Stack cutting is a great method for making a large number of items quickly. If everything works correctly that is. So what is stack cutting you ask? As the name implies, you stack multiple pieces of wood and cut them all out at the same time. There are some limitations and a few things could go wrong if you don't prepare the stack correctly.
You can stack cut a variety of different thickness but the most common is 1/8" or 1/4". This technique is particularly useful for fretwork cutting. You spend the same amount of cut time but instead of one item you end up with up to 8 duplicates.
The magic number isn't necessarily 8 but I would not recommend going beyond that as the chances for things to go wrong increases. Stack 8 layers each 1/8" thick and secure all the layers together. You could tack glue the layers in negative waste areas to keep them all together. If all the sizes are similar, you can run a line of tape along the edges to secure them. You could also use some double sided tape but with intricate fretwork, this method can be long and tedious and tricky to remove the tape without damaging the piece. You could use layers of wood less than 1/8" thick but at that point, the wood becomes very fragile and if the blade catches the piece on the upstroke of a cut and slams it onto the table (always a cringe moment), strong chance that something will break.
So why not 10 layers? Well, the thicker the stack of wood the greater the chance of angled cuts. I've cut many fretwork pieces out where the top layer looks perfect but once you separate the layers, the cuts become less and less accurate and can ruin the bottom few layers. Cutting at 1" or less is a rule that I follow but it can certainly be done on a thicker stack.
One of the other possible occurrences that spell almost certain doom is if the stack of wood shifts. This can happen if the layers are not secured well enough. You want all the layers to be perfectly rigid to each other or cut will not match on the top and bottom layers.
You can sometimes get away with angled cuts on a scroll saw but in the majority of projects, a perfect 90 degree angle is critical. There are a variety of factors that can lead to angled cuts. Some of them can be a quick fix and others will make you tear your hair out. Hopefully after reading this post, angled cuts will be a problem of the past.
1. This may seem obvious but the first thing that you should always check is if the blade is 90 degrees to the table. Even if you set the table at 90 degrees earlier in the week, there is always a chance that it somehow shifted. Leaning or pressing your hand down on the surface can tilt the table a fraction of a degree which can make all the difference. A quick and easy way to check if the blade is a perfect 90 degrees is to take a small piece of scrap wood and make a shallow cut. Turn the saw off and spin the scrap piece of wood around and try to feed the blade backwards into the kerf line. If the blade slides in perfectly, you're all set.
2. Blade tension or really lack there of can be a factor. A blade with too little tension will bow often in the middle creating a cup shaped angled cut. If you notice this, do a few test cuts and increase the tension to see if the angled cut reduces or goes away. Blade tension can be a difficult thing to master and usually needs small adjustments for different types and sizes of blades. Some advice that's given in the scrolling community is when you pluck the blade, if you hear a sharp C note, its tension is correct. If you're anything like me, figuring out what a C note sounds like is like figuring out theoretical physics. I do pluck the blade and listen to the sound but I'm clueless if it's a C note. Practice is the best solution for proper blade tension.
3. Improper blade selection can sometimes result in an angled cut. For instance, If you try to cut a 3/4" piece of purpleheart (a very dense wood) with a number 1 reverse blade, your going to have a difficult time getting cuts without an angle (not mention a good chance of charred wood and a pile of broken blades). The thickness and hardness of the wood are big factors in blade size. In general, we use 3/4" to 7"8 wood for everything we make and I alternate between a #5 and a #7 blade. For very dense woods such as yellowheart, use a #7 blade. Lesser density woods such as American cherry, use a #5 blade. Wood less than 1/2" thick, use a #3 or a #1 blade. Wood thicker than 1" will often create a whole mess of issues. It the cuts are not very intricate, use a #9 blade (This blade is fairly thick and doesn't take kindly to tight curves.)