Drawing on the excellent instructables on the topic, I thought I'd try recycling the many (too many) empty whiskey bottles I've been collecting into drinking glasses. It's been a bit frustrating and I've already put too much time into it with little to show in return. Truthfully, I think I'll just buy a nice set of whiskey glasses and be done with it. Nevertheless, this short instructable is my attempt to share some tips I picked up in the hopes that it will help others find more success.
Step 1: The Cutting Jig
Using scraps, I cobbled together a makeshift cutting jig. I wanted to be able to adjust the length or height of the glass but was too lazy to rout a T-slot or fashion a sliding mechanism. The clamped scrap 2x4 works well enough but if it's not square to the base of the bottle it can effect the score line causing unevenness. There are far more elegant examples to follow if you're not as lazy as I am. The casters are the biggest problem with this jig. Because they're "sloppy" - a lot of play in the wheels - the bottle doesn't turn smoothly and shifts resulting in a score line that's less than perfect. This creates an uneven cut which means more time spent grinding or sanding. If I were to do this again, I'd look for designs that use skateboard wheel bearings or other less sloppy supports.
I still don't know how hard to push down on the glass cutter when turning the bottle. I think I've had most success with the lightest touch that still creates a noticeable score line but I really don't know.
Step 2: Chasing a Smooth Cut
As is often the case, my first attempt at cutting a bottle went well giving me a false sense of hope. After using the jig to score a wine bottle I then pored boiling hot water from a teapot over the score line, immediately followed by cold water from the faucet, back to hot water, etc. The bottle broke cleanly and I was on my way. Successive tries however, sucked. As you can see in a couple of the pictures, bottles simply refused to break evenly along their score lines. (I could use pliers and nibble off the edges but they would still be jagged.)
Granted, I know nothing about glass, its inherent structure, how scoring actually works, etc. but I think the problem was that I was holding the bottle at one end and the weight of the unsupported end influenced how it broke. Once I switched to dunking bottles in hot and then cold water, I had much more success. I also noticed that the longer it took, the better the outcome. Again, knowing nothing, I assume this has something to do with the temperature differential - the process would take longer if I used tap water vs ice water and the result would be more consistent. (I wonder if using a tapper (?) to actually separate the the glass once the break line is initiated would be better?) Of course, this could all be hogwash but give it a try if you're having trouble.
Step 3: Smoothing the Edges
Obviously, an even cut makes all the difference when it comes to smoothing the edges. Even so, there will be imperfections. I have found the quickest way for an amateur like me to address these imperfections is with a belt sander. A belt sander and then hand sanding with successively higher grits leads to a nice even and smooth finish. The whole process takes too long for me to persist though. That, and concern about airborne glass particles (even with a respirator) has me in the market for a nice set of whiskey glasses.
Best of luck.