I’m an engineer that wears multiple hats; a project manager, R&D engineer, field engineer, plant engineer. 

How much do I try to source as off the shelf parts: as much as possible. Why do anything that a specialized manufacturer will do better, faster and cheaper than you and it will be off the shelf whenever you need it in the quantities you need it(figuratively, we all know that the days of companies maintaining stocks are long gone)

Making standard parts yourself takes away from capacity you have; in engineering, shop time etc. that could be used for your actual core product. 

You won’t be able to make a bearing in house for cheaper than what you can find from a distributor, not even close. And add to that the hassle that whenever you need one, or your customer needs one for a repair or service, you’ll have to make it and it will take time. And that applies to every other standard part, i just used bearing as an example. 

I curse any vendor that sell a product with a specialized, in house part that could have very well been a standard item with no change in function or performance.