Accurately measuring distance using ultrasonic rangefinder
Short answer: 

Probably not.

Long answer: 

Ultrasonic sensors are very sensive to the shape and surface of the object distance to which is being measured. If the surface is relatively flat and very large (book, wall etc.) and is parallel to the sensor lineup you could have some good results after calibration and taking the average of many measurements. However not even then an error of 1cm or less can be expected at all times on a distance of 50cm. Additionally, if your surface if uneven with irregularities of +-1cm or so, you will have additional fault introduced. 
From my experience you will have hard time implementing a system of this precision cheaply using ultra-sound. My advice would be to lower the expectations of the precision of the system. You could also try out some cheap infra-red sensors, they can be more accurate under some circumstances. 

A general hint is that the object you want to measure the distance to should have a decent area exposed to the sensor. At a distance of 50cm using a cheap sensor we are talking at least 5-10 centimeters or more in order to get any reasonable reading.