Finding the right fit for your jig is much easier once you get a handle on a drill bushing size chart. If you've ever tried to freehand a hole that needed to be perfectly perpendicular or exactly spaced, you know how quickly things can go sideways. A good drill bushing acts as a guide, keeping your bit exactly where it belongs, but if you pick the wrong size, the whole thing is basically useless.
It's one of those parts of a project that seems simple enough until you're staring at a list of numbers, letters, and decimal points. Whether you're a machinist working with hardened steel or a woodworker building a high-precision jig for a cabinet project, understanding these charts is the difference between a professional result and a pile of scrap.
Why the numbers actually matter
When you look at a drill bushing size chart, it isn't just about the hole in the middle. You've got to balance the Inner Diameter (ID), the Outer Diameter (OD), and the overall length. If the ID is too loose, your drill bit will wander, defeating the purpose of the bushing. If it's too tight, you'll generate a massive amount of heat, ruin your drill bit's temper, or even seize the bit right inside the bushing.
Most people start with their drill bit size. Let's say you're using a standard 1/4" bit. You might think you just buy a 1/4" bushing and call it a day. But if you look at a chart, you'll see that the actual ID is often a tiny bit larger than the nominal size—usually just a couple of ten-thousandths of an inch—to allow the bit to spin without grinding against the walls.
Breaking down the dimensions
The chart is usually broken into a few main columns. Once you know what they mean, it's like reading a map.
The Inner Diameter (ID)
This is the hole the drill bit goes through. Charts will typically list these by fractional sizes (like 1/8", 1/4"), letter sizes (A through Z), or number sizes (1 through 80). If you're doing precision work, you'll probably want to look at the decimal equivalent column to make sure it matches your specific bit's tolerances.
The Outer Diameter (OD)
This is where people often get tripped up. The OD is the part that fits into your jig or fixture. If you're using a press-fit bushing, the OD on the chart is designed to be just slightly larger than the hole you've bored into your jig. This "interference fit" ensures the bushing stays put while you're working. If the chart says the OD is .5000", your hole in the fixture needs to be precisely sized to hold it tight.
Length and Head Thickness
The length of the bushing determines how much support the drill bit gets. Shorter bushings are great for tight spaces, but longer ones provide more stability. If you're using a "Head Type" bushing (one with a shoulder), the chart will also give you the head diameter and thickness. This is important because that head will sit on top of your jig, adding height to your setup.
The different types of bushings you'll see
You can't just pick any bushing from a drill bushing size chart without knowing how you plan to mount it. There are three or four common types that pop up in almost every shop.
Press-Fit (Type P) Bushings are the most common. They're meant to be pressed into a hole and left there forever. They're flush with the surface, making them perfect for jigs where you don't want anything snagging on your workpiece.
Head Press-Fit (Type H) Bushings are similar, but they have a shoulder at the top. This prevents the bushing from being pushed all the way through the jig. If you're going to be applying a lot of downward pressure, the head acts as a mechanical stop.
Slip-Renewable (Type S) Bushings are used when you need to perform multiple operations in the same hole. Maybe you need to drill a pilot hole and then come back with a reamer. You'd use a "Liner" (Type L) bushing permanently installed in the jig, and then you'd swap out different slip-renewable bushings as you go. The chart for these will usually have a separate section for the "Liner" OD and the "Slip" ID to make sure they match up.
How to use the chart without getting a headache
When you first open a drill bushing size chart, don't try to read the whole thing. Start with your drill bit. Find that decimal size first. Once you have that, look across the row to find the available ODs.
It's worth noting that manufacturers often use a coding system. You might see something like P-20-8. This looks like gibberish, but it's actually shorthand. Usually, the "P" stands for the type (Press-fit), the "20" refers to the OD in 1/64ths of an inch, and the "8" refers to the length. Learning the code for your preferred brand makes ordering a lot faster, but always double-check against the actual measurements on the chart just to be safe.
Material choices and why they're on the chart
Most charts assume you're looking for hardened tool steel. This is the standard for a reason—it's tough, it lasts a long time, and it's relatively affordable. However, if you're working with very abrasive materials or doing high-volume production where you're drilling thousands of holes, you might see a "Carbide" option on the chart.
Carbide bushings stay sharp and keep their dimensions much longer than steel, but they're brittle. If your drill bit hits the side of a carbide bushing at a weird angle, the bushing might crack. Also, they're significantly more expensive. For the average shop, hardened steel is usually the way to go, but it's good to know the choice exists.
Common mistakes to avoid
Even with a drill bushing size chart in front of you, things can go wrong. One big mistake is forgetting about the "chip clearance." If you set your bushing too close to the workpiece, the metal chips have nowhere to go. They'll get packed into the flutes of the drill bit, generate heat, and potentially snap the bit. Most veterans recommend leaving a gap about equal to one diameter of the drill bit between the bottom of the bushing and the top of the part you're drilling.
Another hiccup is ignoring the tolerances. If you're buying a cheap, off-brand bushing, the actual size might not match the chart. Stick to reputable suppliers who provide ANSI (American National Standards Institute) or ISO standard parts. This ensures that when the chart says .2500", it really means it.
Setting up your jig
Once you've used the drill bushing size chart to buy your parts, you have to actually install them. If you're working with a press-fit bushing, don't just whack it with a hammer. You'll likely tilt it or mushroom the top.
The best way is to use an arbor press or even a large bench vise with some soft jaws (or pieces of scrap wood) to gently squeeze it into place. Make sure your hole is clean and slightly chamfered at the top to help the bushing find its way in. If you did your math right using the OD column on the chart, it should go in with a firm amount of pressure and stay there for the life of the tool.
Final thoughts on precision
At the end of the day, a drill bushing size chart is just a tool to help you be more accurate. It takes the guesswork out of the equation. Instead of "eyeballing it" or hoping your hand is steady enough, you're relying on calculated dimensions and hardened steel.
Whether you're building a specialized engine stand, a custom furniture jig, or parts for a CNC machine, taking those five extra minutes to cross-reference your bit size with the right bushing OD and length pays off. You'll save money on broken bits, save time on botched workpieces, and—let's be honest—it's just a lot more satisfying when everything fits together perfectly the first time. So, keep a copy of that chart taped to the side of your toolbox; you'll find yourself reaching for it more often than you think.