Making Perfectly Straight Cuts

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Lesson 2: Making Perfectly Straight Cuts
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Introduction: Making Perfectly Straight Cuts

One of the first things any woodworker is going to be faced with is making cuts, either by hand or with a power tool. It might seem like just diving in is the way to go, but there's some finesse in how to make your cuts accurate, square, and as clean as possible.

Before we start cutting wood to the right size, let's learn a very misleading thing about wood dimensions.


Nominal Vs. Actual Dimensions

The wood dimensions listed on lumber are a big fat lie. It's not really a secret, as every carpenter knows this, but it's something you should be aware of before buying the wrong size for a project.

We've all seen lumber dimensions in the home stores, like 2x4's, which is indicating the cross section dimension of the lumber - in this example 2" by 4". However, if you take a tape measure to the wood you'll notice that it's not really 2" by 4" at all, it's about a ½" shorter on both measurements giving you a 1-½" by 3-½".

Part of the reason is that when a tree is felled and chopped up into boards it has a lot of moisture still inside, when the wood is kiln dried it shrinks and warps after losing the moisture. To account for this the mills then process the wood through a plane to smooth down the surfaces and clean up the edges, ideally giving you a straight and smooth board, at the expense of some lumber dimension. This is true for lots of dimensioned lumber, so be aware and always double check the dimensions at the store before buying.

Knowing this will help you plan for your projects and account for the actual dimensions you need, and what you should be shopping for when you pick up lumber.


Kerf

Once you know your lumber size you're going to want to cut it to size. Any cutting will result in some loss of wood that is turned into sawdust, this is called the kerf. The kerf is the divergence between the left and right sides of the saw teeth, and since the teeth of any saw is larger than the blade thickness you'll want to measure kerf from the teeth and not the blade.

When you are cutting you typically want to cut on one side of your marking line so the kerf doesn't cut into your piece and remove some of your measured material.

On a circular saw there's a small indent on the plate, this indent represents the blade kerf. Knowing about kerf will help us work out how we measure and layout or material before cutting. Your saw marking will likely be different! And, the type of blade you're using might have a different kerf. Always make a few tests cuts to see where your notch/marking end up with your blade setup.


Layout + Cutting

Cutting wood is easy, but takes a little skill to make sure your cuts are straight and square.

To cut square (at a 90° angle to the length of the wood) you'll need to make a square marking, align the edge of your square along an edge of the wood where you want to make a cut and draw a line with a pencil. Rotate the square and make corresponding lines along the sides of your cut line, these side cuts will help guide your cut to ensure that your blade isn't wandering off the cut line.

In this video I want to make a cube from this 4"x4" post. I start by using measuring the thickness of the wood, then measure back the same distance from the end, finishing by scribing a pencil line around the post with the square edge.

When marking I use a little check mark that tells me on which side to cut on, if I line the kerf up to the checked side of the line I know that the piece I cut will be exactly the right size. If I cut directly on top of the marked line the kerf would eat into a small portion of the measurement. This might not seem like a big deal, but when your entire project doesn't fit by 1/8" it can be very frustrating.

Of course, you can always use a mitre box with your handsaw to keep things nice and straight if the marking technique doesn't work for you.


Straight Cuts in Plywood

Cutting straight in plywood is a little easier, since the blade doesn't wander as much during a cut.

Measure and mark where you want to make a cut, remember to add a check mark on your line to determine which side you're cutting on. Secure your wood to a workbench or other stable surface with clamps.

You can make a cut without any guides by carefully following the line with your cut notch on your circular saw, but an easy way to get perfectly straight cuts it to set up a fence. A fence is a straight member used to guide the plate of the circular saw. Setting up a fence is easy, you just need to measure the distance from the teeth of the saw blade to the edge of the plate and then set your fence to this distance away from your cut.

Plywood cut set up with fence

Clamp the straight edge to either the wood or the workbench to ensure it doesn't move. Be mindful when cutting that you want to set up your work so that any cuts fall away safely.

The extra setup may take a little longer than just eyeballing the cut. Straight edges on your cuts are the hallmark of a pro.


Plunge Cut

A plunge cut is where a cut starts in the middle of the board as opposed to the ends. Plunge cuts can be a little tricky, but get easier with a little practice.

Before cutting carefully line up your circular saw above where you want to cut, making sure to account for the kerf and which side of your mark the blade is on. Rest the front of the base on the board and lift the back of the saw up so the blade is not touching the wood, start the blade spinning and slowly lower the blade into the wood using the front of the base as a hinge.

As with all cuts, make sure your piece is securely fixed to your workbench and always be aware of what's underneath your cut.


