I lean on that tool to make fine adjustments to the bits I have in my shop. I can pair it with my WoodOwl 5/8” spade by retracting the cutter of the tenon cutter. Or I can match it with my 5/8” OverDrive bit by advancing the tool’s cutter.

Here’s another example. Let’s say you have a 5/8” tenon/plug cutter because you cannot afford (or find for purchase) the adjustable Veritas 5/8” Power Tenon Cutter. In this case you cannot adjust the tenon/plug cutter, so you have to have an adjustable drill bit.

And that’s when you should definitely turn to spade bits. You can easily grind spade bits to match your tenon cutter. It takes seconds to do it, and you can sneak up on the right fit.

Suggestion No. 3: Go for a Snug Fit – at First

I am obsessive about the fit of my joints. Perhaps overly so. If you are like me, here’s how to go overboard. I adjust my tooling so that the tenon is a bit oversized and will not enter the mortise with hand pressure (usually .005” oversized or a little more). Then I use soft-jaw pliers to compress the tenons so they slip in to the mortise easily with almost no pressure.

This process makes assembly easier – the tenons slide right in. And the joinery is sound. The wet glue expands the tenons and locks the joint.

Suggestion No. 4: Use Tape

Once I have matched a tenon cutter and a drill bit, I wrap them each with a piece of brightly colored tape. This prevents me from making a terrible mistake.

— Christopher Schwarz