We’ve all been there. You’re trying to get a bolt through something and it won’t go. You line up the holes and tighten down the nut but that sucker won’t budge. Metal doesn’t bend! If you build anything from scratch, you know what I’m talking about.
Most times, it’s not the tool in your hands that’s causing the problem. It’s the hole you drilled. Clearance holes are an afterthought. As long as that bolt fits, it doesn’t realy matter. Wrong! And that mistake costs you time, money, and your sanity.
How to Choose the Right Hole Size for Bolts
As you can see from the chart above, there is three types of fits: loose, free, and close. So what do I mean by defining the Joint? Well it’s about making enough room for the shank. But it’s also about how the joint act under stress.
A close fit doesn’t leave much gap but keeps everything aligned so that things don’t bind. This happens when you’re working with something that’s precisely made like a dowel pin or machined part. If you use a loose fit in that situation, then those parts will wander. That will cause your final product to be sloppy.
On the opposite end of the spectrum is a loose fit. This gives you plenty of space. This is what you want if you are going to assemble painted parts with thicker surfaces or if the parts is likely to warp during welding. It lets the two parts float together until they lock down.
For general fabrication most folks just assume they will use a free fit. A free fit is adequate for rigidity but has enough tolerance to slip together easy. The graphic indicates that drill sizes are slightly bigger than the bolt size. You want your joints to have structure but be smooth to insert. Drill a hole in your piece using a 17/64″ bit if the bolt size was 1/4″. That small increment makes all the difference between a pro looking joint and one held together with hope.
Knowing those measurements lets you plan rather than guessing. Then there are materials. If you’re building an assembly for outdoor use, aluminum expands a lot when it gets hot. That means bigger gaps is required because it sticks during temperature changes. Wood absorbs moisture and must have even larger openings to prevent cracks/splits as the wood shift. Steel is relatively stable, which means closer tolerances can be maintained while still effectively handling shear loads. These material factors make the difference, and the data shows why. Failure to consider this results in warping assemblies or seizing fasteners.
When you don’t account for material movement and thermal expansion, you fight physics. A center punch help keep the drill bit in line instead of running rampant over slick metal surfaces. For wider holes, use a pilot hole first to guarantee accuracy before committing to your final diameter. That deburring step is one we all skip, but it’s so important to let the washer or head sit flush. Sharp burrs cause irregular pressure points that can weaken the joint over time. Clean ’em up with a quick countersink bit or deburring tool pass and avoid headaches down the road.
Drill Bit Selection: For most uses, high-speed steel is sufficient; however, when drilling stainless, use cobalt alloys since they resist heat better. For sheet metal, a step bit produces clean edges so no further finish work are required. Always double check hole size with calipers post-drill. Drill bit runout causes holes to be slightly too large, and these errors add up as you drill more of them.
Hole clearance is about respect for the materials you are joining. It is about more than simply putting them together. It is about what happens when you have joined them and how they will livig with each other. Whether you align things precisely or accept conditions of the field, it’s all about the right fit. It is the difference between a joint that lasts and one that doesn’t.
Next time you reach for your drill, think about the gap first. It is a small thing, but it matters.
