trefall,
Filling out your profile may greatly help you. It will first let us know where you are, and it will also tell us a bit about you such as age and life experience. There may be a smith near you who will offer some 1084 or other steel, as well as some help and advice.
While buying steel seems more expensive than using something you got for free, or for $2 at the scrap yard, it often takes far more propane/fuel and other products (abrasives, etc.) as well as much more time to convert them into a knife. Aldo sells a single bar of .125X1.5X48" 1084 for less than $15. Shipping adds about $10 to that, so for $25 you get enough steel to make at least six knives. That is about $4 a blade. The steel comes annealed and ready to make into a knife. The scrap comes....well you don't know how it comes....but it will surely need to be transformed into a piece that you can use for a knife. Its past life may have left it with internal issues that will make the knife poor or fail. Transforming it into a piece of knife stock may create more cracks and flaws. So it often comes down to using a $1 piece of God-knows-what or a $4 piece of steel that was made to be a good knife and is in perfect condition. Is your budget really that restricted?
The other side of the coin is that the steel is usually the lowest cost on a knife. You usually spend far more on abrasives, handle material, supplies ( glue, small tools, pin stock, etc.), sheath, and your own time ( it is worth something).
As to using exotic scrap, like a propeller shaft, it is almost always a bad idea. Some scrap types have alloys that are similar enough to allow a second life as a knife. Leaf and coil springs are some of these. Other things, like saw blades, drive shafts, files, and such, can be made of almost any alloy, and there is no guarantee that it will work at all. The charts and lists you will find on some internet sites are usually pure fantasy. There is no assurance that a leaf spring is 5160, or a file is 1095, or a saw blade is L6. Actually, there is a far better chance they are NOT those steels, but what they are is a mystery.
I commend you on wanting to build a foot powered grinder, but the input power from a treadle sewing machine may be far too low to get the sanding belts to grind the steel.
Most foot pump sewing machines only generate a low torque and fractional HP, around .10HP. The foot powered lathes of yore had much different power input methods. You might want to consider making a bicycle powered grinder, as the belt speeds and power input will be far greater. One guy peddles while another grinds.
Getting back to the what you get for what you spend, the small amount of power consumed by a 1HP motor running for about an hour ( about the average total ON time for a knife) is pretty low. That is .75 KilloWattHours. Look at your electric bill and see what you pay for your power, but the average is about $0.12 per KWH, so running a 1HP grinder for an hour costs about a dime. One cigarette or one beer cost more than that. The gas to drive two or three blocks may cost a dime.
The point is that the grinder motor power consumption is probably the lowest cost item in knifemaking. A used 1HP motor can often be found at the scrap yard or a yard sale. Even a 3/4HP motor powering a home built grinder will be far better than a foot treadle.
