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Making Perfectly Straight Cuts
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Lesson 3: Making Perfectly Straight Cuts
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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.


Now that you know how to make great straight cuts let's put this into action and apply the theory.


Project Time - Yard Dice With Perfectly Straight Cuts

With the basics covered on how to achieve straight cuts we can apply the concepts to a fun and easy project.

Here we'll make Yard Dice, which are large cubes of wood cut from a long post with the edges shaved off. There will need to be consistent square cuts along the post at measured intervals.

Tools + Supplies:

Before we start we'll sketch and plan out the project. We already know that a 4"x4" post is the nominal measurement with the actual dimension is 3-½"x3-½", so we can sketch out our plan of attack before committing to any cuts.


Cut Post

Measuring 3-½" along the length of a 4"x4" post and marking with our square we know where our cut line is, this is followed by making matching marks along the sides and bottom of the post and adding a check to pencil line to indicate which side to cut on to allow for the kerf. 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.

With the post securely clamped to a work surface a pass was made with the circular saw, and remember to cut so that the wood can easily separate and fall away after the cut.

The beefy 4"x4" posts were too deep for my circular saw to cut in one pass, so the post was flipped around after the first cut. If you sawed on the check marked side of the post your cuts should line up and a cube of wood should fall away. We'll need 5 dice for this project, so measure up another 3-½" from your cut and repeat sawing until all 5 dice are cut from your post.

Note the tearout at the bottom of the video where the blade exited the cut.


Shaping

After all 5 cubes have been cut the sharp edges and corners can be filed off using a hand rasp.

Gently clamp the cube to a solid work surface so it doesn't move around, but not too hard so that you damage the wood. Using the flat side of the rasp knock down the edges to give these cubes more of a dice shape. We talk more about wood shaping in a following lesson, but for this project we're looking to smooth out the edges with the rasp. Make a diagonal motion with the coarse side of the rasp to remove the sharp edges on all sides of each dice. It's alright if there's some rough patches form using the rasp, we'll smooth those down with sanding after.


Sanding Smooth

We can now smooth down the dice and make them feel better to hold. Start by using an 80 grit on the random orbital sander and go over all surfaces and edges to smooth out the dice shapes.

Sometimes it can be easier to bring the work to the tool instead of the other way around. If you have a bench vise you can try securing your random orbital upside down in your vise and tighten until snug, then you can easily rotate the cubes around on the sander until smooth.

Repeat for all dice until they are of uniform smoothness and shape.


Sketch Dice Pips

Grab a die for reference so you'll know which order to draw the numbers on. The dots on die are called pips. I used a large washer to trace around to create pips for these dice.

Using a ruler I drew a line between the corners on one of the die faces, where the lines crossed is the center part of that face. With this as a guide I could place the pips accurately and repeatably for each die face, and for each die.


Paint

After marking where the pips were I painted in each pip with black paint and erased any remaining pencil marks.


Finishing

To protect the dice I applied a polyurethane coating which will keep them from getting stained with grass or going dull from too much wear.

We'll be covering finishes like polyurethane in more detail as part of a later lesson, for now you're looking for any clear polyurethane. To allow all sides of your dice to dry you can upturn screws and use them as standoffs.

Wearing protective gloves wipe on a coat of polyurethane and set on the upturned screws to dry overnight.


Play!

Your first woodworking project is complete. Congratulations! Take your yard dice to the park and play a game of Yahtzee, or a whole bunch of other dice games.

Something I find really great about making something simple like Yard Dice is that you just made something that actually costs money if you were to buy it.


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"
}

Class Project!

You've taken the first steps on a rewarding craft that is satisfying and has endless applications. Now that you've learned the basics on cutting wood, let's see what we can do when we combine making straight cuts with glue in the next lesson.

Don't forget to share your version of the Yard Dice in the Class Project box below.

CLASS PROJECT

Share a photo of your finished project with the class!

Nice work! You've completed the class project