Wednesday, October 9, 2013

Mini PVC Crossbow Tutorial

A couple of years ago, I wrote a short story about getting distracted while trying to write a short story and building a crossbow instead. I ended up putting that story plus a mini-crossbow tutorial up on Amazon's Kindle and also featured the crossbow in the video below.

Since my regular website is now down and I'm working on a new site, I've moved the original crossbow tutorial here. Enjoy!

Hey everybody! Here’s a little tutorial on how to make the mini-crossbow pictured in my first Story-a-Week short story, Procrastination. The story itself is 99 cents in the Amazon Kindle store, so check it out!
Like I said in the story, I didn't take any pictures of the making of the particular crossbow described as it completely slipped my mind at the time. Rather than put instructions on how to make the same crossbow twice in the same book, this section goes over how to make a smaller, more compact crossbow.
We'll begin with a little bit of information on selecting PVC pipe, as the bow portion will be made of it. Look for clean, unused PVC pipe in schedule 40. New pipe is best as it is less likely to be brittle and crack on you. On that note, PVC does not last forever in such a highly-stressed format and will need to be replaced after some time.

Let's start with a 12” length of 3/4” PVC pipe. PVC gets soft when heated, so heat will be used to flatten this pipe. Using an electric or gas range or heat gun, heat the entire pipe until it is easily flattened with finger pressure. Be sure to wear heat resistant gloves or use oven pads when handling hot pipe as it can cause severe burns.
Once the whole pipe is soft you can flatten it with a board by placing the pipe under the board and stepping on it, using your weight to flatten it. You could also heat half the pipe and flatten that, then heat the other side and flatten it to match the first.

With a straightedge and marker/pencil, mark the pipe 3” in from both ends and 1/2” from the center on both sides. Next, draw two straight lines connecting the bottom edge of the middle of the pipe to the middle of both ends. This will form the tapered shape of your crossbow prod (bow).

Heat up one of the ends of the pipe to the 3” mark. Once it is soft enough to bend but not soft enough to where it begins to puff up, bend I over an 8” pot to form the recurve. Hold it there with a potholder or heat resistant gloves until it sets. Once it sets run it under cold water to help it keep its new shape.

Repeat on the other end. This will give your bow a nice recurve to both limbs. Make sure that your second limb is close to matching the other. If the recurve is too uneven, the bow may end up folding when the crossbow is loaded.

Follow the lines you marked with a saw and cut the taper into the bow. Finish the bow by rounding the bottom edge with sandpaper to match the curve of the top edge. Make sure to not sand the center too much, as a lot of the bow's strength will come from the center being a full tube.

It is easier to see what I mean from the bottom. Notice how there is a slight gap on both limbs but not in the center. Make sure the center stays connected and intact, as the bow will become unstable if the bottom edge is cut through.

Now that the main body of the bow is finished, it is time to cut the nocks upon which the string will rest. Cut a 1/4” square out of each corner of the bow tips, resulting in a square pin like the one in the photo. Sand and smooth all sharp edges so that they won't cut into the string.

String the bow up with a string that is about an inch or so shorter than the bow. You want about an inch to two inches of space between the string and the bow, like in the picture. Paracord or heavy dacron line will work for this bow, just tie a loop into either end. Here's the bow from the bottom.

Here's the bow from the front. Notice how the bow limbs actually twist. This will help reduce the strain on the bow while it is used in the crossbow. It also keeps the string from hopping up, which will help prevent misfires.

Now is a good time to test the bow by pulling it back six inches. This bow should get around a 40 pound pull at six inches. This is what the finished bow should look like at full-draw. If you want a lighter bow (less power but easier to draw), start with a longer piece of pipe.

Let's get started on the tiller or stock of the crossbow. Cut a piece of 2x2 lumber 12” long. Make sure the piece is whole and not full of large knots. A couple knots are okay, just make sure they are only on one half of the board.

Find the center and cut straight into the board 5/16” deep. Then find the side of the board that will become your handle. This is where any funky grain or knots should go. Measure 5/16” on the handle side and cut at a 45 degree angle to meet the bottom of the first cut.

The resulting trigger notch should look like this. The side facing the front of the stock should be square and the side toward the handle should have a nice slope to it. This will keep the string from slipping and misfiring and from locking up.

Here's the trigger notch from the top. You can see how the left side (front) is square and sharp and the back (handle side) is angled. As a final touch, be sure to round the sharp corners but keep the edge sharp. Splinters may lift when the string rolls over sharp corners.

