When should I use a tapered drill bit vs straight drill bit for countersinking wood screws?

Question

I looking to buy a set of countersink bits and I'm seeing both tapered and non-tapered versions. Is this personal preference or is there a reason to use one other the other? I mostly work with hardwoods but do work a bit with ply woods as well.

Answer 1

As I mention in my Comment above, there are no clear reasons to prefer one type over the other for varied materials, except possibly whether you commonly use straight or tapered screws.

So if you prefer old-school tapered wood screws the decision seems to be an easy one, you get the tapered type.

If you mainly use straight screws then the decision again seems easy, you get the other type.

But there are numerous other design considerations to factor in.

These include:

• the number of cutters on the countersink portion (more flutes cut more smoothly, but slower.... FWIW outside of a production environment I don't think speed of cutting should be any consideration);
• the design of the cutting edges (does it scrape more than cut?);
• the design of the flutes (relates to how well chips are cleared);
• the material of the countersink — carbon steel, HSS or carbide;
• the fit of the countersinks to each bit.

Another factor to consider with straight bits is how clean the initial hole is. You might be thinking the countersink will naturally clean the edge of the drilled hole as soon as it reaches the surface, but there's no absolute guarantee a flake won't be lifted that reaches a little beyond the full countersink/counterbore radius¹. This is obviously more of an issue with twist bits than brad-point/lip-and-spur bits.

And equally importantly, brad-point bits by their very nature are easier to precisely locate on pencilled markings or into an awl depression, with almost zero chance of 'walking' like with twist bits.

So, if you choose to buy the straight type I would suggest giving strong consideration to those NOT based on regular twist drills, unless the point of the drill is ground specifically for wood ².

And the elephant in the room...
These bits are by definition used for drilling holes for what we might term 'joinery screws', and in wood joinery traditionally there were three parts to a properly drilled screw hole, the pilot and countersink/counterbore obviously, but also the clearance hole.

Which, if any, of these bits create correctly sized clearance holes?³

Clearance holes of a suitable size are important to say the least, because if the hole in the first piece of wood is engaged by even one or two turns of threading on the screws it can lead to bridging.

This is especially important now with modern (mostly straight) screws which so frequently have threading over the entire shaft. See a previous Answer on wood screws for a bit more on that.

---

Much more useful info as relates to design details of these combo bits at the following links:
Straight vs. Tapered Countersink Drill Bits? on ToolGuyd.
Countersink Bits – More to Them Than You Might Think! on Popular Woodworking. Amana-Tool Carbide-Tipped Countersink Bits on Fine Woodworking.

---

¹ Been there, bought the T-shirt ^_^ This is a continual frustration working with softwoods, but some hardwoods can be prone to the issue too (perhaps most prevalent in coarser-grained species).

² Which, based on all the images I just looked at online, you won't easily find!

³ It is perfectly fine if they don't, as long as you know this going in. These bits still save a step, even if you have to subsequently open up the top hole slightly for your screws of choice.

Answer 2

As mentioned in the comments, this comes down a little to preference and technique.

However, I will say that for me, personally, I've settled on quality tapered countersinking bits when working with engineered wood products. I find the taper works best with an accurate transition to the clearance hole for the shank, and leaves a nice clean, snug place for the head to clamp down into. For your reference I work a lot with so-called "Baltic" veneers, but also a little with OSG material.

But I do mean quality. The rule I have for tooling in general is "buy once, cry once" because some of what appears "good enough" on that web site named after the jungle and the river has turned out to be literal garbage. As in, I couldn't even reuse the steel, and I threw them in the garbage.

(SE sites aren't for shopping, but at the risk of making an Answer that'll age poorly, you could not go wrong by looking at the recommendations and sponsors for YouTube channels like "Stumpy Nubs".)