Grinding a tool for 316 SS

[AllenH59]

I know tool grinding has been discussed ad infinitem.. However this was my idea and it worked like a charm. I ground a left hand cutter to have a bit of front relief and some leading edge relief, and did not grind the top. Then I cut a chip breaker at the edge of the top with a zip cutter in my dremmel. I cut the chip breaker close enough to the left edge that it made a sharp edge with a high rake angle when cutting to the left. It cut very well. My lathe will go as low as .0015" per turn, I tried it at .003" per turn and it was almost bright, with no tearing. I had mostly abandoned HSS tooling for carbide inserts, but this with the coolant running gave excellent cut and excellent life.

[Harold_V]

It's not secret that 300 series stainless alloys respond quite nicely to positive rake. It's tough (due to the high nickel content) so the cut benefits by the extreme rake angle, which lowers cutting pressure. If you balance the relief with the rake angle, the tool will remove metal quite well, albeit with light feed. With excessive relief, the tool may hog (self feed), however, so your attention to that detail was a very good move. You'll also find that the surface speed can be somewhat faster than you had considered, especially if you keep the cut well lubed.

I am confounded by the resistance by many to use HSS, especially on small machines.

For the record, all of the 300 series alloys will cut bright unless you have a badly flawed or dull tool. If the tip fails, the cut will often have tiny spires projecting from the surface. Keep a sharp tool and the finish is quite nice. The free machining grades tend to cut with a slightly dull surface, but it is generally a nice finish, just not as shiny as the other grades.

H

[Steggy]

For the record, all of the 300 series alloys will cut bright unless you have a badly flawed or dull tool. If the tip fails, the cut will often have tiny spires projecting from the surface. Keep a sharp tool and the finish is quite nice. The free machining grades tend to cut with a slightly dull surface, but it is generally a nice finish, just not as shiny as the other grades.

Depending on what I was doing, I would use HSS or carbide with the stainless alloys. Carbide seems to work better on the straight chromium alloys than on the nickel-chromium (3xx) steels. The "free-machining" alloys seem to always come out better with HSS tools.

I was taught to never machine any type of SS dry. In the past when I used to make race car parts, I used a lot of 440C SS for linkages and such. All turning, facing and threading was done with a continuous application of my "secret sauce" cutting fluid (a 90/10 mixture of petroleum-based cutting oil and mineral spirits). Finishes looked as though the piece had been ground.

[Greg_Lewis]

...
I am confounded by the resistance by many to use HSS, especially on small machines.
...

My Atlas 12-inch (a.k.a. the Flexible Flyer) with it's 1/2-horse motor does not like carbide. I just ruined a $30 carbide spotting drill when the tailstock spindle wiggled an imperceptible amount as I was trying to spot a piece in the chuck. I don't know why I bought carbide instead of HSS. Chronic stupidity, I guess. Almost every time I've tried carbide I've been unhappy. The machine just doesn't have the mass, the rigidity, or the power to run it. OTOH I've had great results with properly sharpened, good quality alloy, HSS.

[Harold_V]

I'm not surprised, Greg. That's typical of how carbide responds when not applied under the best of conditions. They're particularly bad on most engine lathes, as the tailstock is rarely in true alignment with the spindle. The spotting drill is not pointed at the center, but at a small area beyond center, so side pressure is applied to the tip. Given enough time for the spotting drill to seek center, it most likely would be a little more forgiving.

The same problem is common with small center drills. To circumvent the issue, I grip them by only a tiny bit of their length, and tighten the chuck only by hand. That way the tip can easily seek center. Once found, light pressure is applied to get the center drill started. I keep the center drill well lubricated to avoid chip packing in the straight flutes.

Folks, today, have it good, as carbide has improved immeasurably. If one goes back to the infancy of carbide, it was dreadful. Light duty machines couldn't use it under almost any circumstance, as it would chip just by looking at it too hard!

I'm the first to confess that carbide has a place on even light duty, flimsy machines. It can serve quite nicely for cutting abrasive or overly hard materials, but for general machining, it's a true waste of time and money. A properly ground HSS tool will generally outperform carbide, assuming it is applied in keeping with good shop practice.

I can't help but wonder how many use insert carbide in an attempt to eliminate the learning curve of grinding HSS tools. If they only knew the injustice they are doing to themselves. If one can grind an acceptable tool, one is also able to determine what might be wrong with a tool that does not perform well. It's all about addressing the basics.

H

[Greg_Lewis]

Words of wisdom from Harold. I've learned the hard way that a quick-and-dirty tool grind is just that. Back in the 1950s a guy named St. Clair wrote a whole book on lathe cutting tools, with a whole chapter on each angle of the tool. (Design and Use of Cutting Tools; Leo St. Clair*) The difference between a properly ground tool and one that is just slapped out is noticeable.

There were two times when carbide worked well for me. Once when drilling 1/8-inch holes in spring steel for my locomotive. I made a fixture to hold both the work and guide the drill. I used a solid carbide spade drill and got several dozen holes drilled before I had to resharpen the drill. The second time was when I found some iron discs at a scrapyard for cheap. I was warned that they were chilled iron and I've never seen anything that hard. The HSS didn't last 60 seconds but the carbide managed to cut it (just barely). But I did chip out some toolbits on the Flexible Flyer before it was done.


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*https://openlibrary.org/books/OL2100258 ... ting_tools.

[AllenH59]

My lathe weighs about 2400 lbs and is 3 hp.. I restrict carbide use to neutral or positive rake, it is not powerful enough to cut with a negative rake, at speeds required for carbide. I have 4 different carbide systems, and generally try them. I have a good selection of HSS tool bits, from 1/8 to 3/8", but lately I have bought a couple 6" long 1/2" square tool bits, I think they are for a shaper. They are long enough that I can sharpen both ends, and they will last many (like hundreds) of sharpenings. They are of course very rigid, and will handle quite a bit of heat. Yes, they cost a bit of money, but I doubt any of us do enough work to wear out 4... ever...

[Harold_V]

Allen,
My lathe is a 12" Graziano, with a natural gap, so it is capable of turning about 18" (in the gap). It is powered by a three horse motor, and weighs about what your lathe does. I truly believe you have enough power and rigidity to use negative rake, and it's an absolute wonder when it comes to roughing. I'd encourage you to try. Works quite well with steel, including chrome moly, and even with free machining stainless. The other grades, not so well. They induce so much heat that the tip tends to fail prematurely. A cut of .300"/.350" off the diameter is not unreasonable with a feed rate of .012". You'd be amazed at how fast you can remove material with that geometry.

I don't use anything special. My holder accommodates a TNMG 322 insert, which is rather inexpensive and commonly available.

H

[AllenH59]

Harold, my lathe is 16x48, it will swing 23 in the gap... I have never had the gap out in 20 years... and 3 hp.. I will give it a try...