Carve a channel from the trigger notch to the front end. This will serve as the channel the bolt will sit in when the crossbow is loaded and fired. It helps to draw a straight line to follow before cutting the groove. The groove can be made with a router, but in lieu of that you can use a sheet of rough sandpaper wrapped around a block with a square edge.

The finished groove should be at least 1/4” wide and 1/8” deep. Too deep and the string can easily hop over the bolt and cause a misfire. Too shallow and the bolt may hop over the string or be ejected from the bow sideways.

Measure and mark 2” and 4” in from the front end (the two crosses). Next mark a curved line from 2” from the back end to the 4” cross you made. This curve should be about 1/2” high. Follow the picture above and mark a groove around the back end of the handle to one inch from the bottom edge. From there mark a curve up to about midway up the handle.

With a saw, cut along your marked lines. I prefer to deepen the curve near the butt end as it gives a more comfortable grip and gives a good place for the trigger lever to fit. Now is also the time to drill two 5/16” holes where your cross marks were. The one closest to the front can be made wider as it does not need to be perfectly round.

Drill a third hole right in the center of the trigger notch, making sure one edge lines up with the square edge. You can even go so far as to have the hole cut in to the square edge slightly. This will prevent the string from binding and getting caught between the trigger notch and the trigger pin.

With a belt sander, files or sandpaper, round the handle, blending it into the front of the stock. The front of the stock should be sanded down on both sides. The handle should fit comfortably in your hand.

It helps to slightly curve the handle inward about mid-handle. This will give a more secure grip when using the trigger, which will add some overall thickness to the handle. I like curving the butt of the handle, but this can be left square or even flared out.

Here's the handle from the bottom. The bottom edges of the handle should be rounded for comfort. Don't round the area at four inches in from the front of the handle or the trigger lever may slip.

Here's the handle from the back. The handle is left more or less square with the corners rounded off as opposed to completely round. This is a place where you can do whatever you want, as a variety of shapes can be had while still being comfortable.

from the front you can see the bolt channel as well as the alight curve on both sides of the stock. The stock can be left completely square if you desire. The curve is mostly for aesthetics as there is little room on this crossbow for a supporting hand under the stock.

Cut a 3/4” dowel 4” long. This will become the trigger lever. Oak, maple, birch and other dense woods make a good choice for this. If the wood is too soft, it could simply break against the pressure of the bowstring when the bow is fired.

Place the pin under the stock, lined up with the hole nearest center. Take note of where the curves in the handle correspond with the pin. It will take a little eyeballing and guesswork to figure out the right shape for the pin.

The front end should have a rounded lump cut into it, as this is where the lever will pivot on the handle. The lump should be rounded from the side, but flat as opposed to spherical on its face. This is a good shape for a trigger lever.

You can see that the lump allows the lever to pivot freely without touching the angled edge on the stock. The trigger lever also curves up to rest flush with the handle and trigger pin hole. The bottom edge of the trigger is curved slightly to give the trigger finger a good grip.

Once your stock and trigger are at this point, sand both of them until they are smooth. If you wish to put a finish or stain, now is the time to do so. If waterproofing the crossbow, make sure to do the next step before applying your finish.

Place the trigger lever under the stock, lining it up to where it will sit when assembled. Mark the spot on the lever that matches the location of the hole in the stock. This will be your pivot point. Cut a v groove or rounded groove, making sure it is deep enough for three strands of cord or rope to rest in without slipping.

With a fairly strong twine or rope, wrap the trigger lever to the stock twice. Pull this tight and make sure the lever will move up and down easily. If the twine slips, make the notch deeper.

Once the lever is tight and secure, bring the two loose ends over and tie them tightly into the groove. Follow up with two or three overhand knots which will lock the lever in place.

Trim the ends of the knot and the lever is finished. If you want, the cord bundle can be slid around and the knot hidden in the drilled hole. That will not only protect the knot but also give a cleaner appearance.

Take a 5/16” diameter dowel and fit it into the trigger hole. It will probably either be very snug or not fit at all. To make it fit, sand the dowel and make sure to keep it round. The trigger pin should slide easily in and out of the hole but without huge gaps that the string can slip into and bind.
Once the dowel fits, press it into the hole until the end lines up flush with the top of the stock. Mark the bottom of the dowel where it exits the bottom of the stock with a pen or pencil.

Drill a small, 1/16” hole into the place where you marked the dowel. This will serve as an anchor point to lash the lever and pin together.

Cut the dowel about 1/8” from the hole and round the bottom edge. The trigger pin is finished.

To attach the trigger pin, start by sliding it into the trigger hole, leaving the small hole you drilled exposed. The trigger lever should rest easily on the pin. If it slide off to one side rather than stay in the center, the lever may need to be flattened in that area.

