what to do with a free scrap dumpster find ?

I did not see anything about that process when picking material. the only surface treatment i seen was this material can be carburized. as is it sits in it's current form it falls somewhere around 25 to 28 on a Rockwell C scale. it's tough material and most of the uses for it are when you want to do away with the heat treating process but still need something tough. on a #30 tapper the pull forces need to be in the 900 to 1200lbs of force range. once you go to a #40 tapper that jumps to about 2200lbs of force and when you get into a #50 tapper you are looking at close to 5000lbs of pull force.

right now with a 3 hp limit and a #30 tapper i think it should be fine as is. the yield strength is 100,000 psi plus in it's current state. i agree there are options to make it stronger i think i would rather sink any extra money in making it more accurate by getting better bearings. i found a set of two NSK 7009 angular contact P4 or ABEC7 class for $165 i still need to look around for two more deep groove bearings for the drive side.

$90 for a piece of material $200 for bearings and $40 for bearing locknuts adds up quick to get costly machine might have been free but the cost of parts adds up quick

machinedude wrote:

it's tough material and most of the uses for it are when you want to do away with the heat treating process but still need something tough.

The only reason to heat-treat would be for wear resistance. I do see some signs of fretting in my own spindle, as there is a lot of cyclic load variation. 
(I mainly see it in the taper of the vertical head adapter, which is clamped into the horizontal spindle for months at a time) 

Not all steels can be nitrided. 

i did look at that site and i did not see 1144SP listed so it might not be something that can be treated that way? 1144SP and 4140 are about 99% identical in make up. the only thing i seen as being a main difference was the high sulfur content of 1144 which makes it not very weldable which is not even a factor for my use. i had a chance to rough the shaft out over the weekend and it is actually different to work with. it cuts better at high feed rates than going slower. it is a little bit easier on carbide but still dulls an insert pretty quickly. and it cuts better dry than with my favorite go to with no flood coolant the can of WD-40 maybe a mist that is water based might work too? weird it cut better dry though

machinedude wrote:

i did look at that site and i did not see 1144SP listed 

You wouldn't, that's a UK web site and we don't (generally) use AISI steel grade codes. 

from what i could gather this process does not have to much impact on sizes to a point but when you get into tenths it can change sizes. so with bearing fits and the spindle tapper you are into those kinds of tolerances. so your back to square one. the best plan would probably be size the not important sized features and leave .015 on the bearing diameters and stock in the tapper to grind in with a tool post grinder. the tapper could be done with a make shift set up with a mounted wheel. then send it out to get carburized. the depth of surface treatment is much deeper with carburization. somewhere around .06 to .08 for a depth from what i gathered in a quick look through. carburizing would land you well into 60's on a Rockwell C scale.

but i just spent well over $200 for spindle bearing and i was broke before that soo.....

machinedude wrote:

from what i could gather this process does not have to much impact on sizes to a point but when you get into tenths it can change sizes. so with bearing fits and the spindle tapper you are into those kinds of tolerances. 

Bearing fits, yes. I would certainly be looking to mask those. But the taper could change by a few tenths, leading to the seating depth of tools changing by a thou or so. Neither here nor there, the tools themselves vary more than that. 

If I was making a spindle I think that I would nitride the taper and any seal tracks, but leave all threads and bearing seats unhardened. I would also look to harden any surfaces that see relative tool drawbar motion.
But that is purely a hypothetical consideration.