Bread and butter

They will do some machining if you ask them, but sheet metal has always been their bread and butter.

Source: wiktionary

“There’s a big lockout at the foundry.” / “Lockout, what!” cried several. / “Sure as blazes, boys; just when the procession was passin’ I takes a squint at the big foundry door, and there I seen it writ, as clear as sunshine with a hole through it, on a bit o’ paper, that the ould foundry’s shut after Sathurday next, till further notice. Divil a bit less, but I’m shure them’s the very words.” / Silence fell on the crowd in the tobacco smoke. Most of them were foundry-men and had families. Their bread and butter were at stake.

Source: wiktionary

Every escort has at least one or two regular clients; they are our bread and butter.

Source: wiktionary

What the nation, like the man, earns for itself by the honest labor of its people—when a man exerts himself to anything in an honorable calling—he is said to be earning his bread and butter, which includes his food and clothes and house-rent and all the rest of the thing [sic] he pays for. Now, if a man does not earn his bread and butter, he must either have it given to him as a gift, or steal it, as our forefathers nearly all did, honest, good people, too, as they were. ... It is worth while examining as to whether we earn all our bread and butter ... We ought to look after the bread and butter on our farms, and see that we eat the best bread and butter.

Source: wiktionary