Well I'm definitely not an acccountant and taking my advice on financial advice would be risky at best. But, since you asked here's what I know, or was told by my accountant.
Basically anything that is tangible in your finished product, (you can see it,touch it, etc...) is a material and intangible things, (plasma consumables, welding gas, solvents etc..) are supplies.
And as far as the practice or junked parts they told me definitely write off materials but after that it gets a little more complicated. If you are paying someone a wage to make those parts then that is a deductible expense. If you are a one ring circus like me they said claiming for your own time can get touchy if you are audited.
And that's my story for what it's worth, which isn't much!