Rolle's Dad's Method

Rolle's Dad's Method (RDM) is a brilliant way to accurately measure the alignment of a spindle to the bed/column, and is extremely useful for aligning the head on the mini mill (with the column removed from the base) and the headstock on the mini lathe. It's great because it takes run-out completely out of the equation and only measures the alignment. However, I haven't found a description which I felt practically explained the method to me, and therefore it took me a little while to figure out what it meant. Here's how I do it:

1. Chuck as straight and smooth of a rod in the spindle as you can. Using RDM the smoothness and straightness of the rod doesn't actually matter, but it does make doing the measurements easier.
2. On a lathe mount a DTI on the carriage so it can indicate the rod. On a mini mill I'll move the head to the bottom of the column and then mount the DTI on the column right below the head so it can indicate the rod. You check the X and Y alignment separately, determined by whether you're indicating the side or the top of the rod.
3. You want to indicate two parts of the rod as far apart as possible. On the lathe this means indicating it with the carriage at the chuck, and then as far to the tailstock as possible. On the mini mill it means indicating with the head all the way down and then all the way up. Generally I can get about 7-8" of separation. I'll mark the two points with a Sharpie so I can easily hit the same spots.
4. With the DTI at the head, I turn the spindle by hand and observe the DTI dial, and adjust the dial so the needle is traveling the exact same amount above and below zero; I call this "average zero".
5. Then, without touching the dial, I go to the opposite end of the rod. With the lathe this means moving carriage and with the mini mill the head.
6. Again, I turn the spindle by hand and observe the DTI. The needle will usually move a lot more, but that's ok. I mentally determine where the new average zero would be. The difference between the old average zero and your new average zero is how far out of the alignment you are. Again, it doesn't matter how much the needle moves, all the matters is where your average zeros are.

For example, I'm going to check the horizontal headstock alignment on my mini lathe. I chuck the rod and mount my DTI so it's indicating the side of the rod and move the carriage with the DTI on it all the way to the chuck. I turn the spindle and see the needle is moving a total of .004", and I turn the dial until the needle is traveling exactly 0.002" above zero and 0.002" below zero. I now have my average zero.

Average zero set at the headstock. You can see the Sharpie on the rod marking where I measured.

Without touching the indicator, I move the carriage to the end of the bed. I again turn the spindle by hand and observe the DTI. It's now traveling 0.004" below zero and 0.008" above zero. Mentally I calculate the new average zero is +0.002" on the dial. That means the headstock is 0.002" out of alignment with the bed. If the needle had actually traveled 0.006" below zero and 0.006" above zero it would have meant my headstock was perfectly aligned with the bed.

The needle at average zero at the far end of the bed.
It's reading +0.002", so my headstock is 0.002" out of alignment with the bed. 

Remember, the key is the difference between your average zeroes.