Steel Selection for Arbor

[Jaguarman]

Hi Folks,

I'm planning on making a small milling machine arbor and would appreciate input as to the best steel to use. I'm presently leaning toward prehardened 4140, but really need some expert advice. Thanks in advance.

 

[Metalguy]

First, how large is this arbor? Do you have a hardness number in mind that you want? How would you machine/grind a hardened part?

Do you need any corrosion resistance?

 

[Jaguarman]

Thanks for the response Metalguy. The diameter of the arbor would be 1 1/4" by 12" long. I believe the hardness of commercially sold arbors is about 52C on the Rockwell scale. My interest in prehardened 4140 is that I do not have the means to harden a 12" piece, and was under the impression that prehardened was Rockwell 28-32C and would therefore still be machinable. Am I wrong about this?

 

[arunmrao]

At 52 Rc trying to work with the material will be frustrating. Rather advise that buy in soft condition and work with your material and the harden and temper. Why do you need such high hardness/ stifness RC52. There will be other problems associated .

 

[Jaguarman]

No, I could never machine 52Rc. My question is whether prehardened 4140 comes with a hardness of 28 to 32Rc, and since I don't have the facility to harden a 12" length, would prehardened 4140 at 28-32Rc be acceptable to use as a milling arbor? Thanks again.

 

[Metalguy]

Jaguarman,
There are quite a few options you can use, depending on various things. *If* you don't require high precision (not too likely for an arbor), OR if you are willing to have some finish grinding done (by you or a shop), you can buy either 17-4 or 17-7 PH stainless steel bar in the solution-annealed condition. After machining, all it needs is an hour at 900 deg. F. and you'll have Rc 44 for the 17-4, or Rc47 for 17-7. But it will warp slightly, so finish grinding will be required. If you can heat it to 900, this may be the way to go.

If not, the softer 4140 *may* be OK-depending on the amount of stress it will see. Another problem would likely be with any threads on it. For many tightening sequences, involving years of use, you'd want the threads to be harder-thus the Rc52 of the comm. arbors.

You should be able to find 4140 bar in various heat treated conditions and hardnesses. You can even save $$ and just buy it "normalized", and it will be about Rc30.



Arunmrao,
He merely stated that Rc52 is what some comm. arbors are made to. BTW, the stiffness isn't affected by hardness or heat treatment-it's a physical property of the material itself.

 

[Jaguarman]

Thanks for the advice Metalman. I'll stick with the 4140 for the arbor, but would like to try the 17-7 PH stainless for some smaller parts that I can harden. One last question, what's the best way to prevent scaling and decarbonization of the 17-7PH during heating? I've seen a product called Tool Wrap, but it's a bit pricey.

 

[Metalguy]

That's another one of the good things about using 17-4 and 17-7 PH--the low 900 deg F HT merely turns them blue/purple-color change only, no scaling! Doesn't get much better than this!

 

[ornerynorsk]

I have made literally hundreds of custom milling arbors, special "one-time use" arbors for special jobs, out of stressproof, 1045, O-1, and yes, even 12L14! It all depends on how heavy a milling job that you intend to do. Pre-hard 4140 will do a fantastic job for an arbor, and yes you are correct, it should come in at about 28 to 32 Rc which is still very machineable.

 

[Jaguarman]

Thanks guys for the speedy responses. Your expert advice is very much appreciated.