If you're a bowhunter and you've done some reading up, you've probably heard the phrase "third-axis adjustment." It's a term for one of several axes on which a bow sight can be rotated and adjusted. Bow sights should be plumb with the bow so that the shooter can be better be assured that the bow is not canted, because inadvertently canting the bow can ruin accuracy at long range. But before getting to the third axis, we should briefly discuss the first and second.
To understand what is called the first axis, picture an imaginary line running through the bow sight from left to right that would allow the bottom of the sight to swing toward or away from the archer like a mud flap on a truck. This axis should be perfectly perpendicular with the bow's riser before the next two axes can be properly leveled. It can be aligned using a bow square, a regular carpenter's square or even a door frame that you know to be plumb. Or, of course, you can take your bow into an archery pro shop and let them do it â€“ for a small fee.
For the second axis, imagine a line running from the target, through the bow sight, to the archer's eye. All of the sight pins should line up vertically on this plane. (In other words, they should form a vertical line that's parallel to the bow's riser.) It's important to get this second axis perfectly plumb with the bow (and level with the ground) so that the archer can use the sight's bubble level while shooting and know that if it is level, then the pins are in line, and so the arrow will strike on the same vertical plane no matter the sight pin chosen. It's very important for precision shooting at longer distances.
The third axis, however, is not important for the average bowhunter who doesn't shoot much past 40 yards and/or at extreme upward and downward angles. Sitting 15 feet up in a treestand and shooting something at 35 yards doesn't constitute an extreme angle or need for precision third-axis adjustment. It's more regarded by western hunters who may shoot at targets 60 yards distant and 40 yards down (or up) a cliff face. Extreme target archery tournaments, like the Total Archery Challenge, make a point of setting up these difficult shots. So, if you're not one of these guys who must shoot at such crazy angles, you probably shouldn't fret over your third-axis adjustment. But if you are an extreme target shooter, a mountain hunter â€“ or just a perfectionist â€“ you can enhance your long-range accuracy by making sure it's perfectly adjusted. It's complicated to explain by mere words, but here goes.
Imagine a plane running vertically down though the bow sight that would allow the bow sight to rotate like a revolving door. If that door (the sight) is slightly pulled slightly open or pushed back (in relation to the bow's riser) and then angled up or down (as if taking an extreme uphill or downhill shot), the relationship between the bow sight and the ground can change, causing it to become out of level with the ground, and therefore causing your arrow to be off.
But it's not as simple as using a square to make sure the sight's third axis is plumb with the riser while it's in a vise; third axis is best evaluated while at full draw. Torque placed on the bow by the shooter's hand and the cable guard can affect the third axis. While there are specialized tools designed to check the third axis, you can check it at home, if you know what you are doing. To check, draw a perfectly vertical scribe line at the bottom of a wall. Place a bubble level on the bow. Then, while facing the wall, draw the bow, and while aiming it down at a 45-degree angle, make sure the bow is level by viewing the bubble level. Next, make sure the sight pins perfectly line up on the scribe line. If they do not, you'll need to make adjustments to the third axis.
Some newer and more expensive bow sights have third-axis adjustments, but most hunting sights do not. For these, a small shim such as a strip of duct tape or a length of business card placed between the sight's mounting bracket and the riser can often tilt the sight's third axis enough to do the trick. But if you are serious about perfection, you are probably best served by buying a sight with an integral third axis hinge that can be easily adjusted.
Fact is, most hunters â€“ especially eastern whitetail hunters â€“ can hunt their whole lives and never worry a third-axis adjustment. But if you're a bowhunter who might shoot a mule deer from a cliff face or plan to bowhunt sheep â€“ or maybe you're just an archery geek â€“ be sure to check your third axis.