Every woodworker who has been around for some time knows the importance of pre-drilling holes. Most people argue that they waste time, but experienced woodworkers will tell you they save wood. They are used to either make a pilot hole or a clearance hole to drive in screws. What confuses most people is the difference between the two of them. What is the use of both these holes and what sets them apart is an important distinction to make? To answer this question, we will compare a pilot hole vs. a clearance hole to clear the confusion once and for all.
A pilot hole is used when you want to drive in a self-threading screw into wood. The hole is as big as the shank of the screw and the threads bite into the wood fastening the screw. A clearance hole has a hole in one piece of wood bigger than the size of the screw’s body and threads but smaller than its head. Then there is another hole the size of the screw’s shank in the base wood. While a pilot hole is used for driving a screw into wood, a clearance hole is used to join two pieces of wood together.
In some cases, it is okay to skip drilling a pilot hole in softer materials. However, if you want precision and a clean job, it is always a good idea to pre-drill a pilot hole. Clearance holes on the other hand are a necessity no matter what materials you are working with. You cannot skip on pre-drilling a clearance hole, and it is very important to make one. It is because both a pilot hole and a clearance hole have different purposes.
Main Differences Between a Pilot Hole and a Clearance Hole
There is a huge difference between a pilot hole and a clearance hole, and it’s the reason why they are both pre-drilled. A pilot hole consists of drilling one hole which is the exact size of the body or shank of a screw. Depending on the type of wood you are drilling, you might want to make the size of the hole either slightly smaller or the exact size of the shank of the screw. For softwood, you drill the size of the pilot hole slightly smaller than the shank. For hardwood, it needs to be the exact size of the body of the screw excluding the threads.
Meanwhile, a clearance hole has two holes of different sizes in two pieces of wood. Depending on which wood will be placed on top and which is going to be the base, the size of the hole will be either bigger or small. The piece of wood on top will have a hole slightly bigger than the threads but smaller than the head of the screw. The piece which will be on the base will have a hole the exact size of the body of the screw excluding threads.
Pilot Hole vs. Clearance Hole Uses
In woodworking, a pilot hole is most commonly drilled for fastening screws into wood without the risk of cracking or splitting. Because when you try to drive a screw into wood normally, it ends up exerting pressure on the fibres. So instead of cutting into wood, it is pushing its way inside, acting like a wedge that splits wood. So to prevent that from happening, you create a hole as big as the body of the screw excluding the threads. There is no obstruction or hurdle for the body of the screw and the threads bite into the wood fastening it tightly.
Another reason for drilling a pilot hole in wood which isn’t as common is to help prevent larger drill bits from walking. Walking refers to sliding or skidding of a large drill bit. This almost exclusively happens with twist bits that are large in size since they don’t have a pointed edge that penetrates into the wood stock before it starts to cut or bore a hole into the wood. So to help ease the way and to give a guide to the larger bit, you can drill a pilot hole with a smaller drill bit.
Clearance holes on the other hand have a single purpose, it is to join two pieces of wood together. When you drill a large hole in the piece of wood which will be on top, you prevent the screw’s threads from biting into it. With the smaller hole in the base wood, the threads cut into the wood, gripping onto it. The head of the screw helps the top piece fasten it to the base wood while the threads of the screw bite into the base and grip them together tightly.
The larger hole in the piece of wood above prevents it from jacking, a phenomenon that occurs when the screw binds itself into the fibres of the top piece. This separates the top piece of wood from the one on the bottom because the threads of the screw do not pull them together.
Pros and Cons of Pilot Holes
Pros
- Prevents wood from splitting apart when driving a screw in it.
- Prevents larger drill bits from sliding or skidding on wood when drilling.
- Helps you fasten screws on the edge of the wood.
Cons
- Time-consuming.
Pros and Cons of Clearance Holes
Pros
- Helps join two pieces of wood together.
- Prevents the screw from jacking into the top piece of wood when joining two of them together.
- Can help join two pieces of wood together without glue.
Cons
- Takes time to drill two holes of different sizes.
- Can be difficult to get right the first time.
FAQs – Frequently Asked Questions
When should you drill a pilot/clearance hole?
You should drill a pilot hole when you are planning on fastening a screw into it. When you are planning on joining two pieces of wood together with a screw, then you need to drill a clearance hole.
Do you need a clearance hole for plywood?
Yes, you need a clearance hole for plywood if you are planning on joining two pieces of them together using a screw.
Final Thoughts
Pre-drilling holes help prevent unnecessary risks in woodworking projects involving screws. However, choosing between a pilot hole or a clearance hole requires knowledge of both. The purpose of the holes will help determine which one you need to drill. Pilot holes are more common in woodworking than clearance holes. However, in some cases, it is absolutely necessary to drill a clearance hole as the project depends on it. Hopefully, after going through the purpose along with the pros and cons of both types of holes, you are better equipped to choose one instead of the other for your project.
