Turning and finishing a mirror finish on mild steel

Jim,

Are you sure the marks are dings and not chatter marks? I'd bet chatter. Try running your lathe at "dead slow" to reduce chatter. Try to keep sandpaper away from your lathe also.

Your not getting what is referred to as BUE are you. BUE is built up edge, and it comes from the material your cutting building up opn the cutting tools edge., Its known to drag or make marks in an overall good looking finish randomly. Try using a diamond hone to polish up your cutting tools edge a bit, and make it more slick. that sometimes helps a great deal....You can get them at Wal MArt in the sporting good section for under $6.00 or so.....

Jim, You're experiencing the "joy" of machining what must be mild steel, or low carbon steel. Without the addition of an element for free machining, that's the nature of the stuff. The only way you can get around the tearing is to take a cut that is deep enough, and also run fast enough. That requires the use of carbide, and depth may have to be more than .025" in order for the material to machine without tearing. Surface finish then comes out with a high luster instead of a dull appearance. The alternative is to run with high speed, fair amount of positive rake, with the cut well lubricated, and accept the finish that you get, polishing progressively after machining until you've removed the intermittent tearing that occurs.

If you want to avoid this condition, use other materials. The materials available for production machines avoid that problem by the addition of lead or other elements, or in the case of stressproof, the material undergoes considerable cold working, improving its properties immeasurably. It generally machines without tearing. For free machining, you can choose from several alloys of mild steel----some of which are 12L14, 1213,1215,1117, &11L17. Stressproof, while free machining, has a higher carbon content and can be heat treated. It would not be classified as a mild steel as a result, but a medium carbon steel.

If, by chance, your flywheel is cast gray iron, there has to be a problem with your tool geometry. It's not known for tearing, due to the nature of the material.

Harold

Yup, it's mild steel. I thought it was a good deal at $.60/lb, but maybe not now...

The frustrating thing is that the tool will cut beautifully for maybe a tenth of an inch and then for no reason dig in. I was hoping that there was some magical tool geometry that might help.

I don't think I can take a .025" cut and hold the tolerance that I want, but I will do some tests on a piece of scrap. I will also play with a greater positive rake and diamond stoning the tool.

Thanks for the advise.

I have the best luck with very sharp HSS toolbits, good cutting oil to prevent a buildup on the toolbit nose and a slow cutting speed of about 50. One time I used a carbide toobit at high speed with smoking hot chips and I got a almost chrome looking finish. I never could duplicate it though!

I have the same problem sometimes with CI flywheels but not as bad as with crummy steel. I use very coarse emery paper (80-100) to get the marks out followed by finer grits to get the finish. It might not be a prefect surface anymore but it doesn't matter for a flywheel.

What Harold and Chuck said, plus I'll repeat some advice I got here when I was having a similar problem: do all the dances to the Rigidity God, (Make sure you are facing the direction for the intended type of Rigidity or you'll get distracted from machining.) including minimum tool/holder overhang, snugged gibs, lock unneeded slides. Made all the difference for me.

I had a similar problem turning some smaller CRS stock with a HSS bit and it was suggested to take a deeper cut using a bit with about a 1/32" radius on the tip. That worked much better for me.

Yep. There's a minimum amount that you can take without tearing, and it's far more than one generally takes for finish cuts, especially if you're working to tight tolerance. Once you go below that minimum, tearing is guaranteed with mild steel. It's the nature of the critter, and always has been. When you work to tight tolerances you learn to turn oversized by a half thou, then polish for size and finish. 'Or, if you're fortunate to be so equipped, you leave .010"/.015" and grind. Unlike many other materials (aluminum, most of the copper alloys, even stainless steel), you can't take light cuts with any success when machining mild steel, not on a lathe. Strangely, it mills quite nicely with end mills cutting on the periphery.

Harold.

One of the "old" rules of thumb for hot rolled steel was to allow 1/16 stock removal for finished size.

Guess that was to sort of allow for what you are referring to, plus HRS has a lot of size variations, etc.

I received an email from a reader who suggested grinding a tool to take a shear cut. The way mine ended up is like a small roughing tool with high rake angles laid on it's side.

I was able to take .005" cuts and get a beautiful finish. I also tightend up the gibs and locked the axis that I wasn't using. I used a slow speed and feed, trying to make sure that I didn't heat things up and work-harden the stock.

Thanks all for the useful suggestions.

Ive found that 800 sfm and a .020 final cut depth works well, and a .003 in/rev feedrate in conjunction with a heavy oil. I'd use HSS for steel as carbide is too likely to chip without going waaaayyyy fast. Just turned a .750 rod that way and it was nearly a mirror finish. Might be different with a larger stock though

About 16 years too late for that though I guess