Mirror finish - what's going on?

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Nick Hacking

H-M Supporter - Gold Member
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I don't know a great deal about the metal on my shelves: where I've been able to identify it, I've written on it but a lot of it is fairly anonymous. The colours that are sometimes applied to the ends of the bars don't really help: different manufacturers seem to use different colour codes.

Anyway, I need a short bar with an M14 thread on the end, so I picked up one of the anonymous rounds and popped it into my Myford. It's a small lathe with a maximum speed of about 500 rpm.

I have some of those cheap cutting tools with a piece of carbide brazed on to the end. Since they need using up I thought I'd use one to get the work-piece down to the nominal diameter. It might save using an insert for an indexed tool.

Well, the facing cut was really juddery even with very light cuts. I checked the tool height several times and made sure that the little lathe was running at top speed.

The first HSS centre drill snapped off. The second one made a hole, eventually, after I'd re-faced the work. The steel seemed to be very tough.

Now, turning with a live centre, there was less chatter but the finish was dreadful, rough as a bear's backside. I tried taking cuts of .001" but the finish still looked awful. Then, I inadvertently turned my cross-slide too far, about .005" - and I got the most incredible finish. It looks absolutely beautiful. I tried again at .002" - rubbish - and then .005" - perfection again.

I guess (I don't know) from the bright appearance of the work, now that it is cleaned up, that I'm dealing with something like chromoly steel which (I've heard) is not the easiest material to machine. Why the heavier cut gave better results baffles me. All I can think is that with very light cuts the tool can skite around on the surface but the slightly heavier cuts are somehow more rigid. I'm speculating.


Can anyone explain what I've encountered? Apart from my (random) choice of steel, am I doing something obviously wrong?

I wish I could a get a finish like this every time!

Kind wishes,

Nick
 
Using a cutting fluid might help on the finish cuts, and be sure that the tool is sharp with a slight radius at the point.
 
Thank you.

I've been told that carbide doesn't like intermittent cooling so, presumably, I'd have to have a constant flow of cutting fluid to avoid thermal shock? Or am I wrong? I really know next to nothing which is why I'm posting in this forum.

Kind wishes,

Nick
 
I suggest a cutting oil, brushed on lightly, I use TapMagic, it's not smelly and does not make much smoke or stain things like sulfur based oils are prone to do.
 
Carbide likes speed or lacking that deep cuts, but only sometimes. Maybe and maybe not, depending on the moon phase or something weird like that. Carbide while it holds an edge, isn't usually as sharp as HSS can be. It is a different animal with different needs of speed and DOC.
 
A lot of the cheap brazed carbide bits need to be ground because they have no front clearance, thus they rub and won't take a fine cut. If you add a bit of front and side clearance you can take fine cuts. But the rubbing does make a nice shiny finish if you get aggressive enough with it.
 
In addition, those chinesium brazed inserts are not designed to cut right out of the box, they need to be ground for the application.
The geometry is usually off as well. Ask me how I know.
I agree on the cutting speeds, carbide likes RPMs and a deeper cut.
500 RPM? that sounds like a job for HSS, IMHO.
 
In my opinion, what you are experiencing is deflection. It isn't the speed so much as it is the fact that carbide needs a minimum depth of cut to even cut. This is especially true if the tool has any kind of nose radius. Once you get past the minimum depth of cut the tool will usually cut reliably. If using brazed carbide, sharpen it as the others suggested and keep any nose radius small. Then experiment to see what the minimum depth of cut for that tool is and write it down somewhere.

If you want to take micro-cuts, switch to HSS.
 
Probably working with something like chromoly steel or a steel that work hardens, and you need to take off more aggressive cuts. I had this with some 4140 3" stock that I was turning, my DOC was 0.080" but the feed was 0.003 IPR and the finish was poor and I also were getting continuous strings of long chips. I dialed up the feed to 0.007-0.008 IPR midway and the finish significantly improved and the strings broke into smaller pieces. Probably needed to push the feed a bit more and have a bit more radius on the nose. Also the relief angle if you are grinding it.
291951
 
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