Rough Finish on 1" Hot Rolled Steel

J

JRT

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Any recommendations of things to try here in regards to turning a better finish? As I was advised in a post about some different material, it's possible that the tool tip is not rounded enough but I don't yet have the knowlede of making one.

current set-up:
- lead screw turning at .0024, as slow as it can turn
- spindle is at 380 RPM.
- steel is a 1" hot rolled bar I had laying around
- I'm using a new HSS insert with some cutting fluid

Maybe it's just meant to be with this particular steel... I'm new at learning feeds and speeds so it's probably also operator error. I'm just looking for any recommendations to try. I did try cutting it at 540 RPM to no improvement. Should I try upping the spindle speed even more? The higher the speed, the more scared I get! Happy Friday! -John



IMG-9458.jpg
 
Some types of steel just turn rough. The finish can be improved somewhat by going with a cutter that has a larger radius at the tip, but that will increase the power needed to make the cut.

I made a vertical shear bit for situations where I want a better finish on stuff like this. It definitely improves the finish but is limited to very shallow DOC's. There have been a number of discussion threads regarding this type of cutter if you're curious.

On the theory that a very large radius would work, I've been wanting to make a tangential style toolholder that accepts round HSS bits but haven't gotten around to it yet. I have a small benchtop lathe with limited power so it definitely would be limited to small DOCs.
 
Some of that is due to the steel and some is due to the cutting tool
A tool with more rake and relief would do better- with a keen, slightly rounded edge
Kennametal CCGT3251HP series carbide inserts work very well on small lathes:
KC730 for ferrous
KC5410 for non-ferrous
 
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Hot rolled is very gummy. +1 on using a vertical shear tool for the final cut to really clean things up. The DOC for a shear tool is 0.0005 to 0.001" only, but it really cleans up a finish like yours.

Here's what my shear tool looks like:

FRONT:SHEAR1.jpg
TOP:
SHEAR2.jpg

WORK FACE:
SHEAR3.jpg

Notice that only two sides of the HSS shank are ground - the rest don't matter. The grooves on the face and front are only there to make honing the bit less tedious. Properly set up, the swarf coming off the tool will look like angel hair (it will even float in the air).
 
I tried with a carbide insert at 835 RPM and it was similar.

I slowed it down to 125 RPM with carbide insert and while it took forever, there is a noticeable difference in shine but overall it’s still not good.

Next I’m going to try 125 & 82 with HSS insert and then I’m going to chalk it up to “gummy” steel. Wish I had something else to turn of quality steel.

What alloy would you all get? I’m trying to make a mill vise stop that attaches to the rear of my mills vise. I know there are easier ways to go about this for sure but none of them will “punish” me quite like this idea that I have!

…and for what it’s worth, I’m turning this on an Atlas 618. Would a larger HP motor help with DOC? It currently bogs down pretty easily.

-John
 
Just a thought but are you using the same insert from start to finish? One thing about hot rolled is the hard mill scale skin will take the edge off a fresh tool mighty quick and then you get crap after that. I'll often knock the scale off first with a carbide insert and then switch to a sharp HSS tool for the real turning to dimension once the skin is gone.

No reason you shouldn't be able to do this on a 618 -- that's what I use all the time and if you stay within a sane depth of cut and feed speed you can do fine. It just takes a bit longer.

Edit: BTW that's about what I would expect from hot rolled in terms of a general finish. Sometimes you get lucky and it finishes up nicer but a lot of the time for me it's kind of ropey like that.

-frank
 
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