We thought we'd answer these questions in two segments. First, we'll explain what each sewing machine does, then we'll talk about when it would be appropriate to purchase each one. And this discussion will be done in the most informative way possible, so if you're looking for a dose of humor, skip along.
So, to answer the first question, "What does each machine do?" let's go way back to the basics. We're not trying to insult anyone's intelligence here, but we do hope that this will be a useful post for people just starting out.
First, a sewing machine.
This one is obvious. It produces a stitch that holds two fabrics together. When you look at the wrong side of the seam though, you'll see these pesky little leftover bits called seam allowances:
The problem with seam allowances is that most fabrics will fray at the edges. At best, you'll be left with something that constantly has strings hanging out, and at worst, the edge of your seam allowance will fray all the way to the stitchline and your garment will fall apart. So, something needs to be done. Now, there are many many ways of dealing with seam allowances, but I'll just mention the two simplest methods.
First, you could use pinking shears to cut the seam allowance. The zig zag cut that it produces interrupts the fraying process because the individual threads of the fabric can no longer be pulled off in a straight line.
Second, you can zig zag stitch the edge, which produces another barrier to the fraying. Here is an example of both:
As you can see though, neither of these methods is ideal. They'll save your garment from falling apart, but you'll still have annoying threads and some fraying at the edges, and of course it just isn't pretty. Which may not be a consideration for most home sewists, but is definitely something to consider if you're going to be giving handmade items as gifts or trying to sell them.
Enter the serger.
In dealing specifically with the problem of seam allowances, a serger is a "finishing" tool. First, the serger's knife cuts a clean edge on the fabric, and then it's multiple threads (depending on the machine, you can use 3, 4, or 5 threads) wrap around the raw edge of the fabric, enclosing it and ensuring it won't fray. The result is a seam that is "prettier" and lays flat, shown below on the right. Sergers can also produce a very narrow rolled hem, as seen on the left.
Most home sewists start out sewing with woven fabrics, and a serger is an excellent way to deal with the seam allowances. First, you'll construct the item using a sewing machine, then finish the seam allowances using a serger, as you can see by the two lines of stitching below.
But let's move beyond finishing seam allowances and discuss construction. Take a look at the side seam on a t-shirt. It was most likely sewn together with just a serger. This is a great option if you are working with knits. In fact, the main difference between a seam sewn with a sewing machine vs. a serger is that a serged seam will stretch. A straight stitch from a sewing machine will not stretch. When putting on a fitted knit shirt, you'll see that it's necessary to stretch the seams a bit. If those seams are done with a straight stitch on a sewing machine, the thread will break and your garment will fall apart.
So using a serger, the construction and the finishing are all done in one handy, stretchy seam:
So, now we've discussed dealing with the internal construction and seam allowances, but what about when it comes time to hem? Well, when using a woven fabric, most likely you'll do a standard double-fold hem with your sewing machine. That way the raw edge of fabric is tucked up neatly inside the hem and won't show or fray. But when using knits, a double-fold hem isn't ideal. It's bulky, it leads to bubbling, and it's also not necessary because knits do not fray. Also, as previously mentioned, the hems need to be able to stretch a bit.
You can hem using a serger. It will look like this (and as you can see, it'll tend to roll up):
This option is fine if you're going for a raw, deconstructed look. Or, if you're using a rolled serged edge to produce a lettuce effect, like this:
And of course, you could stitch the edge with a serger, then fold it up once and stitch it down with a sewing machine. This is fine for wovens if you don't feel like doing a double fold hem. But again, remember that it won't stretch, and thus is not an ideal hem for fitted knit shirts.
If you'll look at the hem of your shirt, either at the waist or the sleeve, you'll see that it looks much neater with a simple double stitch, and it can stretch. And this stitch can only be done with a Coverhem machine.
The reason a serger cannot produce this stitch is because it has those knives that cut off the edge of the fabric as you stitch. A coverhem works more like a sewing machine in that it just stitches through layers of fabric without cutting.
You can refer to Susan's previous post showing all the different coverhem stitches, but the most common that you'll see is this one. Two lines of stitching on the outside of the hem:
And chain stitching on the inside:
Also, as you'll notice from the above picture, a knit hem is only folded once.
If you look at the neck band of a normal shirt, you'll also see the coverhem stitch in use. The neck binding is applied to the shirt by folding it under and topstitching it down on the top of the shirt:
And the inside of the shirt will reveal no fold, just an edge tacked down with the resulting chain stitch:
So, those are just the most basic operations of a sewing machine, serger, and coverhem machine. Obviously there are lots of different options and features in each machine to do all kinds of fancy things.
And of course, there are speciality feet, stitches, and techniques for regular sewing machines that will help you emulate the stitches that can be performed by a serger or coverhem. Adrianna discusses them briefly in her Ringer Shirt tutorial. But basically they are just cheater methods to help you if you don't own a serger or coverhem machine.
But like any cheater method, they're not ideal. Which leads us to segment 2 of this discussion: When is it appropriate to buy each machine?
Obviously the first thing a home sewists needs is a sewing machine. In fact, if you're not a garment sewist, you're probably never going to need anything else. When making quilts, bags, curtains, etc, you're always going to be encasing the raw edges.
After that, and given the capabilities of each machine that we've already discussed, the question becomes: What's harder to do? Deal with seam allowances or hemming? And, in our opinion, the answer is definitely dealing with seam allowances. There are many techniques to deal with hemming without a coverhem machine. But there is nothing a sewing machine can do to equal the seam allowance finishing and stretch stitch provided by a serger.
So, to answer the question of what you should buy in which order, our answer is:
1. Sewing machine
(obviously we're strictly talking about features and capabilities of each machine here, not the considerable topic of finances.)
And of course, we'd both like to mention that the machine you should buy before any of these is this one:
So, there's our not-so-brief discussion about helpful machines for the home sewist. Hope that answered more questions than it raised. Please continue to direct any confusion our way (crafterhours at live dot com).