Tips For Making Cabinets Square With A Circular Saw

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Reader question on how to get square cabinets when cutting plywood with a circular saw and what to do when your cabinets aren't square. A lot of good information on how to buy a circular saw for cutting plywood too.

A couple of days ago I received the following email from Catherine regarding cabinets that aren't square.

Hi,

I just found your site and love it. I'm new to woodworking and want to know what tools you use keep boxes square? I see a lot of 90 degree clamps out there and wonder if you recommend any of them? Also, what do you do if your project isn't square? How can you adjust it? Say a cabinet or bookcase for example.

Thank you, Catherine

In a follow up email I found out she's cutting her plywood with a circular saw and guide with pocket screw joinery. Since this is how I also cut my plywood I'm very familiar with the problems. There are four main problems that can cause your cabinets to not be square. In this article I'll discuss what causes cabinets to not be square as well as how to prevent them. At the end  I'll discuss what to do when things don't line up square.

I had planned to write a separate guide on how to buy a circular saw but Catherine's question brings up a lot of the points I wanted to address so I'm including that here.

Cut Edge Is Beveled Not Square

The cut along the edge of the plywood needs to be square to the top of the plywood. If you get square cuts you'll get 90 degree corners because there's no other way for the plywood to go but square. If however the cut isn't square, when the screws pull the pieces together then they'll join at an angle. Here's an illustration with an exaggerated bevel in the cut to show you what I'm talking about.

There are some things you can do to make sure you get a square cut with your circular saw.

Check That Circular Saw Blade Is Square

The first thing is to check that your circular saw's blade is square to the base of the saw.

1. Unplug or remove the battery from your saw.
2. Set the depth adjustment to it's maximum to get the most blade sticking out through the bottom of the saw base.
3. Retract the blade guard to expose the blade.
4. Use a combination square against the base of the saw and the side of the blade to make sure the blade is square.

If the blade isn't square to the base you won't get a square cut. If the blade is out of alignment hopefully there's a way you can adjust it. On my Milwaukee M18 Fuel Cordless Circular Saw there's a little adjustment screw to make sure the bevel is square when it's at the 0 degree position. The head of the screw is on the bottom of the base but here's a side view so you can see how it works.

I marked to location of the screw with an arrow. Also notice how thick the sliding part of the bevel gauge is. It's not going to bend and go out of alignment easily. It also locks securely. Before making an adjustment to the bevel make sure the blade is flat and it is installed correctly.

Wobble When Cutting

Nothing is perfect and when the blade spins at such a fast speed there is going to be a little wobble in the blade. Better saws and better saw blades minimize this but that's not what I'm talking about here.

You need to keep the base (aka shoe) of the saw flat against the top of the plywood as you're cutting. Here's what it looks like cutting plywood using the DIY Circular Saw Cutting Guide from previous plans.

The saw's base is deeper on one side of the blade. I like to keep this wide side on the guide for better stability. If I'm not using this guide I cut so the wide part of the base is on the keep side and the short side of the base is on the cut side, the side that falls away.

Even if you do this you still have to be careful to make sure the saw doesn't rock from side to side. Get some scrap plywood and practice a lot to get your cutting technique good.

You also need to make sure the base of your saw is flat and sturdy. The base of the circular saw is very important if you want to get good straight cuts. If you think about it, it's like the table of a table saw. The base is now the first thing I look at on a circular saw. Bells and whistles like laser guiding don't help if the base isn't strong.

I got my first circular saw for maybe $25 and it had a stamped metal base. The base was made out of thin sheet metal which wasn't strong at all. Combined with using it as a demolition blade it wound up getting bent a little and I believe there was even a slight bow to it when I first got it.

Many brands have thinner bases in their lines to make the saw more affordable. It's fine for general cutting but if you want to make cabinets out of plywood a better saw is easier to use. The thinner bases still work, you just need to be more careful. Here are close ups of two different bases. You can see that one is noticeably thicker than the other.

There are also cast magnesium circular saw shoes which provide excellent stability with a lightweight material. Here's the cast magnesium base on my Milwaukee cordless saw.

I have the Milwaukee M18 Fuel model 2730-20 saw. There are some improvements between the 2730-20 vs the 2630-20 non Fuel cordless saw. The most noticeable being the cast magnesium base vs an aluminum base.