With a needle and strong thread (35-50 pound test nylon or dacron line), wrap the lever three times, each time going through the hole in the pin.

After wrapping the lever, pull the thread tight and tie it with three or four overhand knots. Go ahead and try the trigger out. The pin should slide smoothly up and down. If it doesn't, simply slide the pin out and sand it until it fits.
The beauty of the lever having a notch instead of a drilled hole is that the lever can be taken off and put back on without untying the bindings.

Go ahead and pull the trigger up all the way until it rests flush with the stock, as if you were shooting the bow. Because of the extra 1/8” you put onto the dowel, the pin should stick out a little and not be flush.

With a file or sandpaper, cut an angle into the trigger pin so that the bottom lip goes over the top of the stock slightly (it's hard to see in this picture as the bow is tilted. The edge of the pin is actually flush with the stock). The angle on the top of the pin keeps the string from being pushed over backwards and binding the trigger pin.

This is what the pin should look like when the trigger is engaged. Because of the angled pin the string should automatically engage the trigger when you pull it over into the trigger notch.

With the trigger assembly finished it's now time to attach the bow to the stock. Hold the bow on the front of the stock by the string so you can see what kind of angle will be needed to seat the bow snugly. The non-tapered edge of the bow goes up.

With a pencil or pen mark the angle you saw, leaving a bit of a lip on the top of the stock. This lip needs to be below the channel so the bow does not get in the way of the bolt when it is fired.

Cut the angle and lip with a saw or file it down. See how the bow fits in the angled area and sits against the lip. The bow should sit like this, as it will twist when loaded. The gap will go away when the bow is lashed in place, so don't try to compensate for it. It is better for the string to sit too tightly into the stock rather than too loose.

Cut a 5/16” dowel 4 inches long. Slide it into the hole closest to the front of the stock. It should be loose so that the pin can be pulled out rather than the lashing undone to remove the bow.

Start lashing the bow to the stock by laying the end (about 10 inches) of your twine upon the pin and the rest of it goes over the bow. You twine should be at least 4 feet long if not longer.

Bring the twine over the front of the bow and under to the other pin. Make sure to keep the twine on the bow and to not block the bolt channel.

Bring the twine around the bottom of the pin and over the top. Repeat the first step you did, bringing the twine over the bow and around the other pin. Keep repeating the wrap, making sure to pull it tight as you go.

Once the bow and stock have been wrapped three or four times, tie three or four overhand knots in front of the bow. Make sure the bow sits flush with the stock with no gaps before tying the final knots. The wrap should be tight.

Here's a view of the drawn bow from the front. You can see the wrap and where it is knotted on the front. If you wanted to you could actually tie the knot behind one of the limbs to hide it from view.

Here's the finished bow. Notice how the string presses against the stock of the bow at rest. That helps the bow from misfiring by keeping the string on the stock throughout the draw.

To draw the bow, simply hold the handle in one hand, hook two fingers on the string on either side of the stock, and pull it up and into the trigger notch. The string should push the trigger pin down and engage the trigger mechanism. Another way to do this is to brace the butt of the crossbow against your hip or some leg and pull the string towards you with both hands.

When fully drawn, the bow should flex evenly in both limbs like this. If one side is slightly stiff, you can sand the underside of the limb to thin it and make both equal. All that's left is to put a bolt in the groove and fire the bow.

There are six inch plastic and aluminum bolts that can be purchased online fairly inexpensively and those will work in this crossbow. If you're like me and only plan on shooting boxes and soft targets for fun, a simple all-wood bolt works just fine. Start by cutting a 6” piece off of a 5/16” dowel. The dowel should be fairly straight, straight grained and free of knots. Next, either use a knife or pencil sharpener to create a point and round it slightly with sandpaper.

The vanes on this bolt are the same as described in the story. Simply take two strips of duct tape 2 and 1/2” long and sandwich the dowel with them, starting 1/4” from the end. Once the tape is smoothed down, cut a 45 degree angle into all four corners with a pair of scissors. Finish the vanes by cutting at a slight angle from back to front, and viola! You have a good little target bolt.

Congratulations! The crossbow is now finished! To use it, simply pull the bow to full draw and make sure the string is securely in the trigger notch. Next, place a bolt into the bolt channel (make sure it is lined up and secure) and take aim. When you are ready to shoot, simply pull the trigger lever up and the trigger pin will push the string up and fire the bow. Be careful with it, don't aim at people or animals as this is not a toy and can cause injury and damage. Enjoy!

No comments: