To build an effective steam engine, you need a precisely ground piston and a precisely bored cylinder, otherwise you can't get useful working pressures. Grinding the piston is laborious but straightforward - you just need two centers and a cutting tool to achieve a rotationally symmetrical part. Boring the cylinder is much more challenging, because you need to cut a very wide, very deep, very straight hole into a huge lump of iron. Cutting that hole requires a very rigid, very powerful machine.
John Wilkinson developed an effective boring machine in 1774 for making cannons, but it was limited in speed and capacity by the water wheel that powered it. The next year, the Boulton & Watt company was founded, building stationary steam engines with cylinders made by Wilkinson. Wilkinson received the second steam engine built by Boulton & Watt, which he used to power a bigger and faster cylinder boring machine, which he used to build more and bigger steam engine cylinders for Boulton & Watt.
