Material choice?

[KellyLee]

Hello, just trying to dive in to all this and learning as I go. I'm a complete novice and have everything to learn.

Anyway, as from my introduction post, I'm working on a project where I need to make an adapter. I was originally thinking of making the part from 6061-T651, and a slightly different overall design, but realized that I could simplify the part making it easier to machine and thinner (for space considerations) using steel. However, material selection is overwhelming to me at the moment and could use some input and guidance.

Looking at my 3D model below, the center hole will be tapped to a M12x1.5 internal thread. This part, when assembled, will be threaded on to a small crankshaft and torqued down to 47ft-lbs. My question, which steel would be the easiest to machine yet strong enough to hold that amount of torque on the threads? Would hot rolled A36 work for this or do I need to step up to something like 4140 or ??? The part is a non-wear piece other than installation and removal but can be spun up to 10,000 rpm and will be used to keep the much heavier flywheel in place when torqued down.

 

[extropic]

Without going through all the numbers, I suggest going with a material with about 120K psi tensile strength (4140 PreHard)

A quick Google search came up with (approximately) these values: Tensile strength for 8.8 grade material = 800 N/mm² = 116K psi.
Recommended torque for 8.8 12mm x 1.5mm = 62ft-lbs (unplated).

If this part is a mounting flange for a 10K rpm flywheel, stripping the threads may not be your biggest design challenge.
At first blush, and without knowing anything else about your design/function, you might want to consider incorporating a generous fillet radius at the transition from the hub to the disk, even if you have to add spotfaces for the fasteners.

 

[KellyLee]

Thank you. I'm just not smart enough for proper material selection...yet! Yes, the 3D model doesn't show that I do plan to add an 8mm radius at the transition point between the main disc section and the stub. I don't know how to do a fillet radius in CAD yet but, again, I will be learning!

EDIT: A little more input on what this part does. It adapts a sprag to a factory flywheel by replacing the M12x1.5 factory shouldered nut with this adapter. Essentially, this adapter becomes the nut. The torque spec for the factory nut is 47ft-lbs. The sprag is roughly 74x11mm.

 

[Illinoyance]

Almost any steel will work. See what you can scrounge.

 

[Chewy]

Almost any steel will work. See what you can scrounge.

With that small of a part, I agree. If you pick the 4140, you can probably turn it in one shot and it will be fine. You can go with A36 (HRS) and do the same. If you go with 1018 (CRS), it will come out whoppy jawed unless you come to an understanding with it. Get an oversized piece and turn in so it is about 1/4" bigger in all dimensions. Once you start cutting on CRS, it relives all of of it's stresses. You will be surprised how crooked it will be. Take the part, heat it up real hot, not cherry red. MAP gas will do. Put it on a big vise or anvil with a good size hammer, teach it a lesson. You don't have to leave big dents in it, but not a love tap either. Do this once or twice and then let cool. There are all sorts of formulas in the books for stress releiving metal, but this will work at home and it is pretty simple. CRS is hit or miss on how straight a piece is after machining it. Some stuff is perfectly usable, some I heat and beat and some needs to pressed flat. I like to let it sit in the sun for a week or two, but that is not always possible. There is a reason most of the recomendations here recommend stress proof steel.