920 spindle speeds & "machining a backplate for concentricity."

[FrJacob]

So I suddenly needed a three-jaw chuck, which I have, but without a back plate. Got one off Amazon of the right size, but one that “must be machined for concentricity” to fit into the back of my chuck.
I have only ever turned briar or cherry. According to the machinist's table on this, the cutting speed for steel is 100 SFPM. And the formula is SFM [100] x 4 = 400, divided by the diameter of what you’re going to cut. In this case that is 2.875” = 139.13 rpm. Right? The lowest speed config. for my 920 is 130 rpm, so I guess that's the spindle speed I should use. I have a cutting tool with a carbide insert, so I guess I'll use that.
Have I messed up yet?
And here is my question to check my understanding: how come a metal lathe starts with such a “high” rpm? Is the explanation that most of the steel stock the 920 is “actually” intended for is of a smaller diameter?
Whenever I’m confused like this I tend to think I’m missing something, which tends to make me nervous as to whether I’m thinking straight or maybe there’s a gap in my am understanding. I’ve learned to be cautious about that where lathes are concerned. Can somebody check me out?

[caskwith]

Did you check the speed for carbide? Usually carbide tools are used at a much higher speed than HSS.

[FrJacob]

Right. So even though I’m cutting steel (semi-steel cast iron I guess), the low speed I’m cutting at means I’m better off with a HHS bit. Thanks.

[caskwith]

You need to check the material of the backplate. Cast iron is more common than steel and you will definitely want to use carbide then as it's tough stuff to machine.