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I couldn’t find the command to flip the parts in the menus of Prusa Slicer, but a right-click on the object allowed me to “Mirror” it. It took a moment’s thought, to make sure I mirrored the hinge box and brace in the right dimension (X, Y, or Z). Once I figured it out and mirrored the two parts, I was able to print a right-folding brace!

Have the Right Tools

A small collection of tools will help you process a new print into a finished item. Power tools are seldom needed.

• Eye protection, and mask or respirator – Eye protection is an absolute must-have. If sanding or grinding polymer, a mask—or better yet, a respirator—will help to protect your lungs.

• Nippers – a small pair of nippers for clipping supports from the project

• Knife – a utility knife is okay, but I find a small, sharp, sturdy fixed blade knife, useful for both cutting and for light prying to pop supports loose. Mine is a kiridashi neck knife that I made from an old file.

• Needle nose pliers – perfect for pulling pieces of filament out of small spaces.

• Narrow wood chisel – an inexpensive chisel will be fine. Use it as a scraper that can reach into tight spaces and shave away filament, so that parts can fit into place.

• Metric and SAE drill bits – for cleaning up/sizing holes in your project. The instructions enclosed with the files should tell you which sizes are needed.

• Non-electric drill – an old school—very old school—hand drilling tool. An old-fashioned brace and bit is a great way to gently drill holes or clean up holes in polymer, while maintaining much greater control than with a handheld electric drill. In my opinion, a brace and bit is the only tool that comes close to substituting for a drill press for making holes in polymer. I started using mine for drilling the safety/selector holes in Polymer 80 AR receivers, and it’s ideal for that task. It can grip small bits, but larger bits I modify on a sanding disk in an electric drill. I grind four flats at the butt end of the large bit, creating a square, tapering shape. Look at some old-style bits for the brace, and you’ll see what I mean. You don’t need to remove a lot of metal from the rear of the bits. Available in antique stores, yard sales, and via eBay.

• Pin vise – a heavy-duty pin vise is another tool that works very well for reaming out holes in polymer. It’s basically a chuck—for holding drill bits, awls or other small tools—at the end of a short handle. Mine handles up to a 1/4-inch bit. An okay substitute is a vise-grip, which is still better than trying to control a handheld electric drill in plastic.

• Caliper – a dial or digital caliper is very useful for measuring or for verifying that something is the right size. Good ones are not inexpensive, but the dial caliper I bought over 30 years ago is still serving me very well.

• A bench block (optional, but useful!) – this is an object for resting parts on when tapping pins in or out and for similar operations. You don’t need to buy one, as Thingiverse has free bench block files, for 3D printing. A bench block is very helpful for gun projects. It’s much better than resting an object on top of a roll of duct tape.

• 1/4-20 tap and tap handle (and drill bit) – the standard AR-15 grip screw has about one inch of 1/4-28 threads. When printing a receiver that takes an AR-15 grip, why bother with the unusual threading spec that is intended for 7075 aluminum? The very common 1/4-20 tap kit is easy to find and inexpensive, and I firmly believe the more coarse thread pattern is a better choice for polymer. I use common 1/4-20 x 1” socketed screws that take a hex key.

• Soldering iron (optional, depending on your project) – used to install metal heat-set inserts used in some projects. Can also be used to stipple a grip, or to gently smooth out bad rough spots on polymer and/or to close small, cosmetic gaps between layers. Practice on some scraps of the same filament type and color, before risking a project that took 40 hours to print. Best used on dark colors.

• Dremel tool (optional) – used with care and caution, a Dremel can be used to cut metal with a cut-off wheel. The sanding drums are also very useful. Grasp it firmly, but use it like a paintbrush or a pen, with a light, gentle touch.

• Needle files, a small, fine file, and fine sandpaper – for cleaning up surfaces roughened by support material.

Don’t Forget Parts Kits, Rails, Etc.

