Bulb Auger attached to a drill, very fast to dig perfect holes.

No you should plant the bulbs apart.

Sharon

Here is a link that might be useful: Bulb Auger

If you have hard clay soil even the bulb auger can be hard to use. It can jam in the soil and take going forward and backing up multiple times. Using one of those broke my cordless drill: 3 holes burned out the motor. I found out that hard way: you have to use a corded drill because it has more power. Also not suitable if you have other bulbs already in the soil, unless you know exactly where those are. I still have it but have never used it since the first time I planted that bed and needed 200 holes for tulips. The soil still fell back in, and I had to use a trowel to scoop it out. But easier than hacking into hard clay with just the trowel.

Lilies like many other bulbs look better in groups of 3-5 plants. I planted about 400 bulbs last fall in heavy clay soil. About 100 Lilies. I made holes with a shovel and placed 3-6 bulbs into each hole spacing them 6-8" apart. Also, 3-7" is not enough. You have to make deeper holes to loosen the soil underneath bulbs. I'd say about 12" would do this.
Planting Lilies

I have the Lee Valley bulb auger, and I aggree that you have to use a corded drill for the power. I also find that I can't seem to tighten the chuck of the drill enough to keep the auger from spinning in the drill rather than in my hard soil! Every year though, I try again thinking that it will work better since the soil is either dryer or wetter perhaps than the year before! Then after much frustration, I go and get my shovel to plant the rest of my bulbs :)!

Janet

Use a 1/2" corded drill, better torque.
File three flat spots evenly spaced on the shaft of the auger that match the three jaws of the drill to keep it from slipping.

and really you should not be planting lilies in hard clay soil, check out lasagna gardening ;)

I dig my bulb planting holes with a post hole digger.