Posted by Steve Clowes on 06/01/2021 22:20:04:
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Looking through some threads has given me some ideas ,so i'm looking at buying some material "stock". Anyway my laboured question is what sizes (round bar, flat bar, sheet), material (steel, brass, aluminium) etc would make a good store of stock for model engineering
Sorry for such a long 1st post. I promise i won't bore you like this again 
Steve
No need to apologise, it's a good question.
Based on my experience and reading what others have said about setting up a workshop, I've decided the best approach is personal. It depends on you Steve!
One way is to strive for perfection from the get go. Purpose built workshop with carefully planned benches and storage plus lathe, mill, pillar drill, compressor, cutters, measuring gear, and a large stock of metal, wood and plastics in all possible sizes. Wonderful!
Not how I approach the hobby. Unsure of what exactly I was going to make (I'm not a modeller or restorer), I buy tools and materials piecemeal as needed for each project. I did decide I was metric rather than imperial, but apart from that kept an open mind. Hand-tools and a pillar drill soon proved inadequate, and I realised I needed a small lathe, then a big lathe, a milling machine and – most important – a band-saw. (Because cutting metal by hand is a boring, tiresome timewaster!)
First mistake was trying to learn on scrap metal. Very disappointing results, which turned out to be because my scrap collection was all unsuitable, and DIY store metal ain't much better. Soft squishy Aluminium Alloys, work-hardening stainless steel and other obnoxious stuff. I was unlucky because all of it was unfit for purpose. Much better to buy known metal where the description says it's suitable for machining.
To make a start and get a feel for the lathe whilst keeping costs down I'd buy a metre each of 10mm-ish diameter Brass, Ordinary Steel 'EN3', Machineable mild-steel 'EN1a' or 'EN1aPb', and a machinable Aluminium alloy.
Having tirned, threaded, faced, tapered and bored the above, make something! I found Stewart Hart's Potty Mill engine to be an excellent beginner challenge, being hard enough to develop new skills and make me think, but not so difficult as to reduce me to tears. No castings, everything is fabricated from stock size metal. The maker has to shape different materials, in some places accurately, and then assemble and debug the engine so it runs. It taught me a lot.
Building the engine involved studying the plan and identifying the metals and sizes needed to make it, A mild-steel base-plate cut from rolled-strip; brass pillars, steel runners, aluminium cylinder, brass piston and bearings, silver steel axle. So having found the cylinder was made of 35mm diameter aluminium rod, I ordered a metre, used about 50mm to make the engine and kept the rest as stock. Four years later, I'm down to my last 150mm and need to buy more. Same with the other sizes; identify what's needed, and over buy moderately to build stock.
Turns out I use a relatively small number of stock sizes and shapes. More rod than strip, and more strip than heavy section. Not much Hex or angle. I'm more likely to turn 10mm and bigger diameters than below, I use more Aluminium than steel, but Brass is popular too. It would have been a mistake for me to order a lorry load of expensive metal because I don't use most sizes.
But that's just me: other workshops operate differently – it depends on what you make, how much you specialise, and how much not having metal and tools to hand annoys you. Clockmakers major in small Brass, motorbike fixers go more for mid-sized steel and aluminium. I have a local metal Emporium and can get metal in an hour or two, so running out isn't a crisis. I'd hold more stock if I had to order everything online and wait for it to be delivered.
Dave