Hi @Amanda2273,
Thank you for your question on cutting straight edges and welcome to the Bunnings Workshop community, it is great to have you with us.
Can I ask what materials you are using for these display boxes? The size and dimensions of the material will dictate the best method for cutting them nice and square.
The best tool for getting straight, square cuts is a mitre saw (drop saw). They are set with a fence that runs perpendicular to the blade, which has been calibrated to cut square. A jig saw could be used in conjunction with other tools to get straight cuts, but it is not specifically designed for this purpose. They are more useful when cutting curves as they allow the blade to be turned while travelling through materials.
If the materials, you are using are too big to be cut on a mitre saw, the best tool you can use is a circular saw. They can be used in conjunction with a speed square or spirit level, tape measure and clamps to get straight, square cuts. This is possible because circular saws are made with a baseplate that is square to the blade, allowing you to run it up against a straight surface to keep the line of cut perfectly straight.
The speed square has an edge that you can run the blade against as well as being manufactured to a perfect 90° angle so your cuts will be square and straight. Outside of a mitre saw, this is the easiest way I've found to get straight, square cuts on materials that aren't too wide.
If the material is too wide to use the speed square, you can use the tape measure to measure your cut on either side of the material and make a mark. You would then use the straight edge of the spirit level to "join-the-dots" giving a straight line for your cut. Once the cut is marked, you can measure from the outside of the saw blade to the outside of the base plate on the circular saw. This measurement will tell you the distance back from the line to clamp your spirit level to give a guide to run your circular saw against.
When cutting with these tools, please remember to use caution and prepare yourself for success before carrying out the cut. Always ensure you are in a comfortable position and are wearing all necessary personal protective equipment including gloves, mask, eye protection and hearing protection.
You might like to check out The D.I.Y. Guide To The Circular Saw and Tools 101: How To Use Power Saws for some more information.
Let me know if you have any further questions, I am more than happy to help.
Jacob