Each firearm project usually comes with a shopping list in the instructions. Depending on the project, this may be a few, simple items from a hardware store (or already in your workshop). It may also be a collection of firearms parts, like the complete upper receiver of a MAC-11 9mm pistol. If the instructions ask for gun parts but don’t suggest a source, do some online searches. There is a whole industry of parts suppliers who support the 3D gun parts printing community.

Be Sure to Use Correct Pin Lengths

A number of 3D firearm projects use AR-15 trigger groups (hammer, trigger and disconnnector, along with two springs and two pins). Polymer receiver walls are usually thicker than those of aluminum AR-15 receivers, so mil-spec hammer/trigger pins may not be fully supported. Ideally, the ends of the pins should be flush with the sides of the receiver. Hammer and trigger pins are under stress. A short pin places that stress on a smaller amount of filament and may cause premature receiver failure. A full-length pin spreads the stress out over a larger area.

If mil-spec pins are too short for your receiver, you can take advantage of the fact that AR-15 hammer/trigger pins are 5/32 inches in diameter. I use long 5/32 drill bits for pin material. I measure the receiver width and cut two pieces of the smooth shank from the back part of one or two drill bits. I grind and polish the ends to get the pins to their final length. For the hammer pin, I use a triangular needle file to cut a very shallow groove around the pin at its center. Look at a set of mil-spec pins for guidance as to groove depth. The trigger pin receives a similar groove, 0.3 inches from its center point. The AR-15 hammer has a spring inside that engages the groove in the center of the hammer pivot pin, and one of the legs of the hammer spring rests in the groove of the trigger pin. This is why properly-installed AR-15 hammer and trigger pins usually don’t “walk” out.

Alternatively, measure the width of your receiver and go online to look for long AR-15 hammer and trigger pins. If you find a set of pins that are a little longer than you need, you could carefully shorten them, working from both ends. KE Arms sells 1.1-inch AR-15 hammer/trigger pins (https://www.kearms.com/KP-15-HammerPin.aspx), for about $10 each. I have no experience with the company, as I made my own pins, as described above.

Don’t Hammer Pins Into Printed Receivers!

Be cautious and gentle installing pins and screws into printed gun parts. Although the polymers are surprisingly tough, you could “punch” a misaligned pin right through a receiver wall, ruining a part that might have taken many hours to print!

When installing AR-15 hammer and trigger pins, for example, I was taught to lightly oil the pins and to press them into place. I use a piece of hardwood to press them in, rather than “tapping” or “hammering” them. The trick is to gently clamp the receiver in a padded vise to free your hands. If it’s an AR lower, there are inexpensive fixtures that go into the magazine well and can be clamped firmly in a vise. When the trigger—and then the hammer—are properly positioned in the receiver, the pins should go in with minimal pressure. I look down into the receiver, to help me position the trigger and hammer, while pressing the pins in. It takes a bit of time and patience—at least at first—but you avoid damaging a print into which you’ve already invested time and filament.

By the way, assembly of AR-15 hammer/trigger groups is a good place to mention eye protection again. If you goof during assembly, the hammer can come flying out of the receiver with surprising force from its strong spring. If you’re looking down into the receiver at that moment, then your eyes are in danger.

Make Sure You Don’t Run Out of Filament During a Print

Losing a print because you ran out of filament near the end should not happen to you. It’s easy to prevent this occurrence.

1. When you receive a new spool of filament, weigh it. Subtract the filament weight (most commonly 1 kilogram) from the total. You now know the approximate weight of that filament brand’s spool.

2. Slicers generally tell you the weight of your project as part of the slicing process. Compare that to the weight of the partial spool of filament, minus the spool weight. You want to have at least a little more on the spool—I like to have at least 10%—than the slicer says is needed.

3. If you’re a little bit short, you can reduce the infill and try slicing and exporting to G-code again, provided it’s a non-critical part.

Just Do It!

The best way to have successful prints is to print out projects, make mistakes, and learn from your mistakes. If you start with small projects, you can learn quite quickly.