Facing Woes

[RV-N8ZG]

Gentlemen -
I have QCTP from LMS and Shars 1/2" indexable carbide cutters for my Jet 920.
I am working to get things set up enough to make a Pitkin donut.
In the interest of practice and check-out, I mounted a 4 x 4 x 1/2" chunk of steel bar of unknown pedigree (liberated from my daughter's welding class scrap) and took several light facing cuts with repeatable, sad results. Moderate speed (didn't write it down, maybe 400 RPM)

Facing

• RV-N8ZG
• Mar 23, 2017

Used cutting oil, tried different cutter shapes and angles, fresh inserts. No joy.
Forum posts seem to indicate that I'm turning too slow.

Mounted the 3/4" piece I intend to use for the donut (also of unknown alloy) and spun it at 1000 RPM - better, smoother finish and blue curlies, until the last inch of diameter. I'm assuming it is heating up and affecting the cut.

Tried 2000 RPM (most I can make) and got burning swarf. Didn't like that so much.

Thoughts?
Is it junk Chinese inserts, my tools not suited for carbide, or am I doing something wrong?

neal

 

[mikey]

Have you tried HSS? I suspect it would work much better for you.

Which insert are you using and what is the nose radius? It would also help to know what kind of steel that is; some steels don't like to finish nicely. 1018 is notorious for this, although it can be faced fairly well with HSS. A36 is also a common plate steel and it can be fussy about finishing. Again, HSS usually produces a finer finish, at least for me.

 

[Bob Korves]

The surface speed is changing with the diameter. It is fast at the beginning of the cut at full diameter, and becomes less and less as you cut. At the center the speed is zero. That is just a fact of life of facing operations. No cutter is going to like that whole wide range of speeds.

 

[petertha]

What Bob said on diameter change when disc is large. You may have alter the setting & some interim diameter. Sometimes steel is just sh*tty stuff, facing or turning, particularly cold rolled plate. What I find helps is lock the carriage down, take the equivalent of a spring pass but select a different feed to alter the track. Generally larger radius tool for finishing pass but this has all sorts of caveats. Your Chinese inserts may not be bad for the application but examine the tip under microscope for any partial chipping or degradation. For example if you used the same tool across interrupted cuts. But if your same setup can finish peachy in say aluminum, the issue may be the steel alloy itself.

 

[RV-N8ZG]

That all make sense, Bob.
What puzzles me is there is a definite point where the color and finish changed, not a gradual shift.
Unfortunately, I didn't get a photo of the second plate.



I tried both of these tools at different angles.

 

[talvare]

My experience has been that obtaining nice surface finishes with carbide inserts usually requires heavy cuts. When making very light cuts, I get better finishes using HSS. Some lathes don't have the power and /or the rigidity for making heavy cuts.
Ted

 

[Bob Korves]

The general rule of thumb with inserts is to bury the entire tip radius of the insert into the work to get a good finish. Shallow final passes are usually a problem. An example: From .060" out, take two .020 passes, measuring before and after each cut. Assuming it came out .020 smaller each time, dial in .020 again and take it to finished size. If you are getting more or less than .020 on the test passes, adjust a bit for the trend, and make the final cut. I suppose we all do it, but trying to sneak up on your number, with, say, cuts of .030, .015, .008, .005, and .002 to get the final .060 off usually ends with a poor surface finish and a final cut that is not to the desired size. That is often even more so when using inserts, and when using materials that work harden, like 4130 and many others. Soft materials can be just as bad, tearing the metal on light cuts. Get some practice, get bold, and make that final pass like you mean it! The job will also be done quicker, with less fooling around.

 

[Silverbullet]

No doubt your materials work hardening , mystery steel with unknown make up. Sounds like the more you cut the harder it is to cut. I would guess the faster speed don't help? Try a bit faster feed with lots of coolant , sometimes you have to out cut the hardening almost force the cut. Even then it will get so hard it will weld to the tool. Some of those new ceramic cutter inserts may help but I doubt it .

 

[4GSR]

That looks like A-36 plate. Very difficult to get a good decent smooth cut on. Back off your RPM's to around 400-600 RPM. Face from the smaller diameter outward on the last few (0.010") thousandths. Always work for me, most of the time. HSS helps and probably give you the best results as others have said.