It looks like Zip A Bag is turning out to be more interactive than I'd imagined! You're asking questions and leaving comments that jump-start discussions, and I consolidate my responses into a single QnA post in between chunks of tutorial posts. I'm happy to do it, so keep those comments and questions coming!
1 What kind of machine do you use?
I cannot remember the exact context of this question, but the accompanying words implied something like, "your bags look so professionally stitched. Do you have a special machine?"
Short answer = No, I don't.
My sewing machine is a Pfaff Classicstyle 1525. It's nothing fancy. It is completely manual and has something like only 8 or 10 stitches, total. However, it is a real workhorse, specifically because it can handle multiple layers of thick fabric, which is why I bought it over the other fancier electronic machines that embroider your family crest while you're mixing a batch of cookie dough somewhere else in the house.
Someday I want to own an industrial lockstitch machine. It certainly will be more fun to sew bags with - if nothing else, because its longer arm will make it easier to maneuver bulkier bag parts. But I don't have the space in the house for it right now.
I don't think a fancier sewing machine helps you make better bags, though. When I was in my teens and early twenties, I made bags in worse fabrics than these (i.e. like outdoor sports bags and huge padded cases for musical instruments) on the family treadle machine (also a Pfaff). I think what's more important is how you get your machine (and tools) to work for you, and how much you practice you get at making bags.
2 How do you sew through all those layers?
While nothing like what I subjected my old treadle to in the old days, I will admit that I still put my modern Pfaff through bag hell. Multiple layers and thick fabrics and batting and sponge and foam and stuff like that are the status quo. Yes, there has been wrestling. Yes, I've broken needles. Yes, I've impaled my own fingers. Yes, there have been times when I've made myself give up, walk away, and come back the next day because I needed a break. There are limits, after all, to stitching commercial-design materials and configurations on a home sewing machine. I share my struggles so you know that putting together a bag that aims to look like you might see it in a shop window is hard work.
That said, here are some things I've found helpful in working with these thick layers.
I seldom use universal needles in bagmaking (unless I'm sewing a feeble lining of quilting cotton or homedec fabric over a negligible-weight sew-in stabilizer). For the robust outer layers of a bag, I replace the universal with a Jeans/Denim Needle (good for sewing canvas and such) or a Leather Needle (good for vinyl and other such sticky fabrics) and leave it in for the whole bag-making process. Both those needles have thicker and stronger shafts than universals, can accommodate thicker thread (i.e. less snapping) and can topstitch.
Another tip: I've noticed that some fabrics with coarse weave and thick fibers show up topstitching horribly, no matter how gorgeous your tension is. Duck cloth is an example - my topstitching often is wretchedly crooked and looks like I've pulled the thread too tight. I know it's not the tension because the minute I lay, say, a piece of quilting cotton on top of that duck cloth and topstitch those two together, the stitches are once again stunningly beautiful. In cases in which I need the topstitching to be on the bare duck cloth (or other coarse weave heavyweight fabric), I use a denim needle and use smaller stitches. It pierces the fibers better and the smaller stitches, while still pushed this way and that by the weave, look less crooked overall, than longer stitches.
(ii) Walking Foot
My Pfaff machine has an integrated walking foot (that's the black thing behind the presser foot).
If your machine has a walking foot, use it whenever you sew bags. There will be less sliding and slipping with the thicker layers with the walking foot engaged.
(iii) Pins, basting and clips
I do not pin along seams because seams need to be eased as one sews, to avoid puckers and other hideous mistakes. Instead, I pin reference points, i.e. points that need to be matched with other points. I also pin particular points that need to align with print, and parts of zipper tapes that need to align on both sides of an opening, and accent fabric layers that need to lay flat without shearing.
Otherwise, I baste (or tack). I hand-baste concave curves to convex curves, and bindings that need to be stitched with identical allowance on WS and RS, and anything that requires easing to fit something else. I machine-baste strap ends onto bag bodies, and multiple layers to each other, stitching within their SA regions so these are hidden away in the final seams and don't need to be unpicked. This way, I can sew over them along the actual stitching lines without worrying about hitting pins or clips.
Sometimes I use those cute little Clover wonder clips for fabrics that aren't hole-friendly and can't be pinned or basted. They are useful for holding layers together as you are assembling them for sewing. But they wouldn't be my first choice, because you can't sew over them, and they get stuck at the side of the presser foot and still have to be moved as you ease the layers together, and sometimes leave indentation marks on puffy materials (like neoprene).
