Low Carbon Steel & Structural Shape Selection
Here is a brief overview of the materials you will use to make most of your non-contact, non-wear parts. You have likely already used several of these even if you're just starting out, so you may be inclined to skip this post, but there are a few niche materials scattered throughout for some specialty applications that you may want to peruse.
1018 Cold Rolled Steel (CRS)
This is generally available in 1/8" increments from 1/8" to 1 3/4" and then 1/4" increments above that. There are some gaps where it jumps by 1/2" increments in larger sizes, so you may want to check. It's available in bar stock, hex stock, and rounds. This is a good general purpose steel for your non-tool steel details with one exception: the cold forming process does establish a skin tension on the material, so if you are going to machine a surface from edge to edge, you can experience some warping and bowing of the material. Holes and slots are not an issue, and it's still not a bad choice even if you are machining a surface on a non-critical detail, but just realize that it could bow .010" [0.3mm] across even a few inches of the surface. If you want something more stable in a non-tool steel, hot rolled steel (HRS) is a better choice (see next paragraph). Also, CRS tends to warp significantly when you weld it, so HRS should be specified for all weldments. Tolerances are usually .003" [.08mm] to .006" [.15mm] under nominal. It is roughly $2-3/lb for full bars or random lengths, with a $3-5 cut charge per piece if you get it cut to length.
A36 Hot Rolled Steel Bar Stock (HRS)
OK, so I had to look up the "A36" part because no one actually types anything more than HRS, but I thought I'd be accurate in the post. This is the most commonly used hot rolled steel. Two downsides with HRS are that it has a scaly skin on it that needs machined or shot blasted off and it is a slightly weaker material than CRS (36,000psi yield strength versus 55,000psi), although the strength is seldom an issue in our business since we don't tend to try to minimize thickness to the absolute theoretical level because of the one-off nature of the parts. It comes in the same sizes as CRS and is roughly half the cost at $1-1.50/lb. It should be specified for ALL carbon steel weldments and, as described above, for details that have a lot of machining that could cause warpage when CRS is used. Size tolerance is typically +/- .005" [.13mm] from nominal until you get into widths larger than 6," which can increase the tolerance band a little.
A36 Hot Rolled Plate
Hot rolled steel is also the primary ferrous material used for large plates. I'm not even sure of the max size available, but I'm willing to bet you will never need to exceed it. Thicknesses start at 1/4," but it's a scaly metal that isn't very flat, so in 90% of the situations, you'll be machining it. For that reason, the practical minimum thickness I'd consider using is 1/2" if any precision is involved, and even that is pretty thin. Max thickness is over 12." See posts on bases and weldments for more application specific information. The plate can be delivered cut to some nominal width by length, or it can be brought in with semi-complex external shapes and internal cutouts via laser cutting, waterjet cutting, or torch cutting, depending on the thickness. Just realize you will be paying for the rectangular piece that encompasses the shape you've requested. Cost is around $0.80-1.00/lb plus there may be a charge for burnout/laser/waterjet of the shape.
Structural Shapes
Structural shapes are a staple in our industry. While there are I-Beams and angles and U-channel shapes available, the primary shape you will use is the square or sometimes rectangular tube. This is usually A36 Hot Rolled "seamless" (in quotes because there IS a seam on the inside, so that makes using the inside of the tube as a guide rather difficult). I would say "typical" is 1/4 wall thickness on 2" tubes and above, but obviously that's application specific. See my posts on weldments and bases for additional implementation thoughts, but just know that in shorter lengths (under 4-5'), this stuff is really stiff and strong. What looks "right" on your screen will likely be built like a tank once you see it on the floor. Beyond running FEA on your designs, you will get a feel for what's appropriate before too long.
Specialty Low Carbon Steels
There are a couple of nice products that you can use that will save a lot of time and machining costs in the right application, so I thought I'd mention them here.
Turned, Ground, and Polished (TGP) Shafting
This is non-hardened shafting that can be ordered in 1018, 1045, and 1141 material. It runs +0/-.001"[.13mm] tolerance for sizes under 1.5" and actually gets more accurate (+0/-.0005" [.01mm]) for larger diameters. As the name implies, it is polished to a mirror finish that is similar to purchased hardened shafting that is used with bearings and bushings. Because it is soft, it would wear too much to be used in a high friction situation (like with a bushing), but is ideal for bearing situations where there is no movement between the inner bearing race and the shaft. Selective areas of it could be hardened for the application as described in my heat treat post.
Drawn Over Mandrel (DOM) Shafting
How do you get a very long, very accurate hole through a shaft without it costing a fortune? Let someone else do it for you! Drawn over Mandrel shafting is the answer. It comes in an almost unlimited combination of OD and ID. Tolerances are slightly oversize (.005" [.13mm]) on the OD and slightly undersize (.005" [.13mm]) on the ID with levels of concentricity within that tolerance band, so if the hole is chased by a reamer, it will stay true to the OD. Again, it's a soft shaft, so it would have to be nitrided or carburized to harden it (see heat treat post), but a nice option for many applications.
"Leaded CRS" (C12L14)
This is a seldom used material, but I've had good success with it for applications like specialty manifolds that require absolute minimum volume but need to be long in order to mount valves and fittings. The addition of lead allows small holes to be drilled over long distances with a reduced chance of the bit "walking" or breaking off. It doesn't have the strength of regular CRS, so it shouldn't be freely substituted unless the situation calls for it.
Keystock
A lot of times, people will just use 1018 CRS for keystock, but remember from the paragraph above, that it actually runs .003-.006" [.08-.15mm] undersized. When you are trying for precision location in a force bearing situation, or it's a key on a shaft with a large hub, that small amount of clearance can actually translate into some pretty big positional errors. So, if it's a non-critical application, 1018 is ok, but they do make specific keystock that is either undersize (+0/-.002 [.05mm]), nominal (+/-.0005 [.01mm]) or oversized (+.002/-0 [.05mm]).