Why is the cut using a jigsaw not a perfect straight line?

My answer to getting clean, straight cuts, in softwood and hardwood is to use a multi-purpose carbide-toothed blade. Using one, either in a bandsaw or jigsaw changes the entire game. These days most blades used in hand-held circular saws or table saws are various versions of carbide-toothed blades. There is a good reason for that. I recommend trying one in your jigsaw too. Also it's best to never force the blade into/through the workpiece, let the saw and blade make their own way through the workpiece, all you should be doing is gently guiding them, to get your best cut.

The blade looks bent. I've drawn lines along the spine of the blade as shown in the photo in the question. If we assume the background is flat, the point where my red lines cross isn't sitting on the surface, and is bent towards the camera. See also the shape of the shadow, and the non-round holes. On a non-flat background, it's possible the blade has been bent towards the teeth, but that would mean pressing very hard backwards without buckling it.

Step 1: don't start with a bent blade, instead buy a new one. Bent blades are more likely to break and won't cut straight. If the blade bent during cutting, you were pushing too hard, and probably sideways.

When replacing it make sure the blade you get is compatible with your saw. I suspect those in your last picture aren't. There are two major fittings for handheld jigsaws, as well as larger reciprocating saws.

Mount the blade very carefully, ensuring it's straight and solid.

If you're cutting against a fence, as I often do for straight cuts, make sure the fence extends far enough beyond the workpiece that the sole plate can be lined up against it before you start to cut.

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Here's the shape I think the bend has taken, mocked up in paper. From the less angular shape of the shadow I'd expect the bend to be a little smoother in the metal blade - but it would be as it's much stuffer and thicker. The location of the bend is where the support stops in my jigsaw, suggesting the blade was either bend before cutting, or the saw was pushed sideways rather hard during the cut (without keeping the soleplate flat to the wood)

I quote from an online article concerning saws of all kinds.

11. Jigsaw

A jigsaw is a handheld powered saw. It has a smaller blade and finer teeth than a reciprocating saw. It moves vertically. You can change the speed to make it go faster or slower. This saw is designed to cut curves and other non-straight lines. https://sawingjudge.com/different-types-of-saws/

This is a useful article to skim through from end to end. It is quite comprehensive.

Each type of saw is described briefly. If you think a particular saw is best for your purposes then it is a good idea to search online for a saw of that name.Very often YouTube will have a video showing how to use it. For example http://bitly.ws/96y6

In the case of a jigsaw you need to distinguish when searching between "jigsaw puzzle" and "jigsaw cutting".

Note that jigsaws are mainly used for cutting (curves) in sheet materials such as plywood. If you want a clean cross-cut (as hoped for in your photo) then try searching online for "cross-cut saw".

You don't have to buy every kind of saw going but it helps to read a little (and perhaps try searching for example for "general purpose saw" etc.).

To avoid a lot of initial expense, it is worth considering buying hand-saws. When they are new and their blades are sharp it is surprising how quickly they can cut. Take note of other answers. When using a drill or a saw, always let the tool do the work, it is rare that a lot of pressure is needed.

If you find yourself working hard, then you may have the wrong sort of blade but, more likely, the saw has become blunt with use.

Arnold Schwarzenegger could probably saw with a blunt blade and I've seen builders with big muscles labouring away with a rusty and blunt saw at a rough plank. They get there eventually.

Hand saws can be reset and sharpened but it's a skilled job. Quite frankly, these days its cheaper to buy a new saw than go to the trouble. For power saws, check the blade for signs of wear after use and consider replacing them if they are bent or damaged. It saves time and money in the end.

A jigsaw is fully capable of making a decent cut. Will it be as nice as a table saw or as a pull saw? No. But the cut you have here doesn't appear to have any straight lines at all.

You want:

• Downward pressure
• High blade speed
• Don't advance the blade into the wood faster than the blade wants to cut the wood

With those three things I don't think you even need a straight edge to get better results than that picture. A straight edge will help further though.

Ever considered why it has its name? It's designed to do just about anything except cut in a straight line!

Its blade is thin, front to back, on purpose. So it can and will go round corners easily. To cut straight lines, the blade needs to be long, front to back, so it's difficult to turn. As in tenon saws, which even have a strengthener to stop them flexing, or handsaws, which have deep blades.

The hand jigsaw, sometimes a coping saw, has an even thinner blade, to get round corners even more easily. One of the reasons it has an adjustable angle handle.

To enable straight cuts, use any saw with a 'deep' blade, so when the front edge may start to wander, the back edge keeps it on the straight and narrow. Where we all belong..!

As a novice-level wood cutter, I will say that what helped me the most with learning how to make straight cuts using a jigsaw was to slow down. You can certainly cut in a straight line, without a guide, if you take it slow, with some practice. More pressure against the blade equals more likelihood of not being straight - let the blade do the cutting, not your force on the wood into the blade. That's an easy mistake to make, especially if you're used to hand-saws.

As others note, it's not entirely intended for straight line cuts - that's what the circular saw is for, and I highly recommend a good one - but it's possible, especially with thinner wood. That wood isn't too thick, so it should be doable with practice. A circular saw will be much easier to do this type of cut with, though. I made a straight cut my second attempt with a circular saw, while with a jigsaw it took five or six tries, and was never quite as perfect.