I need to replace my corded saw and I'm considering purchasing the Porter-Cable PC15CSLK because it's the cheapest corded saw I can find that appears to have a good base for cutting plywood. If my budget allows I'd really like to get the Milwaukee 6391-21 because it has the blade on the left like my cordless and worm drive saws. Hmm... maybe I should just get a worm drive saw?

If you can't seem to get the edge cut square with your circular saw another option is to cut the piece a little long and then clean up the edge using a guide and a router with a straight cutting bit. Or if you have a small table saw that's not good enough for cutting full sheets but can do smaller pieces you can square things up there.

Cut Lines Are Not Straight

The next issue that causes cabinets to not be square is when cut lines are not straight.

This happens when the saw guide is not placed straight but it can also happen because the sheet of plywood may not have been cut perfectly straight at the factory either. This is how to prevent it.

In the above illustration we're going to choose to use the bottom edge as our reference edge. First thing we're going to do is check the right edge to see if it's cut square. If it's not cut square to the bottom reference edge we're going to trim a little off the right so we can make sure the right edge is perpendicular to the bottom edge.

Next we measure the width we want to cut and mark it on two ends (top and bottom in this case) measuring from the right now that we know that edge is square. I'll connect the dots with a large square like a drywall square against the bottom reference edge so I have it marked along the whole cut.

Now we place the cutting side of the guide up against the line and use a combination square to make sure it's at a right angle. Clamp the guide down securely on both ends and recheck to make sure it didn't move when clamping.

When we make this cut we now have 3 sides of the plywood piece that are square (left, right and bottom.) When it's time to make the horizontal cut for length all 4 edges will be square if we square it to one of the 3 square edges.

Opposite Sides Not The Same Size

The next problem arises when two opposing pieces are not cut to exactly the same size. Unlike with a table saw which makes repeat cuts exactly the same size, cutting with a circular saw and guide is prone to error. The pencil line has some width to it and we can make slight errors placing the guide. Here's an exaggerated view of what this looks like.

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We're not machines with laser guided precision. Any way we're going to make a cut there is going to be some margin of error. What's important is that certain pieces are exactly the same size. If we're building a bookcase and we want the top and bottom to be 36" long it's okay if it winds up being 36-1/32" as long as both the top and bottom are off by the same amount. We'll still get a square bookcase.

I do two things that help. The first is taking care with positioning components in the cut plan. Someone recently asked me in a comment what program I use for layout out the cut plans and I don't use any. The main reason is that I want to place items in a way that gives me the best results cutting with a circular saw because right now I don't have a table saw. Here's an example cut plan from my plans to build a desk from one sheet of plywood.

It's important for the two desk Sides to have the same dimensions as it is for the Desk Top and the Shelf. With 2 cuts (the cross cuts in red) I'm able to get even dimensions along one direction for all 4 of those components. Provided I followed my previous advice to get the cross cuts square.

Next we need to cut the 2 Sides from the one cross cut and we need that cut to give us two equal sized sides. But as hard as we try we're not going to get them to be exactly the same. To get all 4 dimensions the same, after we do our best to cut right down the middle we stack both pieces one on top of the other, line up one corner, clamp them together and then make another cut through both pieces.

Now our sides are the exact same size all the way around.

For thinner pieces like the stretchers in cabinets I usually cut them with a miter saw either stacked on top of each other or using a clamp as a stop block to get the right size.

Misalignment

Poor Catherine is probably shaking her head at me because she asked me what she thought was a simple question about right angle clamps and I'm just getting to that part after a long wall of text.

At corners we want the edges to line up but sometimes when driving pocket screws they may decide to move one of the plywood pieces a little because they're at a slight angle.

With pocket screws you don't really need the clamps to make sure you have a 90 degree corner, if you've followed the advice above the screws will pull everything square, but corner clamps can hold everything in place while you're driving screws.

I don't own any right now. I usually put one piece on the floor, kneel or sit on it, prop the other piece next to it, use whatever clamps I have and one hand to hold everything together. It's a pain but it's been working more or less.

I have been looking at two different clamps to buy though. The Wolfcraft 3415405 Right Angle Clamp for the corners because it not only holds the boards at the right angle but also lines up the edges making sure all 3 dimensions are lined up. Also the Kreg KHC-90DCC 90-degree Corner Clamp which can be used in the middle of a cabinet like when you need to attach the bottom of the cabinet just above the toe-kick notch or fixed shelves in tall cabinets or bookshelves.

What Do You Do When Your Cabinet Isn't Square?

Follow the advice above as well as the good ole "measure twice, cut once" axiom and you won't have that problem.

