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

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.

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...)

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.

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..!

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.

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.

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.

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)

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.