Also become familiar with the tools of "cleaning up" after your cuts - shaving a little bit extra off using a hand plane, using a chisel for little bits, etc. - so you don't have to be perfect! And as FreeMan points out, there's also sandpaper, or my preference a file, which may be easier on these end grain cuts if you're not too far off.

Forget the guide block. Seriously.

Jigsaw blades wander for a variety of reasons, and all of them will cause you to have a non-square cut, even if you have a square guide block.

Instead, practice cutting to a line. (Or more accurately, cut most of the line on the waste side.) If the 'top' of the board is supposed to be the nice side, then mark the line on the bottom. (Or get used to cutting upside down, which takes even more time/effort...)

Jig saws are designed for cutting curves.

You can cut straight lines and generally you followed the correct procedure - using a straight edge. Your picture of the cut is pretty blurry, so it's hard to tell for sure, but there are several possibilities:

• The most likely culprit is that you cut too quickly.

  • I have recently made a number of freehand cuts using my jigsaw, and, while my cuts weren't laser straight, they were close enough that a couple of passes of sandpaper were enough to make them quite satisfactory.
  • I cut very slowly, very gently pushing the saw into the wood while following my line.
  • I used my off hand as a guide. I made a fist to keep stray fingers away from the sharp, bitey bit, then used my thumb and the knuckle of my pointer finger as a guide against the edge of the jigsaw base.
  • Again, I cut very slowly, taking my time, making minute adjustments in both the pressure and angle of my guide hand and in my jigsaw hand. If I saw I was wandering off, I immediately backed up, then steered back onto my line.

• The piece of wood you used as a straight edge isn't as straight as you thought. Usually one uses a metal or plastic straight edge - one that is manufactured to be straight. Either a ruler, a level, or a specifically designed cutting straight edge.

  • If you're starting in with tools, wood working and house projects, get yourself a level and plan to clamp it carefully to use as a straight edge.
  • Depending on the design of the level, this might be easy or difficult. Purchase one with the thought of clamping as a straight edge to get more than one use out of your tool.

• You didn't follow the straight edge as carefully as you thought you did.

  • Note my initial comment that jig saws are designed for cutting curves. That means that the slightest variation from dead straight will end up with a wavy line.
  • You have to have a thick enough straight edge, clamped well enough, that you can actually continually pull the base-plate of the jig saw against the straight edge. If you don't, it will tend to wander off because that's what they're designed to do.

• It does appear that your blade has a lower tooth-per-inch count. I'd guess it's between 6 and 9 teeth per inch.

  • These blades are good for "general purpose" cutting and especially for fast cutting. They generally do not make a "pretty" cut. They generally leave a pretty ragged cut edge behind.
  • If you want a smoother cut, a higher tooth-per-inch blade is what you want. Something that has 12 or more teeth per inch. Be aware, however, that these will cut slower, and are often designed for cutting metal. They will cut wood just fine, but they will cut slowly in comparison to the faster cutting wood blades.

Once you get some practice with the saw, you will get to be pretty good a following your pencil line free-hand (without a straight edge), but it's not likely that you'll ever generate dead-straight lines with a jig saw (especially not free-hand). It's just the wrong tool for the job.

Some additional thoughts:

Jig saws are used for curved cuts, but also for notching or stopped cuts where you need to cut into a board, but not all the way through from one edge to the other. You use it to cut straight in from the outside edge of the wood, straight to the corner, then you nibble away at the turn until you've got enough clearance to make a 90° (or whatever is called for) turn, then make the next straight cut. Continue this for as many interior corners as you need.

None of this is to say that you

1. Can't make straight cuts with a jig-saw (everyone has agreed that it's possible, but it takes practice and it's very unlikely they'll ever be laser straight), or
2. Can't use it for making full edge-to-edge cuts. Generally, though, a through cut is made with a circular saw because that's what they are designed for.

Since you've already purchased the jig saw (and may not be able to return it for refund), go ahead and use it. Make some practice cuts to get the hang of how to use it and accept that your cuts will likely never be perfectly straight. If you do need a really neat final cut, adjust your cut to give yourself a smidge (1mm or so) of room to manually clean up the cut using a different technique as noted in Joe's excellent answer.

Finally, it looks like your "option" blade is labeled "extra clean cut". That will help make a nicer cut, but what you really want is a wider blade - one that has more distance between the points of the teeth and the smooth non-cutting side of the blade. This width is what will help the blade track straighter, though it still isn't the magic bullet for straights cuts.

Another, additional thought

Some jig saws have an "orbit" function that causes the blade to move forward and back as well as up and down. The greater the amount of orbit, the faster but rougher the cut will be. This used to be a high-end feature, but seems to have made its way into the mid-range and even some low-end models. Usually, the amount of orbit is adjustable from "lots" to "none". Make sure the orbit function is turned off for a smoother cut

A jigsaw isn't designed to make perfect cuts. Straight cuts can be made but the stock has to be clamped down and the fence, guide block, must be clamped down too. You can't hold down a guide block with one hand and operate the saw with the other, the guide block will move and the saw will jump, as is obvious from your pictures. Clamp everything and use both hands on the saw, practice makes perfect.

Use the proper blades for crosscut or rip cutting. The type of blade is labeled on the packaging.