Straight cuts with an unguided circular saw is difficult to do freehand, but following these methods I discuss will yield a straight cut every time.
I discuss four methods you can use for a straight cut with a circular saw.
Step-by-Step Guide on How to Make Straight Cuts With a Circular Saw
Method 1: Using a rip fence
In the first method, I'll show you how to use a circular saw for making straight cuts with a rip fence.
A rip fence is an attachment that usually comes along with a circular saw. Its useful while trying to make a straight cut along the edge of the wood.
Place the wood on the work surface and draw the mark using a speed square.
Fit the rip fence to the circular saw. Next, adjust the rip fence with the edge of the wood so the saw blade aligns with the mark made on the wood. Then, begin cutting along the mark.
However, since the rip fence follows the edge of the wood while cutting, how straight your cut is will depend directly on how straight the wood's edge is.
This method is easy to execute and prefered when accuracy is not your primary concern.
Method 2: Using a Straight Edge
A straight edge can help make long cuts and, unlike the rip fence, can assure straight cuts irrespective of edge shape.
A straight edge can be any long piece of wood with a perpendicular, smooth edge that helps guide the circular saw throughout the length of the wood.
The length of the cut made with a straight edge is directly dependent on how long the straight edge is. Meaning the longer the straight edge, the longer the cut.
To make a straight edge, take a thin sheet of plywood and a long wooden plank (dimensions doesnt matter as long as its edges are straight).
Glue or nail the wooden plank on top of the plywood while making sure the edges of both are aligned with each other.
Now cut off the excess plywood that projects more than the blade of the circular saw.
With this, your straight edge is done. Use this to make a straight cut along with any wood of varying edges.
Alternatively, you could use a long spirit level (say 48 inches) and clamp it down using two quick grip clamps on either end.
Just be careful as to never tighten the clamps too hard and damage the level.
For cutting using either the straight edge or the level, place the wood on the work surface and mark it for cutting.
Now place the straight edge/level, make sure its aligned with the mark made on the wood, and clamp it in place.
Fix the straight edge/level with two quick grips and saw along the mark.
Additionally, you may use masking tape and mark the line on top of it, so the amount of burr produced is kept to a minimum.
Method 3: Using a Speed Square
This method involves two clamps and a speed square. It's ideally used for making a straight cut along the width (Cross Cut).
Begin by placing the wood on a surface and marking the cut using the speed square.
Measure the distance from the edge of the circular saw to the blade of the saw, make that distance from the edge of the wood to be cut.
Place the speed square at that distance so it may guide the blade while cutting the wood.
Now the blade should align with the mark on the wood. Finally, cut the wood all the way through.
Keep in mind that this particular method has a couple of drawbacks, mainly that it can only help cut wood that is of a particular width or higher, and the length of the speed square limits the length of the cut.
Method 4: Using a Circular Saw Track/Guide
For this method, you need a circular saw track like the Kreg Acc-cut or the Kreg Rip-cut.
Using Kreg Accu-cut
As the first step, measure and mark the width of the wood you need to cut using the circular saw.
Use a tool like a speed square to achieve this.
Place the Acc-cut near the line and place the circular saw on it.
Adjust the Acc-cut such that the blade of the saw is just outside the line you marked earlier.
Make sure the Acc-cut follows the marked line at both ends.
Once the position of the Accu-cut is fixed, lock the circular saw in place in the Acc-cut.
Now, turn the circular saw on and move it along the Acc-cut track to do the ripping.
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Using Kreg Rip-Cut Tool
If you don’t want to mark a line and instead want to use the edge of the wood as reference, you can use the Kreg Rip-cut tool.
However, for this, the edge of the wood must be straight.
Place the rip cut guide with the edge pressed against the wood piece.
Mark the width of ripping using a pencil and tape measure.
Place the circular saw and set its position such that the blade falls just outside the point marked.
Lock the circular saw in place and start the ripping by turning on the circular saw.
Move the saw and the guide together until you complete the cutting.
This method is similar to using a rip fence, but you can set the ripping width a lot higher than in a small rip fence.
With this you have successfully completed the straight cut you need.
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