If you plan on using the cups or the screw in nipples on PVC pipe (or even grey polyethlene pipe and fittings) then go to your local hardware store and get a 1/8 inch pipe thread tap. It is actually sold as 1/8-27npi and uses a 21/64 drill bit. I use a 5/16 drill bit in the plastic which is about 1/64 smaller and makes a more agressive thread in the plastic, if you were doing it in metal you would need the 21/64 so you weren't removing a bunch of metal which can be difficult. The 5/16 isn't bad at all in the plastic.
Speaking of the grey polyethlene pipe, it is sold at Lowe's and there are several different fittings available. Home Depot in my area has discontinued the product for some reason, it must not have been popular. A 100ft roll of 3/4 pipe, item #24166 model #2-75100100 sells for only $18.16 around the Atlanta market area. The pipe can be put together without any kind of clamp as long as you are using a gravity feed or low pressure type system. I use a pair of PVC cutters to cut the pipe and heat the ends with a heat gun (from Harbor Freight) and it quickly and effortlessly slids over the fittings. With a roll of pipe several tees, elbows, end plugs drilled and taped to accept nipples or water cups it doesn't take long at all to build most any water system you can imagine in your coop. I use wire ties to attach the pipe and fittings to wood brackets or walls and it makes for a very inexpensive set-up that will last for years. If something happens it is real easy to repair with a coupling or an end plug to cut water off. Fittings are available with both male and female threads so you can attach valves and other fittings that are PVC or metal. If the temps get real low and a freeze comes along the pipe just expands but doesn't bust and I have seen once it poped off a fitting because of ice build up and was just waiting to thaw out before it could be reattached.