I speed-stitch things like straps and straight seams and basting stabilizers to outer fabric layers. But I go slow when I am assembling layers, and going over bumpy lumpy seams that have straps inserted into them. There are times when I turn the crank and tighten the upper tension by hand, going a stitch at a time, to be sure the needle goes all the way through the layers, catches the bobbin thread, and pulls it up without skipping.
There are also times when I will deliberately hand-stitch a seam, especially one that is in a visually-obvious location on the finished project. Thread tension, even when adjusted to perfection, will still sabotage you when you jump-the-hump, as is often the case around bag openings which have straps inserted on opposite sides. Sometimes, I will stop machine-stitching a couple of inches away from bulky seams and leave long trailing threads, which I'll then thread through an upholstery needle, to finish the seam by hand. Some of those bulky layers are so tough to stitch that I need pliers to pull the needle through them - so it's no wonder that the sewing machine fails.
3 How do you draft a rounded rectangular base?
Here is a rounded rectangular base.
To do this, I first decide how large a rectangle I want.
Let's say that I want it to be 8" x 6".
So I draw that on paper:
Then I round off a corner by tracing around a circular thing. A smaller circular thing gives a sharper, more acute corner, and makes the overall shape more like the original rectangle.
A larger circular thing gives a flatter corner and makes the overall shape more like an oval.
Then I fold that rectangle into quarters and cut the corner out in four layers, to get four identical corners.
Start with the size of the oval you want - we call these dimensions the axes. Let's say that we want an oval that is H high and L wide.
Fold your pattern paper into quarters,
draw those axes along the fold lines,
and draw a curve between the axes. Cut out, unfold and examine.
Adjust by trial, error and preference until you get the oval shape you want.
5 And how do you make the body cylinder fit that oval/rounded rectangular base? Is there a Math formula?
I'm sure there is. For a circle, for instance, it's pi (or 22/7) times the diameter . So a 14" circle has a 44" circumference. But for all other shapes, I just use a measuring tape.
Make the base. Do not add seam allowance.
Measure the perimeter of the base. This will be the actual stitching line.
Draw a rectangle for the body cylinder. Match the width of the body cylinder to the perimeter of the base. And make the height whatever you want your bag's height to be. These lines are the actual stitching lines.
Now add your SA.
When you sew up the side seams and attach the base to the bottom opening of the cylinder, you can be sure that your stitching lines will match exactly, because that's how you drafted them.
6 How do you attach a circular (or any shaped-) base to the bottom of a cylinder?
Ensure the stitching lines match. See the notes to question 5. If the stitching lines (not the actual edges of the fabric, which include the widths of the SA, which throw all your dimensions off) match, your base will fit exactly into the cylinder's opening without fear of puckering.
Make quarter marks on the base and again on the cylinder's bottom edge. Match these up when you sew, to ensure that you are distributing the cylinder fabric evenly around the base.
Snip the SA of the cylinder's bottom opening. When you are attaching a vertical wall (like the cylinder) to a flat base, the wall's SA prevents the stitching lines of both pieces from touching. Snipping the SA lets the bottom edge of the cylinder spread flat on the base so the stitching lines can get close to each other. And when the stitching lines match up, the cylinder will fit perfectly with the base without gathers or puckers.
Sew with the cylinder on top and the base below. The base is a flat piece while the cylinder is a bulky lump of gathers and folds that need to be manipulated (we call this process "easing") around the curve of the base as you sew.
7 Can you provide dimensions/templates/patterns?
So many people have asked for them that I might, yes.
There are 25(? I can't remember) bags and pouches in this series. And they are almost all prototypes, meaning that they are the first samples from the designs, and I sewed them up as I designed them, without a lot of tweaking. I'm thinking that at the end of the series, I could corral my templates (assuming I haven't tossed them out), trace out the ones that have non-easy-geometrical shapes, and measure the dimensions of the others which are simple rectangles and circles, compile them as a collection of templates in a single pdf file and put this file in my pattern store for you to purchase.
Please note that they will ONLY be templates, NOT patterns. This means that there will be NO instructions, only dimensions and measurements and template shapes. You can buy the file, print out the templates, and return to this Zip A Bag series for the free instructions to make the bags and pouches themselves. They will not be as detailed as a typical ikatbag pattern because I don't have the time to write 25 patterns, but I'll still put in a fair bit of work to ensure that the corners match up and the seams align, and so on. Plus you'll then have the option to avoid drafting the templates and eyeballing the measurements yourself.
Would this be something you might be interested in? If so, let me know in the comments.