Having Chris J referred to the specification numbers I quote for my projects I feel I should comment. There are hundreds of steel specifications but only a handful will ever find a place in the home workshop and of the handful two are likely to cover 99% of all steels required in the majority of workshops.
230M07 (very close to EN1A in the earlier spec) is only available in rounds, hexagons and a limited number of square sizes, and is predominantly a free cutting steel intended for use on the lathe. A version of this with a small amount of lead added machines even better and is known as 230M07Pb. Unfortunately, very few of those supplying the home workshop stock this.
080A15( previously known as “good commercial quality&rdquo
is available in the above shapes but also in a very wide range of flat sizes. Because of this it is a material for use on the milling machine. However it machines less well than 230M07 so is not ideal for turning, though of course can be.
070M20 (very close to EN3B in the earlier spec) is a number I now quote in preference to 080A15. This, whilst almost identical for most applications, is because it is more widely available from those supplying the home workshop.
This I feel is all you need to know for the vast majority of work you undertake, that is unless your workshop activities becomes very specialised.
A little more thought will be needed if you get into welding or case hardening which is possible with all the above (except when leaded) but there are preferences and would advise that the subject is researched when such a need arises. Similarly, if you start making parts that will be highly stressed, or need to be hardened, then cross that bridge when you come to it.
I terms of machinability, Macreadys in their catalogue use 230M07 as a base for which they quote a value of 100, for 230M07Pb the value is 130 and for 080A15 and 070M20 a value of 55.
Just to avoid confusion, Macreadys is a large supplier to industry and does not supply small quantities.
I would advise that you do purchase materials to known specifications and mark any left overs with some form of identification. Do Not purchase from suppliers who just state bright mild steel as you have no idea what is being supplied. Worse still, you will end up with some materials that machine well and some that machine poorly with out any knowledge of why.
We have seen questions to this forum where individuals ask why they are not getting a good result but do not know the steel they are using. Comparing 100 with 55 above shows that there can be quite a difference even with commonly available steels.
Hope this help ChrisJ
Harold