I make no claims to the idea behind the actual cutting "magic" as I found the idea on YouTube thanks to Dan Rohas at Greenpowerscience.com. I am sure there are others using this technique too so I credit them also.
Materials you need:
1. Bottles. I say bottles, because it may take a few tries to get the results you want. It took me 3 to get it right.
2. Glass cutter...the manual kind you can find at HomeDepot. Its important that it be made of metal and it should have a nice flat spot behind the head. You need this so that it can be clamped in place.
3. A flat base board approximately 1/2-3/4 thick x 4" wide x 12" long. The size is not critical as all it has to be is a little bigger than the bottle you are trying to cut.. You will also need a few pieces of scrap wood to use as edging for the base board.
4. "C" clamp(s).
5. A bench vise to secure your jig. If you don't have one you can use additional "C" clamps to secure your jig to a strong table.
6. A 2 quart pot and another container for the ice bath plus water and ice.
Step 1: Making the Jig
The jig I built came from scrap lumber I had laying around. It just needs to be able to support the bottle while a reasonable amount of pressure is applied to it.
1. Start with the base board which should be a little larger than the bottle you wish to cut. For a coke bottle its about 3" x 14".
2. Attach another piece of wood to create an edge along the long side of the base board. In my case the back edge extends about 1/2 above the base of the jig. The edge creates a cradle so that you can rotate the bottle during the cut. You want it to be high enough so the bottle does not slip off the jig while you firmly rotate the bottle from the top.
3. Attach the final piece of wood on one of the short edges of the base board. This piece keeps the bottle from moving latterly and will allow you to make 1 continuous cut with the class cutter. I used screws and glue and make it extra secure.
4. With the jig created, you should now be able to push the bottle into the corner of the jig and smoothly rotate it around its axis.
Step 2: Mounting the Glass Cutter
The trick to cutting a Coke bottle is to use one of the 2 smooth areas of the bottle to make the cut. Coke bottles have deep ripples in the glass making it impossible to make the cut using this technique. Fortunately there is a non-ripple section about 1/2" from the bottom of the bottle and another just below the label. I will be cutting at the bottom of these bottles for my project.
You want to position the bottle on the jig so that you have a reasonable idea of the glass cutter location.
1. Clamp the jig securely in a vise or to a table so that you have space for a "C" clamp to secure the glass cutter to the jig's base.
2. Place the bottle so that its in the pushed into the corner of the jig squarely.
3. Place the glass cutter with the "teeth" in the head downward so that the cutter wheel is in the up position against the jig's base.
4. With the bottle in place, you want the cutter wheel to completely support the bottle so that the bottle is not touching the jig's base. The bottle should rest on wheel and against the jig's back edge on the long side.
5. Adjust the location of the glass cutter so that it will score the bottle in the desired location while the bottle is resting in the jig's pocket (created by the end and back edges). Clamp the glass cutter securly in place.
6. Make sure your jig is secure. You don't want it to slip while your are pressing down on a glass bottle.
Step 3: Making the Cut
If you have never cut glass before this may take a few tries before you get it right.
Before starting the cut I used a sharpie to help me identity the start and end of the cuts. Put the marks about 1/4" apart along the circumference of the proposed cut line. These will help you to see when the cut is almost complete and so that you don't "double" score the glass.
Cutting Glass 101
- Put a drop of oil on the glass cutter wheel. Keep the cutter wheel clean and oiled while in storage.
- When cutting glass, score the glass 1 time and 1 time only. Double scoring glass will usually ruin the edge or cause the cut to "run" away at best. At worst the glass could shatter in your hand!
- When you cut glass its called "scoring" and it makes a distinctive "crackling" like sound. This sound will be uniform in intensity if the cut is uniform. This is what you want and it takes practice to get right. If you don't get this either your glass cutter is dull or you are not pressing hard enough.
- Practice. After a couple of tries you should get an idea of the amount of pressure it takes to make a nice uniform cut.
With the bottle firmly in the pocket of the jig and resting on the cutter edge...
- Starting at the marks you put on the bottle, with firm downward pressure, smoothly rotate the bottle.
- If you have to stop, make sure the bottle does not shift position.
- Continue until you have completed the cut by watching the marks.
- Bring the cut all the way around without double scoring the glass.
- When your are done there should be a nice uniform line around the bottle.
Step 4: The Fun Part
The only thing left is actually breaking the glass.
1. Start with a sauce pan on the stove filled about 1/2 full of hot tap water. You want enough water in the pan to cover the score line in the bottle when the bottle is submerged in the pan.
2. While the water is coming to a boil, make a ice bath in another container. Fill it with ice and water so it too will be able to cover the score line when the bottle is submerged.
3. Place the bottle in the cold bath while the water comes up to a boil. You could also place the bottle in a freezer if you have more time.
4. Once the water is at a rolling boil pull the pan off the stove and place it on a soft cloth along with the cold bath.
5. Immediately place the now cold bottle into the boiling water. Leave it there for at least 15 seconds or so. You may hear the glass cracking...this is good.
6. Carefully move the glass back to the cold path for about the same amount of time. I say carefully because eventually the glass is going to break free and you don't want the broken piece hitting the floor mid-move.
7. Repeat this process until the glass breaks cleanly away. In my trials this took 3-4 cycles.
Note: If you are making a score near the center of the bottle it may not be safe or possible to get water deep enough for the hot and cold baths. In that case you will be carefully pouring the hot and cold water over the bottle. If you go this route make sure you put a towel in the sink to cushion the bottle when it breaks. This can and will happen suddenly so a little caution is highly advised.
Step 5: The Result
The cuts I have been able to make are nearly perfect. I noticed I get a little bump on each one near the manufacturing seam or near the start position of the score. This is were the glass fracture deviates from the score line. The cut is still useful for me because this fracture actually came back and rejoined the score line instead of heading off making a new break in the glass. If this happens to you should discard the glass and start over since any fracture in the glass will eventually continue to "run" making it hazardous. This is the type of thing that happens when a stone hits a windshield...eventually a small crack will run and ruin the entire thing.
Your results will vary and it depends on how cleanly your scored the the glass. It may also depend on the type of bottle you are cutting. For me, Coke bottles where tough due to their thickness plus the ripples limit the possible cut locations. For example, thinner bottles may break during the first water bath swap instead of taking 3-4 swaps like it did for me.
As with any project involving glass, make sure you finish the project off by sanding the sharp edges with sandpaper to remove the extremely sharp edges.