If you try to force a cabinet into being square when it wasn't cut properly you're going to compromise the strength of that cabinet. Let's see why.

Fixing Cabinets That Aren't Square

If we use a right angle clamp to make a corner square even though one edge has an accidentally beveled cut this is what it would look like.

Notice how there's very little plywood touching other plywood. The only thing holding those two pieces together are the screws and if you took one side in each hand you'd be able to fold them as if they were hinged. Eventually you would break them apart without much effort.

If you have more plywood or don't mind buying more then I would recommend cutting the piece with the beveled edge again.

If the budget doesn't allow buying more plywood then you can just recut the existing piece to square it up. You'll wind up with a slightly smaller piece but in some cases that's better than spending more money. Because cutting again can screw up the pocket hole placement I always like to dry fit things before I make the pocket holes.

Those are the only two fixes I would recommend to make sure you wind up with something that has the strength that it was designed to have. You don't want a cabinet to fall apart when it's loaded with expensive and heavy china. It could hurt someone.

There might be other options like filling the void with something strong like epoxy or maybe screwing L-Brackets inside somehow but that's expensive, messy, complicated and I don't know how strong that will be.

Same goes for the other types of bad cuts. Try and cut them again from new plywood or cut them down smaller to fix the problem cut. Now that you know what to look for (and how to prevent it) it should be easier.

Example Bookcase Isn't Square

Here's an example of fixing a bookcase that isn't square. We wanted to build a simple box for a bookcase that is 24"H x 36"W x 12"D but when we put it together we got something that isn't square. To make it easy to follow along the I color coded the 4 sides of the bookcase.

• Red - Left Side
• Yellow - Top
• Green - Right Side
• Blue - Bottom

Looking at it from the front we see the following (grey box added to make it easier to see it's not square):

We need to take it apart to see what went wrong. First we'll lay the top on the bottom because they're supposed to be exactly the same size. We check all the cuts and everything matches but when we check the right side of these 2 pieces with a combination square we see that the cuts aren't square, they're beveled.

We don't want to buy new plywood and it doesn't matter if we change the size of the bookcase a little because it's a standalone piece that doesn't have to match up with anything else so we're going to cut whatever we find that's causing us a problem.

In these case we're going to line up the top and the bottom pieces one on top of the other, clamp them down and set our cutting guide on top of the plywood so that we can shave off a little off the right side to square up that edge. Line up the jig as normal to make sure it's square. We don't have to be precise with the placement from the edge as long as we place it far enough from the bad edge to cut off the beveled edge.

That's one problem solved. Now let's look at the left and right sides. We lay them one on top of the each other and line them up. All the cuts are square but for some reason we wound up cutting the green piece about 1/2" shorter than the top. These need to be the same size for the bookshelf to be square.

Clamp the pieces together so the other 3 edges line up, place the guide square on top of both boards and cut off the one end to get everything to match.

Now if we put the bookcase back together everything should be square but, and this is an important but, we may have a problem with the pocket screws where we made the cut on the top and bottom.

If we drove the screws into the existing holes there's a chance that the tips of the screws will poke out the right side of the bookcase. We don't want that. We'll need to make new pocket holes on the right side of the top and bottom where we made the new cut to correct the beveled cut.

When we put the bookcase back together it's going to be about 1/2" shorter and maybe 1/8" narrower than we had planned but we can feel confident placing books in the book shelf. When the joints aren't square they're not strong. It's not just about aesthetics.

In Summary

Cutting plywood with a circular saw has a greater potential for problems and requires you to take a little more time to ensure you're getting good cuts.

If you get good cuts you're project will be square. If your project is square your project will be strong.

Buy some cheap 3/4" plywood or use some scrap and practice making little boxes until you feel comfortable with your technique. 

Inspect your tools to make sure they're not causing you headaches. Circular saw should have blade perpendicular to base and base should be able to rest firmly on plywood without rocking. The guide you're using should also not flex.

If you have trouble making precise cuts with a circular saw but have a router or table saw you can use to square up cuts, cut the plywood a little bigger with the circular saw then trim it to final size on one of the other tools. With practice and patience in setting up your guides this isn't always necessary.

Understand the problems that cause your project to not be square so you can identify them and correct them by either cutting a new piece or trimming away the problem cut. It's better to have a slightly smaller piece than to have something that could fall apart.

Hope that helps and thank you for the kind words about my site,
Tom