Distortion Of 4130 Round Stock?

[fvdbergh]

I have ordered some cast iron for the backplate, and 4130 condition T for the ER20 collet chuck --- I'll post some pictures and feedback when I start with the actual machining. Incidentally, the cast iron is only about 5% more expensive (by volume) than the 4130 in condition T from my supplier.

In the meantime, I have come across an interesting statement in Guy Lautard's "The Machinist's Bedside Reader". In my copy, the article is called "Cold rolled steel - what it is, and how to stress relieve it", which appears on page 23. A quote from this article:

"I said above that 'Cold Rolled Steel' is somewhat of a misnomer. A better term is 'cold finished steel'. The sheet forms are about the only ones that are actually rolled. Square, hex, round and flat sections are made by being lubricated and drawn through dies, after the black scale has been removed, probably by pickling.

As you might well deduce, the surface layer of CRS is stressed by the drawing operation.

If you chuck a piece of 3/8 (inch diameter) CRS and machine it down to 5/16, the stressed surface layer is removed all around the material, and the piece does not warp, because there is no unbalanced stress."​

The article goes on to mention that milling a flat on a CRS bar will result in unbalanced stress, and subsequent warping, requiring a stress relieving treatment.

The salient points:
1. Even if I used cold-drawn steel without any subsequent stress relief, it should not distort (much?), because I will end up removing the stressed skin all around anyway (unlike starting with plate).
2. The condition T material will be stressed isotropically (I hope) after the heat, quench and temper cycle.

 

[fvdbergh]

It took a while to get the stock. Well, it would probably only have taken a week or so, but my supplier forgot to let me know that the stock arrived. Sigh.

First, I made a copy of my spindle using a piece of EN19 in condition T:

I don't think I will ever get all the blue off my faceplate's taper ...

The upshot of the delay in receiving the stock is that I had time to complete my small heat-treatment furnace build. I decided the stress-relieve the cast iron stock for the backplate at 560C for two hours, followed by cooling in the furnace overnight:

Not sure if this was necessary, or even if it helped.

I completed the D1-4 side of the plate first. This proved more time-consuming than I expected, because I overshot the taper diameter (wide end) and had to face back the other surface until I could just fit about a 20 micron thickness of aluminium foil between the back face of the backplate, and the mating surface on the spindle nose. Still, it was worth it, because I ended up with a nice "draw", requiring a swift rap with a mallet to break the taper loose. Here is what the backplate looked like at this point:


Here things got interesting. I installed the partial back plate on the spindle, and proceeded to turn the OD, and face the front part. Then I cut my 20 mm deep 8-degree taper (to accept the collet chuck part illustrated in an earlier post). Unfortunately, I managed to remove my registration mark on the OD, so after I unmounted the back plate, I had no way to check whether I remounted it in the same orientation. So, either all the orientations were the same, or I can simply use a DTI to check which orientation is "best", right?
Well, not any of the three possible orientations seemed to fit at all. I was seeing run-out of 100 micron (4 thou) on the OD in some orientations. The absolute best I could get was about 60 micron TIR. I tried to "indicate in" the D1-4 mounting process by running the DTI on the front face, and carefully tightening the cams to minimise run-out on the face. This worked really well, and I could bring the face to within 10 micron of running true, producing a similar run-out on the OD. Subsequent snugging up of all three cams immediately produced a 60 micron run-out.

So what happened here? Did the cast iron "move on me", or did I simply not mount the backplate properly just before I began cutting?

Fortunately, after ensuring that the cams were snugged equally, I recut the OD, face and 8-degree taper by removing about 75 microns of stock. Now I can re-mount the backplate as many times as I want, and it repeats as intended. Here is a shot showing the high-spot of a TIR of 6 micron (but it looks less because of the parallax error):


I am content with 6 microns of run-out. But I do wonder: was this user error, or an example of cast iron distorting?