Tearout

You may have noticed when cutting wood that there's a ragged edge to your cuts, this is called tearout. Tearout is the ragged edge cause after cutting wood, there's a few reasons why this happens and luckily a few ways to prevent it.

Tearout occurs only on one side of your work piece, the side where the blade exits the cut, for circular saws this will be the side of your wood that is facing up when you cut. Consider flipping your wood to have the best (or "show") side facing down and the ugly (or "hide") side facing up, that way any tearout will be on the less nice side of your wood.

There's a few things you can do to prevent tear out: using a zero clearance board, making a scoring cut, and slowing down your cutting rate / using a different blade.

Zero Clearance

Zero clearance sacrificial board on top

The best way to avoid tear out is to use a sacrificial board to support the wood while the blade exits the cut, this sacrificial board is called zero clearance - meaning there is no clearance between your work piece and the sacrificial board that is abutting your wood. This sacrificial board supports the wood fibres as the tool exits your work and allows a clean cut all the way through.

Scoring Cut

Shallow scoring cut before making deeper cut

Another method of preventing tearout is to make a shallow cut along your cut line to make a groove in the wood. This shallow cut usually doesn't cause any tearout and will prevent the following cuts from creating any tearout on the surface of your work piece.

The plate of a circular saw can be raised and lowered to expose more or less of the blade. The plate height can be adjusted by releasing a tension lever on the back of the saw near the blade and pulling the plate downwards. To make a scoring cut the plate should only expose 1/16" to 1/8" (2-4mm).

Feed Rate + Blade

Different blades have different feed rates

Tearout can also be caused by the type of blade you're using. A large toothed blade will have fewer teeth and will cut wood much faster and aggressively, but can leave a splintered edge from cutting too fast. Switching to a blade that has more teeth will cut less aggressively, will cut slower, and leave less tearout. Slowing down the feed rate (how fast you cut through wood) can also give better results.


Quiz - Straight Cuts

Put your knowledge to the test, try the quiz below and see how you do.

{
    "id": "quiz-1",
    "question": "Wood from the store listed as 2x4 is actually 2 inches by 4 inches",
    "answers": [
        {
            "title": "Yes",
            "correct": false
        },
 {
            "title": "Sometimes",
            "correct": true
        },
        {
            "title": "No",
            "correct": true
        }
    ],
    "correctNotice": "Correct! Lumber is nominally measured as 2 inches by 4 inches is actually 1.5 inches x 3.5 inches.",
    "incorrectNotice": "That's incorrect"
}
{
    "id": "quiz-2",
    "question": "What is blade kerf?",
    "answers": [
        {
            "title": "The thickness of the material removed when cut with a saw blade.",
            "correct": true
        },
{
            "title": "How long a blade can cut before dulling.",
            "correct": false
        },
        {
            "title": "The sharpness of the blade used for cutting.",
            "correct": false
        }
    ],
    "correctNotice": "Correct!",
    "incorrectNotice": "That's incorrect"
}
{
    "id": "quiz-3",
    "question": "A plunge cut is only made in the middle of plywood",
    "answers": [
        {
            "title": "Yes",
            "correct": false
        },
        {
            "title": "No",
            "correct": true
        }
    ],
    "correctNotice": "Correct! A plunge cut can be made anywhere and is perfect for cuts in the middle of plywood.",
    "incorrectNotice": "That's incorrect"
}
{
    "id": "quiz-4",
    "question": "Zero clearance prevents tearout",
    "answers": [
        {
            "title": "Yes",
            "correct": true
        },
        {
            "title": "No",
            "correct": false
        }
    ],
    "correctNotice": "Correct!",
    "incorrectNotice": "That's incorrect"
}
{
    "id": "quiz-5",
    "question": "Always make cuts directly on top of your marked line",
    "answers": [
        {
            "title": "Yes",
            "correct": false
        },
        {
            "title": "No",
            "correct": true
       }
    ],
    "correctNotice": "Correct! Always cut to one side of your marked line to account for blade kerf.",
    "incorrectNotice": "That's incorrect"
}

Project to Try + Next Lesson

Making straight cuts with your hand or power saw is a cornerstone of almost every woodworking project. As with any skill, practice makes perfect and you'll gain more confidence with each cut you make.

An easy project to make while practicing your straight cuts are yard dice. This lawn game is a great way to hone your straight cuts, and then gave a fun project to show off and play with outside. Why not try it out and see what your yard dice come out like?

Now, let's move onto the most helpful tool in the woodworker's arsenal: glue.

CLASS PROJECT

Share a photo of your finished project with the